Level 1 pH Theory
Section 1
What is pH
and How is it Measured?
Why Measure pH?


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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 Basics
Strong Acids dissociate completely in
water releasing hydrogen ions.
HCl
H+ + Cl-
Weak Acids only partially dissociate in
water releasing hydrogen ions.
HAc
H+ + Ac-
Strong Bases dissociate completely in
water releasing hydroxyl ions.
NaOH
Weak Bases only partially dissociate in
water releasing hydroxyl ions.
NH4OH
Water itself partially dissociates releasing
hydrogen and hydroxyl ions. There is an
equilibrium in water between hydrogen
and hydroxyl ions.
H 2O
Na+ + OHNH4+ + OH-
H+ + OH-
What is pH?
pH = - log [aH+]
or
aH+=10-pH
aH+ = g cH+



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 Scale
•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
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 values of everyday solutions
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pH 0
pH 1
pH 2.3
pH 3
pH 4.3-4
pH 4.8
pH 6.5
pH 7
pH 7.36
pH 8.3
pH 13
pH 14
Sulfuric Acid (Battery Acid)
Gastric Juice, 0.5% Sulfuric Acid
Lemon Juice
Vinegar, Coca Cola
Beer, Sour Milk
Pure water air equilibrated
Fresh Milk
Pure Water
Blood
Sea Water
0.4 % NaOH
NaOH (Drain Opener)
Key pH Sensor Components

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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 Electrode
Shield
Glass
Body
The purpose of a pH glass
electrode is to develop a millivolt
potential directly proportional to pH
of an aqueous solution
Ag/AgCl
Internal
Wire
pH Sensitive
Glass
Buffered Fill
Solution
The Reference Electrode
Glass
Body
Ag/AgCl
Internal
Wire
KCl
Fill
Solution
Liquid
Junction
pH17
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 Junctions
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
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 Diffusion
Rate
5K
ohms
6-8 month life
KCl Diffusion
Rate
20K
ohms
1-2 year life
pH Sensor Construction
Basic circuits for a pH measurement
Sensor Potential (E) using a pH
sensor with a Solution Ground.
E = (EpH - ESG) – (ERef – ESG)
A solution ground makes it possible to
measure reference electrode impedance
and use reference impedance as a
diagnostic tool.
ESG
Temp
EpH
ERef
Sensor Potential (E) using a pH
sensor without a Solution Ground.
E = EpH – ERef
Temp
EpH
ERef
Preamplification

pH sensor outputs (mV) are high
impedance signals and can be
susceptible to noise and interference.
Preamp
– To lower the impedance and amplify the
signal, a preamplifier is used:
• In the pH sensor
• In a remote junction box
• In the transmitter
Preamp
Preamp

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.
Temperature Compensation and
the Calculation of pH
The effect of temperature on the
pH sensor must be taken into
account when determining pH to
avoid large errors.
How pH is Calculated with Temperature Compensation:
Sensor potential (mV)
Isopotential pH
Zero offset (mV)
Temperature (K)
Slope (mV/pH K)
pH Changes with Temperature Change
in 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 pH Change with Temperature
pH
9.00
8.90
8.80
8.70
8.60
8.50
8.40
SOLUTION pH CHANGE OF A DETERGENT
WITH TEMPERATURE
Solution Temp.
Coefficient
o
o
Z= -.0242
pH/°C
o
o
o
8.30
8.20
8.10
8.00
o
o
20
25
30
35
40
45
50
55
pH Transmitter Configuration
All 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 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
Question 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 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 3
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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 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 5

pH sensors must be mounted at least 10 degrees
above horizontal.
– True or False?
Answers to Section 1 Test
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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 Solutions
At pH values below 1.0 pH, a bad pH application becomes a good Conductivity application.
Conductivity
(microS/cm)
1,000,000
100,000
10,000
pH vs Conductivity for a Strong Acid
100 ppm
Acid
4%
Acid
1,000
100
10
1
-1.0
1 ppm
Acid
1%
Acid
0.0
1.0
Less Than 0.0 pH
Not Possible w ith pH
2.0
3.0
Acid Error Below
1.0 pH
4.0
5.0
pH
When to use pH -- Basic Solutions
At pH values above 13.0 pH, a bad pH application becomes a good Conductivity application.
pH vs Conductivity for a Strong Base
Conductivity
microS/cm
4%
Base
10%
Base
1,000,000
100 ppm
Base
100,000
10,000
1 ppm
Base
1,000
100
Large
Sodium Error
10
pH Electrode
Destroyed
1
9.0
10.0
11.0
12.0
pH
13.0
14.0
15.0
Special pH Applications
Special 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)

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.
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Pure water pH applications need a special sensor
designed to take these effects into account.
High Purity Water pH System Components
500 ml
Electrolyte
Reservoir
Vent Tube
Reservoir Filter
Reference
Tubing
Air Bleed Screw
Combination
Electrode
Sample OUT
Diffuser (inside flow cell)
Sample IN
Flow Cell
High Temperature Applications

High Temperature
Traditional pH Sensor Lifetime versus 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.
Current High Temperature pH Sensor Ratings
•Sensor 1: Up to 155°C at 400 psig
Process 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 Coating
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
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
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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 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 Poisoning
Reference Poisoning can be a slow process. Nothing happens until the silver ion
concentration in the reference is irreversibly depleted and the potential changes.
Reference Potential, mV
Time
Reference 3: OK
Reference 1: Poisoned
Reference 2: Poisoned
Triple 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
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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
– 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
Question 1

Conductivity is a better measurement than pH for
concentrated acids and bases.
– True or False?
Question 2

Measuring pH of a solution with a conductivity
less than 5mS/cm can requires a special pH
sensor.
– True or False?
Question 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 4

A process flow velocity greater than 5 ft/second
will help minimize sensor coating.
– True or False?
Question 5

pH measurements in processes with high
temperature and pressure require a special pH
sensor.
– True or False?
Answers to Section 2 Test
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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
Diagnostics
pH Buffer Solutions
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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 Calibration

– Verifies Sensor Response to
pH Change
14
12
10
– Determines Slope and Zero
Offset
Buffer 2
8

Zero
6
4
Buffer 1
2
Slope mV/pH
14
12
10
8
6
4
2
0
0
Two Point Calibration
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
pH 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 Calibration
The current live measurements and their status
Begin Buffer or
Temperature Cal or pH
Standardization
Cal Constants from the
last calibration
Set the maximum zero offset limit
Select: Manual or Auto Buffer Cal
Select the buffer type you want to use for Auto Cal
Select Stabilization Span and Time
pH Diagnostics
pH Diagnostic Types
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Sensor Diagnostics
Calibration Diagnostics
Transmitter Diagnostics
Events
Sensor Diagnostics
Electrode 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 Diagnostics

Temperature Diagnostics
– High / Low Temperature
• Temperature is outside the range of the
pH sensor, which can be damaged
Sensor potential (mV)
– 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.
Temperature
A good Temperature measurement
is as important as a good Millivolt
input.
Calibration 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 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
Events

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 Diagnostics
Sensor Diagnostics
Calibration Diagnostics
Events
Transmitter Diagnostics
An Example of a Diagnostic Message in
an Asset Management System
Smart pH Sensors
What 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
Smart pH Sensor Information
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 Sensor : Basic Sensor Information
in an Asset Management System
Smart Sensor : Calibration History
in an Asset Management System
Troubleshooting 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 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
Question 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 2

If a glass pH electrode is broken, can it be
standardized?
– Yes or No?
Question 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 2

The pH of a solution can change with
temperature.
– True or False?
Question 5

When pH sensors consistently fail prematurely,
the following should be considered and
examined:
a)
b)
c)
d)
Process temperature and pressure
Sensor or calibration diagnostic history
The composition of the process
All of the above