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Introduction to Ph INTRODUCTION the MOLAR CONCEPT Ion” Are Used Interchangeably in Ph Measurement Applications

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Technical pH Reference Introduction to pH INTRODUCTION THE MOLAR CONCEPT ion” are used interchangeably in pH measurement applications. pH is a unit of measure which describes A mole of a compound is defined as the degree of acidity or alkalinity of a Avogadro’s number of molecules Some compounds form weak acids or solution. It is measured on a scale of 0 (6.02 x 1023 molecules), which has bases; only a very small percentage of to 14. The term pH is derived from “p,” a mass approximately equal to the such compounds dissociates into its the mathematical symbol for negative molecular weight, expressed in grams. constituent ions, so very few hydrogen or logarithm, and “H,” the chemical symbol For example, sodium hydroxide, NaOH, hydroxyl ions are formed. An example of for Hydrogen. The formal definition which has a molecular weight of this is acetic acid, which forms less than of pH is: the negative logarithm of 23 + 16 + 1 = 40, would have 40 grams one hydrogen ion for every one hundred Hydrogen ion activity. in a mole. Since the atomic weight of the molecules: hydrogen ion (H+) is one (1), there is + pH = -log[H+] H2O + CH3COOH → H3O + one gram of hydrogen ions in a mole of CH COO- pH provides needed quantitative hydrogen. A solution with a pH of 3 information by expressing the degree of -10 Pure water also dissociates weakly, with 10 has 1 x 10 moles of hydrogen ions, -7 -7 activity of an acid or base in terms of its or 10-10 grams in a one liter solution. 10 hydrogen and 10 hydroxyl ions hydrogen ion activity. formed for every water molecule at 25°C: IONIZATION + - The pH value of a substance is directly 2H2O → H3O + OH related to the ratio of hydrogen ion [H+] An ion is a charged particle, created by - an atom or molecule which has either The addition of acid to water increases and hydroxyl ion [OH ] concentrations. the concentration of hydrogen ions If the H+ concentration is greater than gained or lost one or more electrons. - The presence of ions in solution allows and reduces the concentration of the OH , the material is acidic; i.e., hydroxyl ions. A base added to water the pH value is less than 7. If the OH- electrical energy to pass through the + solution as a conductor. Different has the opposite effect, increasing the concentration is greater than its H , the concentration of hydroxyl ions and material is basic, with a pH value greater compounds form ions in solution in + - different amounts, depending on the reducing the concentration of than 7. If equal numbers of H and OH hydrogen ions: ions are present, the material is neutral, ability of the atoms to gain or lose + - with a pH of 7. electrons. They will dissociate (or ionize) H2O + HCI → H3O + CI + + - in solution to form hydrogen (H ) or H2O + NaOH → Na + H2O + OH Acids and bases have free hydrogen hydroxyl (OH-) ions in the solution. and hydroxyl ions, respectively. Since There is a wide variety of applications the relationship between hydrogen Molecules that dissociate easily form for pH measurement. For example, pH ions and hydroxyl ions in a given strong acids or bases when in aqueous measurement and control is the key to solution is constant for a given set of solution (water solvent). Examples of the successful purification of drinking conditions, either one can be determined these are hydrochloric acid (HCI) or water, the manufacture of sugar, by knowing the other. Thus, pH is sodium hydroxide (NaOH): sewage treatment, food processing, + - electroplating, and the effectiveness a measurement of both acidity and HCI + H2O → H3O + CI alkalinity, even though by definition it is NaOH → Na+ + OH- and safety of medicines, cosmetics, a selective measurement of hydrogen etc. Plants require the soil to be within In an aqueous solution, hydrogen ions ion activity. Since pH is a logarithmic a certain pH range in order to grow normally combine with the water solvent function, a change of one unit of properly, and animals can sicken or die to form the hydronium ion (H O+). pH represents a ten-fold change in 3 if their blood pH level is not within the pH measurements of these solutions hydrogen ion concentration. Table correct limits. Figure 1, next page, gives are therefore measurements of the 1 shows the concentration of both pH values for some common industrial hydronium ion concentration. Normally, hydrogen ions and hydroxyl ions at and household products. the terms “hydronium ion” and “hydrogen different pH values. pH MEASUREMENT HYDROGEN ION CONCENTRATION A rough indication of pH can be obtained Table 1 IN MOLES/LITER AT 25°C (77°F) using pH papers or indicators, which pH H+ OH- change color as pH level varies. These indicators have limitations on their 0 (100) 1 0.00000000000001 (10-14) accuracy, and can be difficult to interpret 1 (10-1) 0.1 0.0000000000001 (10-13) correctly in colored or murky samples. 2 (10-2) 0.01 0.000000000001 (10-12) More accurate pH measurements 3 (10-3) 0.001 0.00000000001 (10-11) are obtained using a pH meter. A 4 (10-4) 0.0001 0.0000000001 (10-10) pH measurement system consists of 5 (10-5) 0.00001 0.000000001 (10-9) four components: a pH measuring 6 (10-6) 0.000001 0.00000001 (10-8) electrode, a reference electrode, a high input impedance meter, and a sample 7 (10-7) 0.0000001 0.0000001 (10-7) of the material to be measured. The -8 -6 8 (10 ) 0.00000001 0.000001 (10 ) pH electrode can be thought of as a 9 (10-9) 0.000000001 0.00001 (10-5) battery, with a voltage that varies with 10 (10-10) 0.0000000001 0.0001 (10-4) the pH of the measured solution. The 11 (10-11) 0.00000000001 0.001 (10-3) pH measuring electrode is a hydrogen 12 (10-12) 0.000000000001 0.01 (10-2) ion-sensitive glass bulb, with a millivolt output that varies with changes in the 13 (10-13) 0.0000000000001 0.1 (10-1) -14 0 relative hydrogen ion concentration 14 (10 ) 0.00000000000001 1 (10 ) inside and outside the bulb. To order, call 1-800-826-6342 or shop online at omega.comSM 1 Technical MID-POINTS OF pH RANGES pH VALUES OF SOME FOR PROCESS CONTROL COMMON SUBSTANCES EXTREMELY ALKALINE 14.0 HOUSEHOLD LYE of one electrode and another, as well BOTTLE WASHING as changes in the output of electrodes 13.0 COPPER PLATING over time. Therefore, each system must BLEACH be calibrated periodically. Buffers are 12.0 available with a wide range of pH values, and they come in both in premixed BRASS PLATING AMMONIA liquid form or as convenient dry powder 11.0 capsules. Most pH meters require calibration at several specific pH values. MILK OF MAGNESIA One calibration is usually performed 10.0 near the isopotential point (the signal produced by an electrode at pH 7 is LIME-SODA 0 mV at 25°C/77°F), and a second is SOFTENING BORAX 9.0 typically performed at either pH 4 or pH 10. It is best to select a buffer as close BAKING SODA as possible to the actual pH value of the sample to be measured. SALT WATER AQUARIUM 8.0 SEA WATER TEMPERATURE EFFECTS SWIMMING POOL WATER BLOOD FRESH WATER AQUARIUM 7.0 DISTILLED WATER As previously stated, the pH electrode MILK is temperature dependent, and may be compensated for in the pH meter BREWING PROCESS CORN 6.0 circuitry. The circuitry of the pH meter NICKEL PLATING utilizes the Nernst equation, which is a general mathematical description of FOOD PROCESSING 5.0 BORIC ACID electrode behavior. E = E + 2.3RT log (a ) ORANGE JUICE x _______K i 4.0 nF PICKLE PROCESSING where: PHOTOENGRAVING 3.0 Ex = constant (depending upon VINEGAR reference electrode) LEMON JUICE 2.0 R = constant TK = absolute temperature (Kelvin) n = charge of the ion (including sign) 1.0 F = constant BATTERY ACID a = activity of the ion EXTREMELY ACID 0.0 i Figure 1 For pH measurement, we are interested in the hydrogen ion for H+: The reference electrode output does not temperature sensitive. The temperature 2.3RTK vary with the activity of the hydrogen ion. compensation can be either manual or ________ The pH electrode has very high internal automatic. With manual compensation, = 59.16 mV resistance, making the voltage change a separate temperature measurement nF with pH difficult to measure. The input is required, and the pH meter’s manual impedance of the pH meter and leakage compensation control can be set with where: n = 1 and T = 25°C. This term resistances are therefore important the approximate temperature value. With is commonly known as the Nernst factors. The pH meter is basically a high automatic temperature compensation coefficient. Since pH is defined as the impedance amplifier that accurately (ATC), the signal from a separate negative logarithm of hydrogen ion measures the minute electrode voltages temperature probe is fed into the activity, the general equation at any and displays the results directly in pH pH meter, so that it can accurately temperature can be expressed as: units on either an analog or digital determine the pH value of the sample at display. In some cases, voltages can that temperature. E = Ex – 1.98 TKpH also be read for special applications Changes in the temperature of a solution or for use with ion-select or Oxidation- BUFFER SOLUTIONS will alter the millivolt output of the glass Reduction Potential (ORP) electrodes.
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