For determination of phosphate, the reagent containing ammonium molybdate and potassium antimonyl tartrate reacts in an acid medium with orthophosphate to form a phosphomolybdate complex. The phosphomolybdate complex is reduced by ascorbic acid, causing a blue colored complex to form; the greater the concentration, the deeper the color. A color comparator cube is included for an accurate, side-by-side, visual analysis to determine phosphate concentration. These reagents are designed to be used with samples that have an expected phosphate range of 0 to 5 mg/L (ppm) PO₄^3-.