Page 369 - Physical chemistry understanding our chemical world

336 ELECTROCHEMISTRY

The emf of the concentration cell, therefore, becomes
RT RT
emf = ln[a (O)RHS ] − ln[a (O)LHS ]
nF nF
which, through the laws of logarithms, simplifies readily to yield Equation (7.48).
If we assume that the activity coefficients in the left- and right-hand half-cells are the
same (which would certainly be a very reasonable assumption if a swamping electrolyte
was also in solution), then the activity coefficients would cancel to yield

RT [O] RHS
emf = ln
nF [O] LHS

How do pH electrodes work?

The pH–glass electrode

A pH electrode is sometimes also called a ‘membrane’ electrode. Figure 7.15 shows
how its structure consists of a glass tube culminating with a bulb of glass. This bulb
is filled with a solution of chloride ions, buffered to about pH 7. A slim silver wire
runs down the tube centre and is immersed in the chloride solution. It bears a thin
layer of silver chloride, so the solution in the bulb is saturated with AgCl.
The bulb is usually fabricated with common soda glass, i.e. glass containing a high
concentration of sodium ions. Finally, a small reference electrode, such as an SCE,
is positioned beside the bulb. For this reason, the pH electrode ought properly to be
called a pH combination electrode, because it is combined with a reference electrode.
If the pH electrode does not have an SCE, it is termed a glass electrode (GE). The
operation of a glass electrode is identical to that of a combina-
tion pH electrode, except that an external reference electrode is
Empirical means found
from experiment, rather required.
than from theory. To determine a pH with a pH electrode, the bulb is fully immer-
sed in a solution of unknown acidity. The electrode has fast res-
ponse because a potential develops rapidly across the layer of glass

Thin-walled glass bulb
Buffer solution
(containing chloride ions) Silver wire

Deposit of silver chloride

Figure 7.15 Schematic representation of a pH electrode (also called a ‘glass electrode’)