Can Potentiometric Titration Be Almost as Accurate as 1H NMR to Determine the Degree of Deacetylation of Modified Chitosan?

Chitosan is a linear biopolymer composed of randomly distributed D-glucosamine and N-acetylglucosamine repeating units. Various nucleophilic functionalities within its structure make chitosan easy to modify by chemical synthesis to generate new tailor-made chitosan derivatives with specific properties. One parameter that heavily influences these properties is the degree of deacetylation (DDA), which is the percentage of D-glucosamine units (of the total repeating units) in the polymer chains. Various methods to determine the DDA of unmodified or modified chitosans exist, including 1H NMR and potentiometric titration. 1H NMR is generally considered to be the most accurate method, but is not always readily available. Potentiometric titration relies on more accessible equipment but offers less accurate results. This is particularly the case with heavily modified chitosan derivatives, since the equation generally used to calculate the DDA after the titration does not consider the molecular weight of the modified repeating units that are part of the polymeric chain. To circumvent this limitation, we recently reported a new equation that is better suited to determine the DDA of chitosan derivatives with three types of repeating units. In our previous work, we tested this equation by preparing naphthalene–chitosan derivatives and subjecting them to potentiometric titration. We showed that this new equation gives higher DDA values than the traditional equation, suggesting that classic potentiometric titration underestimates the DDA of modified chitosans. In this paper, we used 1H NMR to calculate the DDA of a series of naphthalene–chitosan derivatives and compared those new DDA values with the results previously obtained by potentiometric titration. As we show here, for our series of naphthalene–chitosan derivatives, our new equation calculates the DDA values with potentiometric titration more accurately (i.e., more in agreement with the NMR 1H result) than the previous equation by up to 5 DDA units (%).