Ion-pair extraction and side reactions

The process of ion-pair extraction is governed by several parameters, such as the nature and concentration of the ionic species in the two phases, the pH of the aqueous phase, the properties of the organic solvent and the specific interactions of the various ions with one another and with the two phases. A good knowledge of the physicochemical parameters that influence the distribution of the ionic compounds will aid in the successful application of the ion-pair technique [29]. 

A charged compound HA, a protonated amine or quaternary ammonium ion, can be distributed between an aqueous and an organic phase with the aid of a counter ion X , the anion of a strong or weak acid. 

The two oppositely charged ions form ion-pairs in the organic phase as summarized by the formula 

The equilibrium constant Kg (HAx lor this process (the extraction constant) is defined by 

It expresses the combined effect of phase transfer and ion-pair formation. Provided that further reactions can be disregarded, the distribution ratio Dha of HA is given by the relationship 

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