Essentials of pH Measurement · Disadvantages •Cost Mercury Free alternative to the Calomel...

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

3 Proprietary

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".


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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


http://en.wikipedia.org/wiki/Hydrochloric_Acid

http://en.wikipedia.org/wiki/Lead-acid_battery

http://en.wikipedia.org/wiki/Gastric_acid

http://en.wikipedia.org/wiki/Lemon

http://en.wikipedia.org/wiki/Cola

http://en.wikipedia.org/wiki/Vinegar

http://en.wikipedia.org/wiki/Orange_(fruit)

http://en.wikipedia.org/wiki/Apple


http://en.wikipedia.org/wiki/Beer

http://en.wikipedia.org/wiki/Acid_Rain

http://en.wikipedia.org/wiki/Coffee

http://en.wikipedia.org/wiki/Tea


http://en.wikipedia.org/wiki/Skin

http://en.wikipedia.org/wiki/Milk

http://en.wikipedia.org/wiki/Water


http://en.wikipedia.org/wiki/Human

http://en.wikipedia.org/wiki/Saliva

http://en.wikipedia.org/wiki/Blood

http://en.wikipedia.org/wiki/Seawater

http://en.wikipedia.org/wiki/Soap

http://en.wikipedia.org/wiki/Ammonia

http://en.wikipedia.org/wiki/Bleach

http://en.wikipedia.org/wiki/Sodium_hydroxide

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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

8 Proprietary


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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The pH Electrode

Sensing Bulb

Internal Fill Solution (Sensing)

Reference

Reference Fill Solution

Junction

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• 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

11 Proprietary

Common Questions: Electrode Types

What is a combination pH electrode?

• A combination pH electrode is one that

has a sensing half-cell and reference

half-cell built into one electrode body

instead of existing as two separate

electrodes.

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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.


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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

14 Proprietary

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

15 Proprietary

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

16 Proprietary

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

17 Proprietary

pH Measurement System - Junctions

• The electrode junction is where

the Outer fill solution (reference)

passes from inside the electrode

body to the sample completing

the “circuit”.

• The type of junction is a good

indicator of how the electrode will

perform in different samples.

• Three basic types of junctions

• Wick

• Ceramic

• Open

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• 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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• 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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• 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.


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.


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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

26 Proprietary

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.


27 Proprietary

• 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%


28 Proprietary

• Temperature affects calibration

slope because it affects the

expected change in the mV

value per pH unit


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%


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


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


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!

33 Proprietary

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

35 Proprietary

• 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




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

40 Proprietary

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+

42 Proprietary

• 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 =


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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 =


44 Proprietary

• Divalent ions generally contribute more

to conductivity than monovalent ions

Ca+2

Na+1vs.

Conductivity =


Charge per Carrier

45 Proprietary

• 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 =


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 =


47 Proprietary

• 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

48 Proprietary

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


K = 1.0 cm-1

49 Proprietary

Cell Constant (K) in cm-1

• The cell constant (K) is the value by which you multiply conductance to

calculate conductivity.

• The cell constant (K) is the

ratio of the distance between the electrodes (d) to the electrode area (A).

Fringe field effects is the amount AR.

K = d / (A + AR)

Conductance = the measured value relative to the specific geometry of the cell

Conductivity = the inherent property of the solution being tested

Conductivity = Conductance x K


K = 1.0 cm-1

50 Proprietary

Cell Constants (K) by Application

51 Proprietary

• 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

52 Proprietary

• 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

53 Proprietary

Electrode

Field

Effect

• A meter applies a current to the electrodes

in the conductivity cell

Measuring Conductivity

54 Proprietary

• 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

55 Proprietary

• 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

56 Proprietary

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


58 Proprietary

• In natural waters, conductivity is often expressed as “dissolved solids”

• Measured conductivity is reported as the concentration of NaCl that

would have the same conductivity

• Total Dissolved Solids (TDS) assumes all conductivity is due to

dissolved NaCl

Comparison of conductivity to TDS:


CONDUCTIVITY (S/cm) DISSOLVED SOLIDS (mg/l)

0.1 0.0210

1.0 0.44

10.0 4.6

100 47

200 91

1000 495

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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




2 Proprietary & Confidential

Laboratory Water Standards

General laboratory standards set by the American Society for

Testing and Materials (ASTM)


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


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


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


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 -


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.


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


• 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


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!


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


Basic Methods of Water Purification

• Distillation

• Reverse Osmosis

• Deionization

• Filtration

• Adsorption

• Ultrafiltration

• UV Oxidation


Thermo Scientific Barnstead Classic Still

• Consistent Type 2 pure

water in one process

• Bacteria, pyrogen

removed


Reverse Osmosis - % Removal Technology

*Removes most impurities

from Tap water


Deionization

•Ion removal only

•Type I water


Electrodeionization (EDI)


Depth Filtration


Adsorption


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


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


Limitations & Capabilities


Applications

ICP, ICP-MS

Environmental testing (BOD for example)HPLC

Chemical synthesis,

extraction and

concentration


Type 1 Systems – POU Type 1


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


Distillation

Glass Distillation:

1.4 LPH – 13 LPH

Classic Tin lined

stills:

0.5 GPH – 10 GPH


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


Simple Cartridge Deionizers


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)


H2O Select

http://www.thermoscientific.com/en/about-

us/promotions/complimentaryh2o-select-analysis.html


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


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


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.


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


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


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


As more CO2 dissolves into UPW,

the pH falls.

Effect of CO2 on pH of Ultrapure Water (1)


The Role of Temperature in Conductivity (1)

The conductivity of degassed ultrapure water rises in a non-

linear fashion with increasing temperature.


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


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


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.


Flow Cell Measurement - Conductivity

UPW “in”

UPW “out”


Grab Cell Measurement – Conductivity


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.



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



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.


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.


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

http://www.thermoscientific.com/waterlibrary


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


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


Resistivity/Conductivity Measures Ions

TIS= Total ionized solids:

Calculated from

conductivity/resistivity

TDS = Total dissolved

solids

Includes dissolved impurities

(organics, colloids)

Measured gravimetrically


Total Organic Carbon (TOC) Measures Organics


Total Organic Carbon Measured with Resistivity


Bacteria - No Displayed Measurement

Biofilm on

chiller filter

bag


Particles - No Displayed Measurement

Turbidity

Meter


Endotoxins and Nucleases - No Displayed Measurement


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.


Cartridges


• 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


Point of Use Final Filter

0.2 micron pore size

Pseudomonas

0.3 micron

Particle

0.6 micron


• Remove endotoxins in water

• <0.001 Eu/ml

• Polysulfone hollow fibers

• Large surface area

Ultrafilters


Recirculation and Dead Legs


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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