Essentials of pH Measurement
Kelly Sweazea, Specialist
Thermo Scientific Water Analysis
& Purification Products
2 Proprietary
What is pH?
“Potential Hydrogen” or “Power of Hydrogen”
pH electrodes are a type of ion selective electrode
(ISE) measuring free hydrogen ion activity
Common Questions: Measuring pH
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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+.
Common Questions: What is pH?
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• 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".
Common Questions: What is pH?
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The pH Scale
• [H+] activity increases by a
factor of 10 for every pH unit.
• Cola pH is about 2.5. Cola
is 10x more acidic than
Orange Juice (pH of 3.5)
• Cola is 100x more acidic
than Beer! (pH of 4.5)
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
Common Questions: What is pH?
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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
Common Questions: Measuring pH
7 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
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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
Sensing Bulb
Internal Fill Solution (Sensing)
Reference
Reference Fill Solution
Junction
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pH Measurement System
• 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.
Reference Electrode
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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.
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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.
Common Questions: Electrode Types
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• 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
pH Electrode Reference Types
Common Questions: Electrode Components
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pH Measurement System – Reference Types
• Recommended for all applications exceptthose involving TRIS buffer, proteins, metal ions, sulfides or other substances that will react with either Ag or AgCl.
Single Junction Silver/Silver Chloride Reference (Ag/AgCl)
• Mid-range cost, Variety of body styles, Refillable or gel-filled, Good Precision (±0.02 pH)
Advantages
• Temperature Hysteresis, complexation in samples such as: TRIS, proteins, sulfidesDisadvantages
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pH Measurement System – Reference Types
• The double junction Ag/AgCl reference isolates the reference, making it ideally suited for all types of samples.
Double Junction Silver/Silver
Chloride Reference (Ag/AgCl)
• Mid-range cost, Variety of body styles, Refillable or gel-filled, Good Precision (±0.02 pH)
Advantages
• Temperature HysteresisDisadvantages
Mercury Free alternative to the Calomel Reference
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pH Measurement System - Reference Types
• Double Junction Iodine/Iodide redox couple
• The ROSS™ reference is ideally suited for all sample types and all temperature ranges
ROSS™ Reference
• Variety of body styles, Unmatched Precision (±0.01 pH), Fast response, Stable to 0.01 pH in 30 seconds over 50 oC temperature change, Drift less than 0.002 pH units/day
Advantages
• CostDisadvantages
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
• Glass fiber, fiber optic bundles, Dacron, etc.
The Wick Junction
• Used in rugged epoxy bodies
• Good for aqueous samplesAdvantages
• Will clog if sample is “dirty” or viscous
• Not as “fast” as other junctions
Disadvantages
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pH Measurement System - Junctions
• Porous ceramics, wooden plugs, porous teflon, etc.
The Ceramic Junction
• Good all-purpose junction
• Ideally suited for most lab applications
Advantages
• Will clog if sample is “dirty” or viscous
Disadvantages
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pH Measurement System - Junctions
• Sure-Flow, Laser Drilled Hole, Ground Glass Sleeve, etc.
The Open Junction
• 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
Advantages
• Sure-Flow Junction has a higher flow rate of fill solution
Disadvantages
“Sure-Flow”
liquid junction
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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.
Common Questions: Electrode Types
ceramic junction
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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.
Common Questions: Electrode Types
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pH Measurement System – Electrode Types
Refillable or Low Maintenance Gel?
• Easy to use
• Rugged epoxy body
• 0.05-0.1 pH precision
• Slower response rate
• 6 month average life
• Gel memory effects at junction
Low Maintenance Gel Electrodes
• Fill/drain electrode
• Wide applicability
• Glass or epoxy body
• 0.02 pH precision
• Faster response rate
• 1 year minimum life
• Replaceable fill solution
Refillable Electrodes
24 Proprietary
pH Measurement System – Electrode Types
Polymer or Low Maintenance Gel?
• Easy to use
• Rugged epoxy body
• 0.05-0.1 pH precision
• Slower response rate
• 6 month average life
• Gel memory effects at junction
Low Maintenance Gel Electrodes
• Low maintenance
• Easy to use
• Glass or epoxy body
• 0.02 pH precision
• Faster response rate
• 1 year minimum life
• Double junction design
Polymer Electrodes
25 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
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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.
Common Questions: Temperature Compensation
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• 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%
Common Questions: Temperature Compensation
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• Temperature affects calibration
slope because it affects the
expected change in the mV
value per pH unit
Common Questions: Temperature Compensation
29 Proprietary
• 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%
Common Questions: Temperature Compensation
30 Proprietary
• 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
Common Questions: Temperature Compensation
31 Proprietary
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
Common Questions: Temperature Compensation
32 Proprietary
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
Common Questions: Calibration
ALWAYS use fresh buffer for each calibration.
Don’t re-use today’s buffer for tomorrow’s calibration!
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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
Common Questions: Stable Readings
34 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
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• 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
Common Questions: Maintenance
Is there a cleaning routine I can follow
to keep my electrode working?
36 Proprietary
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
Would you please tell me what is happening here???
Common Questions…
37 Proprietary
Conductivity Measurement
Kelly Sweazea, Specialist
Thermo Scientific Water Analysis
& Purification Products
39 Proprietary
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
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Common Units and Symbols
Conductance Units
• S (Siemens)
• mS
• S
Conductivity Units
• S/cm
• mS/cm
• S/cm
Resistance Units
• (Ohm)
• k
• M
Resistivity Units
• cm
• kcm
• Mcm
41 Proprietary
Conductivity in Solutions
Conductivity carried by ions is dependent upon:
• Concentration (number of carriers)
• Charge per carrier
• Mobility of carriers
Conductivity =
(concentration) x (charge per carrier) x (mobility of the carriers)
K+
Cl-
SO4-2
Na+
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• 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
Concentration
Conductivity =
(concentration) x (charge per carrier) x (mobility of the carriers)
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• 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
Concentration
Conductivity =
(concentration) x (charge per carrier) x (mobility of the carriers)
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• Divalent ions generally contribute more
to conductivity than monovalent ions
Ca+2
Na+1vs.
Conductivity =
(concentration) x (charge per carrier) x (mobility of the carriers)
Charge per Carrier
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• 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
Mobility
Conductivity =
(concentration) x (charge per carrier) x (mobility of the carriers)
46 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)
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• 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
Conductivity Properties
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Conductivity is defined as the reciprocal of the resistance between opposing faces of a 1 cm cube (cm3) at a specific temperature
Distance (d = 1 cm)
Area (A = 1 cm2)
Conductivity Properties
Conductivity = d/A x conductance
K = 1.0 cm-1
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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
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Cell Constants (K) by Application
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• 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
Temperature Coefficients
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• 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
Non-Linear Temperature Coefficients
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Electrode
Field
Effect
• A meter applies a current to the electrodes
in the conductivity cell
Measuring Conductivity
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• 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
Measuring Conductivity
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• 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
2-Electrode Cells
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A
V
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 is
unaffected by fouling or
circuit resistance
57 Proprietary
• 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
Conductivity Measurement
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• 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 Measurement
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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• 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.
Other Measurement Capabilities
60 Proprietary
The world leader in serving science
Thermo Scientific
Barnstead Water Purification Systems
Kelly Sweazea, Specialist
Thermo Scientific Water Analysis
& Purification Products
2 Proprietary & Confidential
Laboratory Water Standards
General laboratory standards set by the American Society for
Testing and Materials (ASTM)
3 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
4 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
5 Proprietary & Confidential
Water System Types by 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?
• OR
• Replacement – are you replacing
a current system – like for like or
something different?
UV Applications
UV/UF Applications
Water System Types by Application
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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 -
7 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.
8 Proprietary & Confidential
Tap waterRO membrane
Removes ions
and organicsImpurities that will
damage RO membranes:
• CHLORINE
• Particulates (rust)
• Water hardness
•Found in most tap water
•Needs to be removed for RO
membranes >0.1 ppm free
chlorine
•Easily removed with carbonTank
Type 1
system
Chlorine
CARBON to
remove chlorine
9 Proprietary & Confidential
• Ruins RO membranes
• Lab equipment – can leave a deposit,
plug valves
• Affect gravimetric methods
• Prefilters remove particulates
Consumables affected:
• Prefilters
• Reverse osmosis
membranes
• Final filters
Rust
Sediment
Plumbing debris
Typical tap water
Particulates
10 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!
11 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
12 Proprietary & Confidential
Basic Methods of Water Purification
• Distillation
• Reverse Osmosis
• Deionization
• Filtration
• Adsorption
• Ultrafiltration
• UV Oxidation
13 Proprietary & Confidential
Thermo Scientific Barnstead Classic Still
• Consistent Type 2 pure
water in one process
• Bacteria, pyrogen
removed
14 Proprietary & Confidential
Reverse Osmosis - % Removal Technology
*Removes most impurities
from Tap water
15 Proprietary & Confidential
Deionization
•Ion removal only
•Type I water
16 Proprietary & Confidential
Electrodeionization (EDI)
17 Proprietary & Confidential
Depth Filtration
18 Proprietary & Confidential
Adsorption
19 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
20 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
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Limitations & Capabilities
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Applications
ICP, ICP-MS
Environmental testing (BOD for example)HPLC
Chemical synthesis,
extraction and
concentration
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Type 1 Systems – POU Type 1
24 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
25 Proprietary & Confidential
Distillation
Glass Distillation:
1.4 LPH – 13 LPH
Classic Tin lined
stills:
0.5 GPH – 10 GPH
26 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
27 Proprietary & Confidential
Simple Cartridge Deionizers
28 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)
29 Proprietary & Confidential
H2O Select
http://www.thermoscientific.com/en/about-
us/promotions/complimentaryh2o-select-analysis.html
30 Proprietary & Confidential
Brochures/Water Book
https://www.thermofisher.com/us/en/home/
global/forms/life-science/waterbook-
download-request.html
31 Proprietary & Confidential The world leader in serving scienceProprietary & Confidential
Myths and Truths: pH and Conductivity of Ultrapure Water
smart H2O for you,
and your science
32 Proprietary & Confidential
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
33 Proprietary & Confidential
Myth #1 – Ultrapure Water Conductivity
MYTH #1
Ultrapure water conductivity
after dispensing should
match the ultrapure water
system display.
TRUTH #1
Ultrapure water
immediately picks up
carbon dioxide and
conductivity upon
dispensing.
34 Proprietary & Confidential
Myth #2 – Ultrapure Water pH
MYTH #2
The pH of ultrapure water
should be 7.0 after
dispensing
TRUTH #2
The pH of ultrapure water
can quickly decrease
after the water has been
generated
35 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
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As more CO2 dissolves into UPW,
the conductivity rises.
Effect of CO2 on Conductivity of Ultrapure Water (1)
T (°C) 0 ppm (µS/cm) 20 ppm (µS/cm) 300 ppm
(µS/cm)
500 ppm
(µS/cm)
1000 ppm
(µS/cm)
2000 ppm
(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
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As more CO2 dissolves into UPW,
the pH falls.
Effect of CO2 on pH of Ultrapure Water (1)
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The Role of Temperature in Conductivity (1)
The conductivity of degassed ultrapure water rises in a non-
linear fashion with increasing temperature.
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Measurements USP <645> Pure
Water Conductivity
ASTM D1193
Type I Water
ASTM D1193
Type IV Water
Conductivity @
25°C
Stage 1: < 1.3 uS/cm* 0.0555 uS/cm 5.0 uS/cm
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
What to Expect for Ultrapure Water Measurements
*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
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Grab
(USP <645>)
Includes CO2
Compare to USP <645>
Flow Cell
(ASTM D1193)
Excludes CO2
Expect reading similar to UPW
display*
Temperature Compensation
Turn “off” for USP <645>
Use non-linear temp comp for
ASTM D1193 or compare to table
How to Test Conductivity in Ultrapure Water
*when using temperature compensation
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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.
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Flow Cell Measurement - Conductivity
UPW “in”
UPW “out”
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Grab Cell Measurement – Conductivity
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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.
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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
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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.
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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.
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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
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Myth #3 – Ultrapure Water System Display
MYTH #3
Ultrapure water purity system
display measures all of the
impurities in water.
TRUTH #3
Not all impurities will
be measured
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pH and Ultrapure Water
Measurements ASTM Type I ASTM Type
II
ASTM Type
III
ASTM Type
IV
Conductivity @
25°C
<0.0555
uS/cm
<1.0 uS/cm <0.250 uS/cm <5.0 uS/cm
Resistivity @
25°C
>18 MΩ·cm >1 MΩ·cm >4 MΩ·cm >0.2 MΩ·cm
TOC <50 ug/L <50 ug/L <200 ug/L Not specified
pH Not applicable Not
applicable
Not applicable 5.0 to 8.0
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Resistivity/Conductivity Measures Ions
TIS= Total ionized solids:
Calculated from
conductivity/resistivity
TDS = Total dissolved
solids
Includes dissolved impurities
(organics, colloids)
Measured gravimetrically
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Total Organic Carbon (TOC) Measures Organics
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Total Organic Carbon Measured with Resistivity
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Bacteria - No Displayed Measurement
Biofilm on
chiller filter
bag
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Particles - No Displayed Measurement
Turbidity
Meter
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Endotoxins and Nucleases - No Displayed Measurement
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Myth #4 – Ultrapure Water System Accessories
MYTH #4
Accessories added to the
ultrapure water system will
always improve
conductivity/resistivity.
TRUTH #4
Yes and No. Depends
on where it is added to
the system.
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Cartridges
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• Dual 185/254nm and single
wavelength lamps
• 254 nm provides germicidal
action against
microorganisms
• 185 photo-oxidizes
organics into CO2 for
ultralow TOC levels
UV Light
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Point of Use Final Filter
0.2 micron pore size
Pseudomonas
0.3 micron
Particle
0.6 micron
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• Remove endotoxins in water
• <0.001 Eu/ml
• Polysulfone hollow fibers
• Large surface area
Ultrafilters
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Recirculation and Dead Legs
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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
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