Page 378 - Instrumentation Reference Book 3E

Ion-selective electrodes 361
chemistry is changed by the ion being determined
passing from the sample solution across the mem-
brane to the inside of the cell.
An example is an ammonia electrode (Figure
17.15(e)). The sensing surface of a flat-ended
glass pH electrode is pressed tightly against a
hydrophobic polymer membrane which is acting
as a seal for the end of a tube containing ammo-
nium chloride solution. A silverlsiiver chloride
electrode is immersed in the bulk solution. The
membrane permits the diffusion of free ammonia
la1 lbl (NH?), but not ions, between the sample solution
and the film of ammonium chloride solution. The
introduction of free ammonia changes the pH of
the internal ammonium chloride solution which is
sensed by the internal glass pH electrode.

17.6.6 Redox electrodes
In elementary chemistry a substance is said to be
oxidized when oxygen is combined with it and
said to be reduced when oxygen is removed from
it. The definition of oxidation and reduction
t
Ciymlline membrane with may, however, be extended. Certain elements,
internal metal CD~C and membrane
solid COnneCtiOn e.g., iron and tin, can exist as salts in more than
IC) id1 one form. Iron: for example, can be combined
with sulfuric acid in the form of ferrous iron.
AgIAgCI
reference valency 2, or ferric iron; valency 3.
Consider the reaction:
ferrous ferric ferric
sulphate + chlorine = chloride + sulfate
6FeS04 + 3C12 = 2FeC13 + 2Fe2(S04),
The ferrous sulfate is oxidized to ferric sulfate;
chlorine is the oxidizing agent. In terms of the
ionic theory, the equation may be written
6Fe2 + 3C12 = 6Fe3 + 6C1
(el i.e., each ferrous ion loses an electron and so
Figure 17.15 Ion-selective electrodes: (a) glass, gains one positive charge. When a ferrous salt is
(b) crystalline membrane with internal reference electrode,
(c) crystalline membrane with solid connection, (d) liquid oxidized to a ferric salt each mole of ferrous ions
ion exchange, (e) gas sensing membrane. Courtesy Orion gains one mole (1 faraday) of positive charges or
Research Inc. loses one mole of negative charges, the negative
charge so lost being taken up by the oxidizing
the calcium electrode, but other electrodes in this agent (chlorine). Oxidation, therefore, involves
class are available for the determination of Cl-, the loss of electrons; reduction, the gain of elec-
ClO,, NO3, Cu2+, Pb2+, and BF4 ions. The trons. Thus the oxidation of a ferrous ion to
liquid ion exchange electrodes have more restrict- ferric ion can be represented by the equation
ing chemical and physical limitations than the
glass or solid state electrodes, but they may be
used to measure ions, which cannot yet be meas-
ured with a solid state electrode. When a suitable electrode, such as an inert metal
which is not attacked by the solution and which
will not catalyze side reactions, is immersed in a
17.6.5 Gas-sensing membrane electrodes solution containing both ferrous and ferric ions,
These electrodes are not true membrane elec- or some other substance in the reduced and oxi-
trodes as no current passes across the membrane. dized state, the electrode acquires a potential
They are complete electrochemical cells, moni- which will depend upon the tendency of the ions
tored by an ion-selective electrode as the internal in the solution to pass from a higher or lower