The Combination of Wool with Acids–a Quantitative Interpretation in terms of the Donnan Theory of Membrane Equilibrium*

Abstract

A quantitative interpretation of the combination of wool with hydrochloric aoid, sulphuric acid, and hydrochloric acid in presence of different concentrations of potassium chloride has been obtained in terms of the Donnan theory of membrane equilibrium. The course of combination is determined by the pH inside the fibre, which can be calculated although it is incapable of direct measurement. When the amount of combined acid is plotted as a function of the “internal” pH, the mid–point of the titration curve is found to lie between pH 4. 15 and 4. 35 in the case of hydrochloric acid, as would be expected if combination were due to back–titration of the carboxyl groups of salt linkages. Similarly, in the case of sulphuric acid the mid–point is at pH 4. 45, and the unification of the hydrochlorio acid and sulphuric acid titration curves, as well as the agreement between the observed and expected pK values, affords strong evidence in support of the Donnan theory. In addition, it has been shown that the curves obtained by titrating wool with hydrochloric acid in presence and absence of potassium chloride, the amounts of potassium chloride being sufficient to give a constant chloride ion concentration, are unified when the amounts of combined acid are plotted against the calculated “internal” pH values. The Donnan theory also provides a precise interpretation of the manner in which the pH corresponding to half–maximum combination with acid varies with changing salt concentration.