Medium: 0.1 N hydrochloric acid; 900 mL.

Apparatus 2: 50 rpm.

Time: 20 minutes.

Procedure— Determine the amount of timolol maleate (C13H24N4O3S·C4H4O4) dissolved, employing the following procedure: Prepare a Standard solution of USP Timolol Maleate RS in 0.1 N hydrochloric acid having a known concentration of about 11 µg per mL. Filter a portion of the solution under test, and transfer 10.0 mL of the clear filtrate to a suitable separator. Transfer 10.0 mL each of the Standard solution and 0.1 N hydrochloric acid, to provide the blank, to individual separators, and treat each of three separators as follows: Add 20.0 mL of ethyl acetate, mix for 1 minute, allow the phases to separate, and filter the aqueous layer into a suitable vessel, retaining the ethyl acetate layer from the solution under test for the hydrochlorothiazide determination. Determine the amount of C13H24N4O3S·C4H4O4 dissolved from UV absorbances of the aqueous layer from the solution under test at the wavelength of maximum absorbance at about 293 nm in comparison with the aqueous layer from the Standard solution.

Tolerances— Not less than 80% (Q) of each of the labeled amounts of C13H24N4O3S·C4H4O4 and C7H8ClN3O4S2, respectively, is dissolved in 20 minutes.

pH 3.0 Buffer and Mobile phase— Proceed as directed under Assay.

Standard solution— Dissolve an accurately weighed quantity of USP Benzothiadiazine Related Compound A RS in methanol to obtain a solution having a known concentration of about 0.5 mg per mL. Transfer an accurately measured volume of this solution, and dilute quantitatively, and stepwise if necessary, with Mobile phase to obtain a solution having a known concentration of about 0.5 µg per mL.

Test solution— Use the Assay preparation prepared as directed in the Assay.

Chromatographic system— Proceed as directed under Assay, except to chromatograph the Standard solution: the relative standard deviation for replicate injections is not more than 5.0%.

Procedure— Separately inject equal volumes (about 50 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the peak areas. Calculate the percentage of benzothiadiazine related compound A in the portion of Tablets taken by the formula: in which C is the concentration in µg per mL, of USP Benzothiadiazine Related Compound A RS in the Standard solution; L is the amount, in mg, of hydrochlorothiazide in the portion of Tablets taken, based on the labeled amount; and rU and rS are the peak areas of benzothiadiazine related compound A obtained from the Test solution and Standard solution, respectively: not more than 1.0% is found.

(Official January 1, 2007)

pH 3.0 phosphate buffer— Dissolve 13.6 g of monobasic potassium phosphate in 100 mL of water, adjust with phosphoric acid to a pH of 3.0 ± 0.05, and filter.

Mobile phase— Prepare a suitable filtered and degassed mixture of water, acetonitrile, methanol, and pH 3.0 phosphate buffer (38:8:2:2), making adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Transfer about 50 mg of USP Timolol Maleate RS, accurately weighed, to a 500-mL volumetric flask. Add 50J mg of USP Hydrochlorothiazide RS, accurately weighed, J being the ratio of the labeled amount, in mg, of hydrochlorothiazide to the labeled amount, in mg, of timolol maleate per Tablet. Add 50 mL of 0.05 M monobasic sodium phosphate, and 125 mL of acetonitrile, sonicate for 4 minutes, dilute with water to volume, and mix. Pipet 5 mL into a 25-mL volumetric flask, dilute with acetonitrile solution (1 in 10) to volume, and mix.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 20 mg of timolol maleate, to a 1-liter volumetric flask, add about 100 mL of 0.05 M monobasic sodium phosphate, 125 mL of acetonitrile, and 100 mL of water, and mix by mechanical means. Allow to stand for 16 hours, dilute with water to volume, mix, and filter.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 295-nm detector and a 4-mm × 30-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph replicate injections of the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation is not more than 1.5%; and the resolution, R, between hydrochlorothiazide and timolol maleate is not less than 4.0.

Procedure— Separately inject equal volumes (about 50 µL) of the Standard preparation and the Assay preparation into the chromatograph by means of a suitable microsyringe or sampling valve, record the chromatograms, and measure the responses for the major peaks. The relative retention times are about 0.5 for benzothiadiazine related compound A, 0.6 for hydrochlorothiazide, and 1.0 for timolol maleate. Calculate the quantity, in mg, of hydrochlorothiazide (C7H8ClN3O4S2) in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Hydrochlorothiazide RS in the Standard preparation; and rU and rS are the responses of the hydrochlorothiazide peak obtained from the Assay preparation and the Standard preparation, respectively. Calculate the quantity, in mg, of timolol maleate (C17H28N4O7S) by the same formula, changing the terms to refer to timolol maleate.