Spectrophotometric determination of vitamin B1 in a pharmaceutical formulation using triphenylmethane acid dyes

A highly sensitive colour reaction has been developed, based on the fact that vitamin B(1) reacted with a triphenylmethane acid dye such as thymol blue, bromothymol blue, bromophenol blue, bromocresol green, phenol red or cresol red to form an ion-association complex in a weak-base aqueous solution in the presence of some solubilization agents e.g. polyvinyl alcohol, emulgent OP, Triton X-100 or Tween-20. The wavelengths of maximum absorbance of the six ion-association complexes were between 420 and 450 nm, and fading reaction appeared at the longer wavelength and the maximum fading wavelengths were between 550 and 620 nm. The reactions had highly sensitivities and their apparent molar absorptivities of the colour reactions were (0.82-1.65) x 10(5) l mol(-1) cm(-1) and those of fading reactions were (1.26-3.92) x 10(5) l mol(-1) cm(-1) depending on the different dye systems. The composition ratio of the ion-association complex was vitamin B(1):dye = 1:1 as established by Job's and equilibrium shift methods. The method had good selectivity and could be applied to direct spectrophotometric determination of vitamin B(1) in aqueous phase without using organic solvent extraction. Therefore, the method was simple and rapid. The colour reaction mechanism was discussed with the quantum chemistry method.