Neutral extracting agents

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

Conversely, neutral extracting agents possessing nitrogen electron-donor atoms in their structure (soft bases) will easily discriminate between An(III) and Ln(III) even from nitric acid feeds, thanks to covalently hinted An(III)-N interactions, the best example being the terdentate bis-triazinyl-pyridines (BTPs) or the tetradentate bis-triazinyl-bis-pyridines (BTBP). 

Neutral phosphates (RO)sPO, phosphonates (RO)2R PO and phosphinates (RO)R2PO are well known as extracting agents for metal ions.1823 The isolation of their metal complexes as crystalline compounds is, in general, more difficult than the preparation of complexes with other substituted phosphoryl compounds. It is often essential to reflux solutions of the reactants with dehydrating agents such as triethyl orthoformate or 2,2 -dimethoxypropane. In some cases the neutral phosphoryl ligands or triethyl orthoformate by themselves act as the reaction media in the synthesis of the nickel(II) complexes. 

Acetylacetone and analogous 1,3-diketones (32) have been extensively used as extracting agents in their enolic form they behave as monobasic acids and give rise to formally neutral complexes... 

The extracting agents are thus divided into three classes polydentate organic anions A, neutral organic molecules B, or large organic cations RB+. 

The solvate extraction mechanism of trivalent 4/and 5/elements can be described by the following equilibrium, where A- symbolizes the anion of the aqueous phase and E is the neutral solvation agent of the organic phase (overbars account for species in the organic phase, where residual aqueous molecules might also hydrate the... 

Accordingly, Du increases with both the concentration of the neutral solvation agent, E, initially present in the organic phase, and that of the mineral anion, A-, initially present in the aqueous phase. Inversely, the back-extraction of the trivalent element is favored by a decrease of the concentration of A- in the aqueous solution. 

If the source of anions A is a mineral acid, a competitive proton-extraction reaction might occur, which will decrease the affinity of the neutral solvation agent for the target trivalent elements ... 

The metal-ligand stability constants in aqueous solutions for different metal ions, including Am(III) were determined262 using some neutral donors in combination with HTTA as the extraction agent for the uncomplexed metal ion. 

An excess of alcohol is necessary to suppress the hydrochloric acid-induced cleavage of the triester formed. The product is worked up by neutralizing with aqueous alkali and phase separation. (In the case of triethyl phosphate an extraction agent has to be used, due to its solubility in water.) The excess alcohol is distilled off from the organic phase and triethyl- and tributylphosphates are purified by distillation. Dibutyl- and di-(2-ethylhexyl)phosphates, which are formed in small quantities as byproducts, separate from the aqueous phase upon acidification. 

Excluding consideration of a multitude of representatives of solvent extraction classes (water-insoluble alcohols, ethers, acids, esters, ketones and diketones) that have been examined and found to be inadequate in some respect or another, it is the intent here to focus upon only a few representatives of just three classes of extractants (1) neutral phosphorous agents (2) monoacidic orthophosphate and phosphonate esters and (3) primary, secondary, tertiary and quaternary ammonium ion species. 

Uses Fertilizer refrigerant nitriding of steel condensation catalyst neutralizer extraction solvent petroleum industry latex preservative explosives taste and odor control agent in water treatment condensate conosion inhibitor buffer in cosmetics foods (processing aid) phamiaceuticals (alkalizing agenL buffer) microbial fermentation nutrient in food-pkg. applies. 

An extraction efficiency of RTILs over conventional solvents was definitely observed for a series of cations in presence of neutral complexing agents (Dietz, 2006 Murding Tang, 2010). At the same time, unlike for molecular solvents the Dm values for alkali and alkaline earth cations M in numerous RTIL/water systems in the presence of different crown ethers declined as HNQs concentration in aqueous phase increased, then the Dm dependence passed a minimum at c.a. 1 mol/ dmP concentration and started to increase as nitric acid concentration increased up to 3 mol/dm3 (EHetz, 2006 Egorov et al, 2010). Our group has demonstrated that the pH-dependence of crown-ether assisted extraction from water into RTIL phase correlates well with the relative distribution of a crown (Vendilo, et al, 2009), Fig. 1,2... 

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