color development in sample: place 35 mL or less of sample,containing 0.05 to 1.0 mg P,in a 50-mL volumetric flask. Add 10 mL vanadate-molybdate reagent and dilute to the mark with distilled water. prepare a blank in which 35 mL distilled water is substituted for the sample. after 10 min or more,measure absorbance of sample versus a blank at a wavelength of 400 to 490 nm,depending on sensitivity desired (see ¶ 2a above). the color is stable for days and its intensity is unaffected by variation in room temperature.