AOAC Official Method 940.32 Sulfides in Depilatory Powders
Titrimetric Method
First Action 1940
Final Action
Pipet 50 mL 0.1M As2O3 solution, 939.12B (see A.1.03), into 250 mL glass-stoppered volumetric flask. Weigh test sample containing <0.12 g sulfide calculated as H2S and transfer to flask, washing down any material on sides of flask with H2O. Add 20 mL HCl (1 + 1), stopper immediately, and shake vigorously until sample decomposes. (If sample contains CaCO3, slowly add the 20 mL acid through dropping funnel fitted with rubber stopper to fit flask. Shake gently, letting liberated CO2 bubble up through acid. When reaction subsides, drain remainder of acid into flask, remove funnel, stopper flask, and shake vigorously.)
Cool to room temperature and dilute to volume with H2O. Filter through dry paper into dry flask. Pipet 100 mL filtrate (test solution) into 300 mL Erlenmeyer; add 5 mL starch solution (mix ca 1 g soluble starch with enough cold H2O to make thin paste, and 100 mL boiling H2O, and boil ca 1 min with stirring), and enough I2 solution to form blue solution. Make alkaline withNaHCO3, adding 1–2 g excess.
Titrate to permanent blue with 0.1M I, 939.13A (see A.1.07). Subtract mL 0.1M I2 consumed in alkaline titration from mL 0.1M As2O3 present in aliquot. 1 mL 0.1M As2O3 = 0.005411 g CaS or 0.01271 g BaS.
References: JAOAC 23, 437(1940); 25, 113(1942); 27, 112(1944).
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