IONS, CATALYTIC ACTION OF. The fact that many reactions are accelerated by acids and bases has been gener ally associated with the idea that the hydrogen and hydroxyl ions are unique in the possession of catalytic qualities. The term hydrogen ion is here understood to refer to the hydrated form HH20 or oxonium ion OH3. In the case of acid catalysis, the simple hydrogen ion theory met for a long time with almost universal acceptance for the reason that it served to correlate with a fair degree of precision a large body of experimental facts. More recent studies, in which new reactions have been subjected to minute examination indicate, however, that there is in general no simple proportionality between the catalytic activity of an acid and either the concentration or the thermodynamic activity of the hydrogen ion. On the other hand, the observations show clearly that catalytic activity is by no means peculiar to the hydrogen and hydroxyl ions, but that similar qualities must be attributed to other positive and negative ions and also to elec trically neutral molecules. The catalytic effects produced by a weak acid HA, when admixed with the corresponding salt for the purpose of reducing the preponderating influence of the hydrogen ion, require, indeed, for their representation an equation of the form [0H3] +km [HA] +k,, +ka [A] + in in which the total observed catalytic action is expressed as the sum of the joint effects due to the positive hydrogen ion, the neu tral molecules HA and and the negative ions A and OH. The relative magnitudes of the individual catalytic effects depend on the nature of the reaction and also on the solvent medium. As might have been anticipated, the values of k,„, k,,„ and are usually very small compared with the values of and kon• If for a given reaction, the values of k,„ and for a series of different acids are compared, it is found that with increasing ionisation constant of the acid, the magnitude of increases whilst that of diminishes. The high efficiency of the hydroxyl
ion as a catalytic agent is thus connected with the fact that water is an acid with a very small ionisation constant.
If certain secondary effects which depend on variations in the reaction medium are ignored, the results of recent work indicate that the catalytic effects which are produced by the positive and negative ions resulting from the dissociation of the acid HA may be expressed in terms of the hydrogen ion concentration in the form of a general relation which is independent of the nature of the acid and of its concentration. The relation in question, which may be put in the form where r is a measure of the catalytic effect of the ions, and n a measure of the hydrogen ion concentration, is also independent of the temperature, of the nature of the solvent medium and even of the nature of the cata lysed reaction. The catalytic effects associated with the hydrogen and hydroxyl ions conform closely to the requirements of this gen eralisation.
As the result of catalytic observations, it is now recognised that the traditional conception of the relations between acids and bases is inadequate and requires modification. The modern view assigns to the free hydrogen ion or proton the role of the con necting link between the acids and bases as represented by the formula: acid base+H. In accordance with this view