Determine the amount of C8H16O2dissolved by employing the following method.

Potassium phosphate buffer— Dissolve 6.8g of monobasic potassium phosphate and 1.7g of sodium hydroxide in 1.0Lof water.Adjust with phosphoric acid to a pHof 7.4±0.1.

Mobile phase— Prepare a mixture of Citrate buffer,Potassium phosphate buffer,and acetonitrile (35:35:30).Adjust with phosphoric acid to a pHof 3.0±0.1,and mix.Filter and degas.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard solution— Prepare a solution of USP Valproic Acid RSin the pH7.5Phosphate bufferused in the Buffer Stage,having a known concentration of about 0.12mg per mL.[NOTE—Avolume of acetonitrile not exceeding the 10.0%of the total volume may be used to dissolve the USP Valproic Acid RS.]

Test solution— If necessary,dilute a portion of each filtered solution under test with pH7.5Phosphate bufferused in the Buffer Stageto obtain a solution having a concentration of about 0.12mg per mL.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 210-nm detector and a 3.9-mm ×15-cm column that contains 4-µm packing L11.The flow rate is about 1.2mLper minute.Chromatograph the Standard solution,and record the peak responses as directed for Procedure:the column efficiency is not less than 1000theoretical plates;the tailing factor is not more that 2.0;and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 50µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of valproic acid (C8H16O2)dissolved by the formula: in which Cis the concentration,in mg per mL,of USP Valproic Acid RSin the Standard solution;Dis the dilution factor used to prepare the Test solution;and rUand rSare the peak areas of valproic acid obtained from the Test solutionand the Standard solution,respectively.

Mobile phase— Prepare a mixture of Citrate bufferand acetonitrile (7:3).Adjust with phosphoric acid to a pHof 3.0±0.1,and mix.Filter and degas.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Prepare a solution of USP Valproic Acid RSin Mobile phasehaving a known concentration of about 0.5mg per mL.

Assay preparation— Transfer a number of whole Tablets containing the equivalent of about 2500mg of valproic acid into a 250-mLvolumetric flask.Add 150mLof Mobile phase,and sonicate with frequent swirling for 30minutes or until the Tablets completely disintegrate.Allow the solution to cool down to room temperature,and then dilute withMobile phaseto volume.Transfer 5.0mLof the resulting solution to a 100-mLvolumetric flask.Dilute with Mobile phaseto volume,and mix.

Chromatographic system (seeChromatography á621ñ)— The liquid chromatograph is equipped with a 210-nm detector and a 3.9-mm ×15-cm column that contains 4-µm packing L11.The flow rate is about 0.9mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the column efficiency is not less than 1000theoretical plates;the tailing factor is not more than 2.0;and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 15µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of valproic acid (C8H16O2)in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Valproic Acid RSin the Standard preparation;and rUand rSare the peak areas of valproic acid obtained from the Assay preparationand the Standard preparation,respectively.