Weighing

It is generally agreed that pharmaceutical products should be prepared with a low percentage of error. The official compendia allow a tolerance of ± 5% for most formulas, although greater accuracy may be required for very potent drugs with greater toxicity potential. This same degree of accuracy is expected in all extemporaneously compounded products.

Most pharmaceutical products allow for a tolerance of only 5% error, where

If we know the sensitivity of the balance (i.e. the potential error) we can calculate the percentage of possible error when any amount of the substance is weighed.

e.g. The Class A prescription balance has a sensitivity of 6 mg. What % of error would result in weighing 50 mg of a drug on the balance?

Similarly, we can calculate the smallest quantity that can be weighed, on a balance of known sensitivity, to maintain a desired level of accuracy. This weight is referred to as the least weighable quantity (L.W.Q.).

e.g. What is the least weighable quantity that will result in an error of 5% or less on a Class A prescription balance?

You should keep this figure in mind for the remainder of your career. When a prescription formula calls for the incorporation of a component weighing less than 120 mg, special methods must be employed to obtain that weight of the component. If a liquid dosage form (solution, suspension or emulsion) is being prepared, the liquid aliquot method is employed. When the component must be incorporated as a solid into powders, tablets, capsules, or pastes, the trituration method is used.