Relative Affinities of the Anions of Strong Acids for Wool Protein
AbstractThe fastness of wool dyes to washing varies widely not only among different classes of dyes but even among members of a given class. These variations are manifestations of the differences in affinities of the dyestuffs for the fibers. A possible basis for understanding such differences in affinities was described in two earlier papers in which it was shown that wool immersed in hydro chloric acid solutions combines not only with hydrogen ions but with chloride ions as well. As a consequence of this, it appeared that the affinities for wool of the anions of different acids might vary considerably and that therefore the positions of the titration curves with respect to the pH axis might vary by correspondingly large amounts, according to the acid used. The determination of the titration curve of wool with a gvien acid could then be used to measure the affinity of the anion of the acid for the fiber. Measurements have therefore been made by Research Associates of the Textile Foundation at the National Bureau of Standards of the combination of wool with nineteen different acids, including, in addition to some of the common mineral acids, several of the simpler aromatic sulfonic, carboxylic, and phenolic acids and a soluble mono-azo acid dye. Wide differences were found between the positions of the resulting nineteen titration curves. A gen eralized form of the equations previously developed to account for the effect of the concentration of chloride ion on the amounts of hydrochloric acid combined was utilized to calculate the affinity for wool of each of seventeen anions involved. Predictions, based on the equations, as to the effects of variations of anion concentrations and of temperature were tested and confirmed. Measurements of the combination of a number of the same acids with a soluble protein, crystalline egg albumin, were also made. Since qualitatively similar differences in the corresponding titration curves were found with both proteins, it is concluded that the property of combining with anions is not restricted to insoluble proteins, such as wool, but is an inherent property of proteins in general. The affinity of the anions appears to increase with their dimensions, and is higher in aromatic than in aliphatic ions of the same size. These relationships are used as the basis of a consid eration of the nature of the forces involved in the binding by pro teins of anions, including the anions of acid dyes.