The document discusses pH measurement and the components used. It describes that pH is a measurement of hydrogen ion concentration on a logarithmic scale from 0-14. It also discusses Nernst's equation, which relates electrode potential to ion concentration. The key components used for pH measurement are glass electrodes, reference electrodes like calomel or silver-silver chloride, and buffer solutions. The document provides details on the construction and functioning of these different electrode types.

1. pH Measurement
ER. FARUK BIN POYEN, Asst. Professor
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
faruk.poyen@gmail.com

2. Contents:
 What is pH?
 Nernst’s Equation
 Measurement of pH
 Electrode Types
 Buffer Solution
 Buffer Calibration
 Process Effects on The Glass pH Electrode
 Process Effects on Reference Electrodes
 Merits and Demerits of pH Meters
 Application of pH Meters
2

3. What is pH?
 pH is a method of measurement of hydrogen ion concentration.
 The lower-case alphabet “p” in pH denotes negative common (base ten) logarithm,
while the upper-case alphabet “H” denotes the element hydrogen.
 pH is a negative logarithmic measurement of the number of moles of hydrogen ions
(H+) per litre of solution.
 In pure water, hydrogen ion concentration is 10-7 moles per litre under standard
conditions (25°C), giving it a pH of 7.
 The pH scale ranges between 0 and 14. Any acid fully ionizing in water gives a pH of
0.0.
 Acidity describes higher concentration of hydrogen ions and reads below pH 7. Alkali
or base fully ionized in water reads pH 14.0. Alkalinity ranges between 7 and 14,
excluding 7 as it depicts pure water as a neutral solution.
 As pH is measured on a logarithmic scale, one unit increase of pH corresponds to a
decrease in concentration by a factor of 10. For example, the concentration of
hydrogen ions for pH 3 is 10 times greater than that of pH 4.
3

4. pH..
 The pH value is expressed as
𝑝𝐻 =
1
𝑙𝑜𝑔10 𝐶
, 𝐶 = 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐻+ 𝑖𝑜𝑛𝑠
 Solution's pH is measured on its net concentration of hydrogen ions [H+] compared to
concentration of hydroxide ions [OH-].
 Acids dissociates to produce H+ ions whereas bases dissociates to produce OH- ions.
 The product of these two concentrations of H+ and OH – gives the dissociation
constant pH + pOH = pKW. At 25°C under standard conditions, pH + pOH = 14,
which is why the scale for pH usually ranges from 0 to 14.
4

5. Nernst’s Equation
 This equation enables the determination of cell potential under non – standard
conditions.
 It establish the relationship between measured cell potential and reaction quotient
allowing accurate determination of equilibrium constants. This equation relates
potential of electrodes to ion concentration at equilibrium.
 The equation is expressed as
𝐸 = 𝐸0 +
𝑅𝑇
𝑛𝐹
ln(𝑎𝐶)
E = e.m.f of the half-cell; E0 = e.m.f of the half-cell under standard conditions;
R = Gas constant (8.314 J/°C); T = Absolute temperature (K); n = valence of ion;
F = Faraday constant (96493 C); a = activity coefficient (0 ≤ a ≤ 1)
C = molar concentration of ions.
 The pH of any solution is measured on the basis of this equation where the potential
developed across membranes reflects the concentration of H+ ions.
5

6. Measurement of pH
 When very precise and accurate pH measurement is not required, litmus papers that
change colour on coming in contact with solutions of certain pH values are used. But
continuous and process measurement demands more sophisticated measurement
techniques.
 Essentially two electrodes, measuring and reference electrodes are employed for its
measurement.
 Both these electrodes are dipped in the solution whose pH is to be measured.
 These two electrodes form two half-cells and the total potential generated is the
difference between these two electrodes separately produced in each one of them.
 The generation of this potential is dependent on the H+ ion concentration governed by
Nernst equation. This potential is sensitive to the H+ ion concentration, having a
sensitivity of 59.2 mv/pH at 25°C.
𝐸 = 𝐸0 −
2.303𝑅𝑇.
𝐹
𝑝𝐻
6

7. Measuring Electrodes
 Measuring electrodes of specific make (thin ion selective glass) having buffer solution
of constant H+ ion concentration and silver wire inside the glass bulb is dipped in the
unknown solution where a potential is generated across the glass bulb.
 It forms one of the two half-cells. The measurement electrode’s purpose is to generate
the voltage used to measure the solution’s pH.
7

8. Reference Electrodes
 The reference electrode provides continuity to the electric circuit as one half-cell is
unable to measure the potential generated.
 The reference electrodes are commonly of two types i) Calomel (Mercury – Mercurous
Chloride) and ii) Silver – Silver Chloride electrode.
 The two sets of silver wires coming out from both the measuring electrode and
reference electrode complete the circuit measuring the potential generated.
 The reference electrode consists of a chemical solution of neutral (7) pH buffer
solution (usually potassium chloride) allowing exchange of ions with the process
solution through a porous separator.
 The reference electrode’s purpose is to provide the stable, zero-voltage connection to
the liquid solution so that a complete circuit can be made to measure the glass
electrode’s voltage and the electrical connection is maintained through a salt bridge.
8

9. Electrodes
 The resistance offered by the measuring electrode and reference electrode is
substantially high while the voltage produced per scale of pH is of the order of
millivolts.
 The common solution to this problem is to use an amplified meter with an extremely
high internal resistance to measure the electrode voltage, so as to draw as little current
through the circuit as possible.
 The other method is to use a potentiometric “null-balance” voltage measurement setup
to measure this voltage without drawing any current from the test circuit.
9

10. Electrodes
 A new method for pH value measurement is the use of an ion selective field effect
transistor (ISFET).
 The ISFET is a transistor with power source and drain, divided by an isolator. This
isolator (gate) is made of a metal oxide where hydrogen ions accumulate in the same
way as an electrode.
 The positive charge that accumulates outside the gate is 'mirrored' inside the gate by an
equal negative charge generates.
 Once this happens the gate begins to conduct electricity. The lower the pH value the
more hydrogen ions accumulate and the more power can flow between source and
drain.
 The ISFET sensors act according to the Nernst equation.
 The advantage of an ISFET is that they are very small. The disadvantage of using an
ISFET for pH measurements is that they have comparatively short durability and low
long-term stability.
10

11. Electrode Types
 Electrodes are designed to allow H+ ions in the solution to migrate through a selective
barrier, producing a measurable potential (voltage) difference proportional to the solution’s
pH.
 The glass is chemically doped with lithium ions, which is what makes it react
electrochemically to hydrogen ions. All pH electrodes have a finite life depending greatly
on the type and severity of service.
 The pH electrodes come in different types based on different kind of application and use.
They are as given below
I. Glass Electrode
II. Hydrogen Electrode
III. Quinhydrone Electrode
IV. Reference Electrodes - Saturated Calomel Electrode and Silver Chloride
Electrode
V. Combination Electrode
VI. Ion-Selective Electrodes
11

12. Glass Electrode
 It is one type of ion-selective electrode made of doped glass membrane and sensitive to
a specific ion.
 The response may be to H+ ion or it may be to the other cations based on the glass
composition.
 Construction wise, the glass electrode consists of a thin walled bulb of pH sensitive
glass sealed to a stem of non pH sensitive high resistance glass.
 The pH response is limited to the special glass membrane making it independent of the
depth of immersion.
 On the inside of the membrane is a system of effectively constant pH. It is composed
of Ag – AgCl or calomel electrode dipped in HCl acid.
 Changes in the electrical potential of the outer membrane surface are measured by
means of an external reference electrode and its associated salt bridge.
12

13. Glass Electrode
 The complete pH cell is represented as follows:
 Glass Electrodes have two disadvantages and they are
a) Measuring solutions containing particulates can damage the glass membrane
b) The glass membrane is vulnerable towards breaking
13
Internal
Reference
Electrode
Internal
Electrolyte
Glass
Membrane
Test Solution External Reference
Electrode

14. Hydrogen Electrode
 The hydrogen electrode is the primary electrode to which all electrochemical
measurements are referred.
 The performance of all other electrodes is always evaluated in terms of the hydrogen
electrode.
 The hydrogen electrode consists of an inert but catalytically active metal surface, most
frequently platinum, over which hydrogen is bubbled to achieve electrochemical
equilibrium with the hydrogen ions in the solution.
 The following redox reaction takes place: H+ + e- ↔ ½ H2
 The electrode is immersed in the solution under investigation and electrolytic hydrogen
gas at 1 atm pressure is bubbled through the solution and cover the electrode, in such a
way that the electrode surface and the adjacent solution gets saturated with the gas at
all times.
14

15. Hydrogen Electrode
 The choice of platinum for the hydrogen electrode is due to several factors:
• Inertness of platinum (it does not corrode)
• The capability of platinum to catalyse the reaction of proton reduction
• A high intrinsic exchange current density for proton reduction on platinum
• Excellent reproducibility of the potential (bias of less than 10 μV when two well-
made hydrogen electrodes are compared with one another)
 The surface of platinum is platinized (i.e. covered with platinum black) to:
• Increase total surface area. This improves reaction kinetics and maximum possible
current
• Use a surface material that absorbs hydrogen well at its interface. This also
improves reaction kinetics.
15

16. Hydrogen Electrode 16

17. Quinhydrone Electrode
 The quinhydrone electrode is a redox electrode which is used to measure the hydrogen ion
concentration (pH) of a solution in a chemical experiment. It acts as an alternative to the
common glass electrode in a pH meter.
 The electrode consists of an inert metal electrode (usually a platinum wire) in contact with
quinhydrone crystals and a water-based solution. Quinhydrone is slightly soluble in water,
dissolving to form a mixture of two substances, quinone and hydroquinone, with the two
substances present at equal concentration. Each one of the two substances can easily be
oxidised or reduced to the other.
 The potential at the inert electrode depends on the ratio of the activity of two substances
(quinone-hydroquinone), and also the hydrogen ion concentration. The electrode half-
reaction is:
Hydroquinone ↔ Quinone + 2H+ +2e-
 The quinhydrone electrode is not reliable above pH 8. It is also unreliable in the presence of
strong oxidising or reducing agents, which would disturb the equilibrium between
hydroquinone and quinone. It is also subject to errors in solutions containing proteins or
high concentrations of salts.
17

18. Reference Electrode - Calomel
 Reference electrode provides a stable, reproducible voltage to which the working
electrode potential may be referenced.
 A reference electrode acts as a battery whose voltage is dependent on the reaction
taking place between a solid conductor (metal salt) and the electrolytic solution.
 It is necessary that the pH cell be completed by means of a stable reference electrode,
whose potential remains unchanged by changes in the composition of the cell solution.
 The Saturated calomel electrode (SCE) is a reference electrode based on the reaction
between elemental mercury and mercury (I) chloride.
 The aqueous phase in contact with the mercury and the mercury (I) chloride (Hg2Cl2,
"calomel") is a saturated solution of potassium chloride in water.
 The electrode is normally linked via a porous membrane (the salt bridge) to the
solution in which the other electrode is immersed.
18

19. Reference Electrode - Calomel
 SCE consists of a metallic internal element, typically of mercury – mercurous chloride
(calomel) or silver – silver chloride, immersed in an electrolyte, which is usually a
saturated solution of potassium chloride.
 One drawback of calomel electrode is its mercury content which sometimes may create
health hazard.
 Also one cause of malfunction is due to the trapped air bubbles.
19

20. Reference Electrode – Silver Chloride
 This type is used usually in electrochemical measurements.
 The electrode functions as redox electrode where reaction takes place between silver
(Ag) and Silver Chloride salt.
 This type is constructed with glass having a porous ceramic membrane at the interface.
Sodium Chloride is used as the filling solution which is in a semi – solid state.
 A porous reference junction separates the filling solution in the electrode from the
solution whose pH is to be measured.
 The filling solution’s constant chloride ion concentration generates potential at a pure
silver wire with silver chloride on it.
 The silver wire passes a signal from the solution being measured to the electrode’s
cable.
 This configuration of the electrode is called “Single Junction Reference”.
20

21. Reference Electrode – Silver Chloride
 The potential developed is dependent on the effective concentration of Cl- ions as
established by Nernst’s equation.
 The salient features of this type are simple construction, stable potential, inexpensive
and non – toxic components.
 But the issues of concern are it is very sensitive to bromide ion traces and the
electrodes get easily damaged by drying.
21

22. Combination Electrode
 As stated before, here the measuring and reference electrodes are built together.
 Here the internal reference electrode and external reference electrode are identical both
being Ag/AgCl type.
 Inner solution in both the electrodes are also same held at same temperature and
protection against light is provided by ruby red glasses.
 The potential of a combination pH electrode is due to the difference in activities of H+
ions between the test solution and reference solution sides of the glass membrane.
 The potential of the combination electrode is proportional to the pH of the test solution.
22

23. Ion – Selective Electrode (ISE)
 As the name implies, these electrodes are sensitive to the activity of a particular ion in
solution and quite insensitive to the other ions present necessitating different electrodes for
different measurement.
 ISEs work on the basic principle of the galvanic cell.
 By measuring the electric potential generated across a membrane by selected ions and
comparing it to a reference electrode, a net charge is determined.
 The strength of this charge is directly proportional to the concentration of the selected ion.
 An ion selective membrane is fixed at one end so that the external solution can only come
into contact with the outer surface and the other end is fitted with a noise cable or gold
plated pin for connection to the millivolt measuring device.
 Most commonly used ISE:
Cations: Ammonium (NH+), Barium (Ba++), Calcium (Ca++), Cadmium (Cd++), Copper
(Cu++), Lead (Pb++), Mercury (Hg++), Potassium (K+), Sodium (Na+), Silver (Ag+).
Anions: Bromide (Br-), Carbonate (CO3-), Chloride (Cl-), Cyanide (CN-), Fluoride (F-),
Iodide (I-), Nitrate (NO3-), Nitrite (NO2-), Perchlorate (ClO4-), Sulphide (S-), Thiocyanate
(SCN-).
23

24. Ion – Selective Electrode (ISE)
 Advantages of ISE:
a) Relatively inexpensive and simple to use.
b) Robust and durable
c) Rapid operation even in relative dilute aqueous solution viz. lakes or rivers
d) Able for continuous monitoring
e) Measure activity of ions directly rather than measuring the concentration
f) Higher accuracy and precision
g) Can measure both positive and negative ions
h) Unaffected by colour or turbidity
i) Has got a wide temperature range
 Limitations of ISE:
a) Effect of interference with other ions in solution
b) Effect of ionic strength of the solution, reducing activity
c) Drift in electrode potential during a sequence of measurement
d) Vulnerable towards contamination by organic molecules
24

25. Buffer Solution
 A buffer (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution
comprising a mixture of a weak acid and its conjugate base.
 Adding a small amount of strong acid or base changes its pH value very slightly,
therefore it is used to prevent changes in pH of solution, keeping the pH value at a
nearly constant value.
 The consistency of buffer pH value is maintained as it maintains the equilibrium
between acid HA and its conjugate base A-. 𝐻𝐴 ⇋ 𝐻+
+ 𝐴−
 Adding strong acid to an equilibrium mixture of weak acid and its conjugate base, the
equilibrium is shifted to the left. Increase in H+ ion concentration is lesser than
expected for the quantity of strong acid added.
 Likewise happens in case of addition of strong alkali.
 A buffering agent is a weak acid or base applied to maintain the acidity of a solution
near a chosen value even after addition of another acid or base, thus preventing rapid
changes in pH value.
25

26. Buffer Solution
 Buffer capacity, β, is a quantitative measure of the resistance of a buffer solution to pH
change on addition of hydroxide ions. It can be expressed as
𝛽 =
𝑑𝑛
𝑑(𝑝[𝐻+])
 There are three regions of high buffer capacity.
• At very low p [H+] the first term predominates and β increases in proportion to the hydrogen ion
concentration. This is independent of the presence or absence of buffering agents and applies to all
solvents.
• In the region p [H+] = p Ka ± 2 the second term becomes important. Buffer capacity is proportional to
the concentration of the buffering agent, CA, so dilute solutions have little buffer capacity.
• At very high p [H+] the third term predominates and β increases in proportion to the hydroxide ion
concentration. This is due to the self-ionization of water and is independent of the presence or absence
of buffering agents.
26
d n = infinitesimal amount of added base;
d (p[H+]) = resulting infinitesimal change in the co logarithm of the hydrogen ion concentration.
Buffer capacity for a 0.1 M solution of an acid with p Ka of 7

27. Buffer Calibration
 Buffers are standard solutions formulated to preserve a known pH in spite of small
amounts of impurity.
 Buffer calibrations use two buffer solutions, separated by 3 pH units allowing the pH
analyser to evaluate a new slope and zero value to be used for deriving pH from the
millivolt and temperature signals.
 The slope and zero value resulting from a buffer calibration provide an indication of
the state of the glass electrode from the scale of its slope, while the zero value indicates
reference poisoning or asymmetry potential.
 Buffer calibration demonstrates how well the pH sensor responds to pH.
27

28. Process Effects on The Glass pH Electrode
 Temperature Effects: Fluctuating and increasing temperature accelerate the aging of
electrodes. Elevated temperature affects the interior and exterior of the electrodes
resulting in the shift of zero point.
 Sodium Error: Also termed as alkali error occurs in high pH where Na+ concentration
is more than H+ concentration. Under this condition, electrodes start responding to
Na+ ions resulting lower reading than actual. Li+ ion effects are even more prominent
than Na+ whereas K+ ions effect is negligible.
 Components attacking pH Electrodes: High concentrations of hydroxyl ions shorten
the life of pH electrodes. Solutions that reach a pH in excess of 14 pH (equivalent to
4% caustic soda) can destroy a pH electrode within hours.
 Hydrofluoric Acid: This can even dissolve pH glass decreasing the life of electrodes.
HF acids attack glass but not the fluoride ion (F-).
 Alkaline Error: This error can result when cations other than H+ are present in
solution. These cations can exchange for H+ in the gel layer.
28

29. Process Effects on Reference Electrodes
 Reference Poisoning: If instead of Ag-AgCl, other silver compounds viz. bromide, iodide and
sulphide ions are used, they may cause this effect as the salts produced are less soluble than AgCl
leaving behind insoluble particulates in the fill solution. To counter this effect, multiple reference
electrodes are used which slows down the effect of poisoning as they have multiple liquid junctions
and fill solutions. Poisoning can also occur by reducing agent (bisulfite) or complexing agents
(ammonia), which reduce the concentration of silver ion in the fill solution.
 Plugging of the Liquid Junction: Large concentrations of an ion that forms an insoluble precipitate
with silver ion (most notably sulphide ion) will precipitate within the liquid junction and plug it. Metal
ions that form insoluble salts with chloride ion (typically the heavy metals: silver, lead, and mercury)
will also precipitate in the liquid junction. Multiple junction reference electrodes with the outermost
fill solution containing potassium nitrate, rather than potassium chloride are used to counter this effect
as this reduces the concentration of Cl- available for precipitation.
 Liquid Junction Potential: Potassium chloride is chosen for the fill solution because of its ability to
solubilize silver ion and being equitransferent in which case the positive potassium ion and the
negative chloride ion diffuse through a water solution at nearly the same rate maintaining a net zero
charge.
 In case where positive or negative ions diffuse faster than the other ion type, liquid junction potential
is caused, the magnitude of which depends up on the composition and concentration of the solution.
This potential gets added to the potential due to pH resulting in distorted output.
29

30. Merits and Demerits of pH Meters
 Advantages are that they produce reasonably good and reproducible measurements.
Recent advances do provide measurements up to the 4th place of decimal results with
digital outputs.
 Demerits of pH measuring electrodes are they are slow to register devices with little
drifts in final values.
Temperature influences the output results greatly so definite temperature compensation
is required.
Though glass electrodes are selective for H+ ions but not uniquely responsive to them
only leading to response to other ions as well.
Depositions on glass electrodes affect results.
Carbon dioxide absorption influences the output and measurement becomes seemingly
tough with solutions having varying pH.
30

31. Application of pH Meters
 The measurement of pH reflects the effective concentration and activity of H+ and
other ions present in solution.
 For chemical reactors and scrubbers, they provide indications of the solution used
being acidic or basic qualitatively.
 These meters find major application to correct the hypochlorite concentration for an
Oxidation – Reduction Potential (ORP) measurement.
 Water treatment plants, micro-electronics laboratories and pharmaceutical laboratories
are in constant need of pH level monitoring and control for their very accurate and
precise applications.
31

32. Conclusion
 pH measurements are based on the response of a pH sensor to the logarithmic
concentration of hydrogen ions in solution and are a measure of the acidity or
alkalinity of a solution.
 There are a number of factors that should be considered for on-line pH measurements.
These involve the temperature behaviour of the solution pH, the composition of the
process solution, and the potential for fouling of the sensor by undissolved material in
the process.
32

33. References
 Chapter 9: Density, Viscosity and pH Measurement, “Industrial Instrumentation and
Control” by S K Singh. Tata McGraw Hill, 3rd Edition. 2009, New Delhi. ISBN-13:
978-0-07-026222-5.
 Chapter 17: Miscellaneous Instruments in Industrial, Biomedical and Environmental
Applications, “Instrumentation, Measurement and Analysis”. 2nd Edition, B C Nakra,
K K Chaudhry, Tata McGraw-Hill, New Delhi, 2005. ISBN: 0-07-048296-9.
 Chapter 14: pH and Viscosity Measurement, “Fundamentals of Industrial
Instrumentation”, 1st Edition, Alok Barua, Wiley India Pvt. Ltd. New Delhi, 2011.
ISBN: 978-81-265-2882-0.
33