12.1 Displacement values
When a solid is dissolved in a fluid, the volume of the fluid increases. The volume of this increase is called the displacement value. It is important to consider this when preparing medicines, since many medicines are presented as dry powders, to which diluent must be added. For example: amoxicillin 250 mg vial has a displacement value of 0.2 mL. Usually amoxicillin is reconstituted with 5 mL water for injections. After reconstitution the total volume of the solution is 5 mL + 0.2 mL = 5.2 mL. If the prescribed dose is 250 mg, then the practitioner should administer the full 5.2 mL to the patient. The concentration of amoxicillin in the solution is 48 mg/mL.
Perhaps a practitioner is working on a paediatric ward and the dose prescribed is 50 mg. To reconstitute the vial the practitioner should take into account the displacement value. The practitioner should add 4.8 mL water for injections to the vial, so that the concentration is 50 mg/mL. The volume of the dose is 1 mL.
The monographs in Medicine monographs (in alphabetical order) advise how to take into account the displacement value for all drugs in which it is significant.
12.2 Sodium content
Some medicines can have a high sodium content which should be taken into consideration when patients are sodium restricted. Sodium may be included in the medicine because:
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