Ionic Strength and Debye-Hiickel Theory

The concept of ionic strength, which allows individual ionic activity coefficients to be estimated, was developed by Lewis and Randall (1921). The ionic strength of a solution is given by 

The ionic strength of a one-molal solution of CaCl2 is thus 

Clearly, ionic strength corresponds to molality in the case of a uni-univalent strong electrolyte—i.e., for a one-molal solution of NaCl  

A and B in equation 8.31 are characteristic parameters of the solvent (in this case, water), which can be derived in the various T and P conditions of interest by application of 

T range 0 to 300 °C (saturation P), and table 8.5 lists effective diameter values for various ions in aqueous solution. 

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