Chelating Agents:

The chelating agent acts as a weak acid and contains a donor group so as to form a bidentate chelate with the metal. The reagent invariably neutralizes the charge on the metal ion and satisfies the coordination number requirement. If the coordination sites remain vacant, they may be occupied by the solvating molecules which sometimes may be ligand itself. Hydrated metal chelates are poorly extractable in non polar solvents which do not have the tendency to replace the water of hydration.

The above discussion amounts to the fact that the chelating agents which form uncharged metal complexes with the metal ion have at least two functional groups. One of these is either OH or SH not necessarily attached to the carbon atom. The other is a basic functional group capable of coordination by its donor properties. The stable ring formed is usually five or six membered. The stability of the chelate will depend on a number of factors namely basic natures of the coordinating site, the acidity of the OH/ SH group, coordination number and coordination ability of the metal. Both the acidity of the OH/ SH group and the basic strength of the donor atom is affected by the structural factors. Also, the relative position of the active sites in the molecule will affect the extractive effectiveness of the molecule by determining the size of the ring.

The reagents included under this category are weak or very weak acids, therefore the formation of extractable complexes will much depend upon the pH of the aqueous phase. Adjustment of pH of the aqueous is a useful way for the extractive separation of metal ions if the equilibrium constants of the metal ions in question are sufficiently different. It is possible to achieve selectivity by factors other than pH control. One of the most popular approaches is by use of masking agents which form water soluble complexes of higher stability than those formed by the chelating agents.