preferentially with Nn+, reducing αN and hence K'NL relative to K'ML. These ideas will be particularly
useful when handling polyprotic acid and complex forming reactions (p. 205).
Solvent Effects
Assessment of a typical analytical reaction in solution requires consideration of the solvent
participation. It is likely that both reactants and products will undergo significant solvation, and the
solvation energy will materially affect and for the analytical reaction. Solvation energies may
be high, typically in the range –400 to –4000 kj mol–^1 for hydration. Solvent molecules participate in
the formation of ordered species in solution with the reactants and products of the reaction, thus
affecting also. Notably the interpretation of chelating processes depends upon solvent
considerations (p. 50)
Temperature Effects on Equilibrium Constants
If heat is applied to an endothermic reaction Ke will increase and, conversely, heat applied to an
exothermic reaction will result in a decrease in Ke. At the same time heat will tend to increase the
disorder of the system and favour the side of the reaction with the greatest potential disorder. However
the degree of disorder can rarely be assessed easily and the magnitude of the latter effect is difficult to
determine. Overall the variation of Ke is unlikely to exceed a few per cent per kelvin.
Kinetic Factors in Equilibria
The rate at which reactions occur is of theoretical and practical importance, but it is not relevant to give
a detailed account of reaction kinetics, as analytical reactions are generally selected to be as fast as
possible. However, two points should be noted. Firstly, most ionic reactions in solution are so fast that
they are diffusion controlled. Mixing or stirring may then be the rate-controlling step of the reaction.
Secondly, the reaction rate varies in proportion to the cube of the thermodynamic temperature, so that
heat may have a dramatic effect on the rate of reaction. Heat is applied to reactions to attain the position
of equilibrium quickly rather than to displace it.
3.2—
Solvents in Analytical Chemistry
Solvents are conveniently divided into those which promote the ionization of a solute (ionizing
solvents) and those which do not (non-ionizing solvents). As always such divisions are not entirely
satisfactory but Table 3.1 illustrates the classification of some familiar solvents.
Ionizing Solvents
Some ionizing solvents are of major importance in analytical chemistry whilst others are of peripheral
interest. A useful subdivision is into protonic solvents such as water and the common acids, or non-
protonic solvents