This document discusses pH measurement and provides details on: - The definition and scale of pH as a measure of acidity or alkalinity. - Why pH is measured in various industries and applications. - The principles of pH measurement using a glass electrode and pH meter. - Factors that affect pH measurement accuracy such as temperature, ionic strength, and electrode calibration. - The process of calibrating pH electrodes using buffer solutions and adjusting for the Nernstian slope.

1. pH - MEASUREMENT
pH: -
 It is a convenient measure of acidity / alkalinity of a aqueous
solution at a specific temperature.
 It is measured on a continuous scale from 0 to 14
 In a chemical laboratory, pH is commonly measured using an
electronic pH meter with a scale 0.01 pH meter

2. pH - MEASUREMENT
WHY IS pH MEASURED?
 To test a sample against a legal requirement
 To test a chemical against a specification
 As a part of analytical method
 Process control in chemical industry
 Environmental monitoring of waste and effluents
 Monitoring and controlling biochemical reactions, many of
which only take place in a particular and sometimes narrow pH
range.

3. pH - MEASUREMENT
PRINCIPLES: -
 The pH value of a given solution is a measure of the activity of
the hydrogen ion (H+) in that solution
 The scale is logarithmic
 In a solution of pH 5, the hydrogen ion activity is 10 times
higher than it is in a solution of pH 6
 In aqueous solutions, the actual species is the hydronium ion
H3
+, rather than the hydrogen ion H+
 pH = log10 1/a H+ = -log10aH+
 Activity of a species is a measure of its effective or available
concentration rather than its actual or total concentration.
aH+ = γ x cH+
γ = activity of coefficient

4. pH - MEASUREMENT
PRINCIPLES: -
 In dilute solution (< 0.001 mol) of simple univalent electrolyte γ
approximate to unity and activity and concentration are
approximately equal
 In other type of aqueous solution, there is no easily determined
relationship between activity and concentration
 From the theoretical, thermodynamic view point, the activity of
single ion species such as H+ is an exact quantity

5. pH - MEASUREMENT
PRINCIPLES: -
 pH value to be regarded as convenient and comparitative
measure of acidity
 pH is usually determined by electrochemical measurement, in
which the potential of a pH electrode immersed in the test
solution is measured.
 The pH electrode respond quantitatively and specifically to
hydrogen ions even in the presence of other positive ions

6. pH - MEASUREMENT
ELECTROMETRIC DETERMINATION OF pH
 The pH measurement is possible because emf of certain
chemical cell varies with the hydrogen ion concentration of the
solution on the cell.
 pH electrode/test solution to be measured// reference electrode
symbol//signifies the presence of a liquid junction between test
solution and reference electrode
 If other variables in the cells are controlled, emf of the cell can
be correlated with pH
 Pt, H2 (p) H+ (a) reference electrode.
- left hand electrode is the hydrogen electrode
- right hand electrode whose potential is not affected by pH

7. pH - MEASUREMENT
ELECTROMETRIC DETERMINATION OF pH
 Ecell = EH2, H+ + Eref
Ecell - Eref = EH2, H+
Ecell - Eref = RT/nF x 2.303 log10 aH+
R = gas constant (8.314 joule k-1mol-1)
T = Absolute temperature (Kelvin)
F = A electrochemical constant (96487 coulomb mol-1)
N = 1 for univalent species such as H+
Ecell - Eref = 0.0591 pH

8. pH - MEASUREMENT
ELECTROMETRIC DETERMINATION OF pH
 Theoretical slope of a pH electrode is such that change in pH of
1 unit results in a change in the potential (E) of the electrode of
59.1 mV at 25°C
 Mathematically Delta E / Delta pH = 59.1 mV
 The value of ERef include the following
- The standard potential of the pH electrode (i.e. the potential
when a H+ = 1)
- The potential of the reference electrode
- The liquid junction potential
- The asymmetry potential

9. pH - MEASUREMENT
ELECTROMETRIC DETERMINATION OF pH
 The value of Eref cannot be determined in routine
measurements mainly due to the indeterminate
- Nature of the liquid junction potential
- Asymmetry potential
- Value of asymmetry potential tends to drift as the condition of
the electrodes changes
- The equation not used in a basic way to determine pH of a
solution
- The response of the glass electrode is calibrated using
standard aqueous buffer solutions with known reference pH
values. Reading of pH meter adjusted so that they correspond
- to the reference values.

10. pH - MEASUREMENT
ELECTROMETRIC DETERMINATION OF pH
 The pH of a test solution may then be measured
 pH = -log a H+
 pH measurement reflects the activity rather the concentration of
hydrogen ion

11. pH - MEASUREMENT
BUFFERS:-
 The resistance of a solution to change in Hydrogen ion
concentration upon the addition of small amounts of acid or
alkali is termed as buffer action and a solution which posses
this property is called buffer solution.
 The reference pH values for primary pH standards are
established by high accuracy potentiometric measurements
 Using specially designed electrochemical cells and platinum-
hydrogen gas electrode rather than a glass electrode, as the H+
- sensing electrode
 Avoids error in potential measurement arising from liquid
junction potential and the asymmetry potential

12. pH - MEASUREMENT
BUFFERS:-
 Enables the pH of the standard solution to be determined from
the Nernest equation.
 Such specialized measurements done at NIST
 NIST supplies a number of high purity salts as standard
reference materials for pH
 Each of which has a certificate
 Detailed instructions of preparation and use of solution
 The pH values certified to 3 decimal places with typical
uncertainties of ± 0.005 pH unit at a range temperature
 Enables pH meters to be calibrated in a manner that is
traceable to a services of internationally recognized standards

13. pH - MEASUREMENT
 HIGH PURITY SALTS USED AS PRIMARY pH STANDARD
Note: The uncertainty of the tabulated pH values are estimated to be ± 0.01
High purity salts Concentration/g/L pH at stated temperature
15°C 20°C 25°C 30°C
Potassium tetraoxolate 12.61 1.67 1.68 1.68 1.68
Potassium hydrogen phthalate 10.13 4.00 4.00 4.01 4.02
Potassium di-hydrogen
phosphate
Di sodium hydrogen phosphate
3.39
3.53
6.90 6.88 6.87 6.85
Sodium tetra borate
decahydrate
3.80 9.28 9.23 9.18 9.14
Sodium hydrogen carbonate
Sodium carbonate
2.09
2.64
10.12 10.06 10.01 9.97

14. pH - MEASUREMENT
BUFFERS:-
 Buffer solutions to be prepared using salts of the highest purity
available
 Certain of the salts should be dried before use
 Potassium hydrogen phthalate- Dry at 110°C for one hour
 Potassium di-hydrogen phosphate - Dry at 110°C for one hour
 Disodium hydrogen phosphate - Dry at 110°C for one hour
 Sodium carbonate – Dry at 270°C for one hour
 Carbon dioxide-free distilled water to be used to prepare the
solutions
 Important for those buffer solution with a pH > 6

15. pH - MEASUREMENT
BUFFERS:-
 Prepared solutions to be stored in a well stoppered Pyrex or
polythene bottles
 Normally to be replaced after 2 to 4 weeks and sooner if mould
or sediment is observed

16. pH - MEASUREMENT
GLASS ELECTRODE
A – Glass bulb
B – Tube filled with 0.1 mol HCl
C – Silver-Silver chloride
D – Saturated KCl solution
saturated with AgCl
E – Silver-Silver chloride
electrode

17. pH - MEASUREMENT
GLASS ELECTRODE
 Potential is developed in an aqueous solution is proportional to
hydrogen activity or pH of the solution
 Hydrogen ion in the solution forms a dynamic equilibrium with
hydrogen ions that are “bound” to the membrane surface in ion
exchange type process, thereby establishing a potential across
the membrane
 Magnitude of potential is proportional to the pH of the solution

18. pH - MEASUREMENT
CARE OF pH ELECTRODES
 Manufacturer’s instructions to be consulted for specific
guidance on particular electrodes and situation
 For reliable pH measurement, pH electrodes are properly
stored and maintained
 Response of the pH electrode to hydrogen ion activity depends
acutely on such matters as:
- Cleanliness and condition of the glass membrane
- Conditions of the reference electrode liquid junction
- Conditions of reference electrode and its filling solution
- Cleanliness of the electrode body

19. pH - MEASUREMENT
CAR E OF pH ELECTRODES
 On exposure to air and allowing to dry out, glass membrane
becomes dehydrated and liquid junction may also deteriorate
 pH electrode should be stored in an appropriate storage
solution, when not in use
 Mixture of different salts of similar composition to the reference
electrode filling solution, a buffer to provide suitable pH(e.g.4
to7) and mild cleansing agent
 Electrode to be immersed to a sufficient depth to cover both the
glass membrane and the liquid junction
 A pH electrode should not be stored in water
 Periodically the level of the filling solution in reference electrode
should be checked and topped-up as necessary

20. pH - MEASUREMENT
CARE OF pH ELECTRODES
 When not in use, the filling hole of the reference electrode
should be covered with the plug provided for this
 If pH electrode has been left in air and dried out, it should be re-
hydrated by immersion in the storage solution for at least 12
hours
 A pH electrode should be regularly examined for salt crystal
build up and membrane and liquid junction deposits
 For additional cleaning soak the pH electrode in either 0.1
molar HCl or 0.1 molar Nitric acid

21. pH - MEASUREMENT
SELECTION OF pH ELECTRODE:
 For pH measurements in the range 1 to 10, general purpose
glass electrodes suitable
 When the concentration of alkali metal ions become large these
ions seems to permeate the membrane, resulting in a
significant errors in the measured pH
 This is referred to as ‘alkaline error’ or a negative error in pH is
observed
 New glasses rich in lithium have been introduced and gives
reliable results
 At very low pH values the activity of water is reduced due to
salting out type effect arising from very high concentration of
Hydrogen ions
 Nature of glass membrane is altered and values are higher than
true values. This is referred as acidic errors

22. pH - MEASUREMENT
ACCURACY
 Electrodes available that are capable of measuring pH to
typically 0.01, 0.02, 0.02 and 0.1 pH unit
 Possible to record pH to 0.001 pH unit, only done to study small
differences or changes in pH

23. pH - MEASUREMENT
COMPOSITE LIQUIDS
 Composite liquid samples such colloids, suspensions sludge,
slurries, emulsions etc presents number of difficulties for pH
measurements
 The liquid junction easily becomes clogged and contaminated,
leading to poor and inaccurate response
 Electrodes with modified liquid junctions (e.g. sleeve or double
junctions) may over come the problem
 Attention to correct depth of immersion of the electrode in the
liquid and electrode cleaning after use are important

24. pH - MEASUREMENT
CALIBRATION
 Essential to calibrate the response of a pH electrode by using
standard buffer solutions of known pH
 At least two buffer solution should be used
 pH of the test sample is bracketed by two buffers
 The chosen buffers should not be more than 3 pH unit or no
less than 1 pH unit apart
 To be free from contamination, sediment and mould
 Each laboratory to determine the shelf life of their buffer
solutions based on practical experience, frequency of use,
storage conditions and the importance of the pH measurements
being made.

25. pH - MEASUREMENT
CALIBRATION
 When a buffer solutions requirement for use, a suitable quantity
should be transferred to clean dry beaker and close buffer
solution bottle quickly, used buffer solution should not be
returned to the stock bottle and pH electrode should not be
immersed directly into stock bottle
 Ensure that buffer solutions and test solutions whose pH values
are to be measured are equilibrated to ambient temperature

26. pH - MEASUREMENT
USE OF BUFFER SOLUTION:-
 First buffer solution
- Immerse the pH electrode in the first buffer solution ensuring
that the glass membrane and the liquid junction are covered by
the solution
- The depth of immersion should be such that the level of the
filling liquid in the reference compartment is about 25 mm
above the level of the solution being measured
- This will ensure a uniform flow of filling solution through the
liquid junction and a stable liquid junction potential
- Solution should be stirred using a magnetic stirrer at a
moderate speed taking care not to contact and damage the
glass membrane

27. pH - MEASUREMENT
USE OF BUFFER SOLUTION:-
- When the reading has stabilized to within about ± 0.02 pH unit,
adjust the reading using ‘set-buffer’ control so that the
displayed reading is equal to the reference pH value of the
buffer solution
- Discard the buffer solution and re-fill the beaker with a fresh
portion of the same buffer
- Observe the pH reading, if it is not within about ±0.02 pH unit of
the reference value, re-adjust the ‘set-buffer’ control so that the
displayed reading is equal to the reference pH value of the
buffer.
- Repeat this procedure until two successive readings agree to
within about ±0.02 pH unit
- Record details of buffer solution and relevant results obtained

28. pH - MEASUREMENT
SECOND BUFFER SOLUTION
- Rinse the beaker with water and discard rinsing
- Dry the interior of the beaker with a tissue and add the required
amount of the second buffer solution
- Rinse the electrode with water and then the buffer solution and
dry by touching with a tissue
- Immerse the electrode in the second buffer solution, allow the
reading to stabilize to within about ±0.02 pH unit and note the
pH reading
- If the electrode is giving a Nernstian response, the displayed
pH reading will not differ significantly (i.e. by more than about
±0.04) from the reference value of 2nd buffer
- Adjust the displayed reading if necessary to equal the reference
pH values of the second buffer

29. pH - MEASUREMENT
SECOND BUFFER SOLUTION
- Make this adjustment using slope control
- The adjust required should not be unduly large, typically no
more than ±0.3 pH unit
- If the difference between the displayed readings and reference
value for the second buffer solution exceeds +0.3 unit, it
indicates electrode response could be markedly non-
Nernstanian
- That is electrode slope value is significantly different from the
theoretical value of 59.1mV per ph unit
- Manufacturer of electrodes often specify the slope value for an
electrode in good conditions

30. pH - MEASUREMENT
SECOND BUFFER SOLUTION
- Specified slope values are typically in the range of 90 – 105%
- Discard buffer solution, replace it with fresh portion of the
second buffer, immerse the electrode and observe the
displayed pH value. It should be within ±0.02 pH unit of the
reference value

31. pH - MEASUREMENT
THE COMPOSITION OF THE AQUEOUS SOLUTION: -
 Aqueous solution with a relatively high ionic strength is to be
measured, a special pH electrode with a modified reference
electrode, to counter act effects due to liquid junction potential
 Water or aqueous solution of very low ionic strength are to be
measured, response of the glass electrode is slow, noisy,
subject to drift and inaccurate
 These samples have low buffering capacity and absorption of
carbon-dioxide from the atmosphere can cause unsteady
reading
 Use of additive to increase ionic strength of the sample
 Sample contains components that react with silver, the use of a
pH electrode with a silver/silver chloride reference electrode is
prohibited.

32. pH - MEASUREMENT
THE COMPOSITION OF THE AQUEOUS SOLUTION: -
 Reaction products deposited in the liquid junction causing a
slow response or no response
 Sample causing such problems include waste waters and lea
hates containing sulfur compounds
 These samples have low buffering capacity and absorption of
carbon-dioxide from the atmosphere can cause unsteady
reading
 Use of additive to increase ionic strength of the sample
 Sample contains components that react with silver, the use of a
pH electrode with a silver/silver chloride reference electrode is
prohibited.

33. pH - MEASUREMENT
MEASUREMENT OF TEST SOLUTION
- Rinse the beaker with water and discard rinsing
- Rinse it with test solution and add required amount of the test
solution whose pH is to be determined
- Rinse the electrode with water and then with test solution and
dry by touching with a tissue
- Immerse the electrode in the test solution and observe the pH
reading
- Discard the test solution and replace it with fresh portion of the
test solution and re-measure the pH
- For a well buffered test solution duplicate pH result should
normally agree within about ± 0.02 pH unit
- For slightly buffered test solution, replicate results may only
agree within about ±0.1 pH unit

34. pH - MEASUREMENT
MEASUREMENT OF TEST SOLUTION
- Record pH values obtained
- It lengthy series of measurements is made, re-measure the
buffer solutions at regular intervals e.g. every 10 test solutions

35. pH - MEASUREMENT
UNCERTAINTY OF PH MEASUREMENTS
Using a glass electrode
 Below are some possible source of uncertainty in pH measurements
along with the estimate of their magnitude
Source of uncertainty Standard Uncertainty
pH value of the standard buffer solutions ±0.01
The repeatability of pH measurements ±0.01
Drift in the electrode response between the
calibration
±0.01
The liquid junction potential ±0.015
Temperature (where known to ±1°C) 0.005

36. pH - MEASUREMENT
THE STANDARD UNCERTAINTIES IN THE TABLE ARE IN THE
FORM OF STANDARD DEVIATION
 Estimates of the contribution that each of these sources of
uncertainty makes to the overall error of a pH measurement are
some what subjective
 Vary with the precise conditions of the measurement and
equipment used
 Using above data, the total standard uncertainty in a pH
measurement is estimated to be about ±0.025 pH unit
 When temperature difference is ± 5°C, the total standard
uncertainty is about ±0.035 pH units
 For very dilute or concentrated test solutions higher uncertainty
may apply

37. pH - MEASUREMENT
THE ANALYST MUST KNOW:
 The meaning of pH and the pH scale
 Acid/base reactions, the relationship or pH to acidity and
basicity, the Nernest equation
 The different techniques for measuring pH (indicators, pH
papers and pH meters)
 How to operate a pH meter – read the appropriate instruction
manuals
 How to follow written instructions and obtain satisfactory results
– standard test procedures
 How to check that the equipment is working correctly and what
to do if there is a problem

38. pH - MEASUREMENT
THE ANALYST MUST KNOW:
 The effect of temperature on pH
- Information issued with buffer solutions states the temperature
at which the pH should be determined
- Samples should be allowed to equilibrate to the same
temperature as the buffer solutions before measurement
- Some pH meters have a thermometer on the probe, these will
correct for temperature differences
 The importance of a homogeneous solution
 That the solutions used for calibration must be relevant to the
pH range of the sample

39. pH - MEASUREMENT
PRACTICAL TECHNIQUE : ESSENTIAL KNOWLEDGE SHOULD
INCLUDE:
 Knowing how to stir solutions without damaging probe
 Knowing when to decide when a reading is stable
 Knowing how to achieve acceptable precision when measuring
the pH of samples
 Knowing how to clean and maintain a pH electrode
- General cleaning
- Wash with de-ionised water before and after each
measurement
- Removal of deposits
- Storage requirements, electrodes should be stored in a solution
appropriate to the filling medium

40. pH - MEASUREMENT
ANALYSTS SHOULD BE AWARE OF:
 The limitations of the equipment in terms of accuracy
 Interferences:
- Potential problems measuring solutions with very high or very
low pH values
- Measurement of pH in non-aqueous ·solutions
- Matrix effects of the samples under test, e.g., samples with high
solids or high organic solvent content
 Troubleshooting
- Drifting readings - stability of the reading should be obtained
- within 2 minutes
- Slow response
- Erratic readings

41. pH - MEASUREMENT
ANALYSTS SHOULD BE AWARE OF:
 Recording results and logbook entries - accurate transcription
 How to set up control charts
 The level of precision possible to achieve with the available
instrument

42. pH - MEASUREMENT
CALIBRATION :
 Buffer standards
- Know how they work
- Know different types of buffer and how to use them
- Know how to make up buffer solutions for calibration
- Know how to store buffer standard solutions correctly
- Know that it is necessary to use more than one buffer standard
(one either side of the expected pH range of samples to be
tested)

43. pH - MEASUREMENT
CALIBRATION :
 Calibrate the instrument according to the operation manual or
an STP
- Check electrode slope is within manufacturers' tolerance
- Know when to change the electrode, e.g., when the slope
reading is not within the tolerance limits
- Know how often the pH meter needs calibrating
- Check that the pH meter has maintained its performance in the
event of a power cut

44. pH - MEASUREMENT
OBSERVATION SIGNS WHICH INDICATE THAT ALL IS NOT WELL
 Messy bench area where the analyst has been working
 Liquid spilled on the work area
 Tops left off reagent bottles - bottles not returned to the shelf
 Solutions labeled inadequately, etc. - also labeled correctly but
unreadable due to damaged label
 Tops left off buffer solutions or solutions inappropriately stored
 Evidence of incorrect care of pH electrode - build up of deposits
on surface etc
 Dirty glassware left lying about rather than being cleaned and
put away

45. pH - MEASUREMENT
OBSERVATION SIGNS WHICH INDICATE THAT ALL IS NOT WELL
 Lack of notes in work book relating to calibration of equipment
or temperature of measurement if appropriate crossings out in
workbook, indicating that the analyst was having trouble
deciding when a stable reading had been reached
 Only one value recorded - no evidence that a stable reading
had been achieved.

46. pH - MEASUREMENT
PRACTICAL TESTS
 Check that all buffer solutions are labeled with the following
data:
- The analysts initials
- Date of preparation
- Identity and review date
 Store all buffer solutions in an appropriate manner