The solid sample is about 1g, which is dissolved in water, and then to 250ML. Take the solution of 20ml placed in iodine flask, 25ml lmol / L sulfate (12ml) and potassium iodide solution TS-192, immediately plug strictly, set for 5 min in the dark, the starch solution (TS-235 ml) in. Titration with 0.1mol/L sodium and sodium sulfate, and the blank test. Per 0.1mol/L ml sodium sodium sulfate solution is equivalent to sodium nitrite (NaC_lO₂) 2.261mg.