Preparation and purification of Semi-Methylthymol Blue

Abstract

A direct extractive method has been developed for preparation of pure Semi-Methylthymol Blue (SMTB) from a synthetic mixture containing ca. 30% of SMTB in presence of Thymol Blue (TB, ca. 45%), Methylthymol Blue (MTB, ca. 10%) and other components such as acetic acid and iminodiacetic acid (IDA). The method is based on the dependence of the extraction coefficients of the individual components on the pH of an aqueous phase in equilibrium with butanol phase. The separation of TB from the mixture is best at pH 9.5–10.5 and the separation of SMTB is best at pH 2.9–4.5. At pH 10.2 the ratio of the extraction coefficient of TB to that of SMTB is 213 and to that of MTB is 776. At pH 2.9 the ratio of the extraction coefficient of SMTB to that of MTB is ca. 20 and at pH 4.5 is ca. 100, but the absolute values of the extraction coefficient for SMTB are only 3.4 at pH 4.5 and ca. 10 at pH 2.9. This means the quantitative transfer of SMTB to the butanol phase is much easier at pH 2.9. Then a more complete separation of SMTB from the mixture with MTB can be achieved by stripping of the latter into a pH 4.5 aqueous medium. The method for obtaining of anhydrous SMTB is also given. About 70% of the original amount SMTB present can be obtained as the dry product (containing over 99% SMTB).