pH Measurements in Today’s Lab
Midland Scientific
Feb. 19, 2014
Don Ivy
Western US and Canada Sales Manager
Thermo Scientific – Orion Products
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+.
2
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".
3
What is pH?
Representative pH values
• 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
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
Blood
4
pH
6.5 – 7.4
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
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)
5
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
6
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™
7
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.
8
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!
9
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
10
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
11
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
12
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
13
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
14
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
15
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)
16
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
•
•
•
•
•
•
•
17
Fill/drain electrode
Wide applicability
Glass or epoxy body
0.02 pH precision
Faster response rate
1 year minimum life
Replaceable fill solution
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
•
•
•
•
•
•
•
18
Low maintenance
Easy to use
Glass or epoxy body
0.02 pH precision
Faster response rate
1 year minimum life
Double junction design
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?
19
pH Calibration
• The Nernst Equation
E = E0 - RT/nF log aH
E = measured potential
E0 = reference potential
R = Universal Gas Constant
T = Temperature (at 25 °C)
n = Number of electrons
F = Faraday Constant
aH = Hydrogen Ion activity
Slope = RT/nF = 59.16mv @ 25 °C
20
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%
21
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%
22
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
23
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
24
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
25
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
0C
5C
10 C
1.68
1.67
1.67
1.67
3.78
3.86
3.84
3.82
4.01
4.00
4.00
4.00
6.86
6.98
6.95
6.92
7.00* 7.11
7.08
7.06
7.41
7.53
7.50
7.47
9.18
9.46
9.40
9.33
10.01 10.32 10.25 10.18
12.46 13.42 13.21 13.01
*Non-NIST Phosphate Buffer
26
20 C
1.67
3.79
4.00
6.87
7.01
7.43
9.23
10.06
12.64
30C
1.68
3.77
4.02
6.85
6.98
7.40
9.14
9.97
12.30
40 C
1.69
3.75
4.03
6.84
6.97
7.38
9.07
9.89
11.99
50 C
1.71
3.75
4.06
6.83
6.97
7.37
9.01
9.83
11.71
60 C
1.72
70 C
1.74
80 C
1.77
90 C
1.79
4.08
6.84
4.13
6.85
4.16
6.86
4.21
6.88
8.96
8.92
8.89
8.85
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
27
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
28
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
29
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
30
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
31
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
32
Troubleshooting pH Problems
• Common measurement problems
•
•
•
•
•
Readings not reproducible
Slow response
Noisy response
Drifty response
Inaccurate
• Troubleshooting Sequence
•
•
•
•
•
•
33
Meter
Buffers
Reference electrode
pH electrode
Sample
Technique
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
34
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
35
New Technologies
• LogR Temperature Compensation
•
•
•
•
•
36
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
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
37
New Products
Don Ivy
Western US and Canada Sales Manager
Thermo Scientific – Orion Products
Orion Star A and VERSA STAR Benchtop Meters
•
•
•
•
•
•
•
•
•
•
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
39
Orion Star A Portable Meters
•
•
•
•
•
•
•
•
•
•
•
•
•
40
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
Orion High Performance Ammonia ISE
Optimize performance and
maximize efficiency—
• Accurate results down to
0.01 ppm
• Rapid response for increased
sample throughput
Available Now!
41
Orion Dual Star pH/ISE Meter
The interface of the
720/920A
on a Star Meter—
• Simultaneously two
channel measurements
• The user interface makes
running pH and ISE
simple!
Available in Q2
42
ROSS Triode Electrode
The convenience of a Triode with
the accuracy of a ROSS—
• Accurately measure pH and
temperature with one electrode
• Best temperature response
• Longest gel Triode warranty –
1 year
Available in Q2
43
Orion Green Electrodes
Environmentally-friendly
electrodes from Orion—
• Goes beyond RoHS
requirements
• Non-hazardous disposal
• Minimal packaging
Available in Q2
44
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
45
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
46
Why Use ISE’s?
• EPA approved methods
•
•
•
•
•
•
•
47
Acidity
Alkalinity
Ammonia
Bromide
Chloride
Residual Chlorine
Cyanide
•
•
•
•
•
•
Fluoride
Total Kjeldahl Nitrogen (TKN)
Nitrate
Dissolved Oxygen/BOD
pH
Sulfide
What Are ISE’s?
• Electrodes are devices which detect species in solutions
• Electrodes consist of a sensing membrane in a rugged, inert body
48
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
49
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
50
Conductivity Measurement
Don Ivy
Western US and Canada Sales Manager
Thermo Scientific – Orion Products
Conductivity in Today’s Lab
• Overview
• Technology
• Measurement
• Calibration
• Temperature
• Issues??
52
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 Siemens
1 Siemen = 1/ohm = 1 mho =1000 mS = 1,000,000 µS
53
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
54
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
55
Common Units and Symbols
• Conductivity = d/A x conductance
• Conductance Units
• S (Siemens)
• mS
• S
• Conductivity Units
• S/cm
• mS/cm
• S/cm
56
Common Units and Symbols
• Resistivity = A/d x resistance
• Resistance Units
•  (Ohm)
• k
• M
• Resistivity Units
• cm
• kcm
• Mcm
57
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)
58
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=
59
d
(A + AR)
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
60
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
61
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
62
Conductivity in Solutions
Carried by ions and is dependent upon:
• Concentration (number of carriers)
• Charge per carrier
• Mobility of carriers
K+
Cl-
Na+
SO4-2
63
Conductivity in Solutions
• Conductivity = number of carriers x charge per carrier x mobility
of the Carriers
64
Concentration
• As concentration increases, conductivity generally increases
KCl Sample at 25 C
0.0 M/L
0.0005 M/L
0.001 M/L
0.005 M/L
0.01 M/L
0.05 M/L
0.1 M/L
0.5 M/L
1.0 M/L
65
Conductivity, uS/cm
0
73.9
147
718
1,413
6,667
12,900
8,670
111,900
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/cm
1% HCl = 100,000 µS/cm
66
Charge per Carrier
• In general, divalent ions contribute more to conductivity than
monovalent ions
Ca+2
67
vs.
Na+1
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
H+
Na+
K=+
Ag+
OHFClHCO3-
68
Relative Mobility
350
50
74
62
200
55
76
45
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/cm
0.01 M KCl at 25 ºC = 1410 µS/cm
69
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
70
Temperature Coefficients
• 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
71
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
0
10
20
30
50
70
90
72
% per °C
7.1
6.3
5.5
4.9
3.9
3.1
2.4
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
73
Measuring Conductivity
• Instead of using a direct current (DC), the conductivity meter uses an
alternating current (AC) to overcome these measurement problems
74
Two Electrode Conductivity Cell
Field
Effect
Electrode
75
2-Electrode Cells
Uses two electrodes for measuring current
Benefits Include:
 Lower cost than four electrode cells
 Limited operating range with cell constants geared toward specific
applications
Drawbacks Include:




76
Resistance increases due to polarization
Fouling of the electrode surfaces
Unable to correct for surface area changes
Longer cable lengths increase resistance
4-Electrode Cells
Voltage sensed by inner 2 electrodes - a constant current is
supplied regardless of any coating on them
A
V
77
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
78
Advantages of 4-Electrode Cells
•
•
•
•
•
79
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
Conductivity Measurement
• Most conductivity measurements are made on natural waters
WATER
80
CONDUCTIVITY (s/cm)
Ultrapure
0.0546
Good Distilled
0.5
Good R/O
10
Typical City
250
Brackish
10,000
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)
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DISSOLVED SOLIDS (mg/l)
0.1
0.0210
1.0
0.44
10.0
4.6
100
47
200
91
1000
495
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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90
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