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A stock solution is a chemical solution kept as a source from which other chemical solutions are prepared, commonly by dilution. Its known concentration makes repeated preparations traceable to one starting composition. A stock used for measurement calibration must also satisfy the requirements of a standard solution; the word stock alone does not establish that status. The original EPA Method200.8 defines calibration standards as dilutions of stock standards used to relate instrument response to analyte concentration.^[ref]

Suppose an invented stock solution has molar concentration $c_s=10.0\mathrm{mmol}{\mathrm{L}}^{-1}$. To obtain final volume $V_f=50.0\mathrm{mL}$ at $c_f=0.200\mathrm{mmol}{\mathrm{L}}^{-1}$, conservation of dissolved amount of substance givesTransfer that aliquot and add solvent to the final 50.0-millilitre mark of a volumetric flask, then mix. Adding 50.0 millilitres of solvent would specify a different operation: the equation requires final total volume.

The same inventory reasoning underlies serial dilution. A tenfold dilution followed by a fivefold dilution gives the same final concentration reduction of50, provided each transferred portion is representative and the counted solute is conserved. Reactions, losses or incomplete mixing would change those premises.