Minimum weighable quantity & aliquot planner
Determine the smallest quantity a balance can weigh within an acceptable error, and plan an aliquot when the required amount falls below it.
What is this for?
Every balance has a sensitivity requirement — the smallest weight that produces a measurable deflection. Weighing anything below the minimum weighable quantity means the error exceeds the acceptable tolerance, usually 5%. When a formula calls for less than the MWQ, you must weigh a larger amount, dilute it, and take a proportional aliquot.
How to use it
- Enter your balance's sensitivity requirement in milligrams. This is on the balance certificate — do not guess it.
- Enter the acceptable error rate as a percentage. USP <795> conventionally uses 5%.
- Optionally enter the weight the formula requires. If it falls below the MWQ, a step-by-step aliquot plan is generated.
Worked example
A Class III torsion balance has a sensitivity requirement of 6 mg. A formula requires 8 mg of drug. Can it be weighed directly?
Answer: MWQ = 6 ÷ 0.05 = 120 mg. 8 mg is far below that, so an aliquot is mandatory. Weigh 120 mg of drug, dilute to 1800 mg total with 1680 mg of lactose, then weigh 120 mg of the mixture — which contains 8 mg of drug.
Clinical pearls & pitfalls
- Enter the error rate as a percentage (5), not a decimal (0.05) — the calculator converts it. Entering 0.05 as the percentage would give an MWQ a hundred-fold too large.
- The aliquot itself must also be at or above the MWQ. A plan that has you weighing 30 mg of mixture on a balance with a 120 mg MWQ has simply moved the problem.
- Geometric dilution is what makes an aliquot valid. If the mixture is not homogeneous, the aliquot does not contain the calculated amount, no matter how precise the weighing.
- Electronic analytical balances have far lower sensitivity requirements than Class III torsion balances, and correspondingly lower MWQs. Use the figure for the balance you are actually using.
Assumptions & limitations
- Assumes a single-step aliquot. Extremely small targets may require a serial (multi-step) aliquot, which this planner does not generate.
- The plan assumes an inert, compatible diluent that does not interact with the drug. Compatibility is a formulation judgement the calculator cannot make.
- Does not address volumetric aliquots for liquids, which follow the same principle but use a graduate's minimum measurable volume.
References
- United States Pharmacopeia. General Chapter <795> Pharmaceutical Compounding — Nonsterile Preparations.
- United States Pharmacopeia. General Chapter <1176> Prescription Balances and Volumetric Apparatus.
- Allen LV. The Art, Science, and Technology of Pharmaceutical Compounding. American Pharmacists Association.