= Stock solution
{wiki}
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>.<Method 200.8 revision 5.4 by United States Environmental Protection Agency>[§3.2, Calibration Standard; §7.4, calibration standards]{ref}
Suppose an invented <stock solution> has <molar concentration> $c_s=10.0\,\mathrm{mmol\,L^{-1}}$. To obtain final <volume (thermodynamics)> $V_f=50.0\,\mathrm{mL}$ at $c_f=0.200\,\mathrm{mmol\,L^{-1}}$, conservation of dissolved <amount of substance> gives
$$
c_sV_s=c_fV_f,\qquad V_s=\frac{c_fV_f}{c_s}=1.00\,\mathrm{mL}.
$$
Transfer 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 (thermodynamics)>.
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.
Back to article page
