硝酸盐氮经镉柱还原为亚硝酸盐氮，亚硝酸盐氮在中性条件下与酸性显色剂（磺胺-盐酸萘乙二胺）反应生成玫红色的重氮-偶合物，在波长550 nm处测量其吸光度，其吸光度与硝酸盐氮和亚硝酸盐氮质量浓度的和在3.00 mg·L^-1以内呈线性关系。据此原理采用连续流动注射法测定地表水和海水中硝酸盐氮的含量，每小时可测定60个样品。选用pH 7.50±0.05的0.075 mol·L^-1咪唑缓冲溶液作为缓冲剂，所用镉柱为由粒径0.3~0.8 mm的镉粒填装的有效长度为10 cm的镉圈，使用前须按如下顺序激活（于2 mol·L^-1盐酸溶液中1 min，放入0.005 mol·L^-1硫酸铜激活溶液中2 min，再放入2 mol·L^-1盐酸溶液中5 min）。方法的检出限（3s）为0.000 1 mg·L^-1，方法用于两种标准物质的分析，测定值与认定值相符。方法用于水样的分析，并进行加标回收试验，测得回收率在99.0%~102%之间，测定值的相对标准偏差（n=6）小于2.5%。

Nitrate nitrogen can be reduced to nitrite nitrogen by cadmium column, and nitrite nitrogen produced can react with a acidic color reagent composed of sulfanilamide and N-(1-naphthyl)-ethylene diamine in neutral medium to form a rose-red colored diazo compound having its absorption maximum at 550 nm. Linear relationship between values of absorbane of the colored diazo compound and the sum of mass concentration of nitrate nitrogen and nitrite nitrogen was found within 3.00 mg·L^-1. Base on this phenomenon, continuous flow injection method was applied to the determination of nitrate nitrogen in surface water and seawater. Sixty samples could be analyzed in an hour. Imidazole solution (0.075 mol·L^-1, pH 7.50±0.05) was used as buffer in the determination. Granular cadmium with granularity of 0.3 to 0.8 mm packed in a coil tubing to an effective length of 10 cm was used as cadmium column, and it should be activated in the following manner before use:dipping successively in 2 mol·L^-1 HCl solution for 1 min, in 0.005 mol·L^-1 CuSO₄ solution for 2 min and in 2 mol·L^-1 HCl solution again for 5 min. Detection limit (3s) of the method found was 0.000 1 mg·L^-1. Two CRMs were analyzed by the method, giving results in consistency with the certified values. Water samples were analyzed by the method and used as matrixes for recovery by standard addition method, the results obtained were in the range of 99.0%-102%, and values of RSDs (n=6) found were less than 2.5%.