Calculate the %w/v of the drug in the eye drops.

Question

A 2 ml volume of eye drops containing the local anaesthetic proxymetacaine. HCl is diluted to 100 ml and then 5 ml of the dilution is diluted to 200 ml.

The diluted sample was measured by UV spectrophotometry and was found to contain 0.512 mg/100 ml of the drug.

Calculate the %w/v of the drug in the eye drops.

Answer 1

As 0.512 mg is in 100 mL, and this came from a 200 mL flask, the amount in 200 mL is 0.513 mg/100mL x 200 mL = 1.024 mg. This amount came from 5 mL, so the concentration in the 5 mL is 1.024mg/5 mL = 0.2048 mg/mL. As this came from a 100 mL flask, the total amount in 100 mL is 0.2048 mg x 100mL = 20.48 mg. Finally, this amount was in the 2 mL volume, so the concentration of the drug in the original solution is 20.48mg / 2 mL or 10.2 mg / mL