[clevermartin]

In the Molybdate reactive phosphous method, reducing solution made of NaHSO₃, sodium sulfate and 1-amino-2-naphthol-4-sulphonic acid was used to reduce the phosphomolybdate complex into phosphate molybdenum complex for phosphorus content determination.

I want to ask is there any other method or reducing agent which can do the same reduction process correctly?

Thanks

[el13]

DETERMINATION OF PHOSPHORUS (AS P2O5)
Reagents and Equipment
1. Ammonium molybdate stock solution: Dissolve 12.5 g of ammonium molybdate {(NH4)6Mo7O24• 4H2O} in 340 mL of distilled water. Then, add 160 mL of 9 M sulfuric acid.
2. Dilute ammonium molybdate solution: Dilute 20 mL of stock ammonium molybdate solution to 1 L.
3. Stannous chloride solution: Dissolve 0.600 g of stannous chloride {SnCl2• 2H2O} in 25 mL of concentrated hydrochloric acid and dilute to 1 L with distilled water. This solution should be freshly prepared just prior to use.
4. Ultraviolet-visible spectrophotometer, with 1-cm cell.
Procedure
1. Transfer 10-mL aliquots of the blank, standards, and the sample solution to a series of 100-mL polyethylene beakers.
2. Add 25 mL of the dilute molybdate solution, and let the mixture sit for 10 min.
3. Add 25 mL of the stannous chloride solution. The developed color is stable for approximately 40 min.
4. Set the concentration scale of the spectrophotometer to zero for a wavelength of 640 nm with the blank solution in the absorption cell. Then, set the concentration scale with a solution having a known concentration of phosphorus pentoxide (standard silicate reference material), and determine directly the percent phosphorus pentoxide in each of the samples.

OR,
Ammonium molybdate and antimony potassium tartrate react in an acid
medium with dilute solutions of phosphorus to form an
antimony-phospho-molybdate complex. This complex is reduced to an
intensely blue-colored complex by ascorbic acid. The color is proportional to
the phosphorus concentration.This is from Method 365.3.And I have more.