2-The Quinhydrone Electrode ( Pt/Q, H2Q) C6H4O2 . C6H4 (OH)2
↔
C6H4O2 + C6H4 (OH)2Q H2Q
Q + 2 H+
+
2e
=
H2QThe potential of Quinhydrone Electrodeat 25°C is:
EQ/H2Q = E˚ Q/H2Q + 0.0591/2 log[Q][H+]2/[H2Q]
EQ/H2Q = E˚ Q/H2Q + 0.0591/2 log[Q] /[H2Q]+ 0.0591/2 log [H+]2 [Q] = [H2Q] i.e. equimolar 1:1
EQ/ H2Q = E˚ Q/H2Q + 0.0591 log[H+] E˚ Q/ H2Q = 0.6998 V
EQ/H2Q = 0.6998 V + 0.0591 log[H+]= 0.6998 - 0.0591 pH
A schematic diagram of the entire pH measuring electrode system containing a saturated calomel electrode is:
Pt/Q, H2Q/ sample of unknown pH// KClsat ,Hg2Cl2 /Hg/ Pt
Eobserved = EQ/ H2Q - ESCE ESCE =0.244V at 25°C Eobserved = 0.4558 - 0.0591 pH
pH = (0.4558 - Eobserved)/ 0.0591
This electrode cannot be used in solutions of pH higher than 8 because H2Q behaves as a weak dibasic acid and then begins to have an effect on the pH of the solution.
3- The antimony electrode
For some industrial operations an antimony electrode is used to measure hydrogen-ion concentrations. The electrode consists of a rod of antimony which invariably has a coating of oxide, and placed in an aqueous solution the equilibrium:
Sb2O3(s) + 6H + + 6e = 2Sb(s) + 3H2O is established. This gives rise to an electrode potential
E = E˚ Sb2O3,Sb + RT/6F ln[H+]6 = E˚ Sb2O3,Sb -0.0591 pH
since the activities of the solids and of liquid water are constant. This electrode can be used in the pH range 3-8, but each electrode must be calibrated in a series of solutions of known pH. It fails in the presence of:
(a) strong oxidising agents, and
(b) solutions of pH lower than 3 or greater than 8 because the oxide dissolves in acidic and in alkaline solutions. Its great attraction is that it is cheap, simple to use, and is rugged: it is particularly useful for making measurements on slurries and gels.
4- The glass electrode
The glass electrode is the most widely used hydrogen ion responsive electrode.
The basic arrangement of a glass electrode is shown in Fig. 2(a);
*the bulb B is immersed in the solution of which it is required to measure the hydrogen ion concentration, and
*filling the bulb with a solution of hydrochloric acid (usually 0.1 M), and
*inserting a silver-silver chloride electrode.
Provided that the internal hydrochloric acid solution is maintained at constant concentration, the potential of the silver-silver chloride electrode inserted into it will be constant. Hence the potential of the electrode is governed by the hydrogen ion concentration of the test solution.
Fig. 2 Glass electrode(a) and combined with reference electrode(b)
Glass electrodes are now available as combination electrodes which contain the indicator electrode (a thin glass bulb) and a reference electrode (silver-silver chloride) combined in a single unit as depicted in Fig. 2(b).
The nature of the glass used for construction of the glass electrode is very important.
To measure the hydrogen ion concentration of a solution the glass electrode must be combined with a reference electrode, for which purpose the saturated calomel electrode is most commonly used, thus giving the cell:
Ag,AgCl(s)[HCl(0.1M)|Glass|Test solution //KCl(sat'd),Hg2Cl2(s)[Hg
The e.m.f. of the cell may be expressed by the equation:
E = K + (RT/F) ln aH+
or at a temperature of 25˚C by the expression:
E = K + 0.0591 pH
Glass electrodes require soaking in water for some hours before use and it is concluded that a hydrated layer is formed on the glass surface, where an ion exchange process can take place. If the glass contains sodium, the exchange process can be represented by the equilibrium:
On the outside of the bulb, the potential developed will be dependent upon the hydrogen ion concentration of the solution in which the bulb is immersed.
The glass electrode can be used in the presence of strong oxidants and reductants, in viscous media, and in the presence of proteins and similar substances which seriously interfere with other electrodes.
