Molybdovanadic reagent

The limit test for phosphate is based upon the formation of ayellow colour reaction with molybdovanadic reagent (combination of ammonium vanadate and ammonium molybdate) in an acidic medium. However, the exact composition of the molybdovanadophosphoric acid complex is yet to be established. 

Barium Sulphate To 1.0 g add a mixture of 3 ml 2M HNOs and 7 ml DW, heat on a water-bath for 5 minutes, filter, dilute the filtrate to 10 ml with DW, add 5 ml molybdovanadic reagent and allow to stand for 5 minutes. Any yellow colour produced is not more intense than that of a standard prepared simultaneously and in the same manner using 10 ml of phosphate standard solution (= 5 ppm P04) (50 ppm). 

Molybdovanadic Reagent Suspend 4.0 g of finely powdered ammonium molybdate and 0.1 g of finely powdered ammonium metavanadate in 70 ml DW and grind until dissolved. Add 20 ml of HNOs and dilute to 100 ml with DW. 

The test is simply to mix 1 ml of test solution with 2 ml of molybdovanadic reagent R upon which a yellow color is formed. The nature of tiie substance formed has not been fully elucidated and it has been referred to as an acid, a complex, and in some cases a salt. One reference gives the formula below. 

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