Studies On Extraction Equilibria:

The extraction equilibria are usually complicated because we have to take into consideration the equilibria existing in both the phases. It is not only the metal ion which has to be considered but the status of extractant has also to be considered. The existence of different species in the two phases and their partitioning need proper assessment. Generally, the distribution of the metal in the two phases is expressed in the form of an equation which involves certain constants and a few experimental variables. The expression is verified by observing the effect of variables on the distribution of the metal ion. In this circumstance, the common methodology followed is that of slope analysis. In the slope analysis, the experimental condition chosen is such that the different variables are kept constant except for one. The effect of that particular variable on the distribution of metal is observed and the slope of the plot indicates the involvement of that particular variable/ chemical entity in the formation of the extracting species. The spectroscopic methods particularly absorption spectroscopy (IR, UV, Visible) are also helpful in identifying the presence of the different species. Here, it may be worthwhile to point out that usually the infrared spectrum of the organic phase is of little help because of its complexity.

In this section, some typical extraction equilibria are discussed under the following heads:

i) Extraction of metal chelates

ii) Extraction by solvation

iii) Extraction by ion pair formation

iv) Extraction due to synergism

It may be noted that a particular class of extractants or an extractant may behave in a dual capacity. It may act as an acid to neutralize the charge on the metal ion and/or it may solvate the neutral species. It is just possible that the same extracting species may play the dual role.