Abstract: A kit and powder composition are described that can be used to determine a concentration of orthophosphate in a water system. The kit includes a first powder composition including molybdate; and a second powder composition including ascorbic acid. A powder composition is also described that includes molybdate, pyrosulfate, and less than wt. % ascorbic acid. A method for determining the concentration of orthophosphate in a water system using the kit includes adding the first and second powder compositions to a water sample collected from the water system, measuring an absorbance of the water sample after the first and second powder compositions have been added to the water sample, and determining the concentration of orthophosphate in the water system based on the measured absorbance.