The Purification of Methylene Blue and Azure B by Solvent Extraction and Crystallization

Abstract

Detailed schemes are described for the preparation of purified methylene blue and azure B from commercial samples of methylene blue. Purified methylene blue is obtained by extracting a solution of the commercial product in an aqueous buffer (pH 9.5) with carbon tetrachloride. Methylene blue remains in the aqueous layer but contaminating dyes pass into the carbon tetrachloride. Metal salt contaminants are removed when the dye is crystallized by the addition of hydrochloric acid at a final concentration of 0.25 N. Purified azure B is obtained by extracting a solution of commercial methylene blue in dilute aqueous sodium hydroxide (pH 11-11.5) with carbon tetrachloride. In this pH range, methylene blue is unstable and yields azure B. The latter passes into the carbon tetrachloride layer as it is formed. Metal salt contaminants remain in the aqueous layer. A concentrated solution of azure B is obtained by extracting the carbon tetrachloride layer with 45 × 10-⁴ N bydrobromic acid. The dye is then crystallized by increasing the hydrobromic acid concentration to 0.23 N.

Thin-layer chromatography of the purified dyes shows that contamination with related thiazine dyes is absent or negligible. Ash analyses reveal that metal salt contamination is also negligible (sulphated ash < 0.2%).