NOTE—Throughout the following procedures, protect test or assay specimens, the Reference Standard, and solutions containing them, by conducting the procedures without delay, under subdued light, or using low-actinic glassware.

Medium: 0.01 N hydrochloric acid; 1000 mL.

Apparatus 2: 75 rpm.

Time: 60 minutes.

Procedure— Determine the amount of C21H26N2S2·HCl dissolved by employing UV absorption at the wavelength of maximum absorbance at about 262 nm on filtered portions of the solution under test, suitably diluted with Dissolution Medium, if necessary, in comparison with a Standard solution having a known concentration of USP Thioridazine Hydrochloride RS in the same Medium.

Tolerances— Not less than 75% (Q) of the labeled amount of C21H26N2S2·HCl is dissolved in 60 minutes.

(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture of acetonitrile, water, and triethylamine (850:150:1). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Thioridazine Hydrochloride RS in methanol with the aid of sonication, and dilute quantitatively and stepwise, if necessary, with methanol to obtain a solution having a known concentration of about 125 µg per mL.

Assay preparation— Weigh and finely powder not less than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 100 mg of thioridazine hydrochloride, to a 100-mL volumetric flask. Add about 80 mL of methanol, and shake by mechanical means for 30 minutes. Dilute with methanol to volume, and sonicate for 45 minutes with intermittent shaking. Allow the undissolved solids to settle, and filter, discarding the first 20 mL of the filtrate. Transfer 25.0 mL of the clear filtrate to a 200-mL volumetric flask, dilute with methanol to volume, and mix. Filter through a 0.45-µm disk before injecting into the chromatograph.

System suitability preparation— Dissolve 100 mg of mesoridazine besylate in 100 mL of methanol. Mix 1.0 mL of this solution with 9.0 mL of the Standard preparation.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 265-nm detector and a 4.6-mm × 25-cm column that contains packing L1. The flow rate is about 2.5 mL per minute. Chromatograph the Standard preparation and the System suitability preparation, and record the peak responses as directed for Procedure: the resolution, R, between the mesoridazine and thioridazine peaks is not less than 1.0, and the relative standard deviation for replicate injections of the Standard preparation is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of C21H26N2S2·HCl in the portion of Tablets taken by the formula: in which C is the concentration, in µg per mL, of USP Thioridazine Hydrochloride RS in the Standard preparation, and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.