A standard analysis of drug tolnaftate as required in the USP 23 yields the foll
ID: 560908 • Letter: A
Question
A standard analysis of drug tolnaftate as required in the USP 23 yields the following data: Exactly 50.0mg of the tolnaftate sample is quantitatively transferred into a 100mL volumetric flask and adding enough methanol to fill the mark. After mixing well, 1.00mL of this solution is transferred to a 50-mL volumetric flask and mixed with enough additional methanol to 50.00mL of solution. The absorbance of this solution at 258nm is 0.6490. The absorbance of a previously prepared standard tolnaftate solution of exactly 10.0 ug/mL was 0.6555. What is the percent purity of the tolnaftate?
Explanation / Answer
And. Step 1: Given, absorbance of 10.0 ug/mL standard tolnaftate = 0.6555
# Now,
[tolnaftate] in final aliquot = (Abs of aliquot / Abs of std.) x Std. [tolnaftate]
= (0.6490 / 0.6555) x (10.0 ug/ mL)
= 9.9008 ug/ mL
# Step 2: Preparation of final aliquot from solid sample
Let the amount of pure drug in 50.0 mg sample = X ug
# Solution 1: 50.0 mg sample (= X ug drug) is dissolved in 100.0 mL volume.
So, [Drug] in solution 1 = X ug/ 100 mL = 0.01X / mL
# Solution 2 (final aliquot): 1.0 mL of solution 1 is diluted to 50.0 mL to make solution 2.
Using C1V1 (solution 1) = C2V2 (Solution 2)
Or, (0.01X x 1.0 mL) = C2 x 50.0 mL
Or, C2 = (0.01X x 1.0 mL) / 50.0 mL = 0.0002X/ mL
Therefore, [Drug] in solution 2 = 0.0002X/ mL
#Step 3: As calculated in #step 1, [Drug] in solution 2 or final aliquot = 9.9008 ug/ mL
Comparing [Drug] in solution 2 and [Drug] calculated in #step 1-
0.0002X/ mL = 9.9008 ug/ mL
Or, X = (9.9008 ug/ mL) / (0.0002 / mL) = 49504.0 ug
Hence, X = 49504.0 ug = 49.504 mg
Therefore, amount of pure drug in 50.0 mg solid sample = 49.504 mg
Now,
% Purity = (Mass of pure drug / Mass of sample) x 100
= (49.504 mg / 50.0 mg) x 100
= 99.008 %
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