Cambridge Isotope Laboratories, Inc.
isotope.com
August 2014
The Standard
ENVIRONMENTAL CONTAMINANT STANDARDS
Calculating Concentrations of Free Acid
or Base from Salt Form
At CIL, reference standards with concentration based on
gravimetry are carefully weighed, dissolved and diluted by weight,
then packaged. The concentration based on gravimetry is given in
mass of compound per volume of solvent using the density of the
solvent and is adjusted for the purity of the neat material. If the
compound dissolved is a salt, the concentration by gravimetry is
based on the mass of the salt form.
Molecular weight (MW) and chemical formula provided are for the
product as listed. If the product is a salt, the concentration of the
free acid or free base may be calculated as follows:
See the example calculation below for sodium PFOS (sodium
perfluorooctanesulfonate) where the molar concentration and
the free acid concentration are derived from the gravimetric
concentration of the salt form as reported on the certificate
of analysis.
A.Concentration based on gravimetry ÷ MW = molar
concentration
50.0 μg /mL ÷ 522.11 μg /µmol = 0.0958 μmol /mL = 95.8 mM
B. Molar concentration × MWfree = Concfree
0.0958 μmol /mL × 500.13 μg /µmol = 47.9 μg/mL
1. Calculate the MW of the free acid or base (MWfree)
Alternatively, a single-step calculation may be used:
2.From the product’s certificate of analysis, note the MW and the
concentration based on gravimetry (Conc).
3.Divide the Conc by the MW to obtain the molar concentration
of the product (See example A).
4.Multiply the molar concentration with the MWfree to obtain
the calculated concentration of the free acid or base (Concfree)
(See example B).
C.Concentration based on gravimetry × MWfree-MW. =
concentration of free acid or base
Examples
Sodium PFOS
50.0 μg /mL × ,500.13 g /mol - 522.11 g /mol .= 47.9 μg /mL
O,O-Diethylphosphoric acid, potassium salt, D10
100.0 μg /mL × ,163.16 g /mol - 202.25 g /mol .= 80.67 μg /mL
O,O-Diethylphosphoric acid, potassium salt
100.0 μg /mL × ,153.09 g /mol - 192.19 g /mol .= 79.65 μg /mL
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ENV_NEWS_ACID
8/14 Supersedes all previously published literature