Method of assessing the amount of nitrite, the nitrogen oxoanion chemical formula NO2(-), in a solution by treating with a diazotizing reagent, e.g. sulfanilamide (SA), in acidic media to form a transient diazonium salt which is then allowed to react with a coupling reagent, N-naphthyl-ethylenediamine (NED), to form a stable azo compound with an intense purple color that absorbs light at 540 nm. Measurement of nitrite is used to estimate nitric oxide (NO) production in biological systems.