Weighing Dr. Amani M. Elsayed

Learning Objectives Upon completion of this exercise, you should be able to: Define percentage error and state the percentage error tolerated in most prescription formulas. Calculate the potential percentage error for weighing a stated amount of a substance given the sensitivity of the balance. Define least weighable quantity. Calculate the least weighable quantity for any balance, given the allowed percentage error and the sensitivity of the balance. Describe and/or demonstrate the proper techniques for using the Class A prescription balance and corresponding weights to measure solid or liquid substances with an error of measurement not to exceed ± 5%.

Percentage of Error 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.

Percentage of Error If we know the sensitivity of the 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. The Class A Balance has a sensitivity requirement of 6 mg, meaning that as much as 6 mg could be added to or removed from the pan before the balance marker will move 1 division.

Percentage of Error What % of error would result in weighing 50 mg of a drug on the balance?

Least Weighable Quantity 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.).

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

Least Weighable Quantity 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.

Class A Prescription Balance  The Class A Prescription Balance is a 2 pan torsion type balance which utilizes both internal and external weights.  It requires the use of external weights for measurements exceeding 1 g.  This balance is less sensitive than the analytical balances.  No less than 120 mg may be weighed on the balance to maintain an error of < 5%.

Balance Weights  A proper set of metric weights is essential for prescription compounding.  These sets usually contain cylindrical weights ranging from 1-50 g and fractional weights of mg.  Weights should be stored in a special box and must be handled with forceps, NOT with the fingers to prevent soiling and erosion of the weights.  Apothecary weight sets are also available and are convenient to use when the prescription is written in the Apothecary system.

Quiz Complete the following sentence: 1)In weighing, The official compendia allow an error tolerance of )The least weighable quantity is the )The sensitivity of the balance is the