Class of extractants:

The development of this class of extractants, at the initial stage, was closely associated with the objective of finding organic phosphorus compounds for practical solvent extraction applications in the field of processing of nuclear fuel. At an early stage of the use of tributyl phosphate (TBP) for nuclear fuel processing, it was recognized that the acid hydrolysis products of this reagent- the mono and dibutyl phosphoric acids, were themselves very effective extractants. This observation prompted a systematic survey of alkyl phosphorus compounds for metal extraction chemistry. The list of this class of extractants is very big and it may not be possible to mention all of them. Of all the mono esters, mono-2-ethylhexyl phosphoric acid is the most commonly used extractant and among the dialkylphosphates, di-2-ethylhexyl phosphoric acid (DEHPA) has by far received the maximum attention. Besides these two, a number of them like octylphenyl phosphoric acid (OPPA), diisodecyl phosphoric acid (DP-1OR), 2-ethylhexyl phosphonic acid (PC-88A), 2- ethylbenzyl phosphonic acid, dibutyl buylphosphonate (PC-44) and bis (2,4,4- trimethylpentyl) phosphinic acid (Cyanex 272) are well known. In the nineteen eighties, American Cynamid, marketed the mono and disulphur analogues of Cyanex 272, i.e., bis (2,4,4-trimethylpentyl)  monothiophosphinic  acid (Cyanex 302) and  bis (2,4,4-trimethylpentyl)  dithiophosphonic acid (Cyanex 301). The introduction of sulphur in these acids lowers the pKa of the acids and it may be possible to work with them and extract at a lower pH.