THE CASE OF SULFAMETHOXAZOLE

Regarding tablet powder, the reasoning remains the same but excipients must be taken into account.

One Bactrim® tablet weighs 509.4 mg

Looking at the manufacturer’s note, this tablet contains 400 mg of sulfomethoxazole with the remainder consisting of excipients. This must be taken into account when testing a sample of tablet powder.

Thus: for a test sample of 27.4 mg of Bactrim® powder, one should theoretically have 21.5 mg of sulfamethoxazole.
The tablet powder stock solution and the 1/20 dilution resulting in the working solution are prepared.