Practical Pharmaceutical Orientation Liquid measurement Lab (4) Shahen S. Mohammed BSC Pharmacy MSc Pharmaceutics

Liquid measurement Parallax error is one of the main sources of error in liquid measurements. Next figure illustrates that the liquid surface in a container is not even, and a ‘‘meniscus’’ is formed. For most liquids the surface is concave. The uneven surface is due to surface tension, and if not read properly, may result in an error commonly known as ‘‘parallax error.’’ When a container with graduations for volumes has a liquid in it, the reading must always be made at the bottom of the meniscus.

Common containers for liquid measurements include cylindrical and conical graduates for quantities of 500 ml or less, pipettes for quantities of one ml or less, and a medicinal dropper for still smaller quantities.

General Method for Aliquot Measurement of Liquids 1. If a very small volume, which cannot be measured directly, is needed, measure the least measurable quantity of that liquid. 2. Transfer that liquid to a beaker, cylinder or some large container. Find a multiple factor by dividing the volume actually measured with the volume originally desired. By using the multiple factor, determine the amount of diluent needed as follows:

Volume of liquid measured × The multiple number = Total volume of the stock solution Total volume of the stock solution − Volume of liquid measured initially = Volume of the diluent.

3. Mix the liquid and diluent thoroughly to obtain the stock solution. 4. From the stock solution, obtain an aliquot which contains the amount of liquid originally needed. This is determined by the following proportion: liquid quantity measured total volume of stock solution = liquid quantity needed X (a liquot from stock solution)

Procedure How would you measure 0.025 ml of a concentrate which is to be diluted to 50 ml following the measurement? You are provided with a 5 ml pipette with marking in units of 1 mL, a measuring cylinder, and a container to dispense the final product. 1. Since 0.025 ml cannot be measured and 1 ml is the minimum measurable quantity, measure one ml of the drug concentrate using the pipette and transfer this liquid to the measuring cylinder.

2. In order to make a stock solution, find the volume of water needed by making use of the multiple factor. The multiple factor in the present case is 1/0.025 = 40. The total amount of stock solution would be equal to the quantity of measured concentrate multiplied by the multiple number, i.e., 1 × 40 = 40. Therefore, the volume of water needed equals 40 − 1 = 39 ml.

3. Add 39 ml of water to the concentrate in the measuring cylinder or make up the total volume to 40 ml. Mix the solution well. From this solution, withdraw an aliquot which contains the required 0.025 ml of the concentrate and transfer to the final container. The aliquot is determined by using the following proportion: 1 mL of the concentrate 40 mL of the stock solution = 0.025 mL of the concentrate X mL of the aliquot = 1 mL 4. Place 1 mL of the aliquot in the final container, and add sufficient amount of purified water to fill to the calibrated mark of 50 mL.

Homework: 1. If 0.75 ml of a drug is needed and a pharmacist has a 10-mL graduated cylinder with markings from 1 to 10 ml in units of 1 ml, explain how the required quantity of 0.75 ml can be measured. Use water as a diluent. 2. A prescription requires 0.015 ml of a drug concentrate. Using a pipette with markings from one to ten in units of 1 ml and a 100-mL graduated cylinder, explain how you would obtain the required quantity of drug concentrate? Use water as diluents.