Instant Notes: Analytical Chemistry

E(cell) =E(right) -E(left) =E(Cu^2 +, Cu) - E(Zn^2 +, Zn)

Dividing the emf equation into separate electrode equations:

E(Cu^2 +, Cu) =En(Cu^2 +, Cu) +(RT/2F) ln (a(Cu^2 +)/(a(Cu))

E( Zn^2 +, Zn) =En(Zn^2 +, Zn) +(RT/2F) ln (a (Zn^2 +)/(a (Zn))

If these equations are obeyed, then the electrodes show Nernstian response.

Class 1 electrodes

These are the simplest electrodes, but not necessarily the easiest to use. A metal

rod is immersed in a solution of ions of the same metal, for example silver with

a solution containing silver ions. With some ions it is important to prevent

hydrolysis or complexation taking place.

Ag+(solution) +e-=Ag(solid)

E(Ag+/Ag) =En(Ag+, Ag) +RT/Fln (a(Ag+)/(a(Ag))

if a pure silver rod is used, a(Ag) =1, so we may write:

E(Ag+/Ag) =En(Ag+, Ag) +RT/Fln (a(Ag+))

This is therefore an electrode reversible to silver ions. An example of the use

of this electrode is given later. En(Ag+, Ag) =0.800 V at 25∞C is the standard

electrode potentialof the silver electrode.

Class 2 electrodes

When an insoluble salt of a metal (see also Topic C8) is present, the concentra-

tion of the metal ion depends on the concentration of the anion and on the

C3 – Potentiometry 67

Electrode A

Digital voltmeter

Salt bridge

Electrode B

Test solution

Fig. 1. Experimental arrangement with salt bridge junction and DVM.

Table 1. Potentiometric indicator electrodes

Class Description Example

Class 1 Metal/metal ion Ag/Ag+(cation reversible)

Class 2 Metal/saturated metal salt/anion Ag/AgCl/Cl-(anion reversible

Redox Inert metal/redox couple Pt/Ce^4 +, Ce^3 +

Pt/H+, H 2

Membrane Inner electrode/solution/ Glass electrode

ion selective membrane Fluoride electrode

ISFET Coated field-effect transistor pH-sensitive

Gas-sensing electrodes pH-electrode + membrane For CO 2 , NH 3