The FTIR spectrum of CMSS-acid hydrogel showed a new absorption peak at 1741 cm−1 which denoted that the Na in CMSS was being exchanged to H from lactic acid solution.The intensity of the absorption peak depends on the concentration of the acid used. In this study, the concentration of lactic acid was 0.6 M, thus a weak absorption peak was observed and not all of COONa converted to COOH. Similar result has been reported by Takigami, Nagasawa, Hiroki, Taguchi, and Takigami (2012), where the replacement of Na in carboxymethyl group with hydrogen from acid occurred when both are mixed.