-𝐼𝑆 is the ionic strength in mol/L -𝐶 𝑖 is the concentration of the ion in mol/L -𝑍 𝑖 is the charge of the ion Depending on the ionic strength of the ionic solution considered, the relation between activity coefficient and ionic strength will be different, as described in . For wastewater systems, in which the ionic strength should always be <0.5M, the approximation of Davies is always applicable and is expressed as: