Page 144 - Instant notes

Physical chemistry 130

Half-cell reactions often involve soluble reagents and not metals and these are often
called redox electrodes. An example is the ferric/ferrous ion redox electrode (Fig. 2b),
. In these cases a platinum electrode is used as the metal
electrode. The platinum acts at a site for the exchange of electrons between the reductant
and the oxidant, resulting in electrode potential being established. It is also inert and does
not form part of any redox reaction.
When one of the reagents is a gas (a gas electrode), for example the fluorine/fluoride
gas electrode (Fig. 2c), the inert platinum electrode is also used
as a site for the electron exchange, with the gas being bubbled through the aqueous
solution containing the redox active ion and across the electrode surface.

The standard state
The value of the cell voltage, E cell, depends upon the position of the equilibrium for each
half-cell reaction, which depends upon the activities of ions in solution (see Topic E2),
the fugacity (or pressure) of gas in a gas electrode and the temperature. In order to
compare different cells, cell voltage values, , are often obtained at the standard
state (see Topic B3). For biological systems, the biological standard state is used,
symbol (see Topic B2). In electrochemical cells, there is the additional constraint for
both that the inert electrodes and electrical connections are made of platinum. and
values are identical for those half-cells for which hydrogen or hydroxide ions are not
involved in the cell reaction. However, there are significant differences when they are
(see Topic E5).