Solved Problem: How would a pharmacist weigh 40 mg of atropine sulfate on a prescription balance with a SR of 6 mg and a maximum acceptable percentage error of 5 %?

Approach: Preliminary step: Determine the MWQ for the balance.

Step-1: Weigh out a measureable multiple of desired quantity of the drug.

The prescription requires 40 mg of drug. The minimum amount that can be weighed is 120 mg. Choose a multiple factor of 3 and weight out 40 X 3 = 120 mg of drug.

Step-2: Dilute with an inert substance to obtain a mixture.

120 mg is the smallest amount of drug that can be weight out.

120 X 3 = 360 mg of mixture

360 g mixture – 120 mg of atropine sulfate = 240 mg of diluent.

Dilute 120 mg of atropine sulfate with 240 g of lactose to obtain a 360 g mixture.

Step-3: Weigh an aliquot of the mixture that will provide desired quantity of the substance.

Multiple factor used: 3

360/3 = 120 mg

Answer: 120 mg of aliquot contains 40 mg of atropine sulfate.

Solved Problem: A pharmacist needs to weigh 5 mg of a drug for a prescription on a balance with a SR of 6 mg and a maximum acceptable percentage error of 5 %. What is the a) the minimum weighable quantity (if necessary round up to the nearest 5 mg), b) the quantity of drug to be weighed (use a multiple factor of 35), c) the quantity of diluent required and d) the quantity of the aliquot containing the desired amount of drug?

Approach: Preliminary step: Determine the MWQ for the balance.

Step-1: Weigh out a measureable multiple (in this case 35) of desired quantity of the drug.

The prescription requires 5 mg of drug. The minimum amount that can be weighed is 120 mg. Choose a multiple factor of 35 and weight out 5 X 35 = 175 mg of drug.

Step-2: Dilute with an inert substance to obtain a mixture.

120 mg is the smallest amount of drug that can be weight out.

120 X 35 = 4200 mg of mixture

4200 g mixture – 175 mg of drug = 4025 mg of diluent.

Dilute 175 mg of atropine drug with 4025 g of lactose to obtain a 4200 g mixture.

Step-3: Weigh an aliquot of the mixture that will provide desired quantity of the substance.

Multiple factor used: 35

4200/35 = 120 mg

Answer: 120 mg of aliquot contains 5 mg of drug