Carboxyl group determination calcium acetate exchange method

Total acidity (meq/g) and carboxyl groups (meq/g) were determined by the barium hydroxide method and the calcium acetate exchange method, respectively (X4). Phenolic hydroxyl groups (meq/g) were calculated as the difference between total acidity and carboxyl groups. These determinations were carried out under a nitrogen atmosphere to minimize absorption of carbon dioxide. 

Native polysaccharides with acid groups other than the uronic type are not very common except for the sulfate esters. Total acidity may be estimated by direct titration, but erroneous results are obtained if the polysaccharide is alkali-labile as is the case with many oxidized polysaccharides. Addition of calcium acetate (7, 8) or sodium bromide 8, 9) to the polysaccharide solution increases the accuracy of the titration. Other methods for the estimation of carboxyl and other acidic groups involve determination of the amount of methylene blue absorbed, or determination of the amount of silver salt formed by exchange from a solution which contains silver in combination with a very weak acid. The sulfate content of polysaccharide sulfates, such as agar, is obtained by ordinary sulfate analysis of the completely hydrolyzed or ashed polysaccharide. 

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