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

The degree of deacatylation (DDA) of various low molecular weight chitosan (LMWC) species in the hydrochloride and amine forms was determined by direct and back potentiometric titration, respectively. It has been found that the DDA values obtained for the chitosan hydrochloride oligomers by the direct titration are better than 93% for all oligomers (Mwt of about 1.3 to 30.0 kDa) studied. However those DDA values obtained by the back titration of chitosan amine oligomers were significantly low especially for the relatively high molecular weight (30.0 kDa) chitosan amine oligomers. Furthermore, DDA values of chitosan amine have been increased when the chitosan solution was heated up to 60oC before titration. Also, the %DDA values showed a significant decrease with the concentration of the same chitosan oligomer. All these behaviors might be attributed to the conformation changes of chitosan in solution that lead to more inter/intra attractive forces in the case of chitosan amine. Thus, there is a concern about the DDA values obtained for chitosan amine by the back titration method since the results are always significantly lower than anticipated. The DDA values obtained by the potentiometric method were also compared with those obtained by the UV and IR spectroscopic methods.