Chelates acids

This equilibrium value is the same as that found for the chelated acid (Kk = 1.0) (17) so it is likely that the dipeptide carbanion, like 15 (Scheme 15), shows less kinetic discrimination for reprotonation (k-i/ k-2) than does the ester carbanion 18. In D20 H-exchange and epi-merization occur at the same rate, kohs = 6.4 X 10 4 s 1 (pD = 8.4, 34°C), to give the same equilibrium distribution of A-S-S and A-R-S products (24). 

For simple acidic extractants, such as carboxylic or sulfonic acids, the similarity in formation constants does not produce cobalt-nickel separation factors (Sn° 2) sufficiently large for commercial operation (Fig. 11.4). Data for pH versus extraction for some chelating acid extractants does seem to offer the possibility of separation [e.g., for the hydroxyoxime Acorga P50, the pHso for nickel(II) is 3.5 and for cobalt(II) 5.0]. Normally, this pH difference would be suitable for a separation process, but this particular system has hidden complications. The rate of nickel extraction is very slow compared with cobalt and, in addition, although cobalt is initially extracted... 

Extraction with 8-quinolinol derivatives, which can act as either a chelating acid or, after protonation, as an anion extractant. It was observed [35] that palladium extraction was favored at low acidity, but platinum was extracted best at high acidity (Fig. 11.16). At about 2 mol dm acid, the two metals were extracted equally. This suggested the following extraction equations ... 

Sekine, T. Dyrssen, D. Solvent extraction of metal ions with mixed ligands - IV. Extraction of Eu(III) with chelating acids and neutral adduct-forming ligands, J. Inorg. Nucl. Chem. 29 (1967) 1475-1480. 

The hexavalent state of uranium ion is the usually encountered ion in the solution chemistry of uranium and its exceptional stability, relative to its other oxidation states as well as to other hexavalent actinide ions, makes studies with this ion much simple. Though chelating acids, in general, extract U(VI) by the equilibrium,... 

Table 6. Equilibrium constant data for the chelating acid adducts of uranyl chelates...

Moderately strong neutral donors were reported76 to replace the additional chelating acid molecule and give synergism in the extraction of U(VI) by HIPT-B mixtures, according to the equilibrium,... 

Chelating acid Neutral donor Diluent Aqueous medium log Ka log Kt, log. 1 Ref. 

The extraction of Am(III) by the chelating acids HCF or HOx in presence of TPPO was studied138 and the results are given in Table 18. While Am(CF)3 was found to form two adducts Am(Ox)3 was found to form only the first adduct. This may be due to the stronger complexing of Am(III) by oxine, which makes the residual coordination unsaturation in the metal chelate, much less. The steric hindrance could also play a role in preventing the second adduct formation. 

The effect of mixing two chelating acids, HTTA and HOx, on the extraction of U(VI) was studied where synergism was observed197. Though the enhanced extraction was attributed to the mixed chelate formation, many species are likely to get extracted in these systems in view of the ability of the undissociated chelating acids to behave as neutral donors. 

In the extraction of a metal ion by chelating acids, as well as organophosphorus acids, addition of increasing amounts of a neutral donor results in an initial increase in extraction (synergism), followed by a fall in the enhancement (destruction of synergism) and... 

The difunctional Lewis acid character of 1,2-bis(dihaloboryl)ethylenes suggests the possibility that these compounds may behave as chelating acids... 

With both simple and chelating acidic extractants the metal extraction reaction generally follows the fomi... 

The stability constants of Th with malonic (1.2 complexes), succinic (1 1), phthalic (1 2), maleic (1 2), and glutaric (non-chelating) acids have been... 

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