Page 379 - Instrumentation Reference Book 3E

362 Chemical analysis: electrochemical techniques

state of oxidation. If the ions in solution tend to electrode. It has a variety of physical forms,
become oxidized (Le.; the solution has reducing which are discussed below.
properties) the ions tend to give up electrons to
the electrode which will become negatively
charged relative to the solution. If, on the other Mercurylmercurous chloride or calomel electrode
hand, the ions in solution tend to become The metal used is mercury, which has a
reduced (i.e., the solution has oxidizing proper- high resistance to corrosion and being fluid at
ties), then the ions will tend to take up electrons ambient temperature cannot be subject to strain.
from the electrode and the electrode will become The mercury is in contact with either mercurous
positively charged relative to the solution. The chloride or in some electrodes with mercurous
sign and magnitude of the electrode potential, chloride and potassium chloride paste. Contact
therefore, give a measure of the oxidizing or with the measured solution is through a salt
reducing power of the solution, and the potential bridge of potassium chloride whose concen-
is called the oxidation-reduction or redox poten- tration may be 3.8mol per liter or some other
tial of the solution, Eh. The potential Eh may be concentration appropriate to the application.
expressed mathematically by the relationship Contact with the mercury is usually made by
means of a platinum wire, which may be amalgam-
ated. The calomel-saturated potassium chloride
where a, is the activity of the oxidized ion and a, electrode has a potential relative to the hydrogen
is the activity of the reduced ion. electrode of -0.244 V.
To measure the oxidation potential it is neces- Where the use of potassium salt is precluded by
sary to use a reference electrode to complete the the condition of use, it may be replaced by
electrical circuit. A calomel electrode is often used sodium sulfate, the bridge solution having a con-
for this (see Section 17.7 below). centration of 1 mol per liter.
The measuring electrode is usually either plat- Whatever the type of the reference electrode,
inum or gold, but other types are used for special contact must be made between the salt bridge and
measurements: as, for example, the hydrogen the measured solution. Two common methods
electrode for use as a primary standard and the are through a ceramic plug whose shape and
quinhydrone electrode for determining the pH of porosity govern the rate at which the salt bridge
hydrofluoric acid solutions. However, the latter solution diffuses out and the process solution
two electrodes do not find much application in diffuses into and contaminates the bridge solu-
industrial analytical chemistry. tion. If the plug is arranged to have a small
cross-sectional area relative to its length, the rate
of diffusion is very small (say less than
17.7 Potentiometry and specific 0.02 cm3/day), and the electrode can be considered
ion measurement to be sealed and is used until it becomes unser-
viceable. It is then replaced by a similar electrode.
17.7.1 Reference electrodes Where the application warrants it, a high rate
of diffusion from the electrode has to be tolerated
All electrode potential measurements are made (say 1 or 2cm31day), so the relative dimensions
relative to a reference electrode and the e.m.f. and porosity of the plug are changed, or it is
generated at this second contact with the solution replaced by a glass sleeve which permits relatively
being tested must be constant. It should also be fast flow of salt bridge solution, thus reducing the
independent of temperature changes (or vary in a rate and degree of fouling of the junction. In these
known manner), be independent of the pH of the circumstances, the electrode is refilled on a rou-
solution, and remain stable over long periods. tine basis, or a continuous supply of bridge solu-
Standard hydrogen electrodes are inconvenient
(see below) and in practice three types of refer- tion is arranged into the electrode at the
appropriate pressure for the application.
ence are commonly used. A wide range of electrodes is illustrated in
Figures 17.16-17.19. The choice of the appropri-
Silverlsilver chloride electrode This consists of a ate reference electrode for the application is vital,
silver wire or plate, coated with silver chloride, in and consideration must be given to the pressure,
contact with a salt bridge of potassium chloride temperature, and nature of the process stream.
saturated with silver chloride. The concentration The accuracy of the measurement and the fre-
of the potassium chloride may vary from one type quency of maintenance depends upon the correct
of electrode to another but concentrations of 1 .OO choice of electrode. The e.m.f. of the reference
or 4.00mol per liter or a saturated solution are electrode will only remain constant provided
quite common. This saturated type of electrode satisfactory contact is made by the salt bridge,
has a potential of -0.199 V relative to a hydrogen so the junction must not become plugged by