As of June 12, 2018 our Privacy Policy has been updated. For individuals in the European Union, CIL uses cookies on this website. Please review the new privacy statement to see how. By continuing to use this website you agree to us using cookies in accordance with our privacy statement. Click here for the new privacy statement..OK

Corporate Overview

The Standard – August 2014

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:

1.     Calculate the MW of the free acid or base (MWfree)

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).

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

50.0μg/mL ÷ 522.11μg/μmol = 0.0958μmol/mL = 95.8mM


B.         Molar Concentration × MWfree = Concfree

0.0958μmol/mL   × 500.13μg/μmol = 47.9μg/mL


Alternatively, a single-step calculation may be used:

C.          Concentration Based on Gravimetry × ,MWfree-MW.  = Concentration of Free Acid or Base


Sodium PFOS                  50.0μg/mL × ,500.13𝑔/𝑚𝑜𝑙 -522.11𝑔/𝑚𝑜𝑙 .= 47.9μg/mL

O,O-Diethylphosphoric acid, Potassium Salt, D10         100.0μg/mL × ,163.16𝑔/𝑚𝑜𝑙 -202.25𝑔/𝑚𝑜𝑙 .= 80.67μg/mL

O,O-Diethylphosphoric acid, Potassium Salt         100.0μg/mL × ,153.09𝑔/𝑚𝑜𝑙 -192.19𝑔/𝑚𝑜𝑙 .= 79.65μg/mL 


Visit us at:     

Stable Isotope Newsletters | Cambridge Isotope Laboratories
stable isotope, stable isotope labeled compounds, environmental contaminant standards
CIL has been ready to help with the analytical standards critical to the task of defining and resolving any major environmental contamination problems.