Hygienic-chemical Studies on Public Water. VI. : Determination of Nitrate Nitrogen in Water by Sodium Salicylic Acid Method

It was found that the sodium salicylic acid method is more excellent in point of reproducibility and simplicity than the other methods for the determination of nitrate nitrogen in water. Furthermore, the authors improved the precise conditions of procedures of the method, for example, the prevention of the interference of chloride ion and nitrite ion. The interference by chloride ion was prevented by the addition of the excess quantity of sodium chloride and that by nitrite nitrogen was also prevented by the addition of ammonium sulfamate. It was also ascertained that the method is not interfered by 50 ppm of ferrous or ferric ion, 2000 ppm of calcium and magnesium salts, 250 ppm of ammonia nitrogen and 500 ppm of free chlorine. The details of the procedure are as follows. Take V ml of the test solution containing 2∼20 μg of nitrate nitrogen and 2 ml of the standard solution, in which 20 μg of nitrate nitrogen are contained, in the 100 ml beaker respectively. To each solution add 1 ml of sodium salicylic acid-sodium hydroxide solution, which is prepared by dissolving 1 g of sodium salicylic acid in 100 ml of 0.01 N sodium hydroxide solution, 1 ml of ammonium sulfamate aqueous solution (0.1→100) and 1 ml of sodium chloride aqueous solution (0.2→100), then evaporate the solution to dryness on the water bath. After cooling to room temperature, and 2 ml of concentrated sulfuric acid, agitate gently sometimes and allow to stand for ten∼twenty minutes. When the large quantity of the residue remains by evaporation, it might be better that the sulfuric acid solution is heated on the water bath for ten∼twenty minutes. The absorbance is not influenced by heating. After adding carefully 10 ml of distilled water, transfer the solution to the 50 ml Nessler tube and add dropwise 10 ml of the sodium hydroxide solution (40→100) under cooling, then dilute to 25 ml with water. Measure the absorbance A obtained from the test solution and As obtained from the standard solution at the wave length of 410 mμ against the blank solution procedured in the same way. Concentration of nitrate nitrogen (C ppm) in the test solution was given by the following formula. [numerical formula]