SI Analytics Ph Handbook

pH handbook HELPFUL GUIDE FOR PRACTICAL APPLICATION AND USE Welcome to SI Analytics!

We express our core competence, namely the production of analytical instruments, with our company name SI Analytics. SI also stands for the main products of our company: sensors and instruments.

As part of the history of SCHOTT® AG, SI Analytics has more than 75 years experience in glass technology and in the development of analytical equipment. As always, our products are manufactured in Mainz with a high level of innovation and quality. Our electrodes, titrators and capillary viscometers will continue to be the right tools in any location where expertise in analytical measurement technology is required.

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We look forward to presenting the pH handbook to you!

The previous publication "Interesting facts about pH measurement" has been restructured, made clearer and more engaging, and extra information has been added.

The focus has been consciously put on linking general information with our lab findings and making this accessible to you in a practical format. Complemented by the reference to our range of products and with practical recommendations for use with specific applications, the pH primer is an indispensable accompaniment for everyday lab work.

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SI Analytics GmbH

Dr. Robert Reining CEO CONTENTS

SECTION 1 Basic concepts of potentiometry and pH measurement 1.1 Definition of acid and base ...... 7 1.2 Definition of pH value...... 10 1.3 Electrochemical potential...... 13 1.4 Nernst equation...... 17 1.5 Methods of pH measurement ...... 19 1.6 Summary...... 22

SECTION 2 Structure of pH electrodes 2.1 The glass electrode...... 23 2.2 The reference electrode...... 29 2.3 Combined measurement chains...... 38 2.4 Summary...... 40

SECTION 3 Potentiometry of the pH electrode 3.1 Potentials of the pH measurement chain...... 41 3.2 Characteristic curves of the pH measurement chain...... 45 3.3 Summary...... 47

SECTION 4 Buffer solutions and safety in pH measurement 4.1 Definition of buffers ...... 48 4.2 Standard buffers...... 50 4.3 Calibration...... 53 4.4 Working with buffer solutions...... 56 4.5 Measurement uncertainty...... 59 4.6 Summary...... 62

SECTION 5 Measuring equipment and measuring set-up 5.1 Function of the pH meter ...... 63 5.2 The measuring circuit...... 66 5.3 Summary...... 69

SECTION 6 Practical pH measurement 6.1 pH measurement in various applications...... 70 6.2 pH measurement in an organic solution...... 76 6.3 Summary...... 86

SECTION 7 Recommendations for use, maintenance and care of electrodes 7.1 Electrode recommendations...... 87 7.2 Maintenance and care of the electrodes...... 91 7.3 Summary ...... 93

Index of technical terms Bibliography pH handbook SECTION 1 The natural acid level of the skin lies somewhere between pH 4.2 BASIC CONCEPTS OF - 6.7, and plays an important role POTENTIOMETRY AND in the manufacturing of soaps. Fresh milk has a pH value of 6.6 - pH MEASUREMENT 6.8. If the pH value falls to 4.7, it becomes sour and clotting be- This section explains basic con- gins. In cheeses, the pH value in cepts and contexts. The pH value the first few hours determines it´s plays a significant role in many firmness, color and flavor. Bread areas of daily life. For food, the dough manufacturing only rises properly at low pH values, as CO association of certain properties 2 such as taste (acid = fresh, neu- will only form under this condi- tral = bland, alkaline = inedible) tion. and shelf life (the reproduction of harmful bacteria) depend on the Figure 1 contains a diagram pH value. showing various examples of the pH values of well-known, everyday items compared to an acid and a base.

acid neutral alkaline

0 1,1 2,4 4 6,7 7 7,4 9 12,5 13,2 14

cola milk suds limewater HCl blood NaOH 0,1 mol/l wine 0,2 mol/l

Fig. 1 pH values of a range of goods compared to an acid and a base

6 1.1 Definition of acid and base Whether an aqueous solution However, because the concentr- reacts as an acid or a base de- ation of oxonium ions corre- pends on its hydrogen ion con- sponds to the concentration of tent. Even chemically pure, neu- hydrogen ions, in many cases the tral water contains hydrogen ions, first equation can be used. because some of the water molecules are always dissociated: Ionic product of water HOH H+ + OH- The equilibrium of dissociation of water under standard conditions In chemical terms, the positive is on the far left side. In 1 l of pure, H+ ion, the "proton", is usually re- neutral water at 105 Pa and 25 °C ferred to as the "hydrogen ion". there are only 10-7 mol H+- and The negative OH- ion was previ- 10-7 mol OH- ions. In accordance ously known as the "hydroxyl ion", with the law of mass action, the but now the term "hydroxide ion" equilibrium constant KD may be is prescribed internationally. [1] formulated for the dissociation: [H+]·[OH-] Strictly speaking, the hydrogen KD = ion in aqueous solution is not [H2O] present as a free proton, but is hydrated by at least one water The virtually constant concen- molecule. The equation for the tration of undissociated water dissociation of the water is ac- H2O as a result of its extreme- cordingly: ly low dissociation is combined

with KD of the dissociation con- 2H O H O+ + OH- 2 3 stants to give KW the "ionic product": The hydrated proton was previously known as a "hydronium KW = KD · [H2O] ion". Today the term "oxonium ion" is prescribed. [1] + - -7 KW = [H ] · [OH ] = 10 mol/l · 10-7mol/l = 10-14 (mol/l)2

7 pH handbook The ionic product of water An acetic acid solution there- changes with the temperature. fore contains significantly fewer This is because, like all equilibri- hydrogen ions than a hydro- um constants, the dissociation chloric acid solution of the same constant is also dependent on molarity. temperature. The ionic product HCl + H O H O+ + Cl- of water at 0 °C for example is 2 3 0.11·10-14 [mol/l]2, but at 100 °C -14 2 + it is 54.0·10 [mol/l] . CH3COOH + H2O H3O + - CH3COO Instead of 1·10-7mol/l at 25 °C, Some acids can release more the concentration of hydrogen than one hydrogen ion per mole- ions at 0 °C is therefore only cule. Depending on the number 0.34 ·10-7 mol/l, but at 100 °C it is of hydrogen ions which can 7.4 · 10-7 mol/l. This temperature be released, acids are classified dependency must also be taken as either monovalent, bivalent, into account in measuring the pH. trivalent etc. Hydrochloric acid (HCl) for example is monovalent, Acids and Bases sulphuric acid (H2SO4) is bivalent Acids are substances which re- and phosphoric acid (H3PO4) is lease hydrogen ions in aqueous trivalent: solution. Acidic solutions there- + - fore contain more hydrogen ions H3PO4 + H2O H3O + H2PO4 than neutral water. Depending - + 2- on their effect, a distinction is H2PO4 + H2O H3O + HPO4 made between strong and weak acids. Hydrochloric acid, for HPO 2- + H O H O+ + PO 3- example, is a strong acid, as HCl 4 2 3 4 dissociates almost completely in In order to take account of the water, thereby releasing a large valency of the acids, their con- number of hydrogen ions. Acetic centration is not expressed as acid (CH3COOH) for example, is molarity, but as a ratio of valency a weak acid, as it only dissociates and molarity referred to as the partially when dissolved in water. normality. So 1n HCl is 1.0 molar

and 1n H2SO4 0.5 molar.

8 The dissociation constants (KD) Basic solutions were tradition- for the different stages of disso- ally known as "alkalis". Today ciation of a multivalent acid are however the umbrella term is often markedly different. This "base". Even the characterization varying level of dissociation at of basic solutions as "alkaline" is the different stages plays an misleading. After all, it is not just important role, for example in the oxides and hydroxides of the the use of phosphoric acid and alkaline metals which have the phosphates in buffer solutions. property of forming basic solutions. Bases are substances which accept hydrogen ions. When Aqueous solutions are acidic if bases are dissolved in water, they they contain more than 10-7mol/l bind to some of the hydrogen of hydrogen ions at 25 °C, they ions from the dissociation of the are basic, if they contain fewer water. Basic solutions therefore than 10-7mol/l of hydrogen ions. contain fewer hydrogen ions than neutral water. According- Acids and bases neutralize each ly, the concentration of hydroxyl other, resulting in the formation ions in basic solutions is great- of water and salt. [2] er than in neutral water. As with acids, a distinction is made be- For example, hydrochloric acid tween strong and weak bases. and sodium hydroxide So for example sodium hydroxide H+ + Cl- + Na+ + OH- H O + (NaOH) is a strong base, while 2 NaCl ammonia (NH3) is a weak base. Consideration must be given to whether strong acids are being NaOH + H O+ 2H O + Na+ 3 2 neutralized with strong bases, weak bases with strong acids NH + H O+ H O + NH + 3 3 2 4 or weak acids with strong bases. In the first instance, e.g. in the neutralization of hydrochloric acid with sodium hydroxide, neutral table salt is formed (NaCl).

9 pH handbook If a salt like this, which has been 1.2 Definition of pH formed from a strong acid and value base is dissolved in water, the solution reacts neutrally. On the The concentration of hydrogen other hand, if the salt of a strong ions in an aqueous solution is a base and a weak acid, such as measure of how acidic or basic it is. Accordingly, a scale can be sodium carbonate (Na2CO3) is dissolved, the resulting soluti- created for acidity which starts on is basic. If the salt of a strong with the concentration of hydro- acid and a weak base, such as gen ions of 1 mol/l and ends with 10-14 mol/l (Fig. 2). The endpoints ammonium chloride (NH4Cl) is dissolved, the resulting solution of the scale correspond on one is acidic. side to the ideal solution of a 100% dissociated 1N acid and on - - Na2CO3+ H2O OH + HCO 3 the other side the ideal solution + 2Na+ of a 100% dissociated 1N base.

+ - NH4Cl + H2O H3O + NH3 + Cl

pH H+ Activity OH- activity 0 1.E+00 1 0.00000000000001 1 1.E-01 0.1 0.0000000000001 2 1.E-02 0.01 0.000000000001 acid 3 1.E-03 0.001 0.00000000001 4 1.E-04 0.0001 0.0000000001 5 1.E-05 0.00001 0.000000001 6 1.E-06 0.000001 0.00000001 neutral 7 1.E-07 0.0000001 0.0000001 8 1.E-08 0.00000001 0.000001 9 1.E-09 0.000000001 0.00001 10 1.E-10 0.0000000001 0.0001 base 11 1.E-11 0.00000000001 0.001 12 1.E-12 0.000000000001 0.01 13 1.E-13 0.0000000000001 0.1 14 1.E-14 0.00000000000001 1

Fig. 2 Table of H+ / OH - activity depending on pH value

10 However, concentration is not Therefore instead of concentra- used as the measure of acidity. tion [H+], the activity of the hydro- A logarithmic scale, the "pH value " gen ions is measured. The activity is used. The pH value is directly comprises an individual activity proportional to the negative log coefficient (f) and the concentra- base 10 of the hydrogen ion con- tion together. centration. (The term "pH" comes from Latin and is an acronym for a += f·[H+] "potentia hydrogenii" - the power H of hydrogen.) The pH value measured in pH ~ -lg [H+] practice is therefore the negative log base 10 of the hydrogen ion In practice that means that a activity. change in the concentration of pH = -lg a + hydrogen ions by a factor of 10 H creates a change of 1.0 on the pH scale. Conventional pH values Because of the difference be- Concentration and activity tween concentration and activity, Dissolved ions exert electrical the measurement of pH values forces as charge carriers on cannot be directly based on the medium surrounding them. the concentration of hydrogen While the solution is electrically ions in solutions. Conversely, it neutral on a macroscopic scale, is not possible to achieve an on the micro scale the effects absolute calibration of the pH can be drastic. The mobility of scale against the concentration the ions is limited by the recip- of hydrogen ions. Such a calibra- rocal influence, meaning that tion would always be an approxi- significant deviations from ideal mation only. behavior may occur. So as to take account of this fact, the activity Therefore the pH measurement of the ions must be considered, in practice is based on a conven- rather than the concentration. tional pH scale.

11 pH handbook The pH values measured in The temperature dependence practice relate to a series of stan- is specific to each probe and dard buffer solutions created non-linear, so it is not possible to by the NBS (National Bureau of convert the pH value from one Standards) and adopted by the temperature to another. Deutsche Institut für Normung (DIN). Temperature dependence of the pH value Conventional pH values are therefore measured in com- The neutral point of the scale is parison to the pH values of pH 7.00 at 25 °C. The dissociation these standard buffer solutions. equilibrium and ionic product Provided that calibration and of water are temperature measurement is done carefully, dependent and therefore the this makes all pH values compa- neutral point is greater or less rable, regardless of the probe or than 7.00 depending on the measuring equipment used to temperature. A measurement record them. result of e.g. pH 6.82 at 50 °C in no way means that the solution In determining the pH value of is acidic. The temperature aqueous solutions, the reciprocal dependence of the pH value influence of the ions must not be of acidic and basic solutions ignored. A distinction must be differs from that of neutral water, made between the effective and as the activity is temperature therefore measurable concentra- dependent. tion and the nominal concentration. This effective concentration Strongly acidic solutions display is the activity. The pH value is virtually no temperature depen- therefore the log base 10 of the dence, as in acidic solutions the hydrogen ion activity. concentration of hydrogen ions is no longer determined by the All pH values are temperature auto-dissociation of water, but by dependent, so a comparison is the dissociation of the acid. only permissible if the temperatures are the same or similar.

12 The change in the dissociation of This provides no information as the acid with the temperature is to whether the pH value of the so slight that it is not recorded in reaction mixture was at room the pH measurement. temperature before the start of the reaction or even if the pH On the other hand, the pH value value during the reaction was at of basic solutions is quite strong- 80 °C. ly temperature dependent. This is because in basic solutions 1.3 Electrochemical the hydrogen ion activity is determined by the temperature- potential dependent ionic product, i.e. the Electrochemical potential is the auto-dissociation of water. The basic measurement value on pH value of basic solutions gen- which pH measurement is based. erally decreases noticeably with In the next section, the theoreti- increasing temperature (Fig. 3). cal principles will be explored in further detail. pH value and temperature reading Chemical reactions take place in accordance with Comparing pH values without the laws of thermodynamics. simultaneously stating the mea- Thermodynamics describes the suring temperatures is virtually relationships between different meaningless. A synthesis such as forms of energy and can be "The reaction occurs at a pHvalue relied upon to answer the of 10.50 +/- 0.25 with a satisfac- question of whether a reaction is tory yield", is also of little use. energetically favorable, in other words whether it can proceed in terms of energy.

temperature 0 °C 25 °C 50 °C In this respect the decisive pH of water 7.47 7.00 6.63 factor is the Gibbs energy G, pH of 0.001n HCl 3.00 3.00 3.00 also known as free enthalpy. For pH of 0.001n NaOH 11.94 11.00 10.26 changes ΔG during a reaction the following applies: Fig. 3 pH value of three solutions depending on temperature

13 pH handbook • ΔG < 0: an exergonic reaction, When considering mid-range which under the specified condi- concentrations, the activities tions (concentrations) proceeds must be used instead of the con- spontaneously; centrations.

ΔG = n · R · T · ln (a /a ) • ΔG = 0: Equilibrium, no reacti- 2 1 on; The differential change in free enthalpy according to the amount • ΔG > 0: endergonic reaction, of substance under constant which would require energy pressure, constant temperature to be added to proceed in the and constant concentration is specified direction. called chemical potential. Chem- ical reactions can only proceed The change in free enthalpy of if they are associated with a one mole of a substance during change in chemical potential. transition from one concentrati- Absolute values for chemical on to another concentration can potentials are unknown, and be compared to the compressi- so it is only possible to observe on of an ideal gas from one pres- changes in relation to an initial sure p1 to another pressure p2. state. [2] [3 [4] This process can be described with the following equation: If a transition occurs from un- charged particles to ions, this can be compared with the functions ΔG = n · R · T · ln (p /p ) 2 1 of an electrode. The electrode In very dilute solutions, the dis- system is a two-phase system solved particles behave like tho- (metal/solution) in which positive se of an ideal gas, so that a simil- ions from the metal immersed ar formula can be created for the in the solution can be dissolved. transition from one concentrati- In accordance with the number of ions dissolved, the metal be- on c1 to another concentration c2: comes negatively charged. The ΔG = n · R · T · ln (c /c ) 2 1 result is an electrical potential between the negatively charged metal and the positive solution.

14 The solution process continues If a metal is in contact with an until the electrical potential is so aqueous salt solution, this com- great that no further metal atoms bination is therefore an "elec- are released. Strictly speaking, trode". The aqueous salt solution equilibrium is then achieved: is an "electrolyte". The electrode For each unit of time, exactly as metal is a conductor of first order many metal atoms are dissolved and the electrolyte a conductor as ions on the one hand as ions of second order. If the metal is in are discharged and deposited contact with a solution of its own again as metal atoms on the oth- ions, an electrical potential (u) is er hand. This state of equilibrium formed, the so-called "galvanic corresponds to a potential of a voltage". quite specific voltage, which is typical for the metal and ion con- Electrodes of the first kind centration in question. Metal which is immersed in the solution of its own salt is referred A metal which is in contact with to as an electrode of the first a solution of its own ions is a kind. For example a silver wire reduction / oxidation system (Ag) which is immersed in a solu- ("redox" system for short). The tion of silver nitrate (AgNO3), is a donation of electrons by a typical electrode of the first kind. metal atom corresponds to an It is referred to as an Ag/AnNO3 oxidation; the acceptance of electrode (Fig. 4). an electron by a metal ion is a reduction:

Oxidation Ag Me Me+ + e-

Reduction

AgNO - solution 3

Fig. 4 Electrode of the first kind

15 pH handbook Electrodes of the second kind Measuring and reference With electrodes of the second electrodes kind, a metal coated with a slight- Electrodes of the second kind ly soluble metal salt is immersed generally display low polarizabili- in an aqueous solution, which ty. Their potential is very constant, contains a readily soluble, chem- because the anion concentration ically inert salt with the same can be kept constant quite easily. anion. In addition, the solution Electrodes of the second kind also contains the same slightly are therefore widely used as "ref- soluble metal salt as a precipitate erence electrodes" in potentio- (Fig. 5). metric measurements. If however a metal is immersed in a solu- For example, a silver wire coated tion of unknown ion activity, this with slightly soluble AgCl and arrangement functions as a mea- immersed in a solution of KCl is suring electrode when measured an electrode of the second kind, against a reference electrode. the Ag/AgCl/KCl electrode. The potential of an electrode of the After calibration with solutions of second kind does not depend on known ion activity, the unknown the concentration of metal ions, activity can be determined from but on the anion concentration the voltage measured. Today or activity (ax-) in the solution. the silver/silver chloride system is most often used for reference u = u - U · lg a 0(Me/MeX) N X- electrodes. Not only are they universally applicable, they also have no harmful effects on health or the environment. Ag In addition, SI Analytics offers reference electrodes for specific applications using the mercury/ mercury chloride and mercury/ KCl-solution AgCl mercury sulfate and iodine/ iodide systems.

Fig. 5 Electrode of the second kind

16 1.4 Nernst equation In accordance with the theoret- In accordance with Nernst, the ical principles set out above, it galvanic voltage depends on the is possible to come up with an constants u0 and the variables equation which describes the aMe+ and T. U0 is the standard

relationship between the poten- potential of the metal, aMe+ is the tials measured by the electrode activity of the metal ions and T is and the pH value of the solu- the absolute temperature. tion into which the electrode is immersed. This is the Nernst R · T u = u0 + · ln a Me+ equation. n · F

If a galvanic cell is observed in which two metals are immersed The standard potential u0 has in the electrolyte solution , two a typical value for each metal. electrochemical potentials are It can be derived from the so- formed at the phase boundary called "electrochemical series" which is in equilibrium when their of the metals. The galvanic volt- amounts are equal (Fig. 6). For age (u) measured corresponds redox (ORP) systems, the Nernst exactly to u0 if the activity of equation is used to calculate the the metal ions in the solution is voltages of an electrode which 1 mol/l (as ln1 = 0). The expres- occur. [2] sion (R*T) / (n*F) contains the gas constant (R), the oxidation number and valence of the metal ions (n), the Faraday constant (F) and the absolute temperature (T) in metal Kelvin.

Fig. 6 Development of the redox + Me Me+ (ORP) potential by the dissolution Me+ Me+ and deposition of a metal in aque- aqueous solution ous solution.

17 pH handbook Nernst factor Temperature dependence of For redox (ORP) reactions in the electrode potential which a single charge is ex- The Nernst factor is not a constant changed per atom or ion (n = 1) because it contains the absolute and at a standard temperature temperature as a multiplier. The of 25 °C (T = 298K), the entire electrode potential changes in expression can be summarized accordance with this temperature in a single constant . This con- dependence of UN. Compared stant is then multiplied by 2.303, to the standard temperature so that ln aMe+ can be substitut- of 25 °C, the Nernst factor is ed by log base 10 lg aMe+. The approximately 8% smaller at 0 °C resulting factor has a value of and approximately 25% greater 59.16mV at 25° C. It is known as at 100 °C (Fig. 7). the "Nernst voltage" (UN) or simply the "Nernst factor". The equation for the electrode potential is Un (mV) therefore greatly simplified:

u = u0 + UN · lg a Me+ 65 59,16

0 25 50 75 100

temperature °C

Fig. 7 Temperature dependence of the Nernst factor

18 1.5 Methods of pH mea- Photometric pH measure- surement ment There are potentiometric and The color change of the indicator optical methods for determining pigments can also be photomet- the pH value. The potentiomet- rically determined by shining a ric methods relate to the mea- light and measuring the absor- surement of electrical voltag- bance. These methods are re- es on pH-sensitive electrodes. ferred to either as colorimetric Optical methods involve the or spectrophotometric, depend- visual and photometric analysis ing on the equipment and light of pH-dependent color changes. source used. In theory it is possible to take pH measurements Optical methods in this way. However the method is very prone to interference These methods use pH-depen- and the equipment needed is dent color changes of specif- large. [2] ic organic pigments, so-called color indicators. So for example Disadvantages of the optical as the pH value increases, the methods color of methyl red in an aqueous solution changes from red to The area of application for opti- yellow at a pH of 4.9. Phenol- cal pH measurement, be it visu- phthalein for example turns al or photometric, is very limited. reddish at a pH of 9.5. The best If the solution to be measured known of these is the pH indicator is cloudy or has an inherent paper or pH test strips, which are color, the measurements will be prepared with indicator solutions unreliable. Some measurement of these organic pigments. The solutions also contain chemical pH value is estimated by means bonds which destroy the color of a visual comparison of the col- indicators through oxidation or or against a color scale. However reduction and produce incorrect the precision is only sufficient to results. provide a rough estimate.

19 pH handbook Potentiometric determination of the pH value This method uses the electrical The potential (u) of the antimony potential of pH-sensitive elec- electrode (Sb/Sb2O3) is directly trodes as a measurement signal. linked to the pH value: A distinction is made between hydrogen, metal and glass elec- U = U0 (Sb) - UN · pH trodes. The glass electrode is the most commonly used sensor Essentially, the potential of the today. Not having the disadvan- antimony electrode is measured tages of the optical methods, it against the constant potential of can be used almost universally. It a reference electrode. The simp- is one of the most sensitive and le linear relationship between pH at the same time most selective and measurement voltage ho- sensors there is and has an un- wever only applies in the range matched measurement of pH between pH 3 and pH 11. [5] 0 to 14, means from percent to ppq (= parts per quadrillion = Reductive or oxidizing compo- one molecule in one quadrillion nents in the measuring solution other molcules). also interfere with pH values measured using the antimony electrode. pH measurement with the antimony electrode Consequently the antimony On some metals there are redox electrode is only used in special processes which depend direct- cases, if for example the use of ly on the hydrogen ion activity glass electrodes in fluoride-cont- of the solution. For example, the aining solutions is not possible. It oxidation or reduction of antim- should be noted that on the an- ony (Sb), which depends on hy- timony electrode the zero point drogen ion activity, can be used is approximately pH 1 and these as a measurement of pH. days can often only be used with corresponding process measuring amplifiers. m · Me + n · H2O MemOn+ 2nH+ + 2n · e-

+ - 2 Sb + 3 H2O Sb2O3 + 6H +6e

20 Functional principle of the Applications of the hydrogen hydrogen electrode electrode If a fine platinum mesh bathed Theoretically, pH value can be with hydrogen gas is immer- identified very precisely with the sed in an aqueous solution, this hydrogen electrode. In practice, arrangement represents an elec- however, working with the hy- trode of the first kind, the so-cal- drogen electrode is expensive led "hydrogen electrode". Some and cumbersome. High-purity of the hydrogen molecules do- hydrogen and constant hydro- nate electrons to the platinum gen pressure are conditions and are released as hydrogen which are hard to create in a ions in solution: practical setting. The hydrogen electrode will also fail if the so- Pt lution contains heavy metal ions

H2 + 2H2O 2H3O+ + 2e- which contaminate the platinum surface. Reductive or oxidizing The hydrogen electrode is an components in the measuring electrode of the first kind, whose solution also lead to undesired potential at constant hydrogen side reactions and therefore to pressure (1 bar) depends solely errors in measurement. on the activity of the hydrogen ions in the solution. The hydrogen The hydrogen electrode is potential is measured on the consequently only used today platinum, a first-order conductor. under very specific defined For the actual redox process conditions for more scientific of the hydrogen electrode purposes. The same applies the platinum is chemically to the so-called quinhydrone inert. It functions solely as an electrode. As a special form of intermediate: depending on hydrogen electrode, today it is the direction of the current, seldom used. hydrogen can be deposited as a metal or converted to ions.

U = UN · lg aH+ = UN · pH

21 pH handbook 1.6 Summary According to the theory of Brøns- tedt [6], acids are substances which are capable of separating hydrogen ions. Bases on the other hand enable the deposition of hydrogen ions. A distinction is made between weak and strong acids and bases. Strong acids and bases are generally almost completely dissociated, weak ones on the other hand are only incompletely dissociated. In addition, a distinction is made between the molar concentration (previously: molarity) and the equivalent concentration (previously: normality). The molar concentration means moles per liter and the number of dissociable hydrogen ions is still included in the definition of equivalent concentration, i.e. mol/liter divided by the number of dissociable hydrogen ions. [4] The electrode potential can be determined on the basis of the Nernst equation. The measurement voltage is the difference between two electrode potentials. Whether a reaction can proceed depends on the thermodynamic requirements. The decisive factor is the change in free enthalpy ΔG.

22 SECTION 2

STRUCTURE OF pH alkali ions from the gel layer are ELECTRODES exchanged for hydrogen ions from the water. [3] Na+ (glass) + H+ (solution) 2.1 The glass electrode + + What is meant by glass, is H (glass) + Na (solution) an amorphous, i.e. solidified If the alkali concentration of the without crystallization, super- aqueous solution is small, with cooled liquid which softens only certain types of glass a reprodu- gradually when heated, whose cible equilibrium is created bet- atoms have a short-range order ween the solution and the surfa- but no directional long-range ce of the glass, which depends order. [7] only on the concentrations of hydrogen ions in the solution The properties of the glass change and in the gel layer. depending on its composition. H+ (swelling layer) The oldest pH membrane glass is the MacInnes glass with the H+ (solution) composition Na2O : CaO : SiO2 in the proportion 22 : 6 : 72. [3] If two solutions with hydrogen activities a1(H+) and a2(H+) are se- The glass membrane as a pH parated from each other by this sensor type of glass membrane, a surface potential is created on both Under the action of water, alkali sides of the glass membrane. ions are released from the surface of the glass and the oxide Both surfaces of the membrane bridges of the silicate structure are in electrical contact with each are partially converted from H2O other. Due to its ionic structure - to OH groups. This leads to the glass is a second-order conduc- creation of a "gel layer" of appro- tor. A total potential of the mem- ximately 500 nm thick. This gel brane is created which can then layer acts as an ion exchanger of be described using the Nernst hydrogen ions: equation.

23 pH handbook This potential is directly propor- Structure of the glass elec- tional to the difference of the log- trode arithmic hydrogen ion activities. The glass electrode profits from If the hydrogen activity on one the dependence of the poten- side of the membrane chang- tial of the glass membrane on es, a new equilibrium is reached the activity of the hydrogen ions. between the gel layer and the At the end of a glass tube, a glass solution and consequently a membrane is fused on as a pH new potential. It takes just a few sensor. This membrane is filled seconds to reach the new equi- with a buffer solution of a known librium. pH value. Into this buffer solution, which also contains an electrolyte The total time it takes to (usually KCl), the reference elec- measure the changed potential trode is immersed. To measure however depends not only on the pH, the potential difference the gel layer but also on the between the inner and outer sur- resistance of the measuring face of the glass membrane is solution and the electronics used (see Fig. 8). in the measuring equipment. To measure this membrane potential, an electrode of the reference inner buffer second kind is placed in each of electrode with KCl- the two solutions as a reference electrolyte electrode. If the potentials of both reference electrodes are pHmeasuring exactly equal, they cancel each solution other out and the measured pHinside + + voltage (U) now corresponds + + + only to the total potential of the + glass membrane. a 1(H+) Fig. 8 Theoretical design of a glass U = U · lg = N electrode a2(H+)

UN · (lg a1(H+) - lg a2(H+))

24 This potential of the glass electrode (u) is directly proportional head to the pH difference between the internal buffer and the measuring solution:

u = UN · (pHin - pHsolution) Figure 9 shows a cross-section of a typical glass electrode. At the tip of the internal glass tube made from chemically inert, shielding highly-resistive glass is a spherical membrane made from a special pH- sensitive type of glass. The internal tube and sphere contain the internal buffer, e.g. a 3.0 molar KCl solution buffered at pH 7, which simultaneously acts as the electrolyte for the reference system. The reference system in the example shown consists of Ag/AgCl. The elec- inner buffer trical connection to the plug-in head consists of metal wire. The inner tube is covered in a film of metal, which operates as a shield discharge line against stray electrical fields. This shield is connected to the measurement connection shield via the plug-in head. The outer sheath of the electrode consists

discharge element of a chemically inert glass tube.

membrane

Fig. 9 Cross-section of a glass electrode

25 pH handbook Membrane glass and pH range The membrane of modern glass At higher temperatures the flu- electrodes consists of highly- oride attack is accelerated even advanced lithium silicate glass. further. Higher phosphate ion Compared to conventional so- concentrations have a very ag- dium glass, it has fewer alkali gressive effect in conjunction errors and better resistance to with high temperatures. wear due to its improved chemical resistance. [3] Membrane resistance and temperature range However there are no electrodes which are equally suitable at ev- At a sphere diameter of ery temperature over the entire approximately 10 mm, the pH range. The membrane glass wall thickness of the sphere of the glass electrodes is better membrane is approximately optimized either to the acid or 0.5 to 1.0 mm. Its nominal the basic range, which greatly resistance is between 100 and depends on the temperature. 400 MΩ at 25 °C. The resistance increases as the temperature The corrosion resistance of the falls, therefore attention must membrane glass also plays an be paid to the membrane glass important role. Like all glass, the with measurements with low membranes of glass electrodes resistance at 0 °C. corrode under certain conditions. In the basic range, for example, the disintegration of the gel layer accelerates very quickly with increases in temperature. In the acid range, fluoride-containing solutions increasingly attack the glass membrane at pH levels greater than 6.

26 Membrane glass type and shape SI Analytics offers glass elec- H Optimized for the basic range trodes with three different types and for higher temperatures of of membrane glass. This means up to 140 °C, also precise in very that an exact measurement is alkaline ranges. possible at virtually every temperature and pH range. S Resistant to sudden, large changes in temperature, such Glass of type A is suitable for as during sterilization; very almost the entire pH range and consistent measurement values every type of material for analysis in hot alkaline solutions with a at normal temperatures. fast response time.

Membrane glass of type H A Fast response time in drinking, optimized for the basic range has domestic, and waste water and significantly fewer alkali errors for general use (i.e. Na+ errors) and is suitable for temperatures up to +140 °C. In order to ensure optimal They are used in particular in moistening of the glass the basic range where there are membrane with the probe, the high levels of sodium and lithium membrane shape must be ions and for high-temperature optimized for the particular measurements. application. So for example a pH measurement of a flat surface Membrane glass of type S should not be conducted using a is resistant to sudden, large spherical membrane, but with a changes in temperature and flat membrane. produces very consistent measurement values in hot The different types of membranes alkaline solutions with a fast and their features are shown in response time. Fig. 10.

27 pH handbook spear ﬂat cone cylinder dome sphere f or m r for measuring points with automatic ultrasonic cleaning high especially usual for fermenter electrodes high high for most applications constant quality, low resistance because of large surface, especially for pH surface measurements in measuring points with automatic cleaning high resistance, shock-proof, easy to clean and therefore applicable property / application o bust, smooth, easy to clean, universally applicable resistance, resistance, resistance, suitable for penetrating semi - solid media and shock-proof , shock-proof , easy to clean ,

for general application ,

Fig. 10 Types of membrane

28 Inner buffer 2.2 The reference elec- Glass electrodes manufactured trode today generally have an inner Measuring chains buffer at pH 7. For special cases, there are electrodes with inner The potential of an individual buffers at other pH values, e.g. electrode cannot be measured. 4.6, in order to quickly identi- All that can be measured is a fy a short-circuit fault in probes voltage as the difference be- that frequently have a pH of 7. tween two electrode potentials The exact pH value of the in a closed circuit. If for exam- inner buffer is not relevant, so pH ple two electrodes of the same 6.86 is not worse than pH 7.00. metal are immersed in solutions These differences are easily com- of their own salts at various con- pensated for during calibration. centrations, the voltage between What is important is the long- these two redox electrodes can term stability and above all the be measured. The measuring consistency of this pH value at device is simply interconnected all temperatures in the proposed between the electrodes. measurement range. These combined electrodes are termed an "electrode chain" or The electrodes from SI Analytics "measuring chain", when used to also meet stringent requirements measure potentials. in this regard. At temperatures below 0 °C, normal inner buffers Circuit in the measuring chain are unsuitable, as the electrolyte The outer circuit corresponds to begins to freeze below 0 °C. In a first order conductor, because these cases, electrodes with it consists of an electrode metal, inner buffers which contain "frost connection cable and measuring protection" additives such as e.g. device - all ohmic conductors. glycerin are used. The inner circuit corresponds to a second order conductor, it consists of the electrolytes in the measurement solution and the diaphragm - all ionic conductors.

29 pH handbook The voltage measured by the pH measuring chain voltmeter results from the differ- To measure the pH value, a chain ence between the two electrode consisting of one glass electrode potentials. If both solutions in the (= the measurement electrode) example shown (Fig. 11) have and a reference electrode. The the same concentration, the reference electrode is in elec- voltage of the electrode chain is trical contact with the measure- zero (U = u1 - u2 = 0). ment solution, so that the circuit is closed through the measure- However if both solutions have ment solution. different concentrations or activities a and a , metal atoms will be 1 2 Structure of the reference dissolved in the dilute solution, electrode while metal ions will be deposited in the concentrated solution. The reference electrode is an Electrons will therefore flow from electrode of the second kind. the electrode of the lower con- Under normal circumstances an centration via the measurement Ag/AgCl system is immersed in device to the electrode of high- an electrolyte made from a con- er concentration. The voltage of centrated KCl solution (3.0 mol/l). such a measuring chain can be The diaphragm creates the elec- formulated in accordance with trical contact with the measure- the Nernst equation: ment solution. It is only slightly permeable, so that the electro- U = U · lg a /a N 1 2 lyte does not escape too quickly in the case of a liquid electrolyte. 1st order conductor (metal conductor) Figure 12 shows a schematic U diagram of a typical reference st a 1 < a 2 electrode. 1

2 nd order conductor (electrolyte conductor)

a 1 a 2

diaphragm Fig. 11 Circuit in the measuring chain 30 Discharge systems One type is used above all else

head for the conduction system of the glass electrode and the reference electrode: silver/silver chloride (Ag/AgCl/3.0 mol/l KCl). Mercury chloride is very rarely used these days. Thalamid is KCI no longer produced due to the refill opening toxicity of thallium. The iodine/ iodide system was developed as a new conduction system. This KCL solution system combines the advantage of being free from silver ions, with the low temperature curve of the redox potential. There is therefore a new reference system available for potentiometric measuring chains. reference element Silver/silver chloride system The silver/silver chloride system is the conduction system universally used today. As silver is non-toxic for humans, Ag/AgCl electrodes can also be used in diaphragm medicine and food technology, where the poisonous mercury and thallium systems are prohib- ited!

Abb.Fig. 12 12 Cross-section Querschnittszeichnung of a reference einer electrode Bezugselektrode 31 pH handbook Disposal is less critical with Ag/ When these effects are excluded AgCl than with the "poisons" and at room temperature, the thallium and mercury. Ag/AgCl mercury chloride system again has a wide range of application displays the most stable poten- with respect to temperature (up tial in the presence of KCl satura- to 140 °C) and is therefore also tion. Today the mercury chloride suitable for sterilizable elec- system is only used for academic trodes. The sensitivity of the purposes. Ag/AgCl electrode to chelating agents can be compensated for Iodine/iodide with an electrolyte key or double The redox pair iodine/iodide electrolyte electrodes. exhibits important advantages versus other systems, spe- Mercury chloride system cifically low temperature The mercury chloride system dependence, fast response

Hg/Hg2Cl2 is the conduction behavior and freedom from system in use for the longest contaminated metal ions. The time. Today however it is only iodine/iodide system has an used in exceptional cases, and additional interesting feature not only due to the toxicity of versus other redox systems, mercury. The reliable tempera- such as silver/silver chloride or ture range when using mercury mercury/mercury chloride. The chloride is relatively narrow, and temperature dependence, or at temperatures above 60 °C the more specifically the tempera-

Hg2Cl2 begins to degrade. ture coefficient of this reference Mercury chloride displays strong system, is almost zero. [3] temperature hysteresis, meaning A new reference system for po- that during sudden changes in tentiometric measuring chains temperature it takes a relatively with the advantages of being long time until the electrode re- silver ion free and having the low gains the original potential value. temperature curve of the redox Resistance to chelating agents potential is therefore available. (cyanide, thiocyanate, organic It offers a good alternative in the chelating agents) in the measure- presence of fluctuating tempera- ment solution is low. tures and is free from silver ions and other possibly disruptive metal ions. 32 Electrolytes If the KCl concentrations in the The reference electrolyte should reference electrode and the have good electrical conductivity, measurement electrode are in be chemically neutral, not react fact different, an additional po- with the measurement solution, tential is created. This potential and its ions should be as equally has a temperature dependence mobile as possible, as otherwise which cannot be compensated a perturbation potential may be for during room temperature created, a diffusion potential, calibration or for measurements due to the differing speeds of at significantly different tempera- diffusion. tures. The concentration of the reference electrolytes changes Potassium chloride (KCl) meets over time due to the diffusion these requirements. As the of water and evaporation. This temperature behavior of the effect can be balanced out by electrodes is the closest to ideal calibration if the changes are at high electrolyte concentrations, minor and the measurement concentrated KCl solutions are is being conducted at normal temperatures. used. [3] In the event of bigger changes in In the Hg/Hg2Cl2 system, the electrolyte generally consists of concentration, it is recommended a saturated KCl solution, in the that the electrolyte be changed if TI/TICI system a 3.5 mol/l KCl there is a great need for precision solution. For the Ag/AgCl elec- in measurement. trode a 3 mol/l KCl solution is generally used. For measurements at low temperatures, electrolytes of lower KCl concentrations are used with a glycerin additive. For precise pH measurements, it is important not only that the discharge and reference system, but also its electrolyte concentration, are identical.

33 pH handbook Gelatinous and polymer elec- This means it can be used without trolytes special pressure compensation. The disadvantage of gelatinous The use of gelatinous or polymer and polymer electrolytes is their electrolytes has the advantage lower resistance to temperature that no electrolyte can leak out and changes in temperature, during storage. Even in use there which can limit their range is virtually no loss of electrolyte. of use. In addition, the low or vanishingly small rate of outflow Because the traditional diap- in strongly acid and basic test hragm is no longer required, with materials, as well as in those the solid interface instead acting that are low in ions, can lead to as a "diaphragm", electrodes with diffusion potentials (see section polymer electrolytes and KPG 3.1) and consequently to errors diaphragms are less susceptible in measurement. to contamination. On the other hand, the polymer These features mean electrodes electrolyte also has advantages with polymer electrolytes are due to the absence of a largely maintenance-free. traditional rate of outflow. Due to the greatly limited "mobility" of all Polymer electrolytes, such the ions, there is no precipitation as REFERID® and DURALID® of silver at the "diaphragm", and developed by SI Analytics, have the diffusion in of foreign ions the advantage of high pressure ("electron toxins") is virtually and resistance to changes in impossible. All in all, electrodes pressure. with gelatinous or polymeric electrolytes are strongly The difference between the recommended for a whole range DURALID® and the REFERID® of precisely-defined fields of use. system is that the REFERID® On the other hand, electrodes system contains a visible KCl with liquid electrolytes, while reserve. The DURALID® system they are more time-consuming contains finely-dispersed, solid in terms of handling, in many KCl in gel. cases offer better measurement reliability.

34 Types of diaphragms At lower ionic strengths the resis- The diaphragm of the reference tance of the test material may be electrode is of great importance too high for exact measurements. for the measurement precision Both effects are amplified by low of the pH chain. Diffusion volt- outflow rates, and so ceramic ages at the diaphragm are a diaphragms are less suitable in common measurement error. To such cases. Due to the high risk keep these small, the diaphragm of blockage of its pores, it is also must guarantee a relatively large not suitable for solutions contain- and consistent outflow of KCl. Its ing suspended particles. Only electrical resistance should be in measurement solutions that as low as possible and it must contain oxidizing substances is it be chemically inert. A decision clearly superior to the platinum must be made for liquid electro- diaphragm. lytes in accordance with these criteria as to whether a ceramic, Ground-joint diaphragm ground-joint, plastic or platinum The ground-joint diaphragm diaphragm best fits the measure- works with the thin gap of the ment conditions. Figure 13 (see unlubricated ground glass as page 37) gives an overview of an outflow opening for the the various diaphragms and their electrolyte. The outflow rate is features. 3 ml/24 h (p = 1m water column) and greater. Its electrical resis- Ceramic diaphragm tance is very low at 0.1 kΩ. It is The ceramic diaphragm uses suitable for measurements in the porosity of unglazed ce- contaminated solutions, as it is ramic. It's KCl outflow rate is easy to clean. Due to the high approximately 0.2 ml / 24 h outflow rate, it is suitable for (p = 1m water column). Its electri- both high and low-ion solutions. cal resistance is relatively high at In versions without a screw con- 1 kΩ. In measurement solutions nection, the ground gap must be with greater ionic strength, the manually adjusted in order to set concentration gradient at the di- a consistent flow rate. aphragm is very large, meaning diffusion potentials are very easily created.

35 pH handbook Plastic diaphragm KPG or annular gap For special applications there diaphragm are also diaphragms made For solid-state electrodes a con- from plastic fibers. For example, ventional diaphragm is superflu- single-rod measuring chains with ous, as the surface of the solid a plastic shaft often have dia- acts as an interface. In single-rod phragms made from nylon fibers measuring chains, this is capital- so as to avoid contamination of ized on in the form of the "KPG the connection hole. For process diaphragm". It consists of an an- measurements in solutions that nular interface wrapped around contain fluoride, electrolyte keys the sensor between the mem- with PTFE diaphragms are used. brane and the outer tube. Due to the relatively large interface Platinum diaphragm between the electrolyte and the The platinum diaphragm is an test material and their small dis- SI Analytics development. It tance from the sensor, a relatively consists of fine, twisted platinum low resistance is achieved. The filaments between which the annular arrangement around the electrolyte flows out along sensor rules out any disruptive precisely defined channels. The effects based on the geometry. platinum diaphragm does not easily become blocked and therefore features a very constant outflow. With approximately 1 ml / 24 h (p = 1m water column) and approximately 0.5 kΩ electrical resistance, it has advantages over ceramic diaphragms. However it is more sensitive to mechanical stress. It is also less than optimal for strongly oxidizing or reducing solutions due to the occurrence of disruptive potentials.

easy cleaning

ultrapure water , paste , , n o i s l u niversally, short response time, constant, insensible againsts dirt, m

flowing-out deviations by not resproducable handling; in case of pressure overload loosening ground joint, filigree universally, short response time, constant insensible against dirt and chemical reactions, tend to pollution/blockage general applications, robust, low-priced

cleaning only chemically, not mechanically short response time, easy handling u clean and defined flow channels, less diffusion potentials e sample can reach into reference system,

sample can reach into reference system, symmetrical annular gap, easy handling, insensible against dirt

no cleaning of reference system no cleaning of reference system

- -

- - + - + applications / proper ties + + +

d d d l / l / l / m m m

none none 2 1 3 .

flow out

k k k k k

1 5 2 1 1 . . .

0 0 0

resistance pe y annular gap fibre t ceramic ground joint platinum

Fig. 13 Types of diaphragm

37 pH handbook 2.3 Combined measure- Silamid® is a double-diaphragm ment chains system which provides low measurement errors from Single-rod measuring chain disruptive currents as well as a The conduction system of the stable display of values under glass electrode and the reference critical conditions. Long diffusion electrode system must be coor- routes mean there is no need for dinated. It makes sense to com- an additional heavy metal as a bine the measurement electrode silver ion barrier. and the reference electrode into a combined electrode Measuring chains with ("single-rod measuring chain"). electrolyte key This significantly reduces the space required to make mea- For measurements in solutions surements. Figure 14 shows a which could attack or contamina- schematic diagram of a typical te the reference electrode system, single-rod measuring chain. measurements are taken with a so-called electrolyte key. The glass electrode of the single- rod measuring chain no longer This means that the reference requires a metal shield, as the electrode is not immersed in low-resistance electrolyte of the the measurement solution, but reference electrode surrounds instead in a container with the the inner tube with the glass electrolyte solution which is in electrode as a sheath. contact with the mesurement solution via an additional The Ag/AgCl system is suitable for diaphragm. If the electrolyte in all pH uses and for temperatures the electrolyte key is changed of up to 140 °C. Silamid® from frequently, there should be no SI Analytics is increasingly being risk of disruption to the refer- used as the conduction system. ence electrode. For special cases Silamid is a glass tube coated there are reference electrodes with silver, which is half-filled with with an integrated electrolyte key, silver chloride. The connection to so-called "double electrolyte" the reference electrolyte is made electrodes. via a fiber wick.

38 head

KCI refill opening

KCl solution

inner buffer

discharge line

reference discharge Silamid® discharge

diaphragm

discharge element

membrane

Fig. 14 Cross-section of a single-rod measuring chain

39 pH handbook 2.4 Summary measurement solution. Sodium ions are forced into a type of ion The principal components of a exchange due to the higher single-rod measuring chain are affinity of +H ions for terminal the pH-sensitive membrane, the silicate groups. The sodium reference electrode (reference ions can penetrate the gel lay- system) and the diaphragm. All er, but the H+ ions cannot move components must be appropri- from the gel layer to the nega- ately adapted to the application. tive charges of the silicates. As a For example, a measurement result a potential builds at the in which the medium must be interface of the glass and the pierced must not be measured solution. The difference between with an electrode with a spherical the potential of the glass and the membrane. In this case a spear inner electrolyte is measured. membrane must be used. The difference is proportional to the pH value of the solution and The use of the different dia- can be measured against the s phragms depends on the partic- table potential of a reference ular application. The selection of electrode. the right diaphragm for the respective application will also be In glass and reference electrodes, decisive in obtaining good, re- systems consisting of a metal producible measurement values. and its salt are used to conduct A platinum diaphragm can be the signal. used almost universally. A ground-joint diaphragm is One popular system today is good for use suspended solids Ag/AgCl, which is marketed by and in solutions of high and SI Analytics in a special format low ionic strengths. Due to the called Silamid®. low outflow rate of ceramic diaphragms, contamination can occur quickly, for which reason it should only be used in non-critical applications. The outer layer of the membrane glass swells in the solution and H+ ions can penetrate the gel layer from the

40 SECTION 3 If it is possible to keep these potentials constant, they can be POTENTIOMETRY OF electrically compensated during THE pH ELECTRODE measurement by calibration. However as u , u , u and u are 3.1 Potentials of the pH 1 3 5 6 dependent on the temperature, measurement chain electrolyte concentration and on The voltage of a pH measuring the pH value itself, they may in chain (U) consists of six individual some cases become "disruptive potentials together (cf. Fig. 15). potentials". Reliable electrodes are designed in such a way that • the potential in the these potentials are very small. measurement system of the Any fluctuations in the potentials

glass electrode (u1), are consequently of little impact. • the potential on the inside of Once the measuring chains are

the membrane (u2), properly maintained and calibra- • the asymmetry potential of ted, the precision and reproduci-

the glass membrane (u3), bility of the pH measurement is • the potential on the outside virtually unaffected.

of the membrane (u4), • the diffusion potential of the

diaphragm (u5) • and the potential of the reference element of the

reference electrode (u6).

U = u1 + u2 + u3 + u4 + u5 + u6 In order to determine the desi- red membrane potential for identifying the pH value (u2 + u4),

the other potentials (u1, u3, u5, u6) must also be measured at the same time. Fig. 15 Potentials in the measuring chain

41 pH handbook Asymmentry potential Diffusion potential The cause of the asymmetry po- The diffusion potential can pose [3] tential (u3) is the difference bet- difficulties (u5) . This is the result ween the two surfaces of the of the difference in the speed of glass membrane. Their surface, diffusion of different types of ions. shape and the distribution of If a hydrochloric acid solution the gel layer are not comple- adjoins with pure water, the tely identical in each case. Even H+ and Cl- - ions move into the if the pH value on both sides of pure water at different speeds. the membrane is exactly identi- The H+ ions diffuse significantly cal, the overall potential on the faster than the Cl- - ions. membrane is never exactly zero. However if the electrodes have This creates a division between been manufactured with close a positive and negative charge, tolerances and have a well-de- in other words an electrical signed gel layer, the asymmetry potential. With other types of potential is only a few mV. This ions, e.g. with K+ and Cl- - ions, corresponds to a deviation of the differences in diffusion speed a few hundredths of a pH unit, are small, and consequently a which in principle can be correc- much smaller diffusion potential ted by the calibration of the elec- is created. trode. At the diaphragm of the reference electrode there is also an interface between solutions of different ionic concentrations. Generally speaking, in electrodes with liquid electrolytes, some KCl electrolyte constantly flows through the diaphragm into the measurement solution due to the overpressure.

42 The ions in the measurement Chain voltage under ideal solution can therefore only conditions diffuse against this KCl flow. As If electrodes with identical the electrolyte is constantly measurement and reference renewed by the outflow in the systems are used, their poten- diaphragm, a greater diffusion tials u and u are virtually equal. potential of the measurement 1 6 As both potentials have opposite solution ions cannot form there. polarities, they cancel each other out. Potential u can, The K+ and Cl- ions of the 3 as mentioned, be compensated outflowing reference electrolyte for by calibration. U can be kept diffuse between the ions of the 5 negligibly small by ensuring measurement solutions directly sufficient KCl outflow. Under in front of the diaphragm, thereby ideal conditions therefore, the short-circuiting their diffusion chain voltage depends solely on potential. the difference of the potentials between the inside of the mem- If the measurement solution is brane (u ) and the outside (u ). hydrochloric acid, for example, 2 4 the diffusion potential is great-

ly reduced by the KCl outflow. U = u2 – u4 = The diffusion in the opposite direction of K+ ions that are also UN · (pHinner – pH of the measurement solution) positively charged compensates to a large extent for the faster diffusion of the H+ ions. Problems arise if, for example, the KCl outflow is too low due to a blocked diaphragm. The diffusion potential can then become so great that deviations of 0.1 pH units can arise. [8]

43 pH handbook Resistance ratios and polariz- Ideal characteristic curve of ation the measuring chain The Nernst equation only applies The connection between the pH when the processes at the value of the measurement electrodes are in chemical solution and the voltage (U) of equilibrium. If too high a current the pH measuring chain can be flows through the measuring shown in a U/pH diagram. Under chain, it moves out of equilibrium ideal conditions, the voltage of and the measured voltage no the pH measuring chain follows longer matches the theoretical a linear equation. The ideal cha- voltage calculated by the Nernst racteristic curve is there a straight equation. line in the U/pH diagram (Fig. 16). A measurable drop in voltage The incline of the ideal charac- then occurs, the so-called "po- teristic curve at 25 °C is 59.16 larization" of the electrodes. mV/pH, in accordance with the

Polarization can be caused by Nernst factor (UN). The measu- too low an internal resistance in rement voltage is therefore the the measuring device. Both product of UN and the pH diffe- disruptive effects, polarization rence between the inner buffer and diffusion potential, can also and the measurement solution. If play an unwelcome role in pH the pH value of the inner buffer is measurement! 7.00, then when measuring a solution of pH 7.00 at 25 °C the pH voltage U (mV) difference is also 0.00. The ideal + 500 characteristic curve in a measure- + 400 slope 59.16 mV/pH + 300 ment solution of pH 7.00 therefo- + 200 re moves through the 0 mV axis + 100 (U = 59.16 (7.00-7.00) = 0.00). If the measurement solution is at

- 100 pH 13.00, the measurement vol- - 200 tage is -354.96 mV (U = 59.16 - 300 (7.00 - 13.00)). If the measure- - 400 ment solution is at pH 5.00, the - 500 0 7.0 14 measurement voltage is 118.32 pH of the measured solution mV (U = 59.16 (7.00 - 5.00)). Fig. 16 Ideal line of the pH chain at 25 °C

44 3.2 Characteristic curves of the pH measurement chain

Nonlinearity Acid and alkaline errors Even measuring chains whose The cause of the acid error is the zero point and incline match the absorption of acid molecules ideal characteristic curve will into the gel layer and subsequent deviate from the linear course change in water activity [3]. in strongly acidic and strongly basic ranges. The acid error in modern pH The so-called "acid error" causes glasses is however in most cases greater pH values to be displayed negligible. The so-called alkaline at lower pH values and/or the error provides pH readings that voltage of the measuring chain is are too low at high pH values or too negative (Fig. 17). the measurement voltage is too positive. The cause of alkaline voltage U (mV) errors is the exchange of alkali + ions between the gel layer and the measurement solution, which at values above pH 12 compete

noticeably with the hydrogen ions. 0 In modern membrane glasses, alkaline errors now only occur if the measurement solution at

higher pH values contains large - amounts of sodium or lithium 0.00 7.0 14.0 pH of the measured solution ions.

Fig. 17 Acid and alkaline errors

45 pH handbook Characteristic curve of actual Temperature dependence of pH measuring chains the ideal characteristic curve Apart from acid and alkaline The voltage of the pH electrode errors, the other ratios in actual changes with the temperature. measuring chains do not meet This temperature dependence of the theoretical requirements for the measurement voltage affects the exact validity of the Nernst the Nernst factor (UN) in particular. equation. Due to the non-neg- It varies between 54.20 mV/pH ligible potentials u1, u6 and u3, at 0 °C and 74.04 mV/pH at 100 °C. without calibration the zero point This change in the measuring of the measuring chain is almost chain slope caused by the never exactly at the pH value of temperature dependence of the inner buffer. the Nernst factor is balanced out during measurement by The slope of actual measuring so-called temperature compen- chains also rarely corresponds sation (cf. Fig. 18). exactly to the theoretical value of the Nernst factor. The zero point As pH measurement is a mea- and slope of the characteristic surement of the activity of H+ curve of actual measuring chains ions, and activity is a temperature- therefore deviates to an extent dependent thermodynamic val- from the ideal course. These ue, pH values recorded at one deviations are compensated for temperature cannot therefore by calibration. The position of be converted to the pH value at the zero point and the slope of another temperature by a simple electrodes from SI Analytics calculation. which are properly stored are extremely stable over long 0 °C = 54.20 mV/pH-unit periods of time. But even with good electrodes, it should be 25 °C = 59.16 mV/pH-unit noted that they age, i.e. the zero 50 °C = 64.12 mV/pH-unit point and the slope change due 75 °C = 69.08 mV/pH-unit to storage and use. 100 °C = 74.04 mV/pH-unit

Fig. 18 Change in the Nernst factor with the temperature 46 Isotherms 3.3 Summary In very worn electrodes, not only The potential of the measuring is the zero point not exactly at chain consists of six individual the pH value of the inner buffer, potentials combined, with the but even the position of the zero asymmetry potential and the point is temperature dependent. diffusion potential being of In addition, the temperature particular note. The asymmetry dependence of the measuring potential can be determined by chain slope does not exactly calibration and the measurement correspond to the temperature assembly adjusted to the poten- dependence of the Nernst factor tial recorded.

(UN). Consequently, if the voltage The diffusion potential is more of a measuring chain is recorded difficult. This builds up primarily at different temperatures, a dif- on the diaphragm. It can be ferent characteristic curve will be reduced by increasing the elec- obtained for each temperature. trolyte flow at the diaphragm These "isotherms" do not intersect (e.g. using a platinum diaphragm). with the ideal characteristic curve on the 0 mV axis, but at the so-called "isotherm intersection point". The isotherm intersection point therefore deviates noticeably from the zero point of the ideal characteristic curve voltage U (mV) (cf. Fig. 19). +

U 0 T 1

- 0.00 7.0 14.0

pH of the measured solution

Fig. 19 Isotherms are the characteristic curves of actual measuring chains at various temperatures. They never intersect with the ideal zero point. 47 pH handbook SECTION 4 With the addition of the strong acid to weakly dissociated acetic BUFFER SOLUTIONS acid, its dissociation equilibrium AND SAFETY IN pH is shifted and the concentration of hydrogen ions increases by MEASUREMENT a significantly smaller amount (this corresponds in our example 4.1 Definition of buffers to a change of just Δ pH 0.47). Buffers are the aqueous solutions Likewise with the addition of whose pH remains virtually unal- a strong base (e.g. sodium tered by the addition of small hydroxide) to acetic acid, the quantities of acids or bases. Buf- decrease in the concentration of fer solutions are also capable of hydrogen ions due to the release binding hydrogen ions with the of hydrogen ions through the addition of acids and releasing dissociation of the acetic acid is hydrogen ions with the addition less than when added to pure of bases. The easiest way of un- water. A buffer effect, occurs in derstanding is to compare this to the titration of acetic acid with neutral water. If the same quan- sodium hydroxide. tity of a strong acid is added to - neutral water, to a weak acid and CH3COO + HCl CH3COOH to a mixture of a weak acid and + Cl- its salt, the pH value decreases CH3COOH + NaOH very differently in each case. CH3COONa+ H2O Weak and strong buffers pH- after addition difference 1l of 10ml value The change in pH value with 1nHCl Δ pH the addition of strong acids (e.g. neutral Δ pH = pH 7.00 pH 2.01 HCl) to pure water directly corre- water 4.99 sponds to the amount of hydro- acetic acid Δ pH = pH 2.47 ph 2.00 gen ions added. The concentra- 0.1n 0.47 acetic acid tion of hydrogen ions increases Δ pH = 0.1n pH 4.75 pH 4.71 in our example (Fig. 20) from + 0.1m 0.04 10-7 mol/l to 10-2 mol/l (corre- Na-Acetate sponding to a Δ pH of 4.99). Fig. 20 Buffer effect of different solutions and water with the addition of a strong acid 48 The buffer effect of mixtures of Buffer value and dilution weak bases or weak acids with effect their salts is particularly strong, "Buffer value" and "dilution effect" e.g. acetic acid with sodium ace- specify how good the effect of a tate. If a strong acid is added to buffer solution is. The buffer value this buffer solution, its hydrogen (ß) is a measurement of the capa- ions will be absorbed by the city of the buffer. It specifies how acetate ions. On the other hand, much the change in pH (dpH) if a strong base is added, its ef- will be for specified volumes (V ) fect will be compensated for by 0 with the addition of a differential, the undissociated acetic acid. gram-equivalent quantity (dn) of A simple formula can be deri- acid or base. ved from the law of mass action for the pH value of such a buffer 1 dn ß = · solution of a weak acid and its V0 dpH salt In this formula KHA is the dissociation constant of the acid, The dilution effect specifies by [HA] is its weighed concentration what amount (Δ pH) the pH value and [salt] is the weighed salt con- changes with the dilution of the centration. buffer solution with pure water in [salt] the ratio 1:1. pH = - lg KHA + lg [HA]

This formula shows that the pH change is determined by the concentration ratio of salt and weak acid. Only when so much base or acid is added will this ratio change by a factor of 10 does the pH value change by a unit.

49 pH handbook Temperature dependence of 4.2 Standard buffers the buffer Standard buffers in accordance The pH values of buffer solutions with DIN 19266 are used for the are also temperature dependent. calibration of pH measurements. As a general rule, basic buffer The so-called technical buffers solutions exhibit stronger tem- are governed by DIN 19267. perature effects than acidic ones. DIN buffers are manufactured in This should not be overlooked accordance with DIN 19266 and during calibration. For example, can be traced back to primary if calibration is conducted with or secondary reference material. 0.01 m borax solution, an adjust- The primary reference material ment must be made to pH 9.18 (powder form) is manufactured at 25 °C, to pH 9.46 at 0 °C and by NIST (National Institute of pH 8.96 at 60 °C. Modern pH Standards and Technology). meters automatically adjust for the respective temperature profile once the buffer series used has been correctly set.

300 250 1.679 x 3.557 x x 200 3.776 x 4.008 150 100 50 x 6.865

0 x 7.413 - 50 - 100 x 9,180 - 150 voltage (mv) x 10.012 - 200

- 250 x 12.454 - 300

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH Fig. 21 DIN buffer solutions and their pH values at 25°C

50 The pH values of solutions should Technical buffer solutions are be very close to the theoretical based on DIN 19267 and differ pH values and be traceable to in several respects from DIN them. They are the basis of almost buffer solutions manufactured in every practical pH measurement, accordance with DIN 19266. because they represent the official reference system. The They are often colored, so as composition of these solutions is not to be confused during every set by the NBS (National Bureau day use, are based on whole of Standards) and their pH values numbers and are more sta- electrochemically determined. ble. The composition varies depending on the manufacturer. The cells used for this purpose consist of a platinum/hydrogen Accordingly, the temperature and silver/silver chloride electro- responses of the buffers also de. They are the pH values which vary from manufacturer to manu- most closely match the current facturer. When using DIN buffers, thermodynamic definition of these may be used regardless pH value and are fully traceable. of manufacturer, as they are all They can be compared to a based on an identical formula. specification measured against the original meter in Paris. The values stored in the equipment in respect to technical Secondary reference material buffers relate to a specific manu- has an identical composition to facturer. Use of other buffer solu- primary reference material. It is tions generally leads to errors in produced exclusively by accred- measurement. ited manufacturers and the pH values are determined by an In general, it should be noted accredited laboratory (differen- which buffer has been set for tial potentiometry as opposed the device and which one was to primary standard buffer actually used. This should also solutions). be done in the context of the reference temperature, as the pH value is not always given at 25 °C, but sometimes at 20 °C.

51 pH handbook

Therefore they are only with the calibration e.g. certicate from charge PTB for valid. a certain U(pH) = 0.005 between charges.pH(PS) values can di er uncertainty in the measuring T e mol Molalit y solution buﬀer R e m P label f p e 5 4 4 3 3 2 2 1 1 o

T e 5 k r e C

0 5 0 7 0 5 0 5 0 B ra t

- - n 1

z u

oxalate tetra- Potassium- 0 1 1 1 1 1 1 1 1 1 1 P 0 1 . 7 . 7 . 6 . 6 . 6 . 6 . 6 . 6 . 6 . 6 T . 0 A - B 1 0 9 9 8 8 7 7 7 6 0

5 - 2 4 7 4 5 0 6 2 0 8 0

bitatrate Potassium- saturated at T 3 3 3 3 3 3 P A 2 . 5 . 5 . 5 . 5 . 5 . 5 T B - - - - 5 - B 4 4 4 5 5 5

° - 8 4 9 0 3 7 0 C

phthalate hydrogen- Potassium- P 4 4 4 4 4 4 4 4 4 4 H P 0 . 0 . 0 . 0 . 0 . 0 . 0 . 0 . 0 . 0 . 0 T . 0 T C B 6 4 3 2 1 0 0 0 0 0 -

5 0 - 4 9 6 8 5 8 3 1 1 4

P ( ( P N h K H o 0 6 6 6 6 6 6 6 6 6 6 0 H a P p OA s ph a te . 0 . 8 . 8 . 8 . 8 . 8 . 8 . 8 . 9 . 9 . 9 . 0 2 2 T D H H P B 2 3 3 3 4 5 6 8 0 2 5 2

0 P ( P - - 5 3 4 7 1 3 5 0 0 2 0 5 4 0 0

) S 0 4

) )

- values P ( N P ( K h H 7 7 7 7 7 7 7 7 7 7 0 o a P 0 H OB . 3 . 3 . 3 . 3 . 4 . 4 . 4 . 4 . 4 . 5 . 0 2 s ph a te T 0 H 2 E P B 3 8 8 8 9 0 1 3 5 7 0 0

P - 0 - 4 6 9 3 5 6 2 1 4 2 9 0 0

0 4 )

)

B O B o 9 9 9 9 9 9 9 9 9 9 P 0 . 0 . 0 . 0 . 0 . 1 . 1 . 2 . 2 . 3 . 3 - T . 0 F 00B r a B 1 4 7 9 4 8 2 7 3 9

1 - 8 6 6 5 4 4 8 7 1 2 x

citrate hydrogen- Potassium- C I 3 3 3 3 3 3 3 3 3 3 P 0 . 7 . 7 . 7 . 7 . 7 . 7 . 7 . 8 . 8 . 8 T T . 0 H - B 4 5 5 5 6 7 8 0 1 3 0

5 - 8 1 4 7 6 5 7 1 9 7 0

carbonate bi- sodium- carbonate/ Sodium- ( ( N N C A 0 0 10 . 10 . 10 . 10 . 10 .

a . 0 . 0 P 9 9 9 9 9 H R 2 T . . . . . C0 2 2 - C0 8 8 8 9 9 I 0 0 1 1 2 B 0

5 5 3 5 9 1 7 1 6 2 8 4 0 - 3

3 0 6 2 1 0 4 6 1 0 8

)

Fig. 22 Examples of pH values of reference buffer solutions depending on the temperature (cf. DIN 19266 [9] : For a better understanding this non-official document has be translated to English from the official German version).

52 4.3 Calibration Why calibrate? The precision of the measure- Precise and reproducible mea- ment stands and falls with the surements are only possible calibration/adjustment. if calibration has been carried out. How often it is necessary to Adjustment is the setting of the calibrate depends on a num- pH meter to the measuring chain ber of factors. Firstly, the type data gathered through cali- and composition of the probe bration (slope and zero point). and secondly the frequency of During adjustment, the elec- measurement. Whether a cali- trode functions obtained during bration is necessary can easily calibration are balanced out. The be checked by recording the current slope and zero point are measurement value in a buffer identified from the measuring solution. Some applications chain voltages in the reference require daily calibration of the solutions. However as the word measuring equipment, such as calibration is more commonly measurement in low-ion water. used in everyday speech, it will For other applications, weekly also be used here. The pH or even monthly calibration may meter must be aligned with suffice. It is therefore not possible the pH electrodes used, as to give an exact specification. The the design, type of electrolyte principle on which calibration is (gel or liquid electrolyte) or the based will be briefly explained diaphragm (cf. section 2), the below. zero point and slope may differ from electrode to electrode These properties also depend on the age and level of use of the electrode.

53 pH handbook The slope of the electrode Calibration procedure signifies the potential difference For analog measurement devices, in mV when two pH values with the procedure is as follows: a difference of one pH unit are considered. So in an ideal situ- • Bring both measurement buf- ation the pH value of a solution fers to the same temperature with pH 4 should provide a mV value which differs by 59.16 mV • Fill a small container with the from a value recorded at pH measurement buffer 5 and 25° C. The slope is then stated to be 59.16 mV/pH. The • Remove the protective cap zero point of the calibration line from the electrode and open is the intersection of the lines the fill opening, so that elec- with the y-axis and in an ideal trolyte can flow out of the dia- situation should be pH 7. phragm. For gel electrodes this is not necessary. Fig. 23 shows a graph of the calibration lines of an electrode • Rinse the electrode briefly with exhibiting ideal behavior. distilled water.

• Measure the temperature of the calibration buffer.

• Adjust the temperature on the 500 temperature regulator of the 400 300 equipment. This is not neces- 200 sary for measuring devices with 100 0 automatic temperature com- - 100 pensation, when a measuring - 200 voltage (mv) - 300 chain with a temperature sensor - 400 is being used. - 500 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 • Immerse the electrode in pH the buffer and ensure that the Fig. 23 Ideal characteristic curve whole of the tip of the electrode and the diaphragm is immersed in the solution. 54 • If after approximately 30 sec- the processes are begun and onds there is no further change confirmed using buttons. The ad- in the pH value, the pH value justment is calculated inside the displayed is adjusted to the device. It is not necessary to carry nominal pH value of the buffer out the calibration procedure a solution using the zero point second time. For safety, after cali- adjustment knob. Care is neces- bration the value of a used or ad- sary, as pH values are tempera- ditional buffer can be measured. ture dependent and so must be adjusted to the pH value valid at The temperature dependences the respective temperature. of the pH value of the buffers are stored in modern equipment • Remove the buffer from the and are calculated directly. electrode and rinse with distilled water and immerse in the By using new ID electrodes, the second buffer. electrodes can be recognized directly by the device. The • If after approximately calibration values recorded for 30 seconds there is no further this electrode are also stored change in the pH value, directly in the electrode and the pH value displayed is ad- not the device, so that when it is justed to the nominal pH value used the values can be referred of the buffer solution using the to again. Therefore, when using slope adjustment knob. several ID electrodes on the one device or one ID electrode • As a precaution, the buffer is on several devices with ID checked again for the zero point, recognition, it is not necessary as the turning of the slope and to recalibrate every time if these zero point adjustment knobs on electrodes/devices are being analog equipment can have an exchanged. effect on readings.

Modern pH meters are microprocessor controlled. The calibration procedure is in principal the same as with analog measurement equipment, however

55 pH handbook Single point calibration Multi-point calibration With pH measuring equipment, With multi-point calibration, the calibration may be finished main difference to two-point cali- after one buffer. This determines bration is that the calibration line the zero point. For the slope, is calculated by means of a linear the theoretical slope is used. regression. Slope = Nernst slop (- 59.16 mV/ pH at 25 °C). The difference between the The range of use of single-point pH values of the reference buf- calibration is limited. It is possible fer solutions should be Δ pH > to measure only within a range 0.5 if possible. The coefficient of + / - 0.5 pH units from the pH of determination (correlation of the buffer solution used. The coefficient) R² is used to as- measurement probe should sess this. R² is a dimensionless have the same properties as the measure and can accept values buffer solution. The pH value ob- between –1 and +1. At –1 there is tained may be used to compare a negative relationship between to previously obtained mea- the values calculaed, at +1 surement results, but is not an there is a positive relationship. absolute value. The nearerthe value is to 1, the better the match with the linear Two-point calibration assumption. With two-point calibration, asymmetry and slope are identified, 4.4 Working with buffer thereby determining the slope solutions and axis intercept of linear cal- When using buffer solutions, ibration lines. The pH value of there are a few points to observe: the buffer used should ideally differ by two pH units. Basic buf- • Has it reached its expiration fers should not be used as their date? pH value changes by absorbing CO2. For routine measurements • Buffer solutions must not be the use of DIN buffer solutions reused with pH = 6.865 and pH = 4.008 is recommended.

56 • Avoid basic buffer solutions. • Never pour quantities re- If this type of buffer solution moved back into the container. is in contact with the air, a gas Risk of contamination! exchange takes place, which also affects the carbon dioxide • Use opened containers of buf- in the air. In aqueous solutions, fer as soon as possible (neutral carbon dioxide is in equilibri- and acidic buffers within the um with carbonic acid. This has next month, basic buffers within little effect on neutral and acidic the next few days). buffers. In basic buffers however a neutralization reaction occurs, • Use appropriate container meaning that carbon dioxide is sizes! constantly removed from the air. As a result, the pH value of the Many of these problems can be buffer changes. avoided by using buffer vials produced by SI Analytics. If the • If alkaline buffers are nonethe- measuring equipment displays less used for calibration, a sealed a calibration error during calibra- container should be used and tion, this is frequently caused by the storage bottle should only a worn measuring chain. If the be opened briefly. error message is not resolved by a new measuring chain, the pH • Small container sizes are also meter is rarely defective. Usually recommended. the buffer solution used has become contaminated or too old. • Never immerse the measuring This applies especially to buffers chain in the buffer solution con- with basic pH values. The pH is tainers, always take out the vol- ultimately shifted. If a buffer of umes required. That is the easi- pH 10 is left open overnight, its est way to avoid contamination! pH value the next morning will be clearly lower than 10. • Always close the container immediately after use (carbon dioxide, contamination by dust particles!)

57 pH handbook Figure 24 shows the actual pH However there should first be values of an opened technical a brief check of the pH value of buffer solution (pH 10) and a the basic buffer solution of 9.18, standard buffer solution (pH 9). in order to rule out any measure- Over the course of 12 hours the ment errors. pH measurements pH of the pH 10 buffer changes are required in many applica- by 0.22 pH units and that of the tions and accordingly have diffe- pH 9 buffer by 0.02 pH units. If rent requirements. But in general buffers of 6.87 and 4.01 are used the requirements of the respon- for calibration, pH values in the se time and the stability of the basic range can also be identified signal are greater the more pure very well, as the linearity of the and clean the measuring medi- electrodes is excellent. um is. With contaminated media the response time and stability is generally very good, although the service life of the electrode is very short. In these cases the frequency of calibration must also be increased.

t [ h ]

012 345678 9 10 11 12 0,000 -0,020 -0,040 -0,060 -0,080 -0,100 -0,120 -0,140

delta p H -0,160 -0,180 -0,200 -0,220 -0,240

Fig. 24 Change in buffer solutions over time due to entry of CO2

58 4.5 Measurement uncer- Greater differences in tempera- tainty ture may require some minutes Measuring pH values to balance. For high-precision measure- To take a measurement, the cali- ments, care should be taken to brated measuring chain is rinsed ensure that the calibration tem- with distilled water and im- perature and measurement tem- mersed in a sufficient quantity of perature are similar, as otherwise the test material. It is important to significant errors can occur due ensure that no calibration buffer to the temperature dependence solution is introduced during the of the pH measurement. measurement. This would lead to serious measurement errors. When using a measuring chain Measurement uncertainty without a temperature sensor, Different measurement applica- the temperature should be set tions have different measurement on the pH measurement device. uncertainties and consequently The measurement value should there are different requirements only be read when the display to ensure reliable measurement. stops changing. The measuring These requirements include not time must in any event be lon- only the equipment but also the ger than the response time. For type of calibration, buffer solu- intact measuring chains the re- tions used, electrodes used etc. sponse time should be about 30 Different measurement uncer- seconds. If after 1 minute a stable tainties are likewise associated measurement value cannot be with the different materials used, read, this may point to a defec- meaning that a general state- tive measuring chain. ment cannot be made as to how reproducible or difficult a mea- A certain amount of time is re- surement is. One reproducibility quired for the temperatures of may be significantly better, as the measuring chain and the only random errors are identified. test material to balance. If the A systematic error can always temperature difference is a few provide easily reproducible mea- degrees, the adjustment should surement values which do not al- take approximately 1 minute. ways match with reality.

59 pH handbook Some examples should briefly The major factors affecting explain what the possibilities of measurement uncertainty de- measurement uncertainty are pend on the buffer solutions, and how to estimate these measurement equipment and (Fig. 25). The term measure- measurement solution used. ment uncertainty is defined in Temperature plays a significant DIN V ENV 13005 [10] role in measurement uncertainty (Fig. 26). Measurement uncertainty describes a parameter which is Firstly, it affects the properties of accorded to the measurement the measuring chain during cal- result and which characterizes ibration and the adjustment of the dispersion of the values. the measurement device, and secondly it affects the actual measurement itself.

Standard uncertainty of the components of U(k=2)

dimension/ component of the pH-measurement unit 0.3 0.1 0.03 < 0.03 unit referent solution pH 0.05 0.03 0.01 < 0.01

pH-meter pH 0.01 0.01 0.001 0.001

pH-meter mV 1 1 0.1 0.1

pH-electrode, slope (25°C) mV/pH > 57 > 58 > 58 > 58.5

pH-electrode, setting time to dE/dt < 0,1Vm/10s s < 90 < 60 < 30 < 30

pH-electrode, stirring sensitivity Δ mV < 2 < 1 < 0.5 < 0.3

pH-electrode, repeatability Δ mV < 2 < 1 < 0.4 < 0.2

temperature measurement °C < 1 < 0.5 < 0.5 < 0.2 temperature consistency during calibration and measurement °C < 2 < 1 < 0.5 < 0.3

Fig. 25 Standard uncertainty in accordance with DIN 19268 [11]

The table shows the different uncertainties for the different components of the measuring equipment. If the overall calculated error is intended to be no more than 0.03 pH, then the reference solution must have an accuracy of +/-0.01 pH, the pH meter +/- 0.001 pH or +/- 0.1 mV, the electrode slope better than 58 mV/pH etc.

60 If the calibration and measure- by manually inputting the tem- ment temperature are different, perature to the pH meter. If the the measurement device will temperature is manually entered, only be adjusted to the slope it must be recorded beforehand of the calibration lines using the using a temperature sensor (ther- temperature dependence of the mometer, electric temperature Nernst potential. There will be sensor). These measurement de- no adjustment for asymmetry. vices have varying measurement However temperature com- uncertainty, however, which also pensation is only possible if the influences the overall measure- temperature is disclosed to the ment uncertainty. The maximum measurement device. This achievable uncertainty is with the is done either by using an use of technical buffer solutions integrated temperature sen- at 0.1 pH units. sor inside the pH electrode or

E(X) pH(S)

ionic strength CO 2 certified value sample repeatability temperature viscosity temperature

pH(X) liquid junction temperature Potential potential signal stability pH electrode pH meter inflow circumstances temperature measurement E(S), E(X)

pH(S) pH value of the buffer solutions used for calibration pH(X) pH value of the sample solution E(S) potential of the buffer solution E(X) potential of the sample solution

Fig. 26 Components of measurement uncertainty

61 pH handbook The stability control (drift control) 4.6 Summary function of the measurement Buffers are solutions which can device checks the stability of the stabilize the pH value even with measurement signal. The mea- the addition of acids/bases. This surement signal is accepted effect is particularly pronounced as stable when the drift is with buffer mixtures of weak < 0.02 pH units within 15 seconds. acids/bases and their salts e.g. The stability has a significant ef- acetic acid/acetate. The pH value fect on the reproducibility of the of the buffer is temperature de- measurement value. In addition, pendent. At this point, calibration the measuring chain must be is very important. adapted to the application. There are many different measuring The buffer solutions are the chains which due to their design, basis of practical pH measure- the diaphragm used or the type ment, as they serve to calibrate of pH glass used can be used in the pH measurement unit and a variety of different applications. thereby match the measurement unit to the pH measuring chain used. In order to achieve the greatest possible measurement certainty, buffer solutions which meet DIN 19266 should be used, as these provide state of the art levels of certainty in calibration.

Temperature has a decisive influence on pH measurement. It is not possible to measure any more precisely than the precision of the buffer solutions used. Only the user can identify the appropriate time intervals for calibration.

62 SECTION 5 MEASURING EQUIPMENT measuring device must match AND MEASURING SET-UP the actual pH value of the measurement solution, the 5.1 Function of the pH meter amplifier electronics of the measuring equipment must fulfill two conditions: Task of the measuring The electrical zero point and equipment the amplification factor must be The pH measuring equipment adjusted to the zero point and must translate the voltage the slope of the characteristic produced by the electrodes curve of the respective pH used into a pH value. This means measuring chain. For this calibra- that the measuring equipment tion of the measurement elec- must be adapted to the partic- tronics, a calibration buffer of a ular pH measuring chain used. known pH value is used. After In principle it is unimportant this calibration the electrode whether the display is analog characteristics are adapted/ad- with a dial instrument or digital justed. with an LED or LCD. In certain Functional principle of the circumstances the readability analog pH meter and precision of reading may be better with a digital display. Analog pH meters have a What does make a significant high-resistance amplifier, the difference, however, is whether characteristics of which can the measurement voltage is be adjusted with three poten- processed by analog or digital tiometers (variable resistances) means. to the characteristic curve of the measuring chain: for the zero Why calibrate? point, the slope and the tem- The necessity for calibration can perature compensation. With the be seen from the difference be- "zero point potentiometer", the tween the actual characteristic electrical zero point of the am- curve of the pH electrodes and plifier electronics is adjusted to the ideal characteristic curve. the zero point of the measuring If the pH value displayed by the chain.

63 pH handbook With the "slope potentiometer", Functional principle of the pH the amplification factor of the meter microprocessor amplifier is modified so that it Modern microprocessors are matches the slope of the mea- designed with digital electron- suring chain. ics. After a high-resistance input With the "temperature potenti- amplifier, there is an AD convert- ometer" the amplification fac- er, which converts the analog tor is modified in accordance measurement voltage to a digital with the temperature-related value. The microprocessor sets change in the Nernst factor (U ). N off this digitized measurement In addition to this temperature voltage against the values for cal- compensation, a temperature- ibration and temperature com- dependent resistance (e.g. Pt 100 pensation (also digitized). The re- or Pt 1000) with a corresponding sult is then relayed to the digital adjustment may also be directly display. While the amplifier is still incorporated into the amplifica- "turned" on analog measuring tion circuit. This allows tempera- equipment, the internal calibra- ture compensation to take place tion of the microprocessor of the fully automatically. A requirement pH meter is completely different. for this is that the temperature of The microcomputer carries out the measurement solution can the calculation: the pH values be measured simultaneously of common calibration buffers with the pH value (Fig. 27). including their temperature dependences are stored in it. Apart from the button to be pressed to slope temperature zero point compensation carry out the calibration, the microprocessor device carries out

Display the adjustment to the measuring measuring chain chain automatically. 7 amplifier

0 14

Fig. 27 Functional principle of the analog pH meter

64 The microcomputer calculates The microprocessor functions the course of the actual charac- in exactly the same way for teristic curve from the voltages temperature compensation. It of the measuring chain in the offsets the stored temperature calibration buffers used, and dependence of the Nernst factor. compares these mathematically In addition it should always be with the ideal characteristic curve remembered that the micropro- which is stored in the form of the cessor cannot take into account calibration buffer values. non-ideal temperature dependence which deviates from the During calibration the micropro- Nernst equation in the characte- cessor identifies the typical dif- ristic curve, or disruptive poten- ference between the ideal and tials (cf. section 3). Therefore the actual course of the characteristic disruptive potentials should be curve for the measuring chain kept as small as possible through used and offsets this automati- appropriate measuring condi- cally as a correction value during tions, even when working with a measurement. The adjustment microprocessor based pH meter. to the measuring chain in a pH meter microprocessor is therefo- The deviation of the actual iso- re not carried out electrically, but therms from their ideal course mathematically (Fig. 28). is best accounted for by calibra-

temperature sensor tion at the measurement temperature. Only when these requirements are fulfilled does the microprocessor make calibration

ampliﬁer and measurement quicker, easier and more certain. corrections

Un(T) measuring chain

micro processor ampliﬁer converte r /D A storage

pH (T) chart of calibration buffer

Fig. 28 Functional principle of the microprocessor pH meter

65 pH handbook 5.2 The measuring circuit Resistance ratios in the measuring circuit The glass membrane of the mately 10 MΩ, it is clear that con- measurement electrode has the necting a pH glass electrode to greatest electrical resistance in such a measuring device would the measuring chain at approxi- almost cause a short-circuit! mately 100 to 400 MΩ at 25 °C. For precise measurements, how- The electrical resistance of the ever, the chain voltage must not measurement solution depends drop off at the electrode, but on its ion content. Chemically must be fully present at the mea- pure water has an extremely high suring equipment. The input resistance. However as long as resistance of the pH measuring there is not too great a distance device must therefore be signifi- between the diaphragm and cantly higher than the resistance the glass membrane as well of the glass membrane. Modern as a sufficient KCl outflow, the pH measuring equipment has measurement error caused by an input resistance of > 1012 Ω. this, even in low-ion liquids, is A guide value for pH measure- less than 0.01 pH units. The ment is that the input resistance diaphragm of a typical reference of the measuring equipment electrode has a resistance of should be at least 1,000 times up to 5 KΩ = 5 · 103 Ω. When greater than the resistance of using glass electrodes with the glass membrane. At a glass relatively low resistance and pH membrane resistance of e.g. measuring equipment with a 100 MΩ = 0.1 GΩ, the input resistance greater than >1012, resistance of the measuring the voltage drop in normal cases equipment is approximately is negligible. 10,000 times greater. Modern pH meters therefore have Fig. 29 shows a schematic repre- more than adequate reserves. sentation of the resistances in the If this requirement is compared complete measuring circuit of with the input resistance of electrode and measuring equip- modern voltmeters of approxi- ment.

66 phas e zero tion resistance a capacitive coupling with mains so l input resistance i measuring device to ground measuring device

agm r resistance diap h resistance of the measuring solution resistance of the measuring solution to ground

capacitive coupling membrane resistance

Fig. 29 Resistance ratios in the measurement circuit

67 pH handbook Temperature dependence of Therefore if the measuring circuit the membrane resistance is grounded via the test material, it must be galvanically separated The relatively strong temperature from the actual measuring equip- dependence of the membrane ment by an isolation amplifier. resistance must be taken into account. As an ion conductor, the resistance of glass reduces Shielding at higher temperatures and in- Alternating currents can flow into creases at lower temperatures. the measuring circuit by capaci- At 0 °C for example, the resistive or inductive coupling of the tances of the glass membrane is measurement lines with the grid approximately 10 times greater or directly by irradiation of the than at 25 °C. test material (e.g. through a magnetic stirrer or a hot plate) Grounding Closely related to the resistance These alternating currents lead ratios in the measuring circuit is to a measurable proportion of the electromagnetic shielding direct current and unstable dis- and grounding. plays. The measurement errors thereby caused may be in the or- Ideally, both the resistance of der of a few 100ths of a pH unit. the measuring equipment and In case of doubt, measurements the resistance of the test materi- must be taken without mains al to ground would be limitless. power, magnetic stirrer and hot For measurements in industrial plate, with a battery-operated plants, the test material is nor- or rechargeable pH meter. The mally grounded using pipelines. high-resistance glass electrode If the measuring equipment is acts as an antenna for stray elec- grounded in this way, significant tromagnetic fields of all kinds. measurement errors can occur Therefore it must be carefully due to the formation of a ground shielded along with the associat- loop. Grounding the reference ed measurement line. electrode line can even lead to a short circuit, which sooner or Under no circumstances may the later can destroy the reference shielding be interrupted with electrode. "temporary fixes" or repairs.

68 Cables and plug contacts must be of high quality to en- As a result of the above, there sure reliable pH measurements. are significant requirements on Mechanically loose connections, cables and plug contacts for pH contact resistance due to cor- measurement! The cables must rosion, leakage currents from be well shielded and short. If ca- moisture etc. can not only cause bles are connected on an ad hoc errors in measurement but also basis, there is a risk of undesired destroy the electrode. short circuits or interruptions to the shielding. It is therefore al- The original plug-in head sys- ways recommended to use the tem from SI Analytics prevents original cables with the original these errors. The contacts are plug connections of the manu- gold-plated and corrosion re- facturer. sistant. The plug connection between the cable and electrode is With longer cables, e.g. in indus- mechanically secured by a screw trial applications, there is a high connection and the screw con- risk of interference along the ca- nection is sealed against mois- ble. Due to the high resistance ture by an O-ring. The electrical and large capacity of longer ca- shield is seamlessly integrated bles, the response times during into the plug-in head. The isola- measurement are also unusually tion from the inner contact con- long. This effect is difficult to dis- sists of extremely water repellent tinguish from errors in the elec- PTFE. trodes. With longer measuring 5.3 Summary cables, an impedance converter Modern measuring equipment should therefore be positioned is microprocessor controlled. directly behind the measuring The input resistance of the mea- chain. suring equipment should be approximately 10,000 times higher Some of the SI Analytics elec- than the membrane resistance of trodes are supplied with fixed- the pH electrode. In the measur- sealed connection cables. How- ing equipment, the voltage value ever many feature a plug-in head given by the measuring chain is system to connect to the connec- converted to a pH value using tion cable. The plug-in heads of the values obtained during cali- the electrodes and the cables bration (zero point and slope). 69 pH handbook SECTION 6 3. Samples should not be trans- ported and should be measu- PRACTICAL pH MEASU- red on location. If this cannot be REMENT avoided, the sample containers should be filled right up to the 6.1 pH measurement in top, leaving no air. various applications 4. Measurement should be carri- As previously mentioned, there ed out as quickly as possible, par- are different requirements for dif- ticularly with biological samples. ferent applications. The measu- By selecting the appropriate pH rement requirements for waste membrane glass and diaphragm, water are clearly lower than tho- a selection for the application can se for the measurement of drin- be made in advance. king water. There is essentially In the electrodes from SI Ana- one reason for this: the pH value lytics there are four different ty- in strongly buffered solutions is pes of membrane glass available. easier to determine than in less buffered, low-ion media. A-glass has a fast response time in drinking, domestic and Some general points should be waste water. It is used in general set out in advance: applications and in low-ion media. 1. The measuring chain used L-glass can be used at low must be suitable for the applica- temperatures and for general tion in question. applications. H-glass is well suited to high 2. Unless otherwise specified, temperatures, in the acidic the test containers and calibration and alkaline range, and in high containers should be rinsed with concentrations of sodium ions. the buffer solution and samples respectively. S-glass is particularly suitable for hot alkaline media and is therefore primarily used in process electrodes.

70 Measurements in beverages Our electrode recommendation For carbonated beverages (e.g. ScienceLine range e.g. A164 or lemonade, beer) the carbon di- N64 oxide must first be expelled. To do this, the beverage is shaken in Measurements in ground a closed container, vented at in- water, tap water, mineral and tervals and repeated until there is drinking water no more overpressure. It is then Depending on the conductivi- filtered through a fluted filter. ty, it may be useful to take the measurement in the absence Our electrode recommendation of air. Depending on the origi- A7780, N62 or an electrode from nal and regional soil conditions, the BlueLine 11 range. the buffer content of the sample may be very low. Calibration Measurements in water- is conducted with buffer solu- soluble paint tions of pH 6.87 and pH 4.01 or Gel measuring chains are not pH 9.18. suitable for water-soluble paints, as they are difficult to clean. The Our electrode recommendation ideal measuring chains have a ScienceLine range e.g. N64 or high electrolyte outflow and an N62 easy-to-clean diaphragm. There- fore measuring chains with a Measurements in low-ion ground-joint diaphragm should spring water and rainwater be used. Depending on the type The sample bottle must be rinsed of sample, it may be useful to well. Ideally the measurement dilute the sample with distilled should be taken mid-flow. If this water. The immersion depth of is not possible, the measurement the measuring chain should also must be taken in a closed cont- be kept constant. The reference ainer. The use of special measu- electrolyte solution should al- ring chains is usually necessary, ways be filled to the maximum, ideally a measuring chain with in order to avoid penetration of a ground-joint or platinum dia- paint into the measuring chain phragm. due to the hydrostatic pressure.

71 pH handbook Our electrode recommendation Any possible changes in the ScienceLine range e.g. N62 or membrane due to long-term N64 due to the increased de- measurement in the strongly-aci- mand for precision and the diffi- dic range can often be reversed culty of the application by thorough rinsing of the electrode between measurements. Measuring as a means of testing equipment Our electrode recommendation N62 or IL-pH-A120 MF. In contrast to the applications In internal tests, electrodes with listed above, DIN buffer solutions A-glass have proven more re- must be used for calibration. Ba- sistant against small quantities of sic samples must be measured fluorides. in the absence of air. The difference between the measurement temperature and the calibration Strongly basic solutions temperature must not exceed In the range above pH 11, the gel 0.1 °C. The accessible measure- layer is rapidly changed or even ment range is between 1.68 and destroyed, particularly at high 12.45, in accordance with the pH temperatures. This causes strong values of the DIN buffer solutions. asymmetry potentials and slows the response time. If the basic Our electrode recommendation solutions contain sodium or lith- ScienceLine range e.g. N64 or ium ions, then the so-called alka- N62 line error may occur, i.e. the measurement will provide pH values Strongly acidic solutions which are actually too low. To keep these errors to a minimum, For measurements in solutions glass electrodes whose mem- with a pH value of less than pH brane glass has been optimized 1, the acid error may occur, i.e. for the basic range are used (e.g. the pH values measured may SI Analytics Type H or, for hot, ba- be too great. In the acidic range sic solutions, Type S). there is also a risk of corrosion of the glass membrane by fluoride Our electrode recommendation ions and phosphate ions, par- H62 or H64 ticularly at high temperatures.

72 High temperatures Most reference electrodes and For temperatures above 50 °C, single-rod measuring chains are a suitable discharge system for not suitable for use below +10 °C. the electrodes must be ensured. At lower temperatures the KCl in Mercury chloride may only be the reference electrode crystal- used up to 50 °C. At tempera- lizes. These can be retrofitted by tures above 100 °C, the elec- swapping the electrolytes when trolyte may boil. By the use of used at low temperatures (2.0 m additives which raise the boiling KCl solution from 20 °C to -5 °C; point of the electrolytes, certain in 1.5 m KCl with 50% glycerin up glass electrodes, reference elec- to -30 °C and/or pre-manufac- trodes and single-rod measuring tured low-temperature electro- chains can be used up to 110 °C. lyte L 200 from SI Analytics. Pressurizing silver/silver chloride electrodes with overpressure in Our electrode recommendation principle allows temperatures of Electrodes with L glass, e.g. up to 140 °C. However it should L9180. be noted that use at high temperatures shortens the service Extreme ion strengths life of the electrodes. In very high-ion solutions, disruptive diffusion potentials can Our electrode recommendation easily occur at the diaphragm. SteamLine electrodes from the In very low-ion solutions the test process product group. material resistance is relatively high. Therefore diaphragms with Low temperatures a high outflow rate should be At low temperatures the resis- used, e.g. a ground-joint or tance of the glass membrane platinum diaphragm. increases, so that normal glass electrodes may only be used Our electrode recommendation down to -5 °C. Below that, ScienceLine range e.g. N64 or specific low temperature glass N62 electrodes with membrane glass Type L are necessary.

73 pH handbook Chemically-reactive solutions Our electrode recommendation Strongly basic, hydrofluoric acid IL-pH-A120MF. and phosphoric acidcontaining solutions attack the glass mem- Suspensions and emulsions brane, particularly at high In solutions with finely-distribut- temperatures. Regeneration by ed particles or substances with lengthy rinsing of the electrodes high viscosity, diaphragms can in 3 M KCl solution is recom- very easily become blocked. mended to a certain extent. Diaphragms with high outflow Strongly oxidizing solutions (e.g. rates which are not susceptible solutions with chlorine, bromine, in this respect, such as the plati- iodine, chromate etc.) can also num diaphragm, can counteract be problematic. this effect. After measurements Platinum diaphragms cannot be in protein-containing solutions used in them. Reference systems (milk, blood etc.) the diaphragm can also be destroyed. In such cas- should be cleaned with a pepsin es the reference electrode must solution. Another option is the be protected with an electro- use of ground-joint diaphragms, lyte key, or a double-electrolyte which are easier to clean. electrode used. Reference electrodes with the Ag/AgCl system Our electrode recommendation are rarely used in sulfide-contain- ScienceLine range e.g. N64 or ing solutions. Chelating agents N62 contaminate both Ag/AgCl and

Hg/Hg2Cl systems. In this case the For use in the pharmaceutical use of a reference electrode with sector, the IoLine electrodes platinum diaphragm and an elec- such as e.g. IL-pH-A120 MF are a trolyte key can help. In addition, good alternative. the problems may be resolved by the use of the IoLine electrodes, Abrasive test material as the reference system of the Moving test material with solid, IoLine electrodes does not con- hard suspended matter can tain silver and therefore does damage the surface of the not lead to the production of glass electrode. This generally anti-soluble silver sulfide. becomes noticeable due to longer response times.

74 This can be compensated for to a certain extent by frequent regeneration of the glass membrane and repeated calibration.

Our electrode recommendation ScienceLine range N64

Solid test material Solid test material, such as dough or soil samples, is not suitable for the determination of pH value without further adaptation. Com- pliance with precisely defined measurement specifications is crucial, so that the test results are comparable. Soil for exemple are suspended in a specified quantity of water, which must be in a precisely-defined ration to the mass of the test material. Then a suspension additive is used

(e.g. CaCl2 solution). The pH value is only measured after a certain settling period. In such cases the exact DIN standard must be followed (e.g. for soil samples DIN 19684).

Our electrode recommendation ScienceLine range e.g. N64 or N62

75 pH handbook 6.2 pH measurement in Theoretical considerations in an organic solution the measurement of pH in non-aqueous systems Relevance of the transfer of the pH measurement to non- The main difference from classic pH measurement in aqueous aqueous systems solutions is that the pH value The demands, particularly of the in accordance with DIN 19260 pharmaceutical industry, on the is only defined in aqueous feasibility and precision of pH media. [12] measurements and titrations in non-aqueous media for the Statements on the H+ activity of purpose of process and quality the pH in water are therefore control are constantly growing. not applicable to other solvents. These analyses are necessary as However similar observations for many of the substances do not aqueous solvents may be made dissolve in water. and the following equation can be employed: It is therefore important to exam- 2HLy H Ly + + Ly – ine to what extent one can speak 2 at all of a classic pH-measure- Aprotic solvents such as e.g. ment in such analyses and how DMSO or benzene do not dis- the electrodes respond in such a sociate in accordance with this medium. An optimum response equation and are not consid- time, that is as short as possible, + ered in this analysis. H2Ly is the is the basis for achieving a re- protonated solvent molecule producible and precise analysis. and is called the lyonium ion. That means as short a period as Ly- is the deprotonated solvent possible until a stable measure- molecule and is called the ment value is obtained. lyate ion. Water-like solvents are autodissociative, which allows a pH scale for this solvent to be introduced. Fig. 30 provides some values for common solvents.

76 solvent Lyonium-ion Lyat-ion pKLy If the pH electrode is calibrated with the usual aqueous buffer

+ - sulfuric acid H3SO4 HSO4 3.6 solutions and a pH measurement is then performed in an aqueous water H O+ OH- 14.0 3 medium, this corresponds to the proverbial comparison of apples methanol CH OH + CH O- 16.7 3 2 3 and oranges.

ethanol C H OH + C H O- 19.1 2 5 2 2 5 At this point it is necessary to distinguish between the two op- Fig. 30 Common solvents with the tions of pH measurement and titration. resulting ions and the pKLy value at 25 °C [13] In titrations, what is considered is not the exact pH value, but a The length of the pH scale is pH jump. The equivalence point based on the pKLy value. In water is used to calculate the content. it is 14 and for ammonia the scale What matters is not the absolute is 22 units long. The neutral point pH‑ value, but only the consump- of the scale is located at half of tion observed during the jump. the pKLy value and describes In non-aqueous media, only a the point at which the lyonium direct mV measurement can and lyate ion activities are equal. be taken. The main reason for For water for example this is 7 at non-comparability and conver- 25 °C. sion of the measured mV‑ value to a pH‑ value in non-aqueous As the determination of the pH solvents is that the activity of value relates to a conventional the hydrogen ions is not known. aqueous pH scale, a pH scale The measurement is made more must be created for each solvent difficult by the existence of a to ensure correct measurements. phase boundary voltage at the Due to the lack of reference diaphragm during contact of the buffer solutions based on the non-aqueous solution with the particular solvent, it is therefore reference electrolyte of the elec- not possible to convert the actual trode. [3] measured mV value as reported by pH electrodes into a pH value.

77 pH handbook In addition, the measurement is Practical experiments in the made more difficult by the low isopropanol and isopropanol/ conductivity in organic solvents. toluene system The effects of lower conductivity For measurement in non-aque- in terms of very unstable mea- ous solvents, the use of surement values can be felt even electrodes with reference in pH measurements in distilled electrolytes similar to the test or demineralized water. medium is recommended. In order to investigate two com- The electrodes and/or their mon solvents in more detail membranes should therefore be and decide whether an electro- conditioned, i.e. reformed, be- lyte similar to the solvent would fore measurement. make measurement easier, or whether the usual 3 molar KCl The resistance of the glass mem- solution could be used, the brane in the corresponding response times of two electrodes solvent is thereby reduced (N64, N6480eth) are analyzed in ensuring a better and faster acidic or basic isopropanol and response time of the electrode. isopropanol/toluene mixtures. The electrode is already adjusted The only difference between to the non-aqueous medium and the electrodes is the reference a faster measurement can be electrolytes. With the N6480eth, taken. [3] this consists of an ethanol solution saturated with LiCl, while So for example before each the N64 is a 3 molar KCl solution. measurement the electrode is first conditioned in water for 30 The configuration of both seconds and then in a buffer of electrodes is shown in Figure 31. pH 7.00 for 30 seconds, and subsequently in the non-aqueous medium such as isopropanol (100%) or isopropanol/toluene mixtures (50%: 50%).

78 Fig. 31 N6480

79 pH handbook Measurement of response If the electrode is pretreated, i.e. time behavior depending reformed, a response time in a on the water content of the non-aqueous solvent of at least sample 30 seconds is to be expected. A reference electrolyte of a 3 molar Measurements are conducted KCl can also be used for the mea- with a variety of water content surement. proportions. For each measurement, 3 ml of aqueous 0.01 mo- These statements should be lar HCl or 0.01 molar NaOH is reviewed again by determining added to 30 ml of isopropanol/ the mV curve for different water water or isopropanol/toluene contents in non-aqueous sol- mixtures. The ratios of the iso- vents with both electrodes. The propanol/toluene mixtures in the response times of N6480eth measurement solutions were al- in pure aqueous systems and ways 50% : 50%. N64 in non-aqueous systems is 30 seconds. The situation is Figures 32 - 36 show the different for N64 in pure aqueous response time behavior of the and N6480eth in a non-aque- single-rod measuring chains N64 ous system. Here the response and N6480eth depending on times are less than 10 seconds the content of water and at three for N64 and approximately 10 different times (0 sec (directly af- seconds for N6480eth. In the ter introduction of the electrode), isopropanol-toluene system the after 30 seconds and after 60 mV curves are more unstable. seconds). Figures 36 - 39 show the results in In the present case, no pH this system with both electrodes measurements can be taken in and varying water contents. non-aqueous medium with a water content of less than 30%, only mV measurements. Only with a water content greater than this is it possible to speak of a classic pH measurement.

80 This experiment shows that the Measurements in the alka- water content is also important line range are more difficult. for the response behavior of the A response time of approximate- electrode, whether measured in ly 80 seconds should be expect- HCl or NaOH. In pure non-aque- ed in pure non-aqueous systems ous solutions using HCl the using the N64. In pure aque- N6480eth is preferred, as it ous systems a stable final value has a faster response time for the N64 will be reached (~ 40 seconds). A very unstable after approximately 20 seconds. curve is produced with water With the N6480eth, a response content in the range of 30 - 40%. time of approximately 60 sec- For measurement in acidic solu- onds for a pure non-aqueous tions, there is a fast adjustment to a system should be expected. stable measurement value due In pure aqueous systems a to the increased H+ ion activity. waiting time of approximately 50 seconds should be expected for the N6480eth.

360

340

320 mV 0 sec N64 300 mV 30 sec N64 mV 60 sec N64 V 280 m mV 0 sec N6480 eth 260 mV 30 sec N6480 eth 240 mV 60 sec N6480 eth

220

200 0,00 20,00 40,00 60,00 80,00 100,00 water content in [%]

Fig. 32 mV curve of both electrodes in isopropanol/water mixtures, after the addition of HCl

81 pH handbook

390

370

350 mV 0 sec N64 mV 30 sec N64 330 mV 60 sec N64 V

m 310 mV 0 sec N6480 eth mV 30 sec N6480 eth 290 mV 60 sec N6480 eth

270

250 0,00 20,00 40,00 60,00 80,00 100,00 water content in [%]

Fig. 33 mV curve of both electrodes in isopropanol/toluene mixtures, after the addition of HCl

0,00 20,00 40,00 60,00 80,00 100,00

-220

mV 0 sec N64 -270 mV 30 sec N64 mV 60 sec N64 V m -320 mV 0 sec N6480 eth mV 30 sec N6480 eth mV 60 sec N6480 eth -370

-420 water content in [%]

Fig. 34 mV curve of both electrodes depending on the water content in isopropanol/toluene mixtures, after the addition of HCl

82 0,00 20,00 40,00 60,00 80,00 100,00 -170

-220 mV 0 sec N64 mV 30 sec N64 -270 mV 60 sec N64 V m mV 0 sec N6480 eth -320 mV 30 sec N6480 eth mV 60 sec N6480 eth -370

-420 water content in [%]

Fig. 35 mV curve of both electrodes in isopropanol/toluene mixtures, after the addition of NaOH

400

350 mV N64 0% water 300

V mV N64 100% water m 250 mV N6480 eth 100% water 200 mV N6480 eth 0% water

150 0 50 100 150 200

time [sec]

Fig. 36 mV curve of both electrodes depending on the water content in isopropanol/toluene mixtures, after the addition of HCl

83 pH handbook

350,00 340,00 330,00 320,00 100% water 310,00 40% water V 300,00 m 30% water 290,00 280,00 0% water 270,00 260,00 250,00 0,00 50,00 100,00 time [sec]

Fig. 37 mV curve of the N6480eth electrode in isopropanol/toluene mixtures, after the addition of HCl

350,00 340,00 330,00 320,00 100% water 310,00 40% water V 300,00 m 30% water 290,00 0% water 280,00 270,00 260,00 250,00 0,00 50,00 100,00 time [sec] Fig. 38 mV curve of the N64 electrode in isopropanol/toluene mixtures, after the addition of HCl

84 0,00 50,00 100,00 -200,00 -220,00 -240,00 -260,00 100% water -280,00 40% water V -300,00 m 30% water -320,00 0% water -340,00 -360,00 -380,00 -400,00 time [sec]

Fig. 39 mV curve of the N64 electrode in isopropanol/toluene mixtures, after the addition of NaOH

350,00 340,00 330,00 320,00 100% water 310,00 40% water V 300,00 m 30% water 290,00 0% water 280,00 270,00 260,00 250,00 0,00 50,00 100,00 time [sec] Fig. 40 mV curve of the N6480eth electrode in isopropanol/toluene mixtures, after the addition of NaOH

85 pH handbook 6.3 Summary In the systems investigated here, no pH measurements can be taken in a non-aqueous medium with a water content of less than 30 %, only mV measurements.

Only with a water content greater than this is it possible to speak of a classic pH measurement. In the isopropanol/water system, a response time of 30 seconds should be expected, in the isopropanol/ toluene system a response time of 60 seconds should be expected at 0 % water, even when the electrode has been pre-treated, (reformed).

A reference electrolyte of a 3 molar KCl can also be used in measurements with a water content of >30%. It can therefore be said that the water content of the sample and the H+ ion activity are the determining factors in the selection of the most suitable electrode.

Particular emphasis must be placed on the maintenance and care of electrodes, in order to optimize the useful life.

86 SECTION 7 RECOMMENDATIONS FOR USE, MAINTENANCE AND CARE OF ELECTRODES 7.1 Electrode recommendations Due to the large range of The higher electrolyte column electrodes we offer, the and the associated greater following table provides a few electrolyte outflow reduce representative examples of the undesired diffusion potentials at different measurement technol- the diaphragm and rinse it clear. ogies and the recommended For some applications, other areas of use in each case. electrode recommendations may be advisable due to the Accordingly, the BlueLine 11 pH specific test conditions, as even electrode is representative of identical applications can vary the versions 12, 14, 15, 17, 18 significantly due to differences in and 19 pH. For the ScienceLine concentrations or temperatures. and IoLine pH electrodes, in the versions N62 and H62 as well as Please also take note of the ILpHA120MF and ILpHH120MF resistance of the sensor material it should be noted that these are in respect to test medium. also available with longer shaft lengths, which provide a faster and more stable measurement result under the same test conditions, as well as a longer service life for the electrodes.

87 pH handbook

pH measurement ORP Conductivity Electrode series IoLine BlueLine ScienceLine BlueLine ScienceLine

Sensor example IL-pH-A120MF IL-pH-H120MF 7780 A H 62 H 64 32 L 8280 L N 62 N 64 11 pH 22 pH 13 pH Ag 6280 Pt 62 Pt 8280 31 RX 32 RX NFTC T LF 313 T LF 413 T LF 613 T LF 713 Application area Application Application Etching and degreasing baths

Bleach and dyeing solutions

Cutting oil emulsions

Cyanide detoxification

Dispersion paint

Emulsions, water-based

Emulsions, partly water-based

Paint/varnish, water-soluble

Fixing bath

Varnish, water-based

Varnish, partly water-based

Lye, extreme Chemistry

Oil/water-emulsions

Organic percentile high

Paper extract

Acid, extreme

Sulphide containing liquid

Suspension, water-based

Ink

Viscose samples

Beck -

Ground water

Lake water

Seawater surements Field mea Field Rain water

Beer

Fruit juice

Vegetable juice

Lemonades/soda

Mineral water

Juice

Spirits

Wine Beverage production Beverage L 32 L H 62 H 64 N 62 N 64 Pt 62 31 RX 32 RX 11 pH 22 pH 13 pH L 8280 L A 7780 A Pt 8280 LF 413 T LF 413 T LF 613 T LF 713 Ag 6280 IL-pH-A120MF LF 313 T NFTC T LF 313 IL-pH-H120MF

88 pH measurement ORP Conductivity Electrode series IoLine ScienceLine BlueLine ScienceLine BlueLine ScienceLine

Sensor example IL-pH-A120MF IL-SP-pH-A 7780 A A N 1048 32 L 39 L 6880 L 8280 L N 62 N 64 11 pH 22 pH 13 pH 21 pH 27 pH Pt 62 Pt 6140 Pt 8280 31 RX 32 RX NFTC T LF 313 T LF 413 T LF 613 T LF 713 Application area Application Application Creme

Hair dye

Hair gel

Hair mousse

Lotions

Make-up

Mouth wash Cosmetics Shaving cream

Sun lotion

Tooth paste

Ground (extract/slug)

Fertilizer solution

Vegetables

Liquid manure

Agriculture Fruit

Bread/dough/pastry

Vinegar

Grease

Fish

Meat

Honey

Margarine

Coffee extract

Food production Food Jam/marmelade

Mayonnaise

Sausage

Butter

Yoghurt

Cheese

Dairy Milk

Cream

Skin

Leather

Paper

Surface Textiles IL-pH-A120MF IL-SP-pH-A 7780 A A N 1048 32 L 39 L 6880 L 8280 L N 62 N 64 11 pH 22 pH 13 pH 21 pH 27 pH Pt 62 Pt 6140 Pt 8280 31 RX 32 RX NFTC T LF 313 T LF 413 T LF 613 T LF 713

89 121210_oF_SI_Lab_042_111_electro_D+US.indd 52 10.12.12 12:04 pH handbook

pH measurement ORP Conductivity Electrode series IoLine ScienceLine BlueLine ScienceLine BL* ScienceLine

Sensor example IL-pH-A120MF IL-pH-H120MF IL-Micro-pH-A IL-SP-pH-A 157 A 7780 A H 62 H 64 A N 1048 32 L 39 L 6880 L 8280 L N 62 N 64 A N 6000 N 6003 11 pH 22 pH 13 pH 16 pH 21 pH 27 pH Pt 62 Pt 6140 Pt 8280 A Pt 5900 31 RX 32 RX T LF 213 T LF 313 NFTC T LF 313 T LF 413 T LF 613 T LF 713 Application area Application Application Agar-agar gel

Enzyme solution

Infusion solutions

Small vessels/sample quantitiy

Bacteria cultures

Gastric juice

NMR tubes

Precision measurement

Protein containing liquid

Serum

medicine, microbiology medicine, Tris puffer

Urine

Pharmacy, biology, biotechnology, biotechnology, biology, Pharmacy, Vials

Cooling water

Lye, hot cal Acid, hot Techni-

Detergents

Disinfectant

Cleaning agent

Soap solution agents

Washing Washing Dishwashing liquid

Surfactant solution

Waste water, general

Aquarium water

Demineralization/ion exchanger

pH values, extreme

Media containing low ions

Boiler feed water

Water Condensate

Purity water

Salt solution

Drinking water

Drops

* BL = BlueLine IL-pH-A120MF IL-pH-H120MF IL-Micro-pH-A IL-SP-pH-A 157 A 7780 A H 62 H 64 A N 1048 32 L 39 L 6880 L 8280 L N 62 N 64 A N 6000 N 6003 11 pH 22 pH 13 pH 16 pH 21 pH 27 pH Pt 62 Pt 6140 Pt 8280 A Pt 5900 31 RX 32 RX T LF 213 T LF 313 NFTC T LF 313 T LF 413 T LF 613 T LF 713

90 121210_oF_SI_Lab_042_111_electro_D+US.indd 53 10.12.12 12:04 7.2 Maintenance and care of the electrodes Particular emphasis must be • The diaphragm must be placed on the maintenance and immersed in the measurement care of electrodes in order to solution. optimize useful life. We can make the following • For low-maintenance elect- recommendations with respect rodes with gel filling, such as to handling and care: DURALID® or REFERID®, there is no need to refill. • If a watering cap is located above the membrane and dia- • Soaking with electrolyte solu- phragm, it should be removed. tion is particularly important It contains electrolyte solution with these electrodes. L300 (potassium chloride solution 3 mol/l). The electrode is Measuring the pH value ready for measuring. Please also observe the user instructions of the measuring • Dry-stored electrodes are equipment when calibrating soaked for 24 hours in electro- and measuring. In order to lyte solution. minimize distortions in the results, electrodes that are used under • In the electrolyte space of the extreme conditions or at the reference system, any missing limits of the specified application potassium chloride solution ranges must be calibrated more should be refilled. frequently. For precise calibra- tions, we recommend the use • The fill level of the electrolyte of our hot vaporsterilized buffer solutions should always be at vials certified according to DIN least 5 cm above the level of the 19266 . Use only fresh buffer test medium. solutions at all times.

• To calibrate and measure liquid electrolyte electrodes, the cap of the refill opening must be opened.

91 pH handbook Storage and maintenance Cleaning • Electrodes must be stored • Contamination on the mem- between 0 and 40°C. Depend- brane, PT sensor and dia- ing on storage conditions phragm result in measurement (temperature and humidity), deviations. Depositions can be the liquid in the watering cap removed with thinned mineral can dry out prematurely. In this acids (e.g. hydrochloric acid 1:1), case, the electrode must be organic contamination can be soaked for at least 24 hours in a dissolved with suitable solvents, potassium chloride solution of fats can be removed with surfac- 3 mol/l, before it is ready for use. tant solutions, and protein can be dissolved with pepsin solu- • In pH measuring chains and tion (cleaning solution L510). reference electrodes with liquid Rinse the electrode with distilled electrolytes, the electrolyte must water after cleaning, do not rub occasionally be topped off or dry. changed. • Ceramic diaphragms clogged • Crystals in the electrolyte from the outside become us- space of liquid electrolyte elec- able again after careful rubbing trodes can be dissolved by with fine sandpaper or a dia- gently warming in a water bath. mond file. The pH glass mem- The electrolyte solutions should brane must not be scratched! then be changed and the electrode calibrated. • Platinum diaphragms must not be mechanically cleaned. Chemical cleaning (e.g. with diluted hydrochloric acid) can be carried out after unblocking (e.g. by vacuuming).

92 • Ground-joint diaphragms can 7.3 Summary be made functional again be- The recommendations made fore measurement by gently here with respect to cleaning raising and then pushing up the and maintenance should be fol- ground socket to the ground lowed in order to obtain the most core. The refill opening should reliable measurements possible be open during this procedure. and achieve the longest service Warning: electrolyte will flow life for the electrodes. The elec- out more quickly when doing trode must always be matched to so, enabling thorough wetting the particular application in order of the ground surface. The glass to achieve the best test results. membrane can be cleaned by wiping with an ethanol-soaked, lint-free cloth.

Quality Each electrode must meet the quality requirements of our final inspection in order to provide the best measurement reliability, response speed and service life. It is not possible to make a general statement as to the service life of the electrode. The service life is very dependent on the operating conditions. Extreme conditions such as high or frequently changing temperatures, strong acids and alkalis, protein-containing or very contaminated solutions or electrode poisons such as sulfide, bromide and iodide reduce the service life of the electrode. Hydrofluor- ic acid and hot phosphoric acid adversely affect the glass.

93 pH handbook INDEX OF TECHNICAL TERMS Resolution: smallest difference Chain voltage: the electrode voltage U between two measurement values that is the measurable voltage of an electrode the display of a measurement device is in a solution. It is the same as the sum of capable of displaying. all galvanic voltages in the electrode. Its dependence on pH is produced by the Diaphragm: a body in the wall of the electrode function, which is characterized reference electrode or electrolyte key by the slope and zero point parameters. housing which can be permeated by solutions. It transmits the electrical contact Measured property: the measured between the two solutions and hampers property is the physical property the exchange of electrolytes. The term identified by the measurement, i.e. pH or diaphragm is also used for ground-joint conductivity. and diaphragmless transfer. Measurement solution: Term for Adjustment: to intervene in a the sample ready to be measured. A measurement device so that the output measurement sample is usually obtained values (e.g. the display) deviate as little by preparation from the analysis sample as possible from the correct value or a (original sample). Measurement solution value deemed to be correct, or so that and analysis sample are therefore deviations remain within tolerances. identical if no preparation takes place.

Calibrate: comparison of the output Measurement value: is the specific values in a measurement device (e.g. value of a measured property to be the display) with the correct value or identified. It is specified as the product of a value deemed to be correct. The a numerical value and a unit (e.g. 3 m; 0.5 term is also commonly used when the s; 5.2 A; 373.15 K). measurement equipment is adjusted at the same time (see adjustment). Molality: is the amount (in moles) of a dissolved substance in 1000g of solvent. Chain zero point: The zero point of a pH electrode is the pH value at which the Zero point: Term for the offset voltage pH electrode chain voltage is zero for a of a pH electrode. It is the measurable given temperature. Unless otherwise chain voltage of a symmetrical electrode noted, this is at 25 °C. whose membrane is immersed in a solution with the pH of the nominal electrode zero point (pH = 7).

94 Offset voltage: the measurable chain Standard solution: is a solution voltage of a symmetrical electrode whose measurement value is by whose membrane is immersed in a definition known. It is used to calibrate solution with the pH of the nominal measurement equipment. electrode zero point. The zero point is a component of the offset voltage. Slope: the incline of a linear calibration function. pH value: is a measurement for the acidic or basic effect of an aqueous Temperature function: term for a solution. It corresponds to the negative mathematical function which provides log base 10 of the molal hydrogen ion the temperature behavior e.g. of a activity divided by the unit of molality. measurement sample, a sensor or a part The actual pH value is the measurement of a sensor. value of a pH measurement. Temperature coefficient: Value of the Potentiometry: Term for a form of slope of a linear temperature function. measurement technology. The signal of the electrode used that depends on the Temperature compensation: Term measurement property is the electrical for a function which calculates the voltage. The electrical current remains effect of temperature on the measured constant. property. The method of temperature compensation varies depending on the Redox voltage: is caused by oxidizing measured property to be determined. or reducing substances dissolved in the For potentiometric measurements, water, if these become effective at the the slope value is adjusted to the surface of an electrode (e.g. one made temperature of the measurement probe, from platinum or gold). however the measurement value is not converted. Reference temperature: specified temperature for comparison of Resistance: Short term for specific temperature dependent measurement electrolytic resistance. It corresponds to values. In measurements of conductivity, the inverse of electrical conductivity. the measurement value is converted to a conductivity value at 20 °C or 25 °C reference temperature.

95 pH handbook BIBLIOGRAPHY

[1] N. G. Connelly, IUPAC Nomen- [11] DIN 19268 pH Messung von clature of inorganic chemistry, IU- wässrigen Lösungen mit pH Mess- PAC recommendations 2005 ketten, pH-Glaselektroden und Ab- schätzung der Messunsicherheit, [2] Römpp-Chemie Lexikon, J.Falbe, Beuth Verlag Berlin, 2007 M. Regitz 1991, Georg Thieme Verlag, Stuttgart [12] DIN 19260 pH-Messung Allge- meine Begriffe, Beuth Verlag Berlin, [3] H.Galster, pH Messung, VCH Wein- 2005 heim, 1990 [13] T. Mussini et al., Criteria for stan- [4] Greenbook IUPAC 2nd ed. dardization of pH measurements in organic solvents and water + orga- [5] K. Schwabe, pH- Messtechnik, nic solvent mixtures of moderate to Verlag Theodor Steinkopf 1963 high permitivities, Pure and applied chemistry, Vol 57,6, 1085, p. 865-876. [6] J.N. Brønstedt, Zur Theorie der Catquit quodienihil tebate nostraedi Säuren und Basen und der proteoly- fore adeesciti, se, sceres a redieni h tischen Lösungsmittel, Z. Phys.Chem, 169A (1934) 52-74

[7] Hollemann-Wiberg, Lehrbuch der anorganischen Chemie, 102. Auflage, Verlag Walter de Gruyter, Berlin, New York 2007

[8] G. Tauber, Störpotentiale am Di- aphragma, Laborpraxis 6, 1982

[9] DIN 19266, 2000-01, pH-Mes- sung - Referenzpufferlösungen zur Kalibrierung von pH-Meßeinrichtun- gen, Beuth Verlag, Berlin (2000)

[10] DIN V ENV 13005, Leitfaden zur Angabe der Unsicherheit beim Mes- sen, Berlin und Köln, o.J.

96 97 We express our core competence, namely the production of analytical instruments, with our company name SI Analytics. SI also stands for the main products of our company: sensors and instruments.

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