Page 47 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

ELECTROLYTIC DISSOCIATION 2.4

without itself undergoing any net change: it follows that a small amount of the
catalyst can influence the conversion of large quantities of the reactants. If the
reaction under consideration is reversible, then the catalyst affects both the
forward and back reactions, and although the reaction is speeded up, the position
of equilibrium is unchanged.
An example of catalytic action is provided by the titration of oxalates with
potassium permanganate solution referred to above. It is found that even though
the oxalate solution is heated, the first few drops of permanganate solution are
only slowly decolorised, but as more permanganate solution is added the
decoloration becomes instantaneous. This is because the reaction between
oxalate ions and permanganate ions is catalysed by the MnZ+ ions formed by
the reduction of permanganate ions:
Mn04 + 8H+ + 5e- = MnZ+ + 4Hz0
Other examples are the use of osmium(VII1) oxide (osmium tetroxide) as
catalyst in the titration of solutions of arsenic(II1) oxide with cerium(1V)
sulphate solution, and the use of molybdate(V1) ions to catalyse the formation
of iodine by the reaction of iodide ions with hydrogen peroxide. Certain reactions
of various organic compounds are catalysed by several naturally occurring
proteins known as enzymes.
The determination of trace quantities of many substances can be accomplished
by examining the rate of a chemical reaction for which the substance to be
determined acts as a catalyst. By comparing the observed rate of reaction with
rates determined for the same reaction, with known quantities of the same
catalyst present, the unknown concentration can be calculated. Likewise a
catalyst may be used to convert a substance for which no suitable analytical
reaction exists for the conditions under which the substance is present, to a
product which can be determined. Alternatively, the substance to be determined
may be destroyed by adding a catalyst, and the resultant change in some
measured property, for example the absorption of light, enables the amount of
substance present to be evaluated. Thus, uric acid in blood can be determined
by measurement of the absorption of ultraviolet radiation at a wavelength of
292nm, but the absorption is not specific. The absorption meter reading is
recorded, and then the uric acid is destroyed by addition of the enzyme uricase.
The absorption reading is repeated, and from the difference between the two
results, the amount of uric acid present can be calculated.

2.4 ELECTROLYTIC DISSOCIATION
Aqueous solutions of many salts, of the common 'strong acids' (hydrochloric,
nitric and sulphuric), and of bases such as sodium hydroxide and potassium
hydroxide are good conductors of electricity, whereas pure water shows only a
very poor conducting capability. The above solutes are therefore termed
electrolytes. On the other hand, certain solutes, for example ethane-1,2-di01
(ethylene glycol) which is used as 'antifreeze', produce solutions which show a
conducting capability only little different from that of water: such solutes are
referred to as non-electrolytes. Most reactions of analytical importance occurring
in aqueous solution involve electrolytes, and it is necessary to consider the
nature of such solutions.