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Reconstitution
Turn a stated powder preparation into a usable concentration, then calculate the volume containing an ordered dose.
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Medication Preparation
Move from powders and concentrates to medication that is ready to give, without losing track of what stays constant and what changes.
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Reconstitution
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Dilution
Understand what stays constant when a liquid is diluted, interpret final-volume wording, and calculate the new concentration.
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What each Medication Preparation lesson teaches
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ReconstitutionReconstitution turns a powder into a liquid. The amount of medicine in the vial does not change — the diluent gives it a volume, and therefore a concentration. The volume to add and the final volume it produces are both supplied by the product information, never chosen by the person calculating.
Reconstitution turns a powder into a liquid. The amount of medicine in the vial does not change — the diluent gives it a volume, and therefore a concentration. The volume to add and the final volume it produces are both supplied by the product information, never chosen by the person calculating.
Formula
concentration after reconstitution = amount in the vial ÷ final volume stated
Worked example
A vial contains 1 g of powder. The supplied product information states that adding the specified diluent gives a final volume of 4 mL. A dose of 250 mg is prescribed.
- 1Convert so the units match: 1 g = 1000 mg.
- 2Concentration: 1000 ÷ 4 = 250 mg/mL
- 3Volume for the dose: 250 ÷ 250 = 1
250 mg/mL after reconstitution; 1 mL for the prescribed dose
Common mistake
Dividing by the volume of diluent added rather than the final volume stated. Powder occupies space, so the two figures are usually different.
Use the final volume the product information states, and never invent a reconstitution instruction.
DilutionDiluting spreads the same amount of medicine through more liquid. The amount of drug stays constant while concentration falls and volume rises, which is exactly what C₁V₁ = C₂V₂ expresses: the quantity of medicine before equals the quantity after.
Diluting spreads the same amount of medicine through more liquid. The amount of drug stays constant while concentration falls and volume rises, which is exactly what C₁V₁ = C₂V₂ expresses: the quantity of medicine before equals the quantity after.
Formula
C₁ × V₁ = C₂ × V₂
V₂ is the final total volume, not the volume of diluent added.
Worked example
A supplied solution is 50 mg/mL. A preparation of 10 mg/mL in a final total volume of 20 mL is required.
- 1C₁ = 50, C₂ = 10, V₂ = 20.
- 2V₁ = (C₂ × V₂) ÷ C₁ = (10 × 20) ÷ 50
- 3200 ÷ 50 = 4
- 4Sense check: the solution is being made weaker, so V₁ must be smaller than V₂.
4 mL of the supplied solution, made up to a final total volume of 20 mL
Common mistake
Treating V₂ as the amount of diluent to add. It is the total volume you finish with — here, 4 mL of solution plus diluent to reach 20 mL.
Both concentrations must be expressed on the same basis before the equation can be used.