Aliquots

Question 1

What is error, precision, accuracy?

Answer 1

• Error – deviation of the observed from the true value. 
• systematic (determinate) arising from calibration mistakes, impurities etc).
• additive (magnitude independent of sample size ie constant error)
• proportional (error changes with sample size ie incorrect stock solution strength).
• Precision – defined as standard deviation divided by the mean x 100 = %. May be referred to as the coefficient of variation. Little variation in repeat measurements.
• Accuracy – correct or truthful measurement.
• Accuracy and precision in compounding & dispensing are vital.

Question 2

Accuracy in weighing?

Answer 2

• When weighing UP TO 500 mg - an allowance of ± 1% can be made when reading the weight e.g. (100 mg can be 99-101 mg)
• Weights in EXCESS of 500 mg must have an allowance of no more than ± 5mg.

Question 3

Accuracy in measuring?

Answer 3

Do not measure a volume that is less than 20% of the total capacity of the measure.

• The minimum measurable volume using the equipment in the laboratory is 0.2 mL (non- viscous liquids)
• When calculating volumes required, approximations of up to ±1% may be made to allow more convenient measurements.
• Descending order of accuracy of measuring apparatus is: pipette (very accurate) burette, cylindrical measure, conical measure beaker (not accurate)

Question 4

Should all viscous solutions be strained?

Answer 4

Viscous solutions (rule of thumb: >40% syrup) filter too slowly, so should be strained.

• Two layers of gauze are placed between the two halves of a strainer.
• As the gauze retains little product the preparation can be made to volume in a cylindrical measure and strained directly into the container.

Note: Allow sufficient drainage time for viscous fluid to pass through the neck of the strainer

Question 5

Minimum weights that can be measured

Answer 5

Minimum weight that can be weighed = 100mg

Minimum volume of a non-viscous liquid = 0.2mL (using a 1mL pipette)

Minimum volume of a viscous liquid = 2mL (using a 10mL measuring cylinder)

Question 6

Describe Solid in Liquid Aliquot (must dissolve)

Answer 6

1. Check solubility of solids as solid must be able to dissolve in the volume used
2. Weigh 100mg solid
3. Dissolve in liquid make upto the suitable volume (mix throroughly) –> stir/triturate

100mg of solid is dissolved upto 10 mL with liquid, therefore x mg of solid is in y mL of solution

Question 7

Describe liquid in liquid aliquot

Answer 7

1. Check solubility, or that liquids are miscible
2. Measure 0.2 mL liquid (2mL viscous)
3. Mix and make upto a suitable volume with solvent liquid

0.2 mL of liquid is dissolved upto 2mL with liquid solvent, therefore x mL of liquid M is present in y mL of solution

Question 8

What are solid in solid aliquots

Answer 8

1. Calculate weight of small quantity of solid active ingredient
2. Weigh 100 mg of this solid (active ingredient)
3. Weigh a suitable quantity of diluent
4. Mix well by method of doubling

Method of doubling: best chance of achieving random mix if start with equal quantities of powders, when two particles change places, maximise chance that they will be different particles, smaller the content of active ingredient in the mix –> the greater the importance of mixing

Aliquot statement

100mg of solid is made upto 500mg with solid diluent (i.e. added 400 mg diluent), Therefore 40mg of solid is in 200mg of product

> Weight 200 mg which contains 40mg active ingredient and 160 mg diluent

>Active ingredient and solid diluent weighed separately

>Never used another active ingredient as diluent

>Subtract amount of diluent used in final fomrula