Molybdate colorimetry

Soluble, colloidal or particulate forms. Speciation based on molybdate colorimetry or chromatographic separation. 

In river water silica is present in the form of barely ionized silicic acid, H2Si03, in concentrations of 5 to 30 mg-l depending on the soil. The amount can be determined by flame emission spectrophotometry or by ammonium molybdate colorimetry. 

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. 

Elemental composition H 4.58%, P 46.94%, O 48.49%. The hypophosphite ion may he oxidized to orthophosphate by careful oxidation. The orthophophate, PO4 ion, may he measured by colorimetry either hy using ammonium molybdate and vanadium (yellow color), ammonium molybdate and stannous chloride (blue color), or ammonium molybdate, potassium anti-monyl tartrate and asorbic acid (an intense blue color). Absorbances of the solution are read at 400 (or 470), 650 (or 690) and 880 nm, respectively. Hypophosphite ion alternatively may be identified by ion chromatography. 

The orthophosphate anion, P04 can be analyzed readily in aqueous solution by either ion chromatography or colorimetry. The aqueous solution must be diluted for such analyses. In colorimetric measurement, the solution is treated with a reagent mixture containing ammonium molybdate and ammo-... 

Small quantities of phosphorus may be estimated quickly by the molybdate method, the amount of phosphomolybdate being estimated colorimetrieally by comparison in Nessler glasses or test-tubes with a standard prepared under conditions which are made identical as far as possible. An account of this estimation is given under Phosphoric Acid, p. 182. 

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