Fuoss ion pairs and others

Bjerrum s concept of an ion pair and his theoretical development is the most successful treatment used in conjunction with the Debye-Hiickel theory for analysing experimental data, despite all the ambiguities in deciding at what stage to introduce the cut-off distance between ion pairs and free ions. His theory was basically a device to account for the short range electrostatic interactions not included in the Debye-Hiickel theory (see Guggenheim s treatment, Section 10.13.1). 

Fuoss has also attempted to define what is meant by an ion pair and to deduce a theoretical treatment to allow a calculation of the expected tassodation based on his model. He stated that an ion pair formed when the ions come in contact at the distance of closest approach, i.e. to a distance a apart. There are, however, theoretical reasons for saying that this should more correctly read as ions at around the distances close to the distance of closest approach . 

The theoretical basis of his calculation is less secure than Bjerrum s, but his work had the merit of inspiring high precision work on the behaviour of electrolyte solutions over a range of relative permittivities. Bjenum s and Fuoss theories predicted different dependencies of association (sce Section 12.16). 

Other workers have also discussed this concept of a contact ion pair , but detailed analysis indicates that there is a considerable degree of latitude in what physically is meant by a contact ion pair . Fuoss revisited this dilemma in his 1978 conductance theory (see Section 

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