1. Practical Pharmaceutical Orientation
Weighing and Measuring
Lab (1)
Shahen S. Mohammed
MSc Pharmaceutics

2. Weighing and Measuring
Weighing and measuring are two of the most important aspects of dispensing, compounding, and administration of medications.
These laboratories are deal with the
fundamental operation of a prescription
balance, an understanding of sensitivity
requirement, an introduction to various
devices used for measuring volumes of
liquids, and their pertinent calculations.

3. Prescription Balance and Sensitivity Requirements
It is essential to learn proper weighing
techniques, and to be conscious of the
limitations of the balance to weigh certain
quantities.

4. Weighing paper A weighing paper has a glazed surface and
is square in shape. The paper protects the
pans of the prescription balance from chemical
reactions and eliminates the need for repeated
cleaning. It also prevents contamination.
The paper for weighing should be of
reasonable size to fit in the pan of the balance.
It should be creased diagonally before placing
on the pan.

5. It is a good practice to have a paper on each of the pans and to zero the balance before beginning the weighing operation. This zeroing is important because individual weighing papers from the same box may not be of same weight.
For weighing liquids, small beakers or
plastic weighing cups may be used.

6. Sensitivity requirement
Sensitivity requirement is the minimum weight required to move the pointer by one division on the scale. The smaller the weight required to move the pointer by one division, the more sensitive is the balance.
Sensitivity requirement (SR) is related to the percent error or maximum potential error by the equation,
%Error = SR × 100/quantity to be weighed (Q)

7. Aliquot Method for Solids
If the quantity needed is greater than the balance’s capacity, the material may be weighed in portions. For example, if 160 g of an ointment is to be measured on the Class A prescription balance (which has an upper limit of 120 g), 80 g of the ointment can be weighed twice, or 40 g can be weighed four times, depending upon the sensitivity requirement of the balance.
If the desired quantity of ingredients is below the lower limits of balance, the aliquot method of weighing is preferred.

8. An aliquot is a whole number part of a given quantity
An aliquot is a whole number part of a given quantity. For example, five is an aliquot of ten. To be more specific, five is the second aliquot of ten, two is the fifth aliquot of ten, four is the fifth aliquot of twenty, and so on.
Diluents or fillers are bulking substances. These make up the major portion of a tablet or capsule. Sucrose, lactose, mannitol and starch are examples of diluent.

9. In the aliquot method of weighing a small quantity, the minimum weighable amount (also called the least weighable quantity or LWQ) of that compound is weighed and mixed with a certain quantity of an inert material such as lactose or starch to obtain a stock mixture. After thoroughly mixing the drug and the inert material by employing the technique of geometric dilution, weigh an aliquot of the stock mixture which provides the desired quantity of the material.

10. The geometric dilution or mixing technique is useful for uniformly mixing ingredients of unequal amounts. For example, if 2 g of aspirin is to be mixed with 14 g of lactose, mixing should be performed by geometric dilution as follows: mix 2 g of aspirin with approximately 2 g of lactose. To this 4 g mixture, add 4 g of lactose and thoroughly mix the mixture which is now 8 g. Finally, add the remaining 8 g of lactose to the 8 g of mixture, and mix thoroughly.

11. Systems of Weight Measurement
1. Metric System ( e.g. kg, g, mg, µg,…) 2. Apothecaries ( e.g. pound, ounce, grain,…)

18. Procedure
A prescription balance has a sensitivity requirement of 0.2 g. Explain how you would weigh 1 g of sodium chloride with an accuracy of ± 5%, using mannitol as the diluent.
1. Weigh 5 g of sodium chloride.
2. Weigh 15 g of mannitol.
3. Mix sodium chloride and mannitol by geometric dilution.
4. Take an aliquot from the mixture that contain the desired amount of the sodium chloride.