PTCB Certified Pharmacy Technician (PTCE)MedicationsHard
A pharmacy technician is preparing a non-sterile compound that requires the use of a Class A torsion balance. The master formula calls for 100 mg of an active pharmaceutical ingredient (API). The minimum weighable quantity (MWQ) for this balance is 120 mg. Which of the following methods should the technician use to accurately weigh the API?
- ARecalibrate the balance to allow for weighing of 100 mg.
- BWeigh 100 mg directly, as it is close to the MWQ.
- CUse a volumetric flask to measure the API by volume instead of weight.
- DUse an aliquot method to weigh a larger quantity and dilute it.
Show answer & explanationAnswer & explanation
Correct answer: D. Use an aliquot method to weigh a larger quantity and dilute it.
Since the desired quantity (100 mg) is below the balance's minimum weighable quantity (120 mg), direct weighing would not be accurate. The aliquot method involves weighing a larger amount (an aliquot) that is above the MWQ, and then diluting it to achieve the desired smaller quantity, ensuring accuracy.
Why the other options are wrong
- A. A Class A torsion balance has a fixed MWQ; it cannot be recalibrated to weigh below this limit.
- B. Weighing below the MWQ introduces unacceptable error and is not permitted.
- C. APIs are typically weighed, not measured by volume, due to variations in density and powder characteristics; this method is inappropriate for solid ingredients.
Aliquot Method
A technique used in compounding to accurately measure quantities of an ingredient that are below the minimum weighable quantity (MWQ) of a balance.
- Involves weighing a larger quantity (aliquot) that is above the MWQ.
- The aliquot is then diluted with an inert diluent.
- A portion of the diluted mixture is then used to obtain the desired small quantity.
Memory trick: When 'Tiny Amounts' are needed, 'Aliquot' your way to 'Accuracy'.