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Two methods are commonly used for the determination of phosphate.
A. Molybdenum blue method: Discussion. Orthophosphate and molybdate ions
condense in acidic solution to give molybdophosphoric acid (phosphomolybdic
acid), which upon selective reduction (say, with hydrazinium sulphate) produces
a blue colour, due to molybdenum blue of uncertain composition. The intensity
of the blue colour is proportional to the amount of phosphate initially
incorporated in the heteropoly acid. If the acidity at the time of reduction is
0.5 M in sulphuric acid and hydrazinium sulphate is the reductant, the resulting
blue complex exhibits maximum absorption at 820-830 nm.
B. Phosphovanadomolybdate method: Discussion. This second method is
considered to be slightly less sensitive than the previous molybdenum blue
method, but it has been particularly useful for phosphorus determinations
carried out by means of the Schoniger oxygen flask method (Section 3.31).
The phosphovanadomolybdate complex formed between the phosphate,
ammonium vanadate, and ammonium molybdate is bright yellow in colour and
its absorbance can be measured between 460 and 480 nm.
Reagents. Ammonium oanadate solution. Dissolve 2.5 g ammonium vanadate
(NH4VO3,) in 500 mL hot water, add 20 mL concentrated nitric acid and dilute
with water to 1 mL in a graduated flask.
Ammonium molybdate solution. Dissolve 50 g ammonium molybdate, (NH4)6
Mo7024.4H20, in warm water and dilute to 1 L in a graduated flask. Filter
the solution before use.
Procedure. Dissolve 0.4 g of the phosphate sample in 2.5M nitric acid to give
1 L in a graduated flask. Place a 10 mL aliquot of this solution in
graduated flask, add 50 mL water, 10 mL of the ammonium vanadate solution,
10 mL of the ammonium molybdate solution and dilute to the mark. Determine
the absorbance of this solution at 465 nm against a blank prepared in the same
manner, using 1 cm cells.
Prepare a series of standards from potassium dihydrogenphosphate covering
the range 0-2 mg phosphorus per 100 mL and containing the same concentration
of acid, ammonium vanadate, and ammonium molybdate as the previous
solution. Construct a calibration curve and use it to calculate the concentration
of phosphorus in the sample.