Colorimetric analysis by the cadmium reduction 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 that couples with N-1-Napthylethylenediamine dihydrochloride to form a reddish purple azo dye. The dye is subsequently measured colorimetrically at 520 nm on a Lachat 8500 FIA. Acidified samples must be neutralized to a pH between 5 and 9.
(See Method References; High Range: No. 38, Low Range: No. 4 &5, IC: No. 51)