Essentials of pH Measurement
Kelly Sweazea, Specialist Thermo Scientific Water Analysis & Purification Products Common Questions: Measuring pH
What is pH?
“Potential Hydrogen” or “Power of Hydrogen” pH electrodes are a type of ion selective electrode (ISE) measuring free hydrogen ion activity
2 Proprietary Common Questions: What is pH?
The Theoretical Definition: pH = - log aH
• aH is the hydrogen ion activity. • In solutions that contain other ions, activity and concentration are not the same. • The activity is an effective concentration of hydrogen ions, rather than the true concentration; it accounts for the fact that other ions surrounding the hydrogen ions will shield them and affect their ability to participate in chemical reactions. • These other ions effectively change the hydrogen ion concentration in any process that involves H+.
3 Proprietary Common Questions: What is pH?
• The pH of pure water around room temperature is about 7. • pH 7 is considered "neutral" because the concentration of hydrogen ions (H+) is exactly equal to the concentration of hydroxide (OH-) ions produced by dissociation of the water. • Increasing the concentration of H+ in relation to OH- produces a solution with a pH of less than 7, and the solution is considered "acidic". • Decreasing the concentration H+ in relation to OH- produces a solution with a pH above 7, and the solution is considered "alkaline" or "basic".
4 Proprietary Common Questions: What is pH? Representative pH values Substance pH Hydrochloric Acid, 10M -1.0 The pH Scale Lead-acid battery 0.5 Gastric acid 1.5 – 2.0 + • [H ] activity increases by a Lemon juice 2.4 factor of 10 for every pH unit. Cola 2.5 Vinegar 2.9 • Cola pH is about 2.5. Cola Orange or apple juice 3.5 is 10x more acidic than Beer 4.5 Acid Rain <5.0 Orange Juice (pH of 3.5) Coffee 5.0 Tea or healthy skin 5.5 • Cola is 100x more acidic Milk 6.5 than Beer! (pH of 4.5) Pure Water 7.0 Healthy human saliva 6.5 – 7.4 Blood 7.34 – 7.45 Seawater 7.7 – 8.3 Hand soap 9.0 – 10.0 Household ammonia 11.5 Bleach 12.5 Household lye 13.5
5 Proprietary Common Questions: Measuring pH
Calibration •The Nernst Equation
E = E0 + s log aH • E = measured potential
• E0 = reference potential • s = slope = RT/nF = 59.2 mV at 25 oC
• aH = activity
6 Proprietary pH Measurement System
• When two solutions containing different concentrations of H+ ions are separated by a permeable glass membrane, a voltage potential is developed across that membrane. (Sensing electrode) • A voltage potential is also generated from the reference electrode. • The pH meter measures the voltage potential difference (mV) between the sensing electrode and the outside sample (reference electrode) • An algorithm in the meter firmware translates the received mV signal into a pH scale. sensing membrane
reference membrane
7 Proprietary pH Measurement System
The pH Meter • Acts as a volt meter • Translates electrode potential (mV) to pH scale Meter functions • Stores calibration curve • Adjusts for temperature changes • Adjusts electrode slope • Signals when reading is stable Features • mV and relative mV scales • Autocalibration /autobuffer recognition • Number of calibration points • Display information • RS232 or recorder outputs • Datalogging • GLP/GMP compliant
8 Proprietary pH Measurement System
The pH Electrode
Sensing Bulb
Internal Fill Solution (Sensing)
Reference
Reference Fill Solution
Junction
9 Proprietary pH Measurement System
Reference Electrode
• In a two electrode system a reference half-cell electrode is needed to complete the “circuit”.
• The reference wire or element is typically encased in Saturated AgCl or KCl
• The reference must have a “liquid” connection to the sample in order to generate a voltage potential.
10 Proprietary Common Questions: Electrode Types
What is a combination pH electrode? • A combination pH electrode is one that has a sensing half-cell and reference half-cell built into one electrode body instead of existing as two separate electrodes.
11 Proprietary Common Questions: Electrode Types
What is a triode?
• A triode is a combination electrode (sensing and reference cells) together with an ATC (automatic temperature compensation thermistor) built into one electrode body.
12 Proprietary Common Questions: Electrode Components
pH Electrode Reference Types • Calomel reference ++ • Fixed Hg2 activity in contact with solid mercury • Silver reference • Fixed Ag+ activity in contact with silver wire • Single and double junction design • ROSS reference • Redox couple (Iodide/Iodine) • Double junction design
13 Proprietary pH Measurement System – Reference Types
Single Junction • Recommended for all applications except those involving TRIS buffer, proteins, metal Silver/Silver Chloride ions, sulfides or other substances that will react Reference (Ag/AgCl) with either Ag or AgCl.
• Mid-range cost, Variety of body styles, Advantages Refillable or gel-filled, Good Precision (±0.02 pH)
• Temperature Hysteresis, complexation in Disadvantages samples such as: TRIS, proteins, sulfides
14 Proprietary pH Measurement System – Reference Types
Double Junction • The double junction Ag/AgCl Silver/Silver reference isolates the reference, making it ideally Chloride Reference suited for all types of (Ag/AgCl) samples.
• Mid-range cost, Variety of body styles, Refillable or gel- Advantages filled, Good Precision (±0.02 pH)
Disadvantages • Temperature Hysteresis
Mercury Free alternative to the Calomel Reference
15 Proprietary pH Measurement System - Reference Types
• Double Junction Iodine/Iodide redox couple ROSS™ Reference • The ROSS™ reference is ideally suited for all sample types and all temperature ranges
• Variety of body styles, Unmatched Precision (±0.01 pH), Fast response, Advantages Stable to 0.01 pH in 30 seconds over 50 oC temperature change, Drift less than 0.002 pH units/day
Disadvantages • Cost
Mercury Free alternative to the Calomel Reference
16 Proprietary pH Measurement System - Junctions
• The electrode junction is where the Outer fill solution (reference) passes from inside the electrode body to the sample completing the “circuit”.
• The type of junction is a good indicator of how the electrode will perform in different samples.
• Three basic types of junctions • Wick • Ceramic • Open
17 Proprietary pH Measurement System - Junctions
The Wick • Glass fiber, fiber optic Junction bundles, Dacron, etc.
• Used in rugged epoxy bodies Advantages • Good for aqueous samples
• Will clog if sample is “dirty” or viscous Disadvantages • Not as “fast” as other junctions
18 Proprietary pH Measurement System - Junctions
The Ceramic • Porous ceramics, wooden Junction plugs, porous teflon, etc.
• Good all-purpose junction Advantages • Ideally suited for most lab applications
• Will clog if sample is “dirty” or Disadvantages viscous
19 Proprietary pH Measurement System - Junctions
• Sure-Flow, Laser Drilled Hole, The Open Junction Ground Glass Sleeve, etc.
• Junction will never clog • Can be used in all sample types Advantages • Ideal choice for “dirty” or viscous samples • Can be used in non-aqueous samples
“Sure-Flow” • Sure-Flow Junction has a higher Disadvantages liquid junction flow rate of fill solution
20 Proprietary Common Questions: Electrode Types
What is meant by a “single junction?”
• There is one junction in the electrode body. This term applies to Ag/AgCl electrodes that have a silver reference wire and silver ions dispersed in the internal electrolyte fill solution.
ceramic junction
21 Proprietary Common Questions: Electrode Types
What is meant by a “double junction?”
• There are two junctions in the electrode body. This term applies to any electrode that has a ROSS or calomel electrodes and also to some Ag/AgCl electrodes.
22 Proprietary pH Measurement System – Electrode Types
Refillable or Low Maintenance Gel? Low Maintenance Gel Electrodes • Easy to use • Rugged epoxy body • 0.05-0.1 pH precision • Slower response rate • 6 month average life • Gel memory effects at junction Refillable Electrodes • Fill/drain electrode • Wide applicability • Glass or epoxy body • 0.02 pH precision • Faster response rate • 1 year minimum life • Replaceable fill solution
23 Proprietary pH Measurement System – Electrode Types
Polymer or Low Maintenance Gel? Low Maintenance Gel Electrodes • Easy to use • Rugged epoxy body • 0.05-0.1 pH precision • Slower response rate • 6 month average life • Gel memory effects at junction Polymer Electrodes • Low maintenance • Easy to use • Glass or epoxy body • 0.02 pH precision • Faster response rate • 1 year minimum life • Double junction design
24 Proprietary Common Questions: Temperature Compensation
Why is temperature compensation important when measuring pH ?
• Samples / buffers have different pH values at different temperatures • Temperature compensation will contribute to achieving accurate measurements
25 Proprietary Common Questions: Temperature Compensation
The pH electrode slope is the change in mV value divided by the Nernstian theoretical value.
At 25°C, the expected change in mV per pH unit would be 59.2 mV.
26 Proprietary Common Questions: Temperature Compensation
• Newer meters automatically calculate slope • Nernstian calculation of slope at 25°C (59.2 mV/pH unit) • Example: • pH buffer 7 = -10 mV • pH buffer 4 = +150 mV
between these 2 buffers there’s a range of 160 mV 59.2 mV x 3 pH units = 177.6 mV
• Slope = 160 mV / 177.6 mV = 90.1%
27 Proprietary Common Questions: Temperature Compensation
• Temperature affects calibration slope because it affects the expected change in the mV value per pH unit
28 Proprietary Common Questions: Temperature Compensation
• Nernstian calculation of slope at 50°C (64.0 mV/pH unit) • Example: • pH buffer 7 = -10 mV • pH buffer 4 = +150 mV
between these 2 buffers there’s a range of 160 mV 64.0 mV x 3 pH units = 192.0 mV
• Slope = 160 mV / 192.0 mV = 83.3%
29 Proprietary Common Questions: Temperature Compensation
• Temperature compensation will adjust the calibration slope across a wide temperature range
• It is not possible to normalize pH readings to a specific temperature, but it is possible to get an accurate pH measurement for any sample temperature
30 Proprietary Common Questions: Temperature Compensation
Temperature Compensation Strategies • Calibrate and measure at the same temperature • Use automatic temperature compensator (ATC) or 3-in-1 Triode electrode • Manually temperature compensate using temperature control on meter • Use LogR temperature compensation • Record temperature with pH readings
31 Proprietary Common Questions: Calibration
I have small containers on my bench that are labeled and filled with fresh buffer each week. We re-use these buffers all week. Will this practice affect my calibration?
Cal 1, using fresh 7 and 10 buffer: • slope between 7-10 = 96.7%
Cal 2, using fresh 7 and old* 10 buffer: • slope between 7-10 = 93.4% * set on shelf uncovered for 8 hours
ALWAYS use fresh buffer for each calibration. Don’t re-use today’s buffer for tomorrow’s calibration!
32 Proprietary Common Questions: Stable Readings
Why does it take so long to get a stable reading?
• electrode performance and efficiency • junction and bulb function (non-clogged and non-coated) • electrode type (gel effects, open junction, etc.) • meter stabilization settings (if available) • resolution settings
33 Proprietary Common Questions: Stable Readings… continued
Why does it take so long to get a stable reading?
• inner fill-solution freshness • low ionic strength samples • use an electrode with an open junction • stir the samples during measurement • stirred or not? • air bubbles near bulb
34 Proprietary Common Questions: Maintenance
Is there a cleaning routine I can follow to keep my electrode working?
• refresh inner fill solution • use recommended storage solution • close fill hole at end of day • use cleaning remedies if a coated bulb or a clogged junction is the suspected cause of a poor calibration slope
35 Proprietary Common Questions…
Would you please tell me what is happening here??? pH Clinic Program FREE pH Meter / Electrode Diagnostic Service
• Meter and Electrode evaluated individually • Troubleshooting advice • Care and maintenance planning • Electrode selection recommendations • Leave-behind report • Method evaluation and applications support
36 Proprietary 37 Proprietary Conductivity Measurement
Kelly Sweazea, Specialist Thermo Scientific Water Analysis & Purification Products Properties of Conductivity
Current is carried by electrons
• A wire with 1 ohm resistance allows a current of 1 amp when 1 volt is applied • Resistance to the flow of electrons = Voltage/Current
Units of resistance are measured in ohms
Conductance is the reciprocal of resistance • Conductance of the electrons = Current/Voltage
Units of conductance are measured in Siemens
1 Siemen = 1 mho = 1/ohm 1 Siemen = 1000 mS = 1,000,000 µS
39 Proprietary Common Units and Symbols
Conductance Units Resistance Units • S (Siemens) • (Ohm) • mS • k • S • M Conductivity Units Resistivity Units • S/cm • cm • mS/cm • kcm • S/cm • Mcm
40 Proprietary Conductivity in Solutions
Conductivity carried by ions is dependent upon: • Concentration (number of carriers) • Charge per carrier • Mobility of carriers K+ SO -2 4 Na+ Cl-
Conductivity = (concentration) x (charge per carrier) x (mobility of the carriers)
41 Proprietary Concentration
• The tendency of a salt, acid or base solution to dissociate in water provides more carriers in the form of ions • More highly ionized species provide more carriers
Example: 1% Acetic Acid = 640 µS/cm 1% HCl = 100,000 µS/cm
Conductivity = (concentration) x (charge per carrier) x (mobility of the carriers)
42 Proprietary Concentration
• In general, as concentration increases, conductivity increases
KCl Sample at 25 C Conductivity, uS/cm 0.0 M/L 0 0.0005 M/L 73.9 0.001 M/L 147 0.005 M/L 718 0.01 M/L 1,413 0.05 M/L 6,667 0.1 M/L 12,900 0.5 M/L 8,670 1.0 M/L 111,900
Conductivity = (concentration) x (charge per carrier) x (mobility of the carriers)
43 Proprietary Charge per Carrier
• Divalent ions generally contribute more to conductivity than monovalent ions
Ca+2 vs. Na+1
Conductivity = (concentration) x (charge per carrier) x (mobility of the carriers)
44 Proprietary Mobility
• The mobility of each ion species is different. The conductivity of 0.1M NaCl and 0.1M KCl will not be the same. Ion Relative Mobility H+ 350 Na+ 50 K=+ 74 Ag+ 62 OH- 200 F- 55 Cl- 76 HCO3- 45
Conductivity = (concentration) x (charge per carrier) x (mobility of the carriers)
45 Proprietary Temperature Affects Ion Mobility
• Increasing temperature makes water less viscous, increasing ion mobility. • Most meters can do a calculation to show all measurements as if the sample were at 20ºC or 25ºC… conductivity temperature compensation / normalization.
Example: 0.01 M KCl at 0 ºC = 775 µS/cm 0.01 M KCl at 25 ºC = 1410 µS/cm
Conductivity = (concentration) x (charge per carrier) x (mobility of the carriers)
46 Proprietary Conductivity Properties
• Conductivity and Resistivity are inherent properties of a material’s ability to transport electrons • Conductance and Resistance depend on both material and geometry
Conductivity = d/A x conductance (d) distance between the electrodes (A) electrode area
47 Proprietary Conductivity Properties
Conductivity is defined as the reciprocal of the resistance between opposing faces of a 1 cm cube (cm3) at a specific temperature
Conductivity = d/A x conductance K = 1.0 cm-1 Distance (d = 1 cm)
Area (A = 1 cm2)
48 Proprietary Cell Constant (K) in cm-1
• The cell constant (K) is the value by which you multiply conductance to calculate conductivity.
• The cell constant (K) is the ratio of the distance between the electrodes (d) to the electrode area (A). Fringe field effects is the amount AR.
K = d / (A + AR)
Conductance = the measured value relative to the specific geometry of the cell
Conductivity = the inherent property of the solution being tested
Conductivity = Conductance x K
Conductivity = d/A x conductance K = 1.0 cm-1
49 Proprietary Cell Constants (K) by Application
50 Proprietary Temperature Coefficients
• Each ion species has a unique temperature coefficient that can change with changes in concentration • Temperature effects vary by ion type.
Some typical temperature coefficients:
Sample % / °C (at 25°C) Salt solution (NaCl) 2.12 5% NaOH 1.72 Dilute Ammonia Solution 1.88 10% HCl 1.32 5% Sulfuric Acid 0.96 98% Sulfuric Acid 2.84 Sugar Syrup 5.64
51 Proprietary Non-Linear Temperature Coefficients
• Unlike salt solutions, the temperature coefficient for pure water is not linear
Typical temperature coefficients of pure water:
Temp °C % per °C 0 7.1 10 6.3 20 5.5 30 4.9 50 3.9 70 3.1 90 2.4
52 Proprietary Measuring Conductivity
• A meter applies a current to the electrodes
in the conductivity cell Field Effect
Electrode
53 Proprietary Measuring Conductivity
• Reactions can coat the electrode, changing its surface area + • 2 H H2 bubbles • Reactions can deplete all ions in the vicinity, changing the number of carriers • Instead of using a direct current (DC), the conductivity meter uses an alternating current (AC) to overcome these measurement problems
54 Proprietary 2-Electrode Cells
• Two electrodes are used to measure current Benefits: Lower cost than four electrode cells Limited operating range with cell constants geared toward specific applications Drawbacks: Resistance increases due to polarization Fouling of the electrode surfaces Unable to correct for surface area changes Longer cable lengths will increase resistance
55 Proprietary 4-Electrode Cells
• A constant current is sent between two outer electrodes and a separate pair of voltage probes measure the voltage drop across part of the solution A • The voltage sensed by the inner two electrodes V is proportional to the conductivity and is unaffected by fouling or circuit resistance
56 Proprietary Conductivity Measurement
• Most conductivity measurements are made on natural waters
WATER CONDUCTIVITY (s/cm) Ultrapure 0.0546 Good Distilled 0.5 Good R/O 10 Typical City 250 Brackish 10,000
57 Proprietary Conductivity Measurement
• In natural waters, conductivity is often expressed as “dissolved solids” • Measured conductivity is reported as the concentration of NaCl that would have the same conductivity • Total Dissolved Solids (TDS) assumes all conductivity is due to dissolved NaCl Comparison of conductivity to TDS:
CONDUCTIVITY (S/cm) DISSOLVED SOLIDS (mg/l) 0.1 0.0210 1.0 0.44 10.0 4.6 100 47 200 91 1000 495
58 Proprietary Other Measurement Capabilities
• Salinity is the measure of the total dissolved salts in a solution and is used to describe seawater, natural and industrial waters. It is based on a relative scale of KCl solution and is measured in parts per thousand (ppt).
• Resistivity is equal to the reciprocal of measured conductivity values. It is generally limited to the measurement of ultrapure water where conductivity values would be very low. Measured in M -cm.
59 Proprietary 60 Proprietary Thermo Scientific Barnstead Water Purification Systems
Kelly Sweazea, Specialist Thermo Scientific Water Analysis & Purification Products
The world leader in serving science Laboratory Water Standards
. General laboratory standards set by the American Society for Testing and Materials (ASTM)
2 Proprietary & Confidential Barnstead Water Purification Systems
• Ultrapure water for critical laboratory tests and research • HPLC, AA, IC, ICP-MS etc. • Life Science applications • BOD • Microbiology • Water for general lab equipment such as: • Incubators • Water baths • Dishwashers • Chillers and cooling loops • Batteries • Environmental Chambers
3 Proprietary & Confidential What are the pure and ultra pure water concerns?
• A variety of contaminants can interfere with research and testing • Contaminants such as dissolved gases, ions, and particles can foul sensitive equipment
4 Proprietary & Confidential WaterWater System System Types Types by by Application Application
• F.A.V.O.R. • Feed water – tap, house RO, distilled, deionized? • Application/s? • Volume – how much water is needed on an hourly or daily basis? UV Applications • OR • Replacement – are you replacing a current system – like for like or something different? UV/UF Applications
5 Proprietary & Confidential Ion Impurities in Water
Dissolved Ionized Solids Major Calcium - Ca++ Cations Magnesium - Mg++ Sodium - Na+
------Chloride - Cl-
Major Sulfate - SO4= Anions Bicarbonate - HCO3- Silica/Silicate - HSiO3 -
6 Proprietary & Confidential Organics
• Total organic carbon monitoring of all volatile and non-volatile organic compounds: • NOT the same as bacteria! • Plant and animal decay • Agricultural and manufacturing byproducts • Phthalates or plasticizers which are added to plastics to increase their flexibility and transparency. • Best removed from water with UV and carbon.
7 Proprietary & Confidential Chlorine
RO membrane Tap water CARBON to Removes ions Impurities that will remove chlorineand organics damage RO membranes:
• CHLORINE • Particulates (rust) • Water hardness
•Found in most tap water •Needs to be removed for RO Type 1 membranes >0.1 ppm free Tank system chlorine •Easily removed with carbon
8 Proprietary & Confidential Particulates
Typical tap water • Ruins RO membranes Consumables affected: • Lab equipment – can leave a deposit, • Prefilters plug valves • Reverse osmosis • Affect gravimetric methods membranes • Prefilters remove particulates • Final filters Rust Sediment Plumbing debris
9 Proprietary & Confidential Bacteria
Found: Everywhere – air and water
Potentially interferes with: • Lab equipment – clogs filters (HPLC) • Sensitively biological methods such as cell culture, tissue culture, and bacterial studies • Source for pyrogens/endotoxins • Source for nuclease (RNase/DNase)
Removed with final filter, RO, UV • Likes to grow in water systems!
10 Proprietary & Confidential Nuclease and Pyrogens (Endotoxins)
• Endotoxins (pyrogens): Pieces of Gram negative bacteria • Interfere with growth of mammalian cell or tissue cultures • Nucleases: Enzymes RNase and DNase • Degrade and breaks down RNA and DNA • Sensitively biological methods such as cell culture, tissue culture, and bacterial studies
11 Proprietary & Confidential Basic Methods of Water Purification
• Distillation • Reverse Osmosis • Deionization • Filtration • Adsorption • Ultrafiltration • UV Oxidation
12 Proprietary & Confidential Thermo Scientific Barnstead Classic Still
• Consistent Type 2 pure water in one process
• Bacteria, pyrogen removed
13 Proprietary & Confidential Reverse Osmosis - % Removal Technology
*Removes most impurities from Tap water
14 Proprietary & Confidential Deionization
•Ion removal only •Type I water
15 Proprietary & Confidential Electrodeionization (EDI)
16 Proprietary & Confidential Depth Filtration
17 Proprietary & Confidential Adsorption
18 Proprietary & Confidential UV Photo-Oxidation
• Built into systems with UV photo-oxidation: • Type 1 systems – dual wavelength UV: • 185/254 nm organic compounds • 254 nm controls microorganisms • Type 2 systems and storage tanks: • Use 254 nm only to discourage growth • Effective in the flow path • Thermo Scientific UV lamps – up to a 2-year lifetime
19 Proprietary & Confidential Ultrafiltration
• Ultrafilters: • Remove endotoxins in water • <0.001 Eu/ml • Polysulfone hollow fibers • Large surface area
Combination of UV light and Ultrafiltration (UF) . Excellent way to reduce nuclease and endotoxin: . Ultrafilter hollow fiber filters trap impurities . System rinse remove them from the system . UV light – oxidizes bacteria and organics
20 Proprietary & Confidential Limitations & Capabilities
21 Proprietary & Confidential Applications
HPLC Environmental testing (BOD for example)
Chemical synthesis, extraction and concentration
ICP, ICP-MS
22 Proprietary & Confidential Type 1 Systems – POU Type 1
23 Proprietary & Confidential Type 2
•Use RO and DI to produce Type 2 water
•3,7,12,20,40,60 LPH units
•30,60,100 L tank options
24 Proprietary & Confidential Distillation
Glass Distillation:
1.4 LPH – 13 LPH
Classic Tin lined stills: 0.5 GPH – 10 GPH
25 Proprietary & Confidential Reverse Osmosis
•Use RO and DI to produce Type 2 water
•3,7,12,20,40,60 LPH units
•30,60,100 L tank options
26 Proprietary & Confidential Simple Cartridge Deionizers
27 Proprietary & Confidential Features and Technologies • There are many new features / technologies that make using and maintaining a water system much easier • Look for features like: • Smart Reservoirs (always full and can adjust how much is stored in them) • Hand dispensers • Volumetric dispensing (hand’s free, set, go, walk away) • Monitoring of Resistivity or conductivity on the display • Monitoring of the strength of the UV lamp • Quick connects (make cartridge changes quick and easy)
28 Proprietary & Confidential H2O Select
http://www.thermoscientific.com/en/about- us/promotions/complimentaryh2o-select-analysis.html
29 Proprietary & Confidential Brochures/Water Book
https://www.thermofisher.com/us/en/home/ global/forms/life-science/waterbook- download-request.html
30 Proprietary & Confidential Myths and Truths: pH and Conductivity of Ultrapure Water smart H2O for you, and your science
31ProprietaryProprietary & Confidential & Confidential The world leader in serving science Overview: 4 Myths of Ultrapure Water
Myth #1 - Ultrapure water conductivity after dispensing should match the ultrapure water system display Myth #2 - pH of ultrapure water should be 7.0 Myth #3 - Ultrapure water purity system display measures all the impurities in water Myth #4 - Accessories added to the ultrapure water system will always improve conductivity/resistivity
32 Proprietary & Confidential Myth #1 – Ultrapure Water Conductivity
MYTH #1 TRUTH #1 Ultrapure water conductivity Ultrapure water after dispensing should immediately picks up match the ultrapure water carbon dioxide and system display. conductivity upon dispensing.
33 Proprietary & Confidential Myth #2 – Ultrapure Water pH
MYTH #2 TRUTH #2 The pH of ultrapure water The pH of ultrapure water should be 7.0 after can quickly decrease dispensing after the water has been generated
34 Proprietary & Confidential The Role of CO2 in Conductivity and pH
Carbon dioxide in the air quickly dissolves into UPW • The conductivity will rise to near 1 or 2 µS/cm (or 1 to 0.5 MΩ∙cm) • The pH will drop and typically settle near pH 5.7
Reaction of water and carbon dioxide:
+ - CO2 + H2O H2CO3 H + HCO3
At typical ambient pressure (1 atm) and 25°C, there will be about
400 mg/L CO2 in ambient air, which results in:
• Concentration of about 1 x 10-5 M of H2CO3 in UPW • Conductivity near 1 µS/cm (or 1 MΩ∙cm) • pH near 5.7
35 Proprietary & Confidential Effect of CO2 on Conductivity of Ultrapure Water (1)
T (°C) 0 ppm (µS/cm) 20 ppm (µS/cm) 300 ppm 500 ppm 1000 ppm 2000 ppm (µS/cm) (µS/cm) (µS/cm) (µS/cm) 0 0.0117 0.1579 0.6100 0.7875 1.1137 1.5749 5 0.0166 0.1727 0.6662 0.8600 1.2161 1.7197 10 0.0232 0.1863 0.7166 0.9250 1.3079 1.7495 15 0.0315 0.1986 0.7607 0.9817 1.3880 1.9627 20 0.0421 0.2098 0.7984 1.0302 1.4563 2.0591 25 0.0551 0.2203 0.8298 1.0704 1.5129 2.1389 30 0.0711 0.2304 0.8551 1.1025 1.5577 2.2019 35 0.0903 0.2408 0.8741 1.1262 1.5903 2.2474 40 0.1131 0.2519 0.8865 1.1410 1.6100 2.2742 45 0.1399 0.2647 0.8920 1.1464 1.6157 2.2810 50 0.1711 0.2801 0.8906 1.1420 1.6067 2.2663
As more CO2 dissolves into UPW, the conductivity rises.
36 Proprietary & Confidential Effect of CO2 on pH of Ultrapure Water (1)
As more CO2 dissolves into UPW, the pH falls.
37 Proprietary & Confidential The Role of Temperature in Conductivity (1)
The conductivity of degassed ultrapure water rises in a non- linear fashion with increasing temperature.
38 Proprietary & Confidential What to Expect for Ultrapure Water Measurements
Measurements USP <645> Pure ASTM D1193 ASTM D1193 Water Conductivity Type I Water Type IV Water Conductivity @ Stage 1: < 1.3 uS/cm* 0.0555 uS/cm 5.0 uS/cm 25°C Resistivity @ 25°C Not stated 18 MΩ·cm 0.2 MΩ·cm TOC ~500 ug/L 50 ug/L Not specified pH Not applicable Not applicable 5.0 to 8.0 *grab or on-line sample
• pH is not a sensitive indicator of ultrapure water quality • pH of UPW is not expected to be exactly 7.0 • If UPW conductivity meets specifications, UPW pH will be within an expected range
39 Proprietary & Confidential How to Test Conductivity in Ultrapure Water
Grab Flow Cell Temperature (USP <645>) (ASTM D1193) Compensation
Turn “off” Includes CO Excludes CO 2 2 for USP <645>
Use non-linear Expect reading Compare to temp comp for similar to UPW USP <645> ASTM D1193 or display* compare to table
*when using temperature compensation
40 Proprietary & Confidential Conductivity Measurement Equipment
Example of Thermo Scientific Orion Conductivity Measurement Equipment
• Portable conductivity meter • Stainless steel conductivity probe • Glass flow cell (comes with the probe)
See our Application Note, Pure Water Conductivity Measurement, Note 002.
41 Proprietary & Confidential Flow Cell Measurement - Conductivity
UPW “out”
UPW “in”
42 Proprietary & Confidential Grab Cell Measurement – Conductivity
43 Proprietary & Confidential pH of Ultrapure Water
Q: Should I worry about a pH between 5 and 8 if I decide to test my ultrapure water?
A: Probably not. As we have seen, a pH between 5 and 7 or 8 can be attributed to
innocuous exposure to CO2 in the air. In addition, pH is not considered a useful indicator of UPW quality, because of the
expected CO2 absorption.
44 Proprietary & Confidential pH of Ultrapure Water
Q: Will this pH difference affect a solution that I prepare with the UPW?
A: In most cases, we don’t expect there to be a significant effect. CO2 will be present in any solution that we prepare in an air atmosphere.
In general, it may be more useful to monitor/control the pH of the prepared solution than the pH of the UPW used to make the solution.
Exception: a recipe that calls for CO2-free water
45 Proprietary & Confidential pH of Ultrapure Water
Q: Do I need to adjust the pH of the ultrapure water before I use it?
A: Again, in most cases, we don’t expect there is a need to adjust the pH. While the pH may be noticeably different than 7, the acidity of the UPW is negligible. This is not a highly buffered sample.
46 Proprietary & Confidential 5 Reasons to Not Use pH as UPW Quality Indicator
1. Experts and Standards Organizations (e.g. USP, ASTM, EP, JP, etc) agree that pH is not a useful indicator of UPW quality 2. pH of UPW is challenging to read accurately due to low ionic strength
3. pH of UPW changes due to CO2 absorption
4. UPW is an un-buffered liquid: a minute amount of CO2 absorbed will cause a deceptively large pH change 5. UPW at pH < 7 (e.g. pH 5.7) is not significantly acidic
The buffer capacity is minimal, 8 x 10-6 M/pH. Compare to acetate buffer at similar pH, which has buffer capacity of 2 x 10-2 M/pH.
47 Proprietary & Confidential How to Test the pH of UPW (if necessary)
• pH measurements of UPW tend to be unstable and can be inaccurate due to the low ionic strength of UPW • Addition of a neutral salt (KCl) will increase the ionic strength, and minimize drift and bias in the pH measurement • Addition of 0.3 mL of saturated KCl to 100 mL of UPW will stabilize the reading without significantly impacting the pH • The use of a good pH electrode will reduce noise and drift, and increase accuracy
• See our application note for more information: • Measuring pH of Pure Water and Other Low Conductivity Waters, Note 009 • Available in the WAI Online Library at www.thermofisher.com/waterlibrary
48 Proprietary & Confidential Myth #3 – Ultrapure Water System Display
MYTH #3 TRUTH #3 Ultrapure water purity system Not all impurities will display measures all of the be measured impurities in water.
49 Proprietary & Confidential pH and Ultrapure Water
Measurements ASTM Type I ASTM Type ASTM Type ASTM Type II III IV Conductivity @ <0.0555 <1.0 uS/cm <0.250 uS/cm <5.0 uS/cm 25°C uS/cm
Resistivity @ >18 MΩ·cm >1 MΩ·cm >4 MΩ·cm >0.2 MΩ·cm 25°C TOC <50 ug/L <50 ug/L <200 ug/L Not specified pH Not applicable Not Not applicable 5.0 to 8.0 applicable
50 Proprietary & Confidential Resistivity/Conductivity Measures Ions
TIS= Total ionized solids: Calculated from conductivity/resistivity
TDS = Total dissolved solids Includes dissolved impurities (organics, colloids) Measured gravimetrically
51 Proprietary & Confidential Total Organic Carbon (TOC) Measures Organics
52 Proprietary & Confidential Total Organic Carbon Measured with Resistivity
53 Proprietary & Confidential Bacteria - No Displayed Measurement
Biofilm on chiller filter bag
54 Proprietary & Confidential Particles - No Displayed Measurement
Turbidity Meter
55 Proprietary & Confidential Endotoxins and Nucleases - No Displayed Measurement
56 Proprietary & Confidential Myth #4 – Ultrapure Water System Accessories
MYTH #4 TRUTH #4 Accessories added to the Yes and No. Depends ultrapure water system will on where it is added to always improve the system. conductivity/resistivity.
57 Proprietary & Confidential Cartridges
58 Proprietary & Confidential UV Light
• Dual 185/254nm and single wavelength lamps • 254 nm provides germicidal action against microorganisms • 185 photo-oxidizes
organics into CO2 for ultralow TOC levels
59 Proprietary & Confidential Point of Use Final Filter
Pseudomonas Particle 0.3 micron 0.6 micron
0.2 micron pore size
60 Proprietary & Confidential Ultrafilters
• Remove endotoxins in water • <0.001 Eu/ml • Polysulfone hollow fibers • Large surface area
61 Proprietary & Confidential Recirculation and Dead Legs
62 Proprietary & Confidential Summary – Truths!
1. Ultrapure water immediately picks up conductivity upon dispensing 2. Ultrapure water pH can quickly decrease after it has been generated 3. All impurities will not be measured 4. Accessories added to ultrapure water systems could affect purity depending on where it is added to the system
63 Proprietary & Confidential 64 Proprietary & Confidential