The dissociation constants of the carboxyl and hydroxyl groups in some insoluble and sol-forming polysaccharides

Abstract

In order to establish the conditions under which the buffering of insoluble substance may most usefully be studied, suspensions of fibre cellulose, hemicellulose and xylan have been examined to determine the relationship between pH and the degree of dissociation of their carboxyl groups. The materials were suspended in normal potassium chloride solution, in order to confine local gradients of hydrion. concentration to the immediate vicinity of each charged buffer group. Under these conditions the dissociation curves are all of the simple Henderson type with a common constant pK = 2·95 (N /100 hydrochloric acid in normal potassium chloride being taken as pH 2 ). Sols of arabic and pectie acid follow the same dissociation curve, showing that the dissociation of the carboxyl groups is not necessarily influenced by the state of aggregation. Within the accuracy of these measurements, uronic and gluconic carboxyls have the same pK . The dissociation of pectic acid departs from the Henderson curve in the same general way as the dissociation curve of maleic acid differs from that of fumaric acid. It is suggested that some of the carboxyls in pectic acid are linked by hydrogen bridges. To a less extent such linkage may also occur in alginic acid below pH 3 when a gel is formed. The pK . for the first stage of acid dissociation of the hydroxyl groups in wheat starch is close to 13·3. The same value was obtained whether potassium hydroxide, calcium hydroxide or barium hydroxide was used. The alkalis were made up in normal solutions of the corresponding chlorides. The same constant holds for alginate within the limits of experimental error. The dissociation in cellulose up to pH 13·5 is much less than in starch. Using a new method based on a determination of the ratio of chloride ions to hydroxyl ions, the dissociations of starch and cellulose were measured in 5N alkali (approximately pH 14·7). The second pK of starch is estimated at roughly 15·0. The cellulose, largely mercerized at this pH, exhibited a dissociation not much less than that of starch, and agreeing closely with the prediction of Neale. An insoluble buffer substance suspended in an electrolyte solution is a two-phase system. The hydrion dissociation can be determined in two-phase systems over a greater pH range than is possible in one-phase systems.