For determination of ammonia, the Nessler reagent is added to the sample under alkaline conditions to react with ammonia. The reaction between ammonia and the mercury-containing reagent causes a yellow 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 ammonia concentration. These reagents are designed to be used with samples that have an expected ammonia range of 0.0 to 2.5 mg/L (ppm) NH₃-N.