Further investigation on the degree of deacetylation of chitosan determined by potentiometric titration

Abstract

The degree of deacetylation (DDA) of various low molecular weight chitosan (LMWC) species as the hydrochloride and free base (amine form) was determined by direct and back potentiometric titration, respectively. The DDA values obtained for the chitosan hydrochloride by direct titration were greater than 93% for all oligomers tested (Molecular weight (Mwt) between about 1.3 to 30.0 kDa). However, the DDA values obtained for chitosan amine oligomers using back titration were significantly lower, especially for the relatively high molecular weight (30.0 kDa) chitosan amine oligomers. Furthermore, after using the back titration method, greater DDA values were obtained for the same samples of chitosan amine after the chitosan solution had been heated to 60EC before titration. In addition, the DDA values showed a significant decrease with increased concentration for a given chitosan oligomer. Although the effects of hydration time, ionic strength and method specific behavior were not explicitly studied and therefore cannot be entirely ruled out, the results from this study might be attributed to the conformation of changes of chitosan in solution that lead to more inter/intra attractive forces in the case of chitosan amine. The DDA values obtained by the potentiometric method were also compared with those obtained by the FDUV and IR spectroscopic methods. Thus, subject to the caveats mentioned above, DDA values obtained for chitosan amine by the back titration method should be used with caution since the results are significantly lower than those obtained by direct titration, FDUV spectroscopy or FTIR.