ionic strength, I

On a molality basis,

$I m = 1 2 ∑ m B z B 2$

where the sum goes over all the ions B. $z B$ is the charge number of ion B. The ionic strength on a concentration basis is defined analogously

$I c = 1 2 ∑ c B z B 2$ .

Source:

PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 977