Interaction coefficients are determined from equilibrium constant values obtained with solutions at various ionic strengths.
The determination of SIT interaction coefficients also yields the value of the equilibrium constant at infinite dilution.
[3][4] SIT theory was first proposed by Brønsted[5] in 1922 and was further developed by Guggenheim in 1955.
[1] Scatchard[6] extended the theory in 1936 to allow the interaction coefficients to vary with ionic strength.
The theory was mainly of theoretical interest until 1945 because of the difficulty of determining equilibrium constants before the glass electrode was invented.
Firstly, equilibrium constants are determined at a number of different ionic strengths, at a chosen temperature and particular background electrolyte.
The interaction coefficients are then determined by fitting to the observed equilibrium constant values.
[12] It has been shown that the SIT equation is a practical simplification of a more complicated hypothesis,[13] that is rigorously applicable only at trace concentrations of reactant and product species immersed in a surrounding electrolyte medium.