HYDROGEN-ION CONCENTRATION. If diluted solutions of, say, hydrochloric acid and acetic acid are examined, it is found that, whereas the former possesses to a marked degree all the properties generally associated with acids, the latter pos sesses them only to a very limited extent. On the other hand, if each of these solutions contains one gram-equivalent of its acid (36.5 grams of hydrochloric acid and 6o grams of acetic acid) they will both require the same amount of alkali for neutralization (e.g., 4o grams of sodium hydroxide), and are therefore apparently equally acidic.
The reason for this seeming paradox is that, whilst hydrochloric acid is almost completely dissociated (see DISSOCIATION) into hydrogen ions and chlorine ions HCl—* H••Y-Cl', the acetic acid is only very feebly dissociated Since the characteristic properties of acids depend largely on the hydrogen ions, the superiority of hydrochloric acid in this respect is intelli gible. The equality in the neutralizing powers is due to the fact that the acetic acid exists in equilibrium (expressed by above) with the hydrogen and acetate ions, so that, as fast as hydrogen ions are neutralized by alkali, more acetic acid dissociates in order to maintain the equilibrium (see CHEMICAL ACTION) and so supplies more hydrogen ions. Neutralization is therefore a measure, not only of hydrogen ions in solution, but also of the potential source of them (undissociated acid molecules). Now true neutrality is represented by the condition of pure water, which dissociates to such a feeble extent as to give only of an equivalent of hydrogen per litre (at 25° C) and the same con centration of hydroxyl ions: We can therefore express the concentration of hydrogen ions (expressed in gram equivalents per litre) as [H'] = in a neutral solution, and simi larly for hydroxyl ions. The product of these two concentrations is thus and in any aqueous solution, acidic or alkaline, at 25° C the law of mass action (see CHEMICAL ACTION) is ful filled in that [H'] X [OH'] = Many liquids of biological importance, such as soil extracts, blood, cultures, milk, etc., are very nearly neutral as judged by ordinary criteria, but they are very sensitive to extremely slight changes in acidity or alkalinity. Titration of these solutions gives no clue to the degree of acidity, both because it is so slight and because of the reasons already given.
For the adequate study of such solutions, it is convenient to have a less cumbersome method of denoting acidity than the fore going, and for this purpose the symbol PH has been adopted, such that pn = —log [H'] . The neutral solution is therefore of pH =7; an acidic solution has a smaller and an alkaline solution a larger The acidity of, e.g., saliva is more neatly expressed as than as [H'] = or I.26 X and the feeble alkalinity of blood as 7.35 than as 3.6X Similarly, decinormal solu tions of hydrochloric acid and caustic soda have, respectively, and 13 (approx.). (A. D. M.)