AOAC Official Method 931.12 Calomel in Ointments
Titrimetric Method
First Action 1931
Final Action
Accurately weigh ca 1 g ointment, transfer to 250 mL glass-stoppered Erlenmeyer, and treat with ca 50 mL CHCl3. When base is dissolved, decant through dry, closely packed glass fiber filter in Gooch crucible, using light suction. Wash flask and contents several times with 20–30mLportions CHCl3, decanting through glass fiber. Let any residual CHCl3 in flask evaporate, and transfer glass fiber mat and contents to flask, wiping sides of crucible and mouth of flask with damp piece of filter paper and adding it to flask. Add 2.5 g KI and 30 mL standard 0.1M I2, 939.13 (see A.1.07) (standardized against Na2S2O3), stopper, and mix well. Let stand ca 1.5 h or until solution of calomel is complete, agitating frequently and fairly vigorously. Titrate with 0.1M Na2S2O3, 942.27B (see A.1.13), adding 1 or 2 mL excess and using starch indicator, (mix ca 2 g finely powdered potato starch with cold H2O to thin paste, add ca 200 mL boiling H2O with stirring, and discontinue heating. Store under refrigeration. Prepare fresh weekly.) When all traces of I2 disappear, back-titrate with standard I2 solution to blue color. 1 mL 0.1M I2 = 0.02360 g Hg2Cl2.
Reference: JAOAC 14, 312(1931).
CAS-7546-30-7 (calomel)
Revised: March 1996
