Molybdenum blue, adsorption

Yoshimura et al. [193] carried out microdeterminations of phosphate by gel-phase colorimetry with molybdenum blue. In this method phosphate reacted with molybdate in acidic conditions to produce 12-phosphomolybdate. The blue species of phosphomolybdate were reduced by ascorbic acid in the presence of antimonyl ions and adsorbed on to Sephadex G-25 gel beads. Attenuation at 836 and 416 nm (adsorption maximum and minimum wavelengths) was measured, and the difference was used to determine trace levels of phosphate. The effect of nitrate, sulfate, silicic acid, arsenate, aluminium, titanium, iron, manganese, copper, and humic acid on the determination were examined. 

For molybdenum oxide, for example, it is shown that the adsorption of 4% (in mass) of polyanihne is enough to enhance the electrode response, affecting the potential in which the formation of the so-called molybdenum blue is formed. For arsenic oxide, it is shown that the modification of the oxide surface by 13% polyaniline adsorption was responsible for enhancement of the oxide response to apphed potential as well as for a change in the potentials observed for oxidation-reduction processes occurring in the unmodified oxide. 

Many colloids other than gold can be removed from solution by the same method. As a lecture experiment the author uses the adsorption of molybdenum blue by animal charcoal. The dark blue liquid becomes quite colorless on shaking and filtering. As in many other cases the adsorption is quantitative and irreversible. ... 

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