Activities of Dissolved Species, Ionic Strength

Reactions are written in terms of effective concentrations or activities of dissolved species (ai), not their concentrations (m,). By definition the extent that these differ is expressed by the activity coefficient (y,)  

The activities and chemical potentials ( j) of individual dissolved species are also related through the expressions  

In nearly pure water, where all solute ions or molecules contact only water molecules, a, = m,-, and the activity coefficients of all solutes equal unity. As salt concentrations increase, however, individual aqueous species must move closer together and are, therefore, more and more likely to come in contact. Because the interaction between adjacent ions is largely Coulombic, it is also proportional to the charge of the ions involved. These effects are embodied in the definition of ionic strength (/) which is given by 

Example calculations of ionic strength for some simple pure salt solutions are straightforward. Thus, for the solution of a monovalent-monovalent salt such as NaCl, we find from the charge balance that mNa+ = mCL = mNaCl, so that 1 = 2[mNa+ x (+1) + mCL x (-1) ] = (2mNaCl) = mNaCl. 

The most accurate value of ionic strength is obtained from a total water analysis, which includes all ionic species. However, partial analyses are more commonly available than complete ones. Under such conditions the molal ionic strength can be estimated from the total dissolved solids or specific conductance of the water (cf. Langmuir and Mahoney 1985 Polemo et al. 1980). Expressions that permit such an approach are  

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