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?

Answer 1

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 %