Level 1 pH Theory

Section 1

What is pH and How is it Measured?

Why Measure pH?Why Measure pH? Final product quality depends on pH.

– Pharmaceutical– Paper– Metal plating– Drinking water– Food and Beverages– Alternative fuel

Chemical reaction rates are often a function of pH – Corrosion– Scaling– Precipitation (salts)– Fermentation

Environmental Monitoring

Acid and Base BasicsAcid and Base Basics

HCl H+ + Cl-

NaOH Na+ + OH-

H2O H+ + OH-

Strong Acids dissociate completely in water releasing hydrogen ions.

Weak Acids only partially dissociate in water releasing hydrogen ions.

HAc H+ + Ac-

Strong Bases dissociate completely in water releasing hydroxyl ions.

Weak Bases only partially dissociate in water releasing hydroxyl ions.

NH4OH NH4+ + OH-

Water itself partially dissociates releasing hydrogen and hydroxyl ions. There is an equilibrium in water between hydrogen and hydroxyl ions.

What is pH?What is pH?

pH is the “unit of measure” for the acidity of a solution. It is defined as the negative logarithm of the hydrogen ion activity,aH+

Activity is related to the concentration of Hydrogen Ion, H+ by an activity coefficient.

In general, pH is used as the measure of acidity and rarely used as a direct measurement of concentration by users.

pH = - log [aH+]

aH+=10-pH

or

aH+ = g cH+

pH ScalepH Scale

pH Hydrogen Ion [H+] Hydroxyl Ion [OH-] 0 Acidic 1 2 3 4 5 6 7 Neutral 8 9 10 11 12 13 14 Basic

1.0 0.1 0.01 0.001 0.0001 0.00001 0.000001 0.0000001 0.00000001 0.000000001 0.0000000001 0.00000000001 0.000000000001 0.0000000000001 0.00000000000001

0.00000000000001 0.0000000000001 0.000000000001 0.00000000001 0.0000000001 0.000000001 0.00000001 0.0000001 0.000001 0.00001 0.0001 0.001 0.01 0.1 1.0

• pH scale is based on dissociation constant of water Kw

• Kw = aH+aOH- = 10-7·10-7 = 10-14 mol/liter at 25°C (and ONLY at 25°C)

pH values of everyday solutionspH values of everyday solutions pH 0 Sulfuric Acid (Battery Acid) pH 1 Gastric Juice, 0.5% Sulfuric Acid pH 2.3 Lemon Juice pH 3 Vinegar, Coca Cola pH 4.3-4 Beer, Sour Milk pH 4.8Pure water air equilibrated pH 6.5 Fresh Milk pH 7 Pure Water pH 7.36 Blood pH 8.3 Sea Water pH 13 0.4 % NaOH pH 14 NaOH (Drain Opener)

Key pH Sensor ComponentsKey pH Sensor Components

Measuring Electrode– Develops a millivolt potential directly proportional to pH in an

aqueous solution Reference Electrode

– Maintains a stable reference potential regardless of changes in solution pH or other ionic activity

Reference Electrode Liquid Junction– Maintains electrical contact between the pH measuring electrode

and the reference cell via the process solution Temperature Compensator

– Corrects for changes in the millivolt output of the pH sensor due to process temperature change

pH-Glass ElectrodepH-Glass Electrode

The purpose of a pH glass electrode is to develop a millivolt potential directly proportional to pH of an aqueous solution

Shield

GlassBody

Buffered FillSolution

Ag/AgClInternalWire

pH SensitiveGlass

The Reference ElectrodeThe Reference Electrode

pH17

GlassBody

KClFill Solution

Ag/AgClInternalWire

LiquidJunction

The purpose of the reference electrode is to provide stable and reproducible potential to which glass electrode potential may be referenced. It completes the circuit by contacting the sample solution through a liquid junction.

The liquid junction allows diffusion of the electrolytes (ions) into and from the process, to maintain electrolytic contact.

Most reference electrodes are termed ‘non-flowing’, because contact with the process is by ionic diffusion and not flow of the filling solution.

Liquid JunctionsLiquid Junctions Liquid Junction

– A porous plug that allows liquid contact between the internal KCl solution and outside process solution but restricts the flow.

– The larger the porosity is, the lower the electrical resistance is and higher the diffusion rate is.

– The smaller the porosity is, the higher the electrical resistance is and the lower the diffusion rate.

There is a tradeoff between high flow with good measurement accuracy, and low flow with longer reference life and less stable junction potentials.

Potassium and Chloride ions diffuse out the reference at the essentially same rate.– Positive and negative Ions in the process can diffuse

into the reference at different rates, which leads to a build up of an electric charge across the liquid junction.

– This is called Liquid Junction Potential, which normally causes a small error in the pH measurement, which can be calibrated out by standardization

KCl DiffusionRate

20K ohms

1-2 year life

KCl Diffusion Rate

5K ohms

6-8 month life

pH Sensor ConstructionpH Sensor Construction

Basic circuits for a pH measurement

Sensor Potential (E) using a pH sensor with a Solution Ground.

Temp

ESG

EpH ERef

EpH ERef

Temp

E = (EpH - ESG) – (ERef – ESG)

Sensor Potential (E) using a pH sensor without a Solution Ground.

E = EpH – ERef

A solution ground makes it possible to measure reference electrode impedance and use reference impedance as a diagnostic tool.

PreamplificationPreamplification pH sensor outputs (mV) are high

impedance signals and can be susceptible to noise and interference.– To lower the impedance and amplify the

signal, a preamplifier is used:• In the pH sensor• In a remote junction box• In the transmitter

Certain (rare) pH sensors use a 2nd glass electrode as a reference.– Most pH transmitters can be configured

to accommodate the high reference impedance of these sensors.

Preamp

Preamp

Preamp

Temperature Compensation andthe Calculation of pHTemperature Compensation andthe Calculation of pH

How pH is Calculated with Temperature Compensation:

Isopotential pH

Sensor potential (mV)

Zero offset (mV)

Slope (mV/pH K)

Temperature (K)

The effect of temperature on the pH sensor must be taken into account when determining pH to avoid large errors.

pH Changes with Temperature Changein Strong Acid and Base Solutions

The pH of certain solutions can themselves change with temperature. The degree to which this happens is a function of the composition of the solution.

In application where this is an issue, a second, solution temperature compensation is provided to correct the measured pH to 25 C.

Solution Temp.Coefficient

Z= -.0242 pH/°C

8.80

8.70

8.60

8.508.40

8.00

8.10

8.20

8.30

504520 25 403530 55

SOLUTION pH CHANGE OF A DETERGENTWITH TEMPERATURE

8.90

9.00

pH

oo

oo

o

oo

Solution pH Change with Temperature

pH Transmitter ConfigurationAll the Configuration Needed for pH MeasurementspH Transmitter ConfigurationAll the Configuration Needed for pH Measurements

Location of Preamp

Impedance of Reference Electrode (can choose High for special electrodes)

Temp Comp On/Off

Temp Unit

Manual Temp Value

Solution Temp Type

Coefficient for Linear Solution Temp Comp

Isopotential pH (can be changed for special electrodes)

Mounting pH SensorsMounting pH Sensors pH Sensors

should be mounted at least 10 degrees above horizontal.

Otherwise, the air bubble in the glass electrode can cover the inner surface of the glass bulb.

Section 1 Test Section 1 Test

Question 1Question 1 pH is proportional to:

a) The concentration of hydrogen ion

b) The concentration of hydroxyl ion

c) The logarithm of the concentration of hydrogen ion

d) The negative logarithm of the concentration of hydrogen ion

Question 2Question 2 A neutral solution has an equal concentration of

hydrogen and hydroxyl ions and its pH:a) is always 7.00 pH

b) is only 7.00 pH at 25 deg C

c) depends on the temperature of the solution

d) b and c

Question 3Question 3 The millivolt output of a pH sensor:

a) changes with temperature

b) remains constant with temperature changes

c) can be easily compensated for by measuring temperature

d) a and c

Question 4Question 4 The actual pH of a solution can:

a) change with temperature

b) change with temperature and does not require special temperature compensation

c) can be compensated by using special temperature compensation

d) a and c

Question 5Question 5 pH sensors must be mounted at least 10 degrees

above horizontal.– True or False?

Answers to Section 1 TestAnswers to Section 1 Test 1 – d: pH is proportional to the negative logarithm of

hydrogen ion concentration 2 – b and c: The pH of a neutral solution changes with

temperature and is only 7.00 pH at 25C 3 – a and c: The millivolt output of a pH sensor changes

with temperature and can easily be compensated for by measuring temperature

4 – a and c: The actual pH of a solution can change and can be compensated by using special temperature compensation

5 – True: pH sensor need to be mounted 10 degrees above horizontal

Section 2

When to Use pH and Special pH Applications

When to Use pH -- Acidic SolutionsWhen to Use pH -- Acidic Solutions

1

10

100

1,000

10,000

100,000

1,000,000

-1.0 0.0 1.0 2.0 3.0 4.0 5.0

Conductivity (microS/cm)

pH vs Conductivity for a Strong Acid

pHAcid Error Below 1.0 pH

Less Than 0.0 pHNot Possible w ith pH

1 ppm Acid

100 ppm Acid

1% Acid

4%Acid

At pH values below 1.0 pH, a bad pH application becomes a good Conductivity application.

1

10

100

1,000

10,000

100,000

1,000,000

9.0 10.0 11.0 12.0 13.0 14.0 15.0

pH

Conductivity microS/cm

pH vs Conductivity for a Strong Base

LargeSodium Error

pH Electrode Destroyed

1 ppm Base

100 ppm Base

4% Base

10% Base

When to use pH -- Basic SolutionsWhen to use pH -- Basic SolutionsAt pH values above 13.0 pH, a bad pH application becomes a good Conductivity application.

Special pH Applications Special pH Applications

Special pH ApplicationsSpecial pH Applications Most pH applications involve weak solutions or

simply water at near room temperature.– A general purpose pH sensor can be used

• The sensor should have a long, worry free life

In some pH applications, the temperature, pressure, and composition of the process can create issues with the pH measurement– pH sensors with special features need to be chosen to

best deal with these issues

Pure Water (Conductivity < 5 mS/cm)Pure Water (Conductivity < 5 mS/cm) Liquid Junction Potential (LJP)

– In normal applications, a potential at the liquid junction of the reference electrode is set up by the unequal diffusion of ions from the process into liquid junction.• This is usually no more than 15 to 20 mV (~ + 0.3 pH) and can

be calibrated out by standardization. In high purity water application this effect can

become large and unstable resulting in major errors and drifting.– This can be made worse by fluctuations in flow and static

buildup on the pH sensor due to the low conductivity of the water.

Pure water pH applications need a special sensor designed to take these effects into account.

High Purity Water pH System ComponentsHigh Purity Water pH System Components

Air Bleed Screw

Diffuser (inside flow cell)

Reservoir Filter

500 mlElectrolyte Reservoir

Flow Cell

Sample OUT

Sample IN

Reference Tubing

Combination Electrode

Vent Tube

High Temperature ApplicationsHigh Temperature Applications High Temperature

– Accelerates the ageing of pH sensor materials.

– Increases the impedance of the glass electrode.• Results in a slower

response time.– Can quickly destroy a pH

sensor not designed for it.– A pH Sensor designed for

High Temperature must be used.

Traditional pH Sensor Lifetime versus Temperature

• Sensor 1: Up to 155°C at 400 psig

Current High Temperature pH Sensor Ratings

Process Effects on Glass ElectrodesProcess Effects on Glass Electrodes Chemical Erosion and Attack

– Hydrofluoric Acid (HF)• Dissolves glass

– If fluoride is present you need to know its concentration and the pH range.» A special sensor may be needed or the application may not be

possible with pH.

– Sodium and Potassium Hydroxide (NaOH and KOH)• > 4 % (14 pH) will dissolve glass within 8 hours at elevated

temperature – there’s no remedy -- go with conductivity– Solutions containing Abrasives

• To prevent electrode damage or breakage, protect the electrode from the direct impact of the process flow.

– Glass electrodes crack or break in these cases.

Sensor CoatingSensor Coating A sample velocity > 5 ft/sec will help

minimize coating Cleaning Solutions for:

– Alkaline or Scale • 5 % HCl or vinegar

– Acidic Coatings• Weak caustic < 4 %

– Oil, grease, or organic compounds• Detergent / sensor friendly solvents

Use a pH Sensor designed to resist coating

In line Cleaning can be done using a jet spray cleaner.

Coating increases sensor response time and can cause instability in pH control applications.– Severe coating shuts down the pH

measurement altogether.

Reference Electrode ContaminationReference Electrode Contamination Plugging

– Precipitation of silver in the reference fill solution by ions in process.• Typical villains are sulfide, bromide, and iodide ions• Use a triple junction electrode with a potassium nitrate outer fill.• Plugging causes the pH measurement to drift.

Poisoning– Depletion of the silver in the reference solution by precipitation or

complexation of free silver ion.• Precipitation: sulfide, bromide, and iodide• Complexation: ammonia; cyanide is deadly to references• Check the concentration of poisoning ions and use a triple junction

electrode, or in extreme cases, a special electrode will be needed.• Poisoning causes a large zero offset (> 60 mV).

What Happens During Reference Cell PoisoningWhat Happens During Reference Cell Poisoning

Time

Ref

eren

ce P

ote

nti

al, m

V

Reference 1: Poisoned Reference 2: Poisoned

Reference 3: OK

Reference Poisoning can be a slow process. Nothing happens until the silver ion concentration in the reference is irreversibly depleted and the potential changes.

Triple Junction Reference ElectrodeTriple Junction Reference Electrode Reference technology

– Single, Double, Triple Junction are used.• Triple junctions slow the diffusion of harmful

ions into the innermost portion of the reference.

Reference electrode material– Silver-silver chloride wire in potassium

chloride solution• Standard concentrations of silver and chloride

ions maintain a standard potential• Poisoning ions disrupt these concentrations

Junction Materials

– Ceramic, Teflon, quartz fibers Electrolyte Fill Solutions

– Gelled fill solution – Resists the transport of harmful ions by thermal convection

Liquid Junctions

Applying pH Applying pH – You need Information:– Process Pressure and Temperature

• Don’t forget to include transients—a short expose to high temperature or pressure can kill a sensor not designed for it.

– Process Composition• Not such a concern in common, well-known applications.• In certain applications, knowing the process composition is essential.

– Choose the Sensor for Application• In benign applications go with a sensor easy to mount and maintain• Specially Designed pH Sensors are needed for:

– High Purity Water– High Temperature Processes– Processes that Coat– Process with components that Poison

Section 2 Test Section 2 Test

Question 1Question 1 Conductivity is a better measurement than pH for

concentrated acids and bases.– True or False?

Question 2Question 2 Measuring pH of a solution with a conductivity

less than 5mS/cm can requires a special pH sensor.– True or False?

Question 3Question 3 When choosing a pH Sensor the following

should be considered:a) Process temperature and pressure

b) The conductivity of the process

c) The composition of the process

d) All of the above

Question 4Question 4 A process flow velocity greater than 5 ft/second

will help minimize sensor coating.– True or False?

Question 5Question 5 pH measurements in processes with high

temperature and pressure require a special pH sensor.– True or False?

Answers to Section 2 TestAnswers to Section 2 Test 1 – True: Conductivity is a better measurement of

concentrated acids and bases, due to problems with pH in these solutions.

2 – True: Measuring the pH low conductivity solutions requires a special pH sensor.

3 – d: The temperature, pressure, and composition of a process, including transients needs to be considered when applying pH.

4 – True: a high sample velocity does help prevent sensor coating.

5 – True: High sample temperatures and pressures require a special pH sensors; standard pH sensors have a short life in these applications.

pH Calibration and DiagnosticspH Calibration and Diagnostics

pH Buffer SolutionspH Buffer Solutions Solutions of known pH that can withstand moderate contamination or

dilution without significant pH variation. The more concentrated a buffer solution is the more resistant it is to dilution and acid or base contamination.

Buffers 4 and 7 or 10 are usually used – a difference of 3 pH between buffer values is recommended for (two point) buffer calibrations

Rules of buffering– Use fresh buffer– Rinse between buffers– Allow reading to stabilize

The of a buffer solution can and does change with temperature– This needs to be taken into account during a buffer calibration

pH Buffer CalibrationpH Buffer Calibration

0

2

4

6

8

10

12

14

Slope mV/pH

Two Point Calibration– Verifies Sensor Response to

pH Change– Determines Slope and Zero

Offset Transmitter Automatic Buffer

Calibration Features– Identifies the Buffer Value– Compensates for Changes in

Buffer pH with Temperature – Accepts Calibration only upon

Stabilization of the Millivolt Signal

Buffer 2

Buffer 1

Zero

pH StandardizationpH Standardization Performed on-line by grab

sample evaluation– Use a calibrated portable

analyzer– Take sample at the or near the

sensor installation point– Analyze grab sample

immediately for best results Calibrates the sensor in the

process environment– Compensates for minor coatings– Compensates for small offsets

due to liquid junction potential But…Even a broken pH

electrode can be standardized.

Configuring pH CalibrationConfiguring pH Calibration

Select: Manual or Auto Buffer Cal

Select the buffer type you want to use for Auto Cal

Select Stabilization Span and Time

Begin Buffer or Temperature Cal or pH Standardization

Cal Constants from the last calibration

The current live measurements and their status

Set the maximum zero offset limit

pH DiagnosticspH Diagnostics

pH Diagnostic TypespH Diagnostic Types

Sensor Diagnostics Calibration Diagnostics Transmitter Diagnostics Events

Sensor DiagnosticsElectrode Impedance DiagnosticsSensor DiagnosticsElectrode Impedance Diagnostics Glass Electrode Impedance

– Range: 10’s to 100’s of Mohm– Glass impedance is highly

temperature dependent and uses impedance temperature compensation.

– Best use is detecting cracked or broken electrode (R < 1Mohm)

– The pH of a broken or cracked electrode is a constant pH near 7.00 pH.• It can easily go undetected without

diagnostics Reference Impedance

– Needs a pH sensor with a solution ground for measurement

– Range: 1 to 100’s of kohm– Detects plugged or coated reference

electrodes

With a Solution Ground

Without a Solution Ground

More Sensor DiagnosticsMore Sensor Diagnostics Temperature Diagnostics

– High / Low Temperature• Temperature is outside the range of the

pH sensor, which can be damaged– Temperature Open / Shorted

• The measured temperature will appear either extremely high or low– The measured pH will be about 7 pH

» This error can go undetected without diagnostics

Solution Ground Open– The solution ground input is an open

circuit• The millivolt input is out of range• The solution ground lead must be

connected• The solution ground on the sensor must

be in contact with the process solution• If there is no solution ground the

solution ground input must be jumpered to the reference electrode input.

Sensor potential (mV)

Temperature

A good Temperature measurement is as important as a good Millivolt input.

Calibration DiagnosticsCalibration Diagnostics pH Slope Low

– Usual low limit is 40 mV/pH (Ideal slope is 59.19 mV/pH)• Indicates that a pH electrode is worn out• The sensor is worn out (usually has a high impedance short)• The sensor is coated• An error was made during calibration

pH Slope High– Usual high limit is 62 mV/pH

• There could be a sensor problem – check for instability• An error was made during calibration (most likely)

Zero Offset to high– Default setpoint is + 60 mV

• Can indicate a poisoned reference electrode• An error was made during calibration

Transmitter DiagnosticsTransmitter Diagnostics Electronic Errors

– Can be an input out of range (A to D Converter Overrange)• Likely a sensor problem

– Can indicate a fatal error (Ground > 10% Off)• Transmitter must be replaced

Memory Errors– Usually fatal errors requiring transmitter replacement

EventsEvents Alerts user and control system that certain events

are or have taken place, such as:– Buffer calibration– Standardization– Temperature standardization– Transmitter Out of Service

While these events appear on the transmitter’s display, their most important role is in control systems using smart transmitters, where they provide important information in batch and control system records.

An Example of pH DiagnosticsAn Example of pH Diagnostics

Transmitter Diagnostics

Sensor Diagnostics Calibration Diagnostics

Events

An Example of a Diagnostic Message inan Asset Management SystemAn Example of a Diagnostic Message inan Asset Management System

Smart pH SensorsSmart pH Sensors

What Smart pH Sensors DoWhat Smart pH Sensors Do Smart pH Sensors have a preamplifier with a memory

chip Upon connection to a Smart enabled pH Transmitter they

upload their latest calibration data, which is used by the transmitter to measure pH– This makes it possible to buffer calibrate a sensor in the shop or

laboratory and simply install it in the process. Historical data acquired during calibration is also

uploaded to the transmitter for display by it or an asset management system

New calibration data and maximum and minimum temperature measured are downloaded from the transmitter to the Smart sensor

Sensor Information Serial number Manufacturer date code

Calibration Data Slope Zero offset Temp cal offset Glass impedance Reference impedance Sensor run time at each

calibrations

Historical Information Last 5 calibration data sets for

troubleshooting

Plug and Play

Smart pH Sensor InformationSmart pH Sensor Information

Smart Sensor : Basic Sensor Informationin an Asset Management SystemSmart Sensor : Basic Sensor Informationin an Asset Management System

Smart Sensor : Calibration Historyin an Asset Management SystemSmart Sensor : Calibration Historyin an Asset Management System

Troubleshooting pH ApplicationsTroubleshooting pH Applications

– What do you do when sensors fail prematurely?• Get the application information:

– Process temperature and pressure (and transients)

– Process composition (and transients)• How is it failing? Broken glass...poisoned

reference…coating…• How often is it failing? • What do the diagnostics say?• If calibration information is available, get it• Have sensors always failed in this application or

is it a new phenomenon?

Troubleshooting pH ApplicationsTroubleshooting pH Applications Tools for diagnosing problems

– Use the transmitter’s diagnostics – Smart pH sensors capture calibration data, and

min/max temperature; this is very useful for troubleshooting.

– If asset management software is used, there is a lot of information captured an audit trail.

– Data Logging• Look at measurement data, including temperature, millivolts,

and impedances if they are available• Check events, such as diagnostics alarms, if they are also

recorded.

Section 3 Test Section 3 Test

Question 1Question 1 A two point buffer calibration is the only way to

tell if a glass electrode is adequately responding to changes in pH.– True or False?

Question 2Question 2 If a glass pH electrode is broken, can it be

standardized?– Yes or No?

Question 3Question 3 When choosing a pH Sensor the following

should be considered:a) Process temperature and pressure

b) The conductivity of the process

c) The composition of the process

d) All of the above

Question 2Question 2 The pH of a solution can change with

temperature.– True or False?

Question 5Question 5 When pH sensors consistently fail prematurely,

the following should be considered and examined:a) Process temperature and pressure

b) Sensor or calibration diagnostic history

c) The composition of the process

d) All of the above