Measurement of pH

1. pH metry
Presented by:
Dr. Sharayu M. Thorat
Associate Professor
Shri Shivaji College of Arts, Commerce and
Science, Akola (Maharashtra)

2. pH Metry
Measurement of pH of a solution by constructing a suitable electrochemical cell
and measuring its EMF is defined as pH metry.
pH is the unit of measure that describes the degree of acidity or alkalinity.
It is measured on a scale of 0 to 14.
pH is defined as negative logarithm ( to the base 10) of activity of H+ ions
i.e. pH= - log10aH+. For dilute solution the activity of H+ ions may be replaced
by molar concentration of hydrogen ions , hence pH= - log10 [H+].
pH of a solution can be experimentally determined by combining an indicator
electrode which is reversible to H+ ions with a reference electrode and measuring
the EMF of cell with the help of potentiometer or pH meter.

3. Determination of pH of Solution using Hydrogen Electrode
Construction of Hydrogen Electrode :
The hydrogen gas electrode is
reversible with respect to H+ ions
hence it can be used as an indicator
electrode for pH measurement. It
consists of a small strip of platinum
metal which is activated by coating
with the platinum black. The platinum
foil is welded at the end of a platinum
lead fused in a glass jacket provided
with a side tube for the entry of pure
hydrogen gas. The platinum electrode
is immersed in a solution whose pH is
to be determined and pure hydrogen
gas at 1 atm pressure is bubbled across
the surface of the platinum foil in such
a manner that the electrode is
continuously in contact with both
solution and the gas.

4. The half cell reaction and single electrode reduction potential
2H+ + 2e–  H2(g)(1 atm)
Platinum does not take part in the electrochemical reaction and it acts only as the
site for the transfer of electrons. The reduction potential of hydrogen electrode
is given by the Nernst equation.
For this electrode
At 25 0 C,
The electrode potential is given by
, n=2, PH2 =1 atm
E(H+,H2) = E0( H+, H2) -
E(H+,H2) = 0-

= -
= 0.0591 log[H+]
= -0.0591pH as pH = -log[H+]

5. Method
The pH of the solution can be determined by coupling the hydrogen electrode
(pH is unknown) with a Normal Hydrogen electrode (Primary reference
electrode) by KCl salt bridge (to eliminate the liquid junction potential).
Pt(s) | H2(g)(1atm) H+
(aq) (pH = ?) || H+
(aq) (1M ),H2(g) (1atm) | Pt (s)
E.M.F. of this cell is determined by a potentiometer
According to IUPAC rule,
E cell = E (Right) – E (Left)
E cell = 0.0 – (-0.0591 pH) at 25°C
E cell = 0.0591 pH
pH =
Thus simply by measuring the E.M.F. of cell at 25°C we can find out the value
of pH
- +

6. Advantages of Hydrogen Electrode
It can be used over the entire pH range.
It is highly accurate.
It does not exhibit salt error.
It has low internal resistance and hence ordinary potentiometer can be used for
emf measurement
Disadvantages
It can not be used in a solution containing ions of metals that are below
hydrogen in the electrochemical series. (arrangement of elements in increasing
order of reduction potential values). Interaction with the hydrogen will occur
and the metal will be deposited on the electrode surface.
The platinum black coating deteriorates and hence it should be renewed from
time to time. Impurities in solution poison the platinum black foil.
It is not easy to get pure and dry hydrogen gas.
It is difficult to maintain the pressure of hydrogen gas at a fixed value (1 atm)
due to mechanical difficulties. It can not be used in presence of oxidizing and
reducing agents.

7. Determination of pH of solution by using Quinhydrone
electrode
This electrode was introduced by E. Billmann in 1921. Many of the
experimental difficulties can be overcome by the use of this electrode.
Quinhydrone is a 1:1 molecular compound of quinone (Q) and hydroquinone
(H2Q) and in solution it provides equimolecular quantities of these two
substances.
Construction and Single Electrode Potential (reduction) :
The quinhydrone electrode may be set up easily by adding a pinch of
quinhydrone to the solution whose pH is to be determined. A bright platinum
electrode is immersed in a solution to establish electrical contact.
Quinone, hydroquinone and hydrogen ions form a reversible redox system with
the following half cell reaction.
C6H4O2 + 2H+ + 2e– C6H4(OH)2
Or, Q + 2H+ + 2e– H2Q

8. The reduction potential of the quinhydrone electrode is given by the Nernst
equation.
For this electrode
E0
Q = 0.6996 V at 25°C
n = 2
At 25°C,
Now in aq. Solution of quinhydrone, [H2Q] = [Q]
At 25°C, EQ = 0.6996 – 0.0591 pH
Thus, reduction potential of Quinhydrone electrode is a function of pH of
the solution.
2.303 RT/ F = 0.0591

9. Method
The pH of the solution can be determined by coupling the quinhydrone electrode
with a calomel electrode (Secondary reference electrode) by a salt bridge. The
electrochemical cell is represented as
Hg | Hg2Cl2(s) | KCl (sat) || H+ (unknown) Q, H2Q | Pt
In this cell the oxidation takes place on the calomel electrode while reduction
take place at the quinhydrone electrode. Using reduction electrode potential
values we have,
E cell = E quinhydrone – E calomel
At 25°C, E quinhydrone = (0.6996 –0.0591 pH) V and
E calomel = 0.2415 volt
E cell = (0.6996 –0.0591 pH) – 0.2415
= 0.4581 – 0.0591 pH
 pH =

10. =
=
=
=
=
Advantages of Quinhydrone Electrode –
It is very simple to set up and gives accurate values of pH because equilibrium
is attained quickly.
Small quantity of solution is sufficient for pH measurement.
It gives accurate results even in presence of oxidizing ions.
It can be used for measuring pH of solution containing Zn2+, Pb2+, Cu2+ etc
where hydrogen electrode is unsuitable.
Ordinary potentiometer can be used for emf measurement.
Disadvantages
It can not be used in alkaline solution above pH = 8, because H2Q acts as a
weak dibasic acid in more alkaline solution. Atmospheric oxygen slowly
oxidizes H2Q, therefore it should be freshly prepared.
The potential of this electrode is affected by even small concentration of
neutral salts. This is known as salt error.

11. Determination of pH by using Glass Electrode :
Haber and Klemensiewiez (1909) observed that when two solutions of different
pH are separated by a glass membrane of low melting and high conductivity
glass, potential is developed across the membrane. When such a glass bulb is
filled with 0.1 M HCl and is immersed in another solution containing H+ ions,
the potential of such electrode depends on the pH of outer solution. Thus, such
a glass electrode can be used for the measurement of pH of outer solution.
Construction of Glass Electrode and Single Electrode Potential
Glass electrode is made of a thin glass bulb or a special quality glass which is
attached to a glass tube. The glass bulb is filled with 0.1 M HCl solution and a
silver wire coated with AgCl (or Pt wire) is immersed in it.
Single electrode potential or a glass electrode is given by
EG = E°G + 0.0591 log [H+]
or EG = E°G - 0.0591 pH
Where E°G = A constant for the given glass electrode called as asymmetry
potential. Value of E°G may be determined by using a buffer solution of known
pH.

12. Method
Glass electrode is dipped in a solution of unknown pH and is coupled with a
reference electrode (like calomel electrode). Both the electrodes are connected to
an electronic potentiometer (called as pH meter). The pH meter displays a pH
scale (pH = 0 to 14) and hence we can read directly the pH of the unknown
solution. Before this measurement glass electrode must be conditioned and pH
meter should be calibrated using buffer solutions of known pH (commonly pH =
4 and pH = 9). The cell is represented as
-Ag|AgCl |0.1 M HCl |Glass |unknown solution(pH=?) || KCl(Sat)|Hg2Cl2(s) |
Hg(l)+
Or
Pt | 0.1 M HCl | Glass | Experimental solution || KCl(salt) | Hg2Cl2(s) |
Hg(l)

13. The emf of cell is given by equation
E cell = ER – EL
= E Calomel – E Glass
E calomel = 0.2415 volt
E Glass = E°G – 0.0591 pH
E cell = 0.2415 – (E°G – 0.0591 pH)
= 0.2415 – E°G + 0.0591 pH
0.0591 pH = ECell – 0.2415 + E°G

14. By using above formula we can determine pH of unknown
solution using combination of glass electrode and saturated
calomel electrode.

15. Advantages of glass electrode :
It may be used in the presence of strong oxidizing, reducing and alkaline
solution.
It can be used for solution having pH values 2 to 10 with some special glass
values can be extended up to 12.
It is simple to operate.
It is immune to poisoning.
It has no salt or protein error and the equilibrium is reached quickly.
It can be used in colored , turbid and colloidal solution also.
Disadvantages
In glass electrode the bulb is very fragile and therefore has to be used with great
care.
As the glass membrane has a very high electrical resistance hence it cannot be
used with ordinary potentiometer.
It cannot be employed in pure ethyl alcohol, acetic acid and gelatin.