The orthosphosphate level in mg/L (or ppm) is determined by a colorimetric method. Ammonium molybdate and potassium antimonyl tartrate react in acid medium with orthophosphate to form a phosphomolybdate complex, that is reduced to intensely colored molybdenum blue by ascorbic acid. The color intensity of the solution determines the phosphate concentration. Phosphates can be classified as ortho, condensed or organically bound. As with existing test kits on the market, the HANNA Phosphate Test Kit will only determine ortho phosphate levels.