Metal Extractant.Chemorex D2EHPA
Di(2-ethylhexyl) phosphoric acid

Chemical Structure
Mol wgt. 322.4
CAS. No. 298-07-7

Extraction Mechanism.
Chemorex D2EHPA is described as an acidic extractant, which can deprotanate to form an anion,viz:

                                                      (C8H17O)2POOH(C8H17O)2POO- + H+

This anion, being present in the organic phase, can chelate with a cation from an aqueous phase, during a liquid-liquid extraction process and the resulting metal ion chelate is then soluble in the organic phase. It has been suggested that the chelation by the D2EHPA anion is assisted by co-ordination with non-deprotonated species. The extraction of Zn2+ is a typical example of D2HEPA chelation and co-ordination,viz:

              {4(RO)2POOH}org+ {Zn2+ }aq {[(RO)2POO]2Zn[HOPO(OR)2]2}org+ {2H+}aq

As shown by the equation above, the extraction is an equilibrium which can shift in either direction dependent on the [H+]. e.g., at low hydrogen ion concentrations the extraction process is favoured whereas at high hydrogen ion concentrations the zinc loaded organic phase is forced to release the metal ion to the aqueous phase. Consequently, D2EHPA is a pH dependent extractant. Yet, the pH alone is not the only consideration. Structure and valence also play an important part in extraction.

Furthermore the liberation of H+ reduces the degree of extraction and can be overcome by neutralisation with bases during the extraction process or by using the D2EHPA in the form of the Na+ or NH4 salt, which can be achieved by pre-treatment of the organic phase with NaOH or ammonia. The extraction is then carried out via the replacement of the cation to be extracted with Na+ or NH4+ ions. 

Typical Data

Composition (wt./ %)  
Di(2-ethylhexyl)phosphoric acid (mol.wt.322.48) 95% min.
Mono(2-ethylhexyl)phosphoric acid less than 2%
Physical data  
Appearance Clear to colourless to yellow liquid
Texture oily
Odour oudourless
Flash point (DIN 51376) 196o C
Solidifying point -50oC approx
Density @25oC 0.97 g/cm3

Typical Batch Analysis (Samples analysed by Independent Laboratory)

% Str. by Titration %Str. by31P NMR* mol wt. by
Control 16/12/02 95.4 1.3 >97 322
CP204/3/1 04/02/03 95.2 1.4 >97 322
ZD204/1 15/04/03 97.7 0.2 >98 322
 *Phosphorous NMR accounts for only for phosphorous species present (D2EHPA/MEHPA/TriEHPO) therefore is not   true strength. This is determined by titration and will account for small amounts of diluent as well as TriEHPO and 2-ethyl hexanol present as manufacturing impurities.