Determination of the degree of substitution of sodium carboxymethylcellulose by potentiometric titration and use of the extended henderson-hasselbalch equation and the simplex method for the evaluation

Determination of the degree of substitution, D.S., of purified sodium carboxymethylcellulose, CMC, by potentiometric titration has been studied. Different mathematical descriptions of the acid-base properties of polyelectrolytes were tested on the titration curve. It was found that the extended Henderson-Hasselbalch equation reproduced the measured data well. It was therefore used as a basis for evaluating D.S. by the simplex method. The influence of ionic strength, I, on the shape of the titration curve was investigated and the effect of I on the precision of the titrations was evaluated. The titrations showed a precision of +/- 0.004 D.S.-units and the results were in agreement with those obtained from a dry-ashing method, within +/- 0.02 D.S.-units. The whole method, including preparation of solutions and titration, showed a precision of +/- 0.01 D.S.-units.