Instant Notes: Analytical Chemistry

C3 – Potentiometry 69

Contact

(a) (b) (c)

Contact

AgCl/Ag

electrode

Buffer solution

Glass membrane

Contact

Selective surface

Fill hole

Hg/Hg 2 Cl 2

paste

KCI solution

Porous plug

Fig. 2. (a) Calomel reference electrode; (b) glass electrode; (c) solid-state electrode.

The SHE is rather inconvenient to use, since it requires a supply of inflam-

mable hydrogen and has a tendency to change emf slightly as the bubbles of

hydrogen cover the metal.

Reference electrodes should have a constant potential, should not react with

the sample, and should have a very small liquid junction potential with the

sample solution. Two reference electrodes are commonly used.

The calomel reference electrode is shown in Figure 2(a). This is an electrode of

Class 2, with liquid mercury in contact with mercury(I) chloride, or calomel, in a

solution of potassium chloride of high, fixed concentration.

The electrode reaction is

Hg 2 Cl 2 (solid) +2e-=2Hg (liquid) +2Cl-(aq)

The electrode potential is given by:

E(cal) =En(cal) -RT/2Fln (a(Cl-)^2 )

If a concentrated solution of KCl is used, either saturated or 3.5 M, then this elec-

trode has a constant potential at 25∞C. Changes in the solution outside the electrode

have a very small effect on the potential of this electrode, since the chloride con-

centration is high and is not altered significantly by the external solution.

Additionally, the concentrated KCl acts as a salt bridge.

E(cal, sat)=0.244 V (saturated calomel electrode, SCE)

E(cal, 3.5M)=0.250 V (calomel electrode, 3.5M KCl)

The silver/silver chloride reference is similar, having a silver wire coated with

silver chloride and in contact with concentrated KCl solution.

E (AgCl, 3.5M) =0.204 V