COLORIMETRIC DETERMINATION OF CARRAGEENANS AND OTHER ANIONIC HYDROCOLLOIDS WITH METHYLENE-BLUE

Methylene blue interacts with carrageenans and other anionic hydrocoll oids to form water-soluble metachromatic complexes at low concentratio ns of the reactants. The complexation results in a color change of the dye from blue (absorption maxima at 610 and 664 mn) to purple (absorp tion maximum at 559 nm). The anionic sites of the hydrocolloids appear to be primarily responsible for the dye-binding. The interaction betw een methylene blue and polyanions is reversible, electrostatic, and st oichiometric (1:1 ratio between the anionic sites and the bound dye mo lecules). The proportionality of the absorbance of the complex at 559 nm to polyanion concentrations allows a simple quantitative determinat ion of the polymers. The high sensitivity of the assay (i.e., (0.2-2.0 ) x 10(-3)% polyanion) may require a high dilution of samples. Consequ ently, potentially interfering compounds (e.g., acids, sugars, salts, milk, proteins, dyes, emulsifiers, and neutral hydrocolloids) may be d iluted out and become compatible with the assay. In addition, a select ive dye-binding inhibition of carboxylated polymers by phosphate allow s the analysis of mixed carboxylated and sulfated hydrocolloids. Becau se the colorimetric assay is simple, highly sensitive, fast, and repro ducible, it can be useful for routine quantitative analysis.