โ Scribed by F.W.E. Strelow; C.R. Van Zyl; C.R. Nolte
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The separation of the elements of the alkaline earth group, Mg(II), CafII), Sr(I1) and Ra( IL) by ion-exchange chromatography has received considerable attention during the last decade. Eluants employed include hydrochloric acidi, hydrochloric acid-methanol@, ammonium chloridea, ammonium acetate*es, ammonium formate@, ammonium acetate or forrnate plus methanol 7, the ammonium salts of citrate*, lactate@ and cw-hydroxyisobutyratel@*il, and EDTA*g and DCyTArQs*4 for cation cxchangc ; ammonium citrate *6 and nitric acid-methanol or ethanol mixtureslogi7 have been used for anion-exchange chromatography. Hydrochloric acid would be the most attractive of these eluauts because it can be removed most easily, but the separation factor for the calcium(II)-strontium{II) pair is rather small (cw w 1.4 for the concentration range I N to 2 AT hydrochloric acid with Dowex sow-X8 or an equivalent resin); thus only small amounts of strontium can be separated quantitatively and fairly large columns are required. The best eluants for calcium(II)~trontium(Il)barium(U) separations probably are lactate, or-hydroxyisobutyrate, EDTA and DCyTA, but the destruction of these reagents in the eluates is rather tedious and the magnesium(II)-caIcium(II) separations are not very satisfactory. With bc-hydroxyisobutyrate, magnesium forms an insoluble precipitate when present in larger than trace amounts, and with EDTA andDCyTAit appearsin the cluate between calcium(I1) and strontium(II). Anion exchange in citrate media or nitric acid-alcohol mixtures provides good Mg(II)-Ca(II)-Sr(I1) separations; but the strontium(II)-barium(I1) separation is not satisfactoryin citrate because the distribution coefficients for both strontium(I1) and barium(II) are too low, and in nitric acid-alcohol because barium and to a lesser extent strontium tend to form insoluble nitrates in the resin matrix.
In ~monium acetate reasonable separations for ~g(II)-pa-Sr~II)-Ba~II) mixtures are possible", but the separation factors are small (between I ,g and 2.6 in the relevant region of concentration) and fairly large columns have to be employed for the separation of major amounts. The addition of an organic solvent such as alcohol leads to considerably increased separation factors 7, but this is accompanied by slower exchange rates, peak broadening and increased tailing.
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