Obtaining the activity coefficient of an individual ion

We have already shown that the experiment only allows us to determine the mean activity coefficients. If we could determine the activity coefficient of a single ion, we could deduce that of all other ions step-by-step. To do this, Mac Innes benefitted from the fact that in potassium chloride, the chloride and the potassium ion have the same charge in absolute value, the same electronic stmcture and therefore the same size and the same mobility but slightly different masses, and therefore worked out that they should have, in the same solution, the same activity coefficient. From the mean activity coefficient of potassium chloride, we can deduce those of individual ions using the relation  

The results show that monovalent ions have roughly the same activity coeffident and that the activity coefficients are influenced by the electrovalence of ions and by the presence of other ions giving rise to what we call ionic strength. 

We have just seen that the activity coefficient of ions was influenced by the presence of other ions. The experiment shows that it is not necessarily the concentration that is influenced but a variable introduced by Lewis and Randall the ionic strength. 

Note that if the solution only contains a salt By, the ionic strength is proportional to the concentration, i.e.  

This introduction allowed Lewis and Randall to determine the values of the activity coefficients of some individual ions, in different solutions. Some are given in Table 3.1. designates a bivalent cation with a metal origin such as alkahne earth metals, magnesium, copper, cadmium or zinc. 

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