ionic strength, I

https://dx.doi.org/10.1351/goldbook.I03180
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