Determining Carrageenan Sulfate Groups Using Ion Association with Alcian Blue Dye

The purpose of this study is to develop a simple and reliable method to measure sulfate content and determine the purity of carrageenan raw materials. The method uses cationic Alcian Blue dye that binds strongly to anionic carrageenan macromolecules, resulting in an insoluble ion associate. At a specific carrageenan-to-dye ratio, the hydrophobic ion-associated particles form macroscopic flocs and precipitate quickly. The resulting deep sedimentation leaves almost colorless supernatants, indicating the stoichiometric charge ratio. The critical polymer-to-dye ratio has been found to be independent of the dye concentration. The Alcian Blue reagent has been standardized against a synthetic polymer, poly(sodium styrene sulfonate). The method of ion-associate precipitation was used to determine the content of sulfate groups in commercial carrageenans of different types: kappa, iota, and lambda. The results obtained agree well with the IR spectra of the tested carrageenans.