## Wikipedia - Ionivahvuus ionic strength, $$I$$

https://doi.org/10.1351/goldbook.I03180
On a molality basis, $I_{m}=\frac{1}{2}\ \sum m_{\text{B}}\ z_{\text{B}}^{2}$ where the sum goes over all the ions B. $$z_{\text{B}}$$ is the @C00993@ of ion B. The ionic strength on a concentration basis is defined analogously $I_{c}=\frac{1}{2}\ \sum c_{\text{B}}\ z_{\text{B}}^{2}$
Sources:
Green Book, 2nd ed., p. 50 [Terms] [Book]
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 977 [Terms] [Paper]