PTCB Certified Pharmacy Technician (PTCE)MedicationsMedium
A pharmacy technician is preparing a non-sterile compound that requires the use of a Class A torsion balance. The minimum weighable quantity (MWQ) for this balance is 120 mg. If the formula calls for 10 mg of an active ingredient, what is the most appropriate method to accurately weigh this ingredient?
- AUse the aliquot method by weighing a larger quantity and diluting it.
- BRequest a more sensitive electronic balance for the measurement.
- CWeigh the 10 mg directly on the balance, as the sensitivity is sufficient.
- DRound the quantity up to 120 mg to meet the MWQ.
Show answer & explanationAnswer & explanation
Correct answer: A. Use the aliquot method by weighing a larger quantity and diluting it.
Since the desired quantity (10 mg) is less than the balance's minimum weighable quantity (MWQ) of 120 mg, direct weighing would lead to unacceptable error. The aliquot method involves weighing a larger, accurately measurable amount of the ingredient, diluting it with an inert substance, and then taking a portion (aliquot) of the mixture that contains the desired amount of the active ingredient.
Why the other options are wrong
- B. While a more sensitive balance might be an option if available, the aliquot method is a standard and acceptable technique when the required quantity is below the MWQ of the current equipment.
- C. Weighing 10 mg directly would be inaccurate as it is below the MWQ, leading to a significant percentage of error.
- D. Rounding the quantity up would significantly alter the dose of the active ingredient and is an unacceptable compounding practice, leading to an incorrect strength.
Aliquot Method
A technique used in compounding to accurately measure quantities of ingredients that are below the minimum weighable quantity (MWQ) or sensitivity limit of the available balance.
- Involves weighing a larger, measurable amount of the ingredient.
- Diluting it with an inert diluent (e.g., lactose) to create a mixture.
- Taking a calculated portion (aliquot) of the mixture that contains the desired quantity of the active ingredient.
Memory trick: Small amounts need careful plans, aliquot lends a helping hand.