Micellar effect on the sensitivity of spectrophotometric Mo(VI) determination based on the formation of gallic acid complex providing evidence for the polyoxoanion structure of molybdate ions.

In this study effects of anionic (sodium dodecyl sulfate, SDS), cationic (cetyltrimethylammonium bromide, CTAB) and nonionic (Triton X-100, TX100) micelles on the sensitivity of spectrophotometric molybdenum(VI) (Mo) determination based on the formation of a binary complex with gallic acid (GA) were investigated. Micellar CTAB was found to enhance the formation of Mo-GA complex. SDS micelles exerted an inhibitory effect while TX100 micelles had no effect on the complex formation. By the optimization of experimental conditions, the determination limit of the method suggested in the literature was lowered from 5.2x10(-5) to 4.6x10(-6) and to 5.7x10(-7) M, in the absence and presence of CTAB, respectively. The mechanism of the effect of CTAB was investigated by spectrophotometric titrations and it was concluded that CTAB did not form a ternary complex with Mo and GA. The stoichiometry of the complex, deduced from the results of spectrophotometric titrations, provided evidence for the formation of para-Mo7O4(6-) polyanions at pH 4.5, indicating to the formation of a charge transfer complex between these ions and GA in micellar medium.