A. Place 1 g, previously dried at 120°C for 4 hours, in a solution of 100 mg of strychnine sulfate R in 50 mL of water and shake for 5 minutes; filter, rejecting the first 10 mL of filtrate. To a 10-mL portion of the filtrate add 1 drop of hydrochloric acid (~420 g/l) TS and 5 drops of potassio-mercuric iodide TS; no turbidity is produced.
B. To each of two glass-stoppered 100-mL flasks transfer 50 mL of methylthioninium chloride (1 g/l) TS. To one of the flasks add 0.25 g, accurately weighed, of the test substance, insert the stopper in the flask and shake for 5 minutes. Filter the contents of each flask, rejecting the first 20 mL of each filtrate. Transfer 25-mL portions of the filtrates to two 250-mL volumetric flasks. Add to each flask 50 mL of sodium acetate (60 g/l) TS, mix, and add from a burette 35.0 mL of iodine (0.05 mol/l) VS, swirling the mixture during the addition. Stopper the flasks and allow them to stand for 50 minutes, shaking them vigorously at 10-minute intervals. Dilute each mixture with water to volume, mix, allow to stand for 10 minutes, and filter, rejecting the first 30 mL of each filtrate. Titrate the excess iodine in a 100-mL aliquot of each filtrate with sodium thiosulfate (0.1 mol/l) VS, adding 3 mL of starch TS towards the end of the titration. Calculate the number of mL of iodine (0.05 mol/l) VS consumed in each titration; the difference between the two volumes is not less than 0.7 mL.