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 \[I_{c}=\frac{1}{2}\ \sum c_{\text{B}}\ z_{\text{B}}^{2}\]