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? Does meet the purity specification for use in a drug product as specified by the USP 23?

Answer 1

Ans. #Step 1: Let the tolnaftate content in 50.0 mg tablet = X ug

The solid drug is dissolved and diluted to a final volume of 100.0 mL (label it as solution 1).

So,

            [Drug] in solution 1 = X ug / 100 mL = 0.01X ug/mL

# 1.00 mL of solution 1 is diluted to 50.0 mL (label it as solution 2).

Now, using    C1V1 (solution 1) = C2V2 (solution 2)

            [Drug] in solution 2 = (0.01X ug mL^-1 x 1.0 mL) / 50.0 mL = 0.0002X ug mL^-1

# Step 2: Given- Abs of soln. 2 = 0.6490

            Abs of standard soln. = 0.6555      , [Drug] in standard soln. = 10.0 ug mL^-1

So,

            [Drug] in soln. 2 = ([Drug] in std. soln. / Abs of std. Soln.) x Abs of soln. 2

                                                = (10.0 ug mL^-1 / 0.6555) x 0.6490

                                                = 10.1001541 ug mL^-1

# From #step 1, [drug] in soln. 2 = 0.0002X ug mL^-1

            So, 0.0002X ug mL^-1 = 10.1001541 ug mL^-1

            Or, X = 10.1001541 / 0.0002 = 50500.7705

Therefore, total tolnaftate content of the solid sample = 50500.7705 ug = 50.5 mg

# Step 3: % purity of tolnaftate = (Experimental mass / Theoretical mass) x 100

                                                            = (50.5 mg / 50.0 mg) x 100 = 101.0 %

#Since the purity of drug is greater than 100%, the product is as pure as specified by the USP 23.