AOAC Official Method 941.21 Iodine in Ointments Titrimetric Method

(a) Free iodine.—Weigh (to 1 mg) ca 2 g ointment, and transfer to 250mLI flask. Melt onH2Obath (£70°C), add 30mLCHCl3, mix well, and then add 30 mL H2O. (All of base should be dissolved in CHCl3 before H2O is added.) Titrate with 0.1M Na2S2O3, 942.27 (see A.1.13), using starch indicator (mix ca 2 g finely powdered potato starch with cold H2O to thin paste; add ca 200 mL boiling H2O, stirring constantly, and immediately discontinue heating. Store under refrigeration. Prepare fresh weekly.) Approach end point dropwise, shaking flask vigorously to ensure that all I2 has been extracted from CHCl3 layer. 1 mL 0.1M Na2S2O3 = 0.01269 g I.
(b) Potassium iodide.—Pour liquids from free I2 determination,
(a), into 500 mL I flask, rinsing flask with 200 mLH2O, added in several portions. (It is desirable to maintain this volume within rather narrow
limits.) Add 0.5mL0.2% alcoholic p-ethoxychrysoidin indicator and 1–4 drops 0.1M NaOH (4 g/L) (to neutralize). (Aqueous layer should now be clear yellow.) Titrate with 0.1M AgNO3, 941.18 (see A.1.11), approaching end point dropwise and swirling frequently. (AgNO3 solution causes turbidity due to formation of colloidal AgI and development of reddish-brown color similar to that observed in over-titratedd Volhard determination. End point, which is produced by 1 drop AgNO3 solution, is characterized by flocculation of colloidal AgI and complete disappearance of reddish brown tinge, leaving almost clear, pale yellow supernate.) mL 0.1M AgNO3 – mL 0.1M Na2S2O3, (a) =mLconsumed by iodide originally present. 1mL0.1M
AgNO3 = 0.0166 g KI.

Reference: JAOAC 24, 833(1941).