IONIC THEORY Relative Strengths of Acids and Bases.—Although much recent work has been concerned with the precise significance of "active mass," the classical interpretation of this as synonymous with molecular concentration has yielded results of the greatest importance in the interpretation of the manifold phenomena as sociated with aqueous solutions of acids, bases and salts (see SOLUTION). Such substances are, in general, electrolytic con ductors, and in accordance with the theory of Arrhenius are ionized to a greater or less extent. The traditional view, that salt formation corresponds with the neutralization of opposite quali ties which characterize the acids and bases respectively, finds simple expression in the ionic theory, for, according to this, acids are substances which increase the hydrogen-ion concentration of water, whilst bases have the opposite effect. In aqueous solutions, the concentration of the hydrogen ion is directly connected with the hydroxyl-ion concentration, by the mass-action expression In all dilute aqueous solutions however the concentration of the un-ionized water is nearly constant (55.5 moles per litre). Accordingly in all dilute aqueous solutions, whether acid, alkaline or neutral, the product of the two ion concentrations has a nearly constant value at 25° C). In neutral solutions hydrogen ion (sometimes called hydrion) and hydroxyl-ion are present in equal concentrations gram-ions per litre at 25° C) . In acid solutions the concentration of hydrogen-ion exceeds that of hydroxyl-ion ; in alkaline solutions the reverse is true. Yet hydroxyl-ion is never completely absent from acid solutions, nor hydrogen-ion from alkaline solutions, for the concentrations of the two ions must always be so related that the ion-product, [H+] • has the constant value at 25° C. At higher temperatures the ion-product increases very rapidly, attaining a value of about 6o X at loo° C.
Equivalent solutions of different acids show very great differ ences in the hydrion concentration. These are exhibited very clearly when the electrical conductivities of such solutions are compared or when a comparison is made of the velocities with which certain reactions take place when these are subjected to the catalysing influence of the various acids. The two series of numbers show a close parallelism which finds a simple interpreta tion in terms of the view that the differences between the acids are primarily due to differences in their respective degrees of ionization. The conductivity is indeed proportional to the con centration of the ionized fraction of the acid and the catalytic effect is determined for the most part by the hydrion concentra tion. The application of the mass law to the equilibrium between the un-ionized acid and the corresponding ions, as represented by for the ionization constant, c being the molar concentration of the acid and a the fractional degree of ionization. In spite of the charges on the ions, this relation between the concentration of the acid and its degree of ionization is in very close agreement with the actual behaviour of weak or slightly ionized acids, as was clearly demonstrated by Wilhelm Ostwald. The results ob tained in this connection constitute in fact the strongest and most comprehensive evidence as yet available in favour of the quan titative application of the law of mass action to states of equi librium. The Ostwald dilution law does not hold for solutions of salts or more active (largely ionized) acids or bases. Many chemists have indeed arrived at the conclusion that most salts are completely ionized, at all concentrations. (See also ELECTROL