Determination of molybdenum(VI) with o-hydroxyhydroquinonephthalein

o-Hydroxyhydroquinonephthalein (Qn. Ph.)-molybdenum(VI) (Mo(VI)) complex in the presence of nonionic surface-active reagent (LT-221) can be used for the spectrophotometric and fluorometric determination of a minute amount of Mo(VI). A maximum absorption wavelength of the complex is 520 nm in a strongly acidic medium, where the apparent molar absorption coefficient is 130000. The sensitivity is 0.0007 μg/cm2 Mo(VI) for an absorbance of 0.001. This method is applicable to the analyses of iron and steel for Mo(VI). The mole ratio of Mo(VI) and Qn. Ph. in the complex was estimated to be 1 : 3 by the continuous variation and mole-ratio methods. The recommended procedure for Mo(VI) is as follows. Spectrophotometry - To a 10 ml measuring flask an aliquot portion of the sample solution containing less than 9.2 μg of Mo(VI), 2.0 ml of 3.5% LT-221, and 1.0 ml of 1.0 x 10-3M Qn. Ph. are added, and the pH of the mixture is adjusted to 1.8 by the Walpole buffer solution. After diluting the solution exactly to 10 ml with water, the absorbance is measured at 520 nm against the reagent blank. Fluorometry - To a 10 ml measuring flask, an aliquot portion of the sample containing less than 2.1 μg of Mo(VI), 1.0 ml of 3.5% LT-221 and 0.5 ml of 1.0 x 10-3 M Qn. Ph. are added, and the pH of the mixture is adjusted to 1.8 by the Walpole buffer solution. After diluting the solution exactly to 10 ml with water, the fluorescence intensity of Qn. Ph.-Mo(VI) solution is measured at 520 nm. (auth.)