A method of testing the composition or quality of water includes dissolving about two parts by weight of a 2-hydroxybenzyl alcohol and about one part by weight of a sodium nitroprusside in about seventy-five parts by weight of a polyethylene glycol; adding the sodium nitroprusside solution to a water sample to catalyze an indophenol monochloramine reaction; and detecting the concentration of monochloramine in the water sample. The polyethylene glycol may be a polyethylene glycol 300, a PEG-400 or a PEG-1000. The method may also be accomplished with a solution made by dissolving a 2-hydroxybenzyl alcohol and a sodium nitroprusside in a mixture of propane-1,2-diol and water.