Comparison of Carboxyl-Content Determination in Cellulose Half-Acid Ester Derivatives by Calcium Acetate and Silver Absorption Methods

Carboxyl content of cellulose half-acid ester derivatives was determined by the calcium acetate method and by the absorption of silver from silver meta-nitrophenolate solutions. Both methods utilize the acidity of the carboxyl proton in an ion-exchange reaction, thereby producing a salt. With both the calcium acetate and the sitver absorption methods, the neutralization of the carboxylie acid groups is incomplete below pH 6.5. Tp compensate for this effect, a correction curve was derived for cellulose half-acid ester derivatives, which agreed fairly closely to a curve previously derived for cellulose half-acid etlter derivatives. No correction curves could be accurately applied to the silver-absorption test, but 0.6 g was selected as the optimum sample size because larger sample sizes usually resulted in a decrease in the carboxyl determination due to lowering of pH and/or exhaustion of the silver, which had orfly limited solubility. Nevertheless, the free degree-of-substitution values obtained with the calcium acetate method, using the new correction curve, and with the silver absorption method utilizing 0.6 g samples, differed on the average by only 0.011.