Hs. Soedjak, COLORIMETRIC DETERMINATION OF CARRAGEENANS AND OTHER ANIONIC HYDROCOLLOIDS WITH METHYLENE-BLUE, Analytical chemistry, 66(24), 1994, pp. 4514-4518
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.