Why did you have to treat the solution with ammonium vanadomolybdate to use this method?

This ammonium vanadomolybdate reagent will react with phosphates to produce a yellow heteropoly acid compound with an uncertain formula (though it is thought to be a (NH4)3PO4?NH4VO3?16MoO3.) This is the compound which is used colorimetrically for analytical work.This characteristic yellow color gives the indication of the presence of phosphate content. Phosphate is colorless and cannot be determined easily from water samples that contains the usual spectrophotometric techniques, which dictates the concentration based on color intensity.