Aliquot method and minimum weighable quantity
Minimum weighable quantity from balance sensitivity and acceptable error, and an aliquot plan for amounts below it.
Inputs
2 of 3mg
%
mg
Optional; at least the minimum weighable quantity
mg
When to use
- The amount to weigh is below the balance's minimum weighable quantity.
Pearls and pitfalls
- A Class A prescription balance commonly has a 6 mg sensitivity requirement; at 5% error the minimum weighable quantity is 120 mg.
- Use an inert diluent compatible with the drug and mix by geometric dilution.
Why use
Weighing less than the minimum weighable quantity gives an error larger than allowed; the aliquot method keeps every weighing above it.
MWQ = sensitivity requirement ÷ acceptable error; factor F ≥ MWQ ÷ desired amount; weigh F × desired, dilute to F × aliquot, weigh one aliquot