Concentration is quantity divided by volume. Both halves are variables, and a great many preparation errors originate in treating one as fixed. Five milligrams in one millilitre and five milligrams in two millilitres are the same quantity of material at half the concentration, and any calculation downstream that assumed otherwise is wrong by a factor of two.
The working relationship
- Concentration (mg/mL) = quantity (mg) ÷ diluent volume (mL).
- Volume required (mL) = target quantity (mg) ÷ concentration (mg/mL).
- Doubling the diluent halves the concentration; the quantity in the vial has not changed.
Where assayed content enters
The label states nominal strength. The certificate states what the laboratory found. Where the two differ — and they usually do, if only slightly — the assayed figure is the one that belongs in the calculation. This matters most for compounds consumed stoichiometrically rather than acting catalytically, where content variance translates directly into experimental variance.
Recording the preparation
A preparation record needs four fields to be reconstructible: batch number, diluent, volume added, and date of reconstitution. Any one missing and the concentration becomes an assumption rather than a measurement — which is a difficult position to be in when a result needs defending months later.
