Instant Notes: Analytical Chemistry

If the sample solution might react with chloride ions, for example silver or

lead salts, then a double junction reference electrode may be used, with an

additional liquid junction of KNO 3.

Redox electrodes

If an inert wire, usually platinum, is placed into a solution containing both

oxidized and reduced species, it will take up an equilibrium potential depen-

dent on the ratio of their concentrations:

Ox +ne-=Red

E(Ox, Red) =En(Ox, Red) +(RT/nF) ln (a(Ox)/a(Red))

Ion-selective electrodes (ISE)

All of the above electrodes are sensitive to particular ions, but have certain

disadvantages, such as loss of response due to poisoning, or a tendency to

mechanical or other failure.

A large variety of special electrodes have been developed to test for a very

wide range of ions. Many involve an insoluble membrane, capable of electrical

conduction, whose surface potential depends chiefly on the concentration of a

particular ion.

The glass electrode for measuring pH, shown in Figure 2(b)is an early

example. A thin membrane of a conducting glass separates the inner solution of

a silver–silver chloride electrode in a chloride-containing solution from the

sample solution outside. Both sides of the glass membrane become hydrated,

and the outer surface exchanges its cations with the hydrogen ions of the sample

solution. The potential of the glass electrode depends on the ratio of the activi-

ties of H+ions inside and out, on the potential of the inner silver halide electrode

and on the asymmetry potential,characteristic of each particular electrode.

This gives a total electrode potential:

E(glass) =E*(glass) +RT/Fln (a(H+))

=E*(glass) -(2.303 RT/F) pH

The unknown constant E*must be determined by calibration with known

buffers, as described in Topic C4. Modern glass electrodes for pH measurement

often incorporate a reference electrode.

Although these electrodes work very well in the pH range 1-9, at high pH

they suffer from an alkaline errordue to the effect of other cations. This is

discussed in the next section. Glass electrodes are also fragile and may suffer

from slow response times.

Crystalline membrane electrodes, constructed either as the glass electrode,

or with a direct contact as shown in Figure 2(c),have an outer crystal surface

which responds to particular ions. The fluoride electrodehas a crystal of LaF 3 ,

treated with Eu(II) to increase conductivity, which responds selectively to the

adsorption of free F-ion on its surface. The selectivity is very good, due to the

ability of the small F-on to fit to the LaF 3 crystal lattice to the exclusion of larger

ions. However, OH-ions are also small and can interfere, and F-may also form

undissociated hydrofluoric acid. Therefore, it is necessary to use this electrode

in a buffer solution at about pH 6.

Interferences occur with metals such as Al^3 +, Fe^3 +and Ca^2 +, which combine

with the fluoride. With a SCE reference electrode, the cell emf is:

70 Section C – Analytical reactions in solution