Nitrates are extracted by shaking 300 mg of dried plant material with 30 mL 0.1 m CaSO4 solution for 30 min. Add 0.85 cc of prewashed charcoal to each sample and continue shaking for 5 additional minutes. Samples are centrifuged or filtered through a Whatman No. 42 filter paper. Nitrate concentrations in the filtrate are determined colorimetrically by the cadmium reduction method. In this method, nitrate is reduced to nitrite in a copperized cadmium column. The nitrite ions react with sulfanilamide under acidic conditions to form a diazo compound. This couples with N-1-Napthylethylenediamine dihydrochloride to form a reddish purple azo dye which is measured at 520 nm on an Lachat 8500 FIA.
(See Method References No. 56 and 5)