Assay—
[NOTE—Use low-actinic flasks in preparing the solutions,and otherwise protect the solutions from unnecessary exposure to bright light.Complete the assay without prolonged interruption.
]Place a number of Tablets,equivalent to 500mg of pyrvinium,in a 500-mLvolumetric flask,and add 25mLof water and 25mLof acetone.Completely disintegrate the tablets by heating on the steam bath for 10minutes with frequent mixing,allowing part of the acetone to boil off slowly.To the hot mixture add 250mLof glacial acetic acid,and mix occasionally for 5minutes without further heating,to dissolve the pyrvinium pamoate.Dilute with methanol to volume at room temperature,and mix.Centrifuge a portion of the mixture until a clear solution is obtained.Transfer 3mLof the clear supernatant to a 500-mLvolumetric flask,dilute with methanol to volume,and mix.Dissolve an accurately weighed quantity of
USP Pyrvinium Pamoate RSin glacial acetic acid,using 1mLfor each 3mg taken,and dilute quantitatively and stepwise with methanol to obtain a Standard solution having a known concentration of about 9µg per mL.Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 505nm,with a suitable spectrophotometer,using methanol as the blank.Calculate the quantity,in mg,of pyrvinium (C
26H
28N
3+)in the portion of Tablets taken by the formula:
83.3C(0.6644AU/AS),
in which
Cis the concentration,in µg per mL,of
USP Pyrvinium Pamoate RSin the Standard solution,calculated on the anhydrous basis,0.6644is the ratio of the molecular weight of pyrvinium to one-half the molecular weight of pyrvinium pamoate,and
AUand
ASare the absorbances of the solution from the Tablets and the Standard solution,respectively.