pH Measurements in Today’s Lab
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Transcript of pH Measurements in Today’s Lab
pH Measurements in Today’s Lab
Midland Scientific Feb. 19, 2014
Don IvyWestern US and Canada Sales ManagerThermo Scientific – Orion Products
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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+.
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What is pH?
• pH = “Potential Hydrogen” or Power of Hydrogen• The pH of pure water around room temperature is about 7. This 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".
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What is pH?
• The pH Scale
• Each pH unit is a factor 10 in [H+]• pH of Cola is about 2.5. This is
10x more acidic than Orange Juice (pH of 3.5).
• Cola is 100x more acidic than Beer!
Substance pH
Hydrochloric Acid, 10M -1.0
Lead-acid battery 0.5
Gastric acid 1.5 – 2.0
Lemon juice 2.4
Cola 2.5
Vinegar 2.9
Orange or apple juice 3.5
Beer 4.5
Acid Rain <5.0
Coffee 5.0
Tea or healthy skin 5.5
Milk 6.5
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
Representative pH values
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pH Measurement System
• When two solutions containing different concentrations of H+ ions are separated by a glass membrane, a voltage potential is developed across the 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)
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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
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pH Measurement System
• The pH Electrode• Combination• Sensing Half-Cell• Reference Half-Cell
• Internal filling solution (Sensing)• Buffer solution
• Outer Filling solution (Reference)• Saturated AgCl, KCl
• Common References• Calomel• Ag/AgCl• ROSS™
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pH Measurement System – Reference Electrode
• In a two electrode system a reference electrode is needed to complete the “circuit”.
• Combination electrode has the reference built in.
• 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.
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pH Measurement System – Reference Types
• Calomel Reference (Hg/Hg2Cl2)• Calomel electrodes is very stable and is ideally suited for use with TRIS buffers
and sample solutions containing proteins and other biological media. • Also used where samples contain metal ions, sulfides, or other substances that will
react with Ag or AgCl .• Advantages
• Low Cost, Good Precision (±0.02 pH)• Disadvantages
• Limited body styles, Temperature Hysteresis, Contains Mercury!
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pH Measurement System – Reference Types
• Single Junction Silver/Silver Chloride Reference (Ag/AgCl)• Recommended for all applications except those involving TRIS buffer, proteins,
metal ions, sulfides or other substances that will react with either Ag or AgCl. • Advantages
• Mid-range cost, Variety of body styles, Refillable or gel-filled, Good Precision (±0.02 pH)
• Disadvantages• Temperature Hysteresis, complexation in samples such as: TRIS, proteins, sulfides
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pH Measurement System – Reference Types
• Double Junction Silver/Silver Chloride Reference (Ag/AgCl)• The double junction Ag/AgCl reference isolates the reference, making it
ideally suited for all types of samples. • Advantages
• Mid-range cost, Variety of body styles, Refillable or gel-filled, Good Precision (±0.02 pH)
• Disadvantages• Temperature Hysteresis
• Mercury Free alternative to the Calomel Reference
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pH Measurement System – Reference Types
• ROSS™ Reference• Double Junction Iodine/Iodide redox couple• The ROSS™ reference is ideally suited for all sample types and all
temperature ranges. • Advantages
• Variety of body styles, Unmatched Precision (±0.01 pH), Fast response, Stable to 0.01 pH in 30 seconds over 50 °C temperature change, Drift less than 0.002 pH units/day
• Disadvantages• Cost
• Mercury Free alternative to the Calomel Reference
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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
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pH Measurement System - Junctions
• The Wick Junction• Glass fiber, fiber optic bundles,
Dacron, etc.• Advantages
• Used in rugged epoxy bodies• Good for aqueous samples
• Disadvantages• Will clog if sample is “dirty” or
viscous• Not as “fast” as other junctions
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pH Measurement System - Junctions
• The Ceramic Junction• Porous ceramics, wooden plugs,
porous Teflon, etc.• Advantages
• Good all-purpose junction• Ideally suited for most lab
applications • Disadvantages
• Will clog if sample is “dirty” or viscous
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pH Measurement System - Junctions
• The Open Junction• Sure-Flow, Laser Drilled Hole,
Ground Glass Sleeve, etc.• Advantages
• Junction will never clog• Can be used in all sample types• Ideal choice for “dirty” or viscous
samples• Can be used in non-aqueous
samples• Disadvantages
• Sure-Flow Junction has a high flow rate of fill solution (2 ml/day)
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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
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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
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pH Measurement System - Electrode Selection
• Select proper reference for application• ROSS™, Single or Double Junction Ag/AgCl• Remember that Calomel contains Mercury!
• Select proper junction for application• Wick, Ceramic, Open, Sure-Flow, etc.
• Select appropriate body style• Standard, semi-micro, micro, rugged bulb, spear tip, flat surface
• Select appropriate body type• Glass body, epoxy body
• Other considerations• Refillable, Gel, or Polymer?• Built in Temperature Probe?
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pH Calibration
• The Nernst Equation
E = E0 - RT/nF log aH
E = measured potentialE0 = reference potentialR = Universal Gas ConstantT= Temperature (at 25 °C)n = Number of electronsF = Faraday ConstantaH = Hydrogen Ion activity
Slope = RT/nF = 59.16mv @ 25 °C
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pH Calibration
• When you are calibrating, you are determining the electrodes slope as it relates to the theoretical slope defined by the Nernst Equation
• Newer meters automatically calculate slope• Check slope manually by reading mV in buffers and comparing to
Nernstian response (59.2 mV/pH unit)• Example:
• pH 7 = -10 mV• pH 4 = +150 mV• Slope = 160 mV/177.6 mV = 90.1%
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pH Calibration - Guidelines
• Always calibrate with at least 2 buffers• Check calibration drift with 1 buffer• Always calibrate with buffers that bracket the expected measurement
range• Calibrate with buffers that are no more than 3 pH units apart• Track calibration slope on a daily basis• Calibration frequency
• Electrode type• Sample type• Number of samples
• Electrode slope guidelines• Ideal range: 95% - 102%
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Effects of Temperature
• Temperature can have a significant effect on pH measurements
• Electrode• Calibration• Buffers• Samples
• Temperature Compensation Techniques
• Calibrate and measure at same temperature
• Manually temperature compensate using temperature control on meter
• Use automatic temperature compensator (ATC) or 3-in-1 Triode electrode
• Use LogR temperature compensation
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Effects of Temperature – Electrode Effects
• Temperature Hysteresis• AgCl or Hg2Cl2 references drift
with temperature changes• 0.05 pH unit error with 4 °C
difference• ROSS™ electrodes stabilize
within seconds
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Effects of Temperature – Calibration Effects
• Calibration Effects• Theoretical slope of electrode is
59.16mv at 25 °C• Temperature changes the
calibration slope• Temperature compensation
adjusts the calibration slope for temperature effects
• The point at which temperature has no effect on mV is referred to as the isopotential point
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Effects of Temperature – Buffer Effects
• Buffer Effects• Buffers have different pH values at different temperatures• Use the value of the buffer at the calibration temperature• New meters have NIST calibration tables pre-programmed• NIST Certified Values only at 25°C
25 C 0 C 5 C 10 C 20 C 30C 40 C 50 C 60 C 70 C 80 C 90 C 1.68 1.67 1.67 1.67 1.67 1.68 1.69 1.71 1.72 1.74 1.77 1.79 3.78 3.86 3.84 3.82 3.79 3.77 3.75 3.75 4.01 4.00 4.00 4.00 4.00 4.02 4.03 4.06 4.08 4.13 4.16 4.21 6.86 6.98 6.95 6.92 6.87 6.85 6.84 6.83 6.84 6.85 6.86 6.88
7.00* 7.11 7.08 7.06 7.01 6.98 6.97 6.97 7.41 7.53 7.50 7.47 7.43 7.40 7.38 7.37 9.18 9.46 9.40 9.33 9.23 9.14 9.07 9.01 8.96 8.92 8.89 8.85
10.01 10.32 10.25 10.18 10.06 9.97 9.89 9.83 12.46 13.42 13.21 13.01 12.64 12.30 11.99 11.71 *Non-NIST Phosphate Buffer
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Effects of Temperature – Sample Effects
• Sample effects• Temperature compensation corrects for changes in electrode slope not
sample pH• It is not possible to normalize pH readings to a specific temperature• pH of samples will change with temperature changes • Record temperature with pH readings
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Electrode Care and Maintenance
• Electrode Storage• Short-term storage
• Use electrode storage solution• Alternatively, soak in 100 ml pH 7 buffer with 0.5 g KCl
• Long-term storage• Fill electrode, close fill hole, store with storage solution in protective cap
• Cleaning Solutions• Soak electrode in solvent that will remove deposits
• Example: 0.1 M HCl for general cleaning• Example: 1% pepsin in HCl for proteins• Example: Bleach for disinfecting• Example: detergent for grease & oil
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Electrode Care and Maintenance
• When do you need to clean your electrode?• Check slope range
• Ideal range: 95% - 102%• Cleaning range: 92% - 95%• Replacement range: below 92%
• Check response times in buffers• Electrode stability within 30 seconds
• Check precision of electrode by reading buffers as samples• Check for any drift of electrode in pH buffer
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Electrode Care and Maintenance
• General electrode bulb cleaning• Soak in Cleaning Solution for 30 minutes• Replace electrode fill solution• Soak in storage solution for at least 2 hours
• Electrode junction cleaning• Soak in 0.1M KCl for 15 minutes at 70 °C• Replace electrode fill solution when needed• Soak in electrode storage solution for 2 hours
• Check junction by suspending in air for ten minutes• Observe KCl crystal formation
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Keys to Accuracy
• Always use fresh buffers• Check bottle expiration and date
opened• pH 4 and pH 7 buffers expire within 3
months of being opened• pH 10 buffer expires within 1 month of
being opened• Fresh buffer for each calibration
• Calibrate only once in buffer… don’t re-use buffer
• Replace the fill solution in the electrode every week
• Fill solution concentration is maintained
• KCl crystallization is prevented• Make sure to use the correct fill
solution• Ross electrodes cannot use silver fill
solutions
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Keys to Accuracy
• Make sure level of fill solution is high
• Gently stir buffers and samples• Shake any air bubbles out of the
electrode• Use insulation between stir plate
and sample container to minimize heat transfer
• Blot electrodes between samples• Uncover fill hole during
measurement
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Troubleshooting pH Problems
• Common measurement problems• Readings not reproducible• Slow response• Noisy response• Drifty response• Inaccurate
• Troubleshooting Sequence• Meter• Buffers• Reference electrode• pH electrode• Sample• Technique
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Troubleshooting pH Problems
• Troubleshooting pH Meters• Use meter shorting strap• Reading should be 0 mV +/- 0.2
mV• Use meter self-test procedure
• Troubleshooting Buffers• Use Fresh Buffers for calibration• Verify expiration date• Stir buffers during calibration
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Troubleshooting pH Problems
• Troubleshooting pH Electrodes• Clean bulb, junctions• Replace Fill solution• Uncover fill hole• Check for scratches on sensing
bulb• Troubleshooting Samples
• Proper sample preparation• Stir samples
• Troubleshooting Technique• Treat samples and buffers the
same• Clean and blot electrode
between samples
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New Technologies
• LogR Temperature Compensation• Meter reads the resistance (R) from the bulb of any pH electrode• Resistance measurement is inverse to temperature: LogR = 1/T• Calibrate pH electrode for temperature• Direct temperature compensation without using ATC• Use PerpHect Electrodes for ideal Temperature response and improved
accuracy and precision
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Thermo Scientific – Orion Products
• Contact us for any technical questions!• Technical Service: (800) 225-1480• Technical Service fax: (978) 232-6015• Web site: www.thermoscientific.com/water• WAI Library: www.thermoscientific.com/WAI-Library
New Products
Don IvyWestern US and Canada Sales ManagerThermo Scientific – Orion Products
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• Star A111 pH Meter• Star A112 Conductivity Meter• Star A113 DO Meter• Star A211 pH Meter• Star A212 Conductivity Meter• Star A213 RDO/DO Meter• Star A214 pH/ISE Meter• Star A215 pH/Conductivity Meter• Star A216 pH/RDO/DO Meter• VERSA STAR Modular Meters
• Select up to 4 modules per meter• pH Module• ISE Module• LogR pH Module• Conductivity Module• RDO/DO Module
Orion Star A and VERSA STAR Benchtop Meters
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Orion Star A Portable Meters
•Star A121 pH Meter•Star A122 Conductivity Meter•Star A123 DO Meter•Star A221 pH Meter•Star A222 Conductivity Meter•Star A223 RDO/DO Meter•Star A321 pH Meter•Star A322 Conductivity Meter•Star A323 RDO/DO Meter•Star A324 pH/ISE Meter•Star A325 pH/Conductivity Meter•Star A326 pH/RDO/DO Meter•Star A329 pH/ISE/Conductivity/RDO/DO Meter
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Orion High Performance Ammonia ISE
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Orion Dual Star pH/ISE Meter
The interface of the 720/920A on a Star Meter—
• Simultaneously two channel measurements
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ROSS Triode Electrode
The convenience of a Triode with the accuracy of a ROSS—
• Accurately measure pH and temperature with one electrode
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Orion Green Electrodes
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ISE Analysis in Today’s Lab
• Consider Electrode Advancements• New probes-Ammonia and Sure-Flow Ion Plus
• Consider Automation Capabilities• Autosampler capabilities• New Techniques Using MKA and Serial Calibrations
• Consider Broad Applications and Accurate Performance• Take Advantage of Ease of Use and Relative Low Cost
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Why Use ISE’s?
• Responsive over a wide concentration range
• Not affected by color or turbidity of sample
• Rugged and durable
• Rapid response time
• Real time measurements
• Low cost to purchase and operate
• Easy to use
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Why Use ISE’s?
• EPA approved methods• Acidity• Alkalinity• Ammonia• Bromide• Chloride• Residual Chlorine• Cyanide
• Fluoride• Total Kjeldahl Nitrogen (TKN)• Nitrate• Dissolved Oxygen/BOD• pH• Sulfide
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What Are ISE’s?
• Electrodes are devices which detect species in solutions
• Electrodes consist of a sensing membrane in a rugged, inert body
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How Do ISE’s Work?
• The reference electrode completes the circuit to the sensing electrode (ISE)
• Reference electrodes have a small leak to establish contact with the sample
• The reference solution (usually KCl) in contact with the reference keeps the reference potential constant
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ISE Meters
• ISE meters report concentrations• No manual calibration curves are required
• ISE meters generate sophisticated curves which are held in the meter’s memory
• Run standards• Run unknowns• Read results
Don IvyWestern US and Canada Sales ManagerThermo Scientific – Orion Products
Conductivity Measurement
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Conductivity in Today’s Lab
• Overview• Technology• Measurement• Calibration• Temperature• Issues??
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Properties of Conductivity
In a metal, current is carried by electrons• Measures the Resistance to the flow of electrons • A wire with 1 ohm resistance allows a current of
1 amp when 1 volt is applied• Resistance = Voltage/Current• Units of resistance are measured in ohms
• Conductance is the reciprocal of resistance• Conductance = Current/Voltage• Units of conductance are measured in Siemens1 Siemen = 1/ohm = 1 mho =1000 mS = 1,000,000 µS
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Measuring Conductivity
• A meter applies a current to the electrodes in the conductivity cell
• 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
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Properties of Conductivity
Conductivity and Resistivity are inherent properties of a materials ability to transport electrons
While
Conductance and Resistance depend on both material and geometry
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Common Units and Symbols
• Conductivity = d/A x conductance• Conductance Units
• S (Siemens)• mS• S
• Conductivity Units• S/cm• mS/cm• S/cm
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Common Units and Symbols
• Resistivity = A/d x resistance• Resistance Units
• (Ohm)• k• M
• Resistivity Units• cm• kcm• Mcm
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Conductivity Theory
Conductivity is defined as the reciprocal of the resistance between opposing faces of a 1 cm cube (cm3) at a specific temperature (K = 1.0 cm-1)
Distance (d = 1 cm)
Area (A = 1 cm2)
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Cell Constant
• The cell constant (K) is defined as the ratio of the distance between the electrodes, (d) to the electrode area (A). Fringe-field effects the electrode area by the amount AR.
K = d (A + AR)
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Cell Constant
• The cell constant can be determined by placing the cell in a solution of known conductivity at a known temperature and adjusting the meter to read the correct value
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Cell Constant (K) in cm-1
• Cell constant is the value by which you have to multiply the conductance to calculate conductivity. This converts conductance to conductivity.
• Conductance = the measured value relative to the specific geometry of the cell (actual meter reading) Conductivity = the inherent property of the solution being tested
• Conductance x K = Conductivity
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Cell Constants by Application
Cell Constant (K) Application
K = 0.1 / cm Pure Water
K = 0.4 to 1.0 / cm Environmental water and industrial solutions
K = 10 / cm Very high conductivity samples
Cell Constant (K) Range
K = 0.1 0.001 µS/cm to 300 µS/cm
K = 0.475 10 µS/cm to 1000 mS/cm
K = 1.0 10 µS/cm to 200 mS/cm
K = 10 10 µS/cm to 2000 mS/cm
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Conductivity in Solutions
Carried by ions and is dependent upon:• Concentration (number of carriers)• Charge per carrier • Mobility of carriers
K+Cl-
SO4-2
Na+
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Conductivity in Solutions
• Conductivity = number of carriers x charge per carrier x mobility of the Carriers
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Concentration
• As concentration increases, conductivity generally increases
KCl Sample at 25 C Conductivity, uS/cm0.0 M/L 00.0005 M/L 73.90.001 M/L 1470.005 M/L 7180.01 M/L 1,4130.05 M/L 6,6670.1 M/L 12,9000.5 M/L 8,6701.0 M/L 111,900
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Concentration
• The tendency of a salt, acid or base’s 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/cm1% HCl = 100,000 µS/cm
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Charge per Carrier
• In general, divalent ions contribute more to conductivity than monovalent ions
Ca+2 Na+1vs.
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Mobility
• The mobility of each ion is different, so that the conductivity of 0.1M NaCl and 0.1M KCl will not be the same
Ion Relative MobilityH+ 350Na+ 50K=+ 74Ag+ 62OH- 200F- 55Cl- 76HCO3- 45
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Mobility According to Temperature
• Increasing temperature makes water less viscous, increasing the mobility of ions
• Most meters refer temperatures back to 20 ºC or 25 ºC - the correction is approximate: the degree of ionization and activity may also change with temperature
Example:0.01 M KCl at 0 ºC = 775 µS/cm0.01 M KCl at 25 ºC = 1410 µS/cm
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Temperature Coefficients
• Each ionic species has its own temperature coefficient… and these can change with changes in concentration
• The temperature coefficient can be determined experimentally• For any given solution, the temperature coefficient needs to be determined
only one time
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Temperature Coefficients
• Temperature effects vary by ion type. Some typical temperature coefficients:
Sample %/°C (at 25 °C)Salt solution (NaCl) 2.125% NaOH 1.72Dilute Ammonia Solution 1.8810% HCl 1.325% Sulfuric Acid 0.9698% Sulfuric Acid 2.84Sugar Syrup 5.64
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Non-Linear Temperature Coefficients
• Unlike salt solutions, pure water’s temperature coefficient is not linear• Typical temperature coefficients of pure water at different temperatures:
Temp °C % per °C0 7.110 6.320 5.530 4.950 3.970 3.190 2.4
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Measuring Conductivity
• A meter applies a current to the electrodes in the conductivity cell• 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
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Measuring Conductivity
• Instead of using a direct current (DC), the conductivity meter uses an alternating current (AC) to overcome these measurement problems
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Two Electrode Conductivity Cell
Electrode
FieldEffect
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2-Electrode Cells
Uses two electrodes for measuring currentBenefits Include:
- Lower cost than four electrode cells- Limited operating range with cell constants geared toward specific
applicationsDrawbacks Include:- Resistance increases due to polarization- Fouling of the electrode surfaces- Unable to correct for surface area changes- Longer cable lengths increase resistance
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4-Electrode Cells
Voltage sensed by inner 2 electrodes - a constant current is supplied regardless of any coating on them
AV
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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
• The voltage sensed by the inner two electrodes is proportional to the conductivity and unaffected by fouling or circuit resistance
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Advantages of 4-Electrode Cells
• Resists fouling• Resistance to polarization errors• Eliminates effects of cable resistance• Very wide operating range which will cover a number of applications • Wastewater testing for Federal or local agencies
• Seawater testing • Freshwater testing for environmental agencies• Beverage testing for manufacturers
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Conductivity Measurement
• Most conductivity measurements are made on natural waters
WATER CONDUCTIVITY (s/cm)Ultrapure 0.0546Good Distilled 0.5Good R/O 10Typical City 250Brackish 10,000
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Conductivity Measurement
• In natural waters, conductivity is often expressed as “dissolved solids”
• The measured conductivity is reported as the concentration of sodium chloride that would have the same conductivity
• Total Dissolved Solids (TDS) assumes all conductivity is due to dissolved NaCl
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Total Dissolved Solids
• 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
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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.
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Other Measurement Capabilities
• Conductivity is an excellent way of measuring concentrations
• It can be used for any solution that has only a single ionic species
• An individual calibration curve must be prepared, but it is a “permanent” calibration curve
• Curves can take various forms
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Applications for Conductivity Cells
• Glass(epoxy)/platinum, platinized - general lab • Glass/platinum, platinized and scintered - pastes, creams etc.• Stainless Steel (K= 0.1cm-1) - ultrapure water• Epoxy/graphite - durable, 4-electrode cell and
2-electrode cells used where glass fears to tread• Weighted, protective sleeves (stainless steel) -for field use, provides
weight and protection for cells
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Conductivity Applications
• Water purity• Boiler/cooling water• Chemical manufacture• Drinking water• Pharmaceutical injection grade water• Biotech
• Education• Used as a tool in teaching analytical chemistry
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Conductivity Applications
• Environmental• Incursion of seawater into aquifers• Monitor pollution due to industry output
• Industrial• Pulp/paper for bleaching process • Industrial feed water for heating and cooling systems• Refineries
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Conductivity Applications
• USP 645• Specifies the use of conductivity measurements as criteria for qualifying
purified water for injection• Sets performance standards for the conductivity measurement system
• Validation and calibration requirements for the meter and conductivity cell
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USP 645
• The conductivity cell constant must be known within +/- 2% using NIST traceable standards
• Meter calibration is verified using NIST traceable precision resistors (accurate to +/- 0.1% of value)
• The meter must have a minimum resolution of 0.1 S/cm on lowest range• Meter accuracy must be +/- 0.1 S• Conductivity values used in this method are non-temperature compensated• A flow through type cell may function better
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