Studies on chitosan‐induced metachromasy, 1. Metachromatic behaviour of sodium 2′‐hydroxy‐1,1′‐azonaphthalene‐4‐sulfonate in the presence of chitosan

Solutions of chitosan in dilute acetic acid are shown to induce a metachromatic effect in the visible spectrum of the anionic dye sodium 2′‐hydroxy‐1,1′‐azonaphthalene‐4‐sulfonate (3) (C.I. Acid Red 88). The presence of chitosan reduces the intensity of λmax at 505 nm, whilst a new absorption band appears at 450 nm. This change may be observed visually as a change in colour from red to orange, suggesting that it may be of use in histological staining tests. A 1:1 stoichiometry is shown to exist between protonated amine groups on the polymer chain and negatively charged dye ions, at the point where the decrease in λmax reaches a minimum. Quantitative use of the metachromatic effect enables the determination of either the concentration in solution of a chitosan sample of known structure, or the amine group content of a chitosan sample of unknown structure.