Extraction by Solvation:
An important group of solvent extraction systems is based on the solvation of neutral inorganic molecules or complexes by electron donor containing extractants. Solvation increases the solubility of inorganic species in the organic phase. The oxygenated solvents like ethers, esters, alcohols and ketones and organophosphorus esters solvate the proton and neutral inorganic species making the extraction of the acids and salts feasible. The simple examples are Ce(NO3)4. (ether)2, HNO3. TBP, VO2(NO3). (TBP)2 and H+ (TBP)4. ReO - where TBP is tri-n-butyl phosphate.
The large variety of solvating molecules can be grouped into two distinct categories as follows:
i) Reagents containing oxygen bonded to carbon such as ethers, esters, alcohols and ketones.
ii) Reagents containing oxygen or sulphur bonded to phosphorus e.g. alkylphosphates or alkylthiophosphates.
Despite the similarity between the two categories, several features emphasize the differences. It is the strong polar character of compounds of category (ii) which is responsible for the differences.