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

Methylene blue interacts with carrageenans and other anionic hydrocolloids to form water-soluble metachromatic complexes at low concentrations of the reactants. The complexation results in a color change of the dye from blue (absorption maxima at 610 and 664 mn) to purple (absorption maximum at 559 nm). The anionic sites of the hydrocolloids appear to be primarily responsible for the dye-binding. The interaction between methylene blue and polyanions is reversible, electrostatic, and stoichiometric (1:1 ratio between the anionic sites and the bound dye molecules). The proportionality of the absorbance of the complex at 559 nm to polyanion concentrations allows a simple quantitative determination 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. Consequently, potentially interfering compounds (e.g., acids, sugars, salts, milk, proteins, dyes, emulsifiers, and neutral hydrocolloids) may be diluted out and become compatible with the assay. In addition, a selective dye-binding inhibition of carboxylated polymers by phosphate allows the analysis of mixed carboxylated and sulfated hydrocolloids. Because the colorimetric assay is simple, highly sensitive, fast, and reproducible, it can be useful for routine quantitative analysis.