Limit of nitrates
Standard stock solution
Dissolve about 815 mg of potassium nitrate in 500 mL of water, and mix.
Dilute 1 mL of the Standard stock solution with water to 10 mL, and mix. Dilute 1 mL of this solution with water to 50 mL to obtain a solution containing 2 ppm of nitrate.
Transfer 5 g of Adipic Acid to a 50-mL volumetric flask. Dissolve in water, with heating, and dilute with water to volume. Allow to cool and crystallize, then pass through a sintered-glass filter. Wash the filter with water, and collect the filtrate and washings until a volume of 50 mL is obtained. [NOTEThis solution is to be used in the tests for Chloride, Sulfate, Iron, and Heavy metals.]
To 1.0 mL of the Test solution and to 1.5 mL of the Standard solution, separately add 2 mL of concentrated ammonia, 0.5 mL of manganese sulfate solution (1 in 100), and 1 mL of sulfanilamide solution (1 in 100), and dilute with water to 20 mL. Add 100 mg of zinc powder, and cool in an ice bath for 30 minutes, shaking periodically. Filter 10 mL of the solution, cool in an ice bath, and then add 2.5 mL of hydrochloric acid and 1 mL of a naphthylethylenediamine dihydrochloride solution (1 in 100). Allow to stand at room temperature for 15 minutes: the color of the Test solution is not darker than a concomitantly prepared Standard solution; the limit is 0.003%. The test is invalid if a concomitantly prepared blank solution (prepared using 1 mL of water instead of 1 mL of the Test solution) is darker than a solution containing 2 mg of potassium permanganate per L.