Separations with a Complexing Eluent

The ligand selected and the concentration and pH of the eluent should be chosen so that complexation of the metal ions is only partial. U the metal ions are too strongly complexed, they will move too rapidly and no separation will occur. 

For effective elution of divalent metal ions a divalent eluent cation such as the ethy-lenediammonium cation is also needed. Thus, divalent metal cations are eluted nicely by an eluent of 2.0 x lO M ethylenediammonium tartrate, but not by an eluent containing sodium or ammonium tartrate. 

In Chapter 5, the logarithm of the retention factor (log k) was shown to be a linear function of the logarithm of an ionic eluent (log E) as well as the logarithm of the exchange capacity (log C) (Eq. 5.19). When a complexing ligand is incorporated into the eluent, an additional term must be added to the equation  

As predicted by Eq. 7.1, a plot of log k or log adjusted retention time versus log tx is linear for the trivalent lanthanides (Fig. 5.2) as well as for most of the divalent metal ions. By adjusting pH and the concentration of complexing ligand, log oli can be varied and the retention time either increased or decreased. 

Sevenich and Fritz found an eluent containing 2.0 x 10 - M ethylenediammonium tartrate to be effective for separating several divalent metal cations [13]. Calculations from ionization constants showed ethylenediamine to be fully protonated (EnH2 ) at pH 5.0 or below. The adjusted retention times for several metal ions were obtained as a function of eluent pH (Table 7.6). The retention times increased in almost every case between pH 5 and 6. Furthermore, some minor extraneous peaks appeared in this pH region. A buffer pH of 4.5 was selected as giving generally the best results. 

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Because formation and breakup of metal chelates is slower than a simple ion-exchange equihbrium, it is essential to select chelating resins with fast kinetics. When a complexing eluent is involved, the kinetic situation may become more difficult. Now we can envision a still slower equihbrium between the metal chelate, (RL)2M, and the eluent chelate, ME2, As we shall see, this does not necessarily prevent effective separations with a complexing eluent, but it may stiU be an inhibiting factor. 

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