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
50.0 μg/mL ÷ 522.11 μg/μmol = 0.0958 μmol/mL = 95.8 mM
B. Molar Concentration × MWfree = Concfree
0.0958μmol/mL × 500.13 μg/μmol = 47.9 μg/mL
Alternatively, a single-step calculation may be used:
C. Concentration Based on Gravimetry × ,MWfree-MW. = Concentration of Free Acid or Base
Examples
Sodium PFOS 50.0 μg/mL × ,500.13𝑔/𝑚𝑜𝑙 -522.11𝑔/𝑚𝑜𝑙 .= 47.9 μg/mL
O,O-diethylphosphoric acid, potassium salt, D10 100.0 μg/mL × ,163.16𝑔/𝑚𝑜𝑙 -202.25𝑔/𝑚𝑜𝑙 .= 80.67 μg/mL
O,O-diethylphosphoric acid, potassium salt 100.0 μg/mL × ,153.09𝑔/𝑚𝑜𝑙 -192.19𝑔/𝑚𝑜𝑙 .= 79.65 μg/mL