A: Chloramine T–trichloroacetic acid reagent—Prepare as directed forIdentification testBunderDigoxin Elixir (Oral Solution,Official June 1,2005).

B: The retention time of the major peak in the chromatogram of theAssay preparation corresponds to that in the chromatogram of theStandard preparation,as obtained in theAssay.

Medium: 0.1Nhydrochloric acid;500mL.[NOTE—Use the same batch of Dissolution Mediumthroughout the test.]

Apparatus 1: 120rpm.

Time: 60minutes.

Ascorbic acid–methanol solution— Prepare a solution containing 2mg of ascorbic acid per mLof methanol.

Hydrogen peroxide–methanol solution— On the day of use,dilute 2.0mLof recently assayed 30percent hydrogen peroxide with methanol to 100mL.Store in a refrigerator.Just prior to use,dilute 2.0mLof this solution with methanol to 100mL.

Standard solutions— Transfer about 25mg of USP Digoxin RS,accurately weighed,to a 500-mLvolumetric flask.Dissolve in a minimum amount of alcohol,add dilute alcohol (4in 5)to volume,and mix.Dilute 10.0mLof this solution with dilute alcohol (4in 5)to 100.0mL,and mix.Just prior to use,dilute suitable aliquots of the resulting solution with Dissolution Mediumto 50.0mLto prepare Standard solutionsequivalent to 20%,40%,60%,80%,and 100%,respectively,of the labeled amount of digoxin in 500mL.

Test solution— Promptly after withdrawal,pass a portion of the solution under test through a filter having a 0.8-µm or finer porosity,discarding the first 10mLof the filtrate.This is the Test solution.

Procedure— Transfer to individual glass-stoppered flasks duplicate 1.0-mLportions of the Test solution,1.0-mLportions of each of the Standard solutions,and 1.0mLof the Mediumto provide a blank.Begin with the Standard solutions,and keep all flasks in the same sequence throughout,so that the elapsed time from addition of reagents to reading of fluorescence is the same for each flask in the set.Treating one flask at a time,add the following three reagents,in the order named,in as rapid a sequence as possible,swirling after each addition:1.0mLof Ascorbic acid–methanol solution,5.0mLof hydrochloric acid,and 1.0mLof Hydrogen peroxide–methanol solution.Insert the stoppers in the flasks,and after 2hours,measure the fluorescence at about 485nm,the excitation wavelength being about 372nm.To check the stability of the fluorometer,repeat the measurement of fluorescence on one or more treated Standard solutions.Correct each reading for the blank,and plot a standard curve of fluorescence versus percentage dissolution.Determine the percentage dissolution of digoxin in the Test solutionby reading from the standard graph.

Tolerances— Not less than 80%(Q)of the labeled amount of C41H64O14is dissolved in 60minutes.The requirement is met if the quantities dissolved from the Tablets tested conform to the accompanying acceptance table instead of the table shown under Dissolution á711ñ.

Mobile phase,Chromatographic system,and System suitability preparation— Proceed as directed in theAssay underDigoxin.

Standard preparation— Dissolve an accurately weighed quantity of USP Digoxin RSin diluted alcohol,and dilute quantitatively and stepwise with diluted alcohol to obtain a solution having a known concentration of about 40µg per mL.Use a sonic bath to aid dissolution.

Assay preparation— Weigh and finely powder not fewer than 20Tablets.Transfer an accurately weighed portion of the powder,equivalent to about 1mg of digoxin,to a glass-stoppered,50-mLconical flask.Add 25.0mLof diluted alcohol with swirling,sonicate for about 30minutes,and cool.Filter a portion of this solution through a 0.8-µm porosity membrane filter,discarding the first 10mLof the filtrate.

Procedure— Separately inject equal volumes (about 50µL)of theStandard preparation and theAssay preparation into the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C41H64O14in the portion of Tablets taken by the formula: in whichCis the concentration,in mg per mL,of USP Digoxin RSin theStandard preparation;andrUandrSare the responses for the digoxin peaks obtained from theAssay preparation and theStandard preparation,respectively.