Strongly-Retained Cations

The strongly retained cations in soils include many of the essential microelements and also the toxic cations. The concentrations of these ions in the soil solution are low and they are apparently retained by two means. One group is the cations that in aqueous solutions precipitate as insoluble oxides and hydroxyoxides. The root zone of a typical agricultural soil might contain as much as 300 000 kg ha-1 of Fe and Al, but their plant availability is only a few kg ha-1. 

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Again, a mixture of hydrochloric acid and DAP was employed as the eluent. Since the concentration of the eluent components is increased by about one order of magnitude during the run, their purity is of essential importance. In the field of cation analysis, the gradient technique is used predominantly for screening purposes. In comparison with isocratic techniques, a significantly better peak shape is observed for strongly retained cations such as ethylenediamine. 

Benzene sulfonic acid Si-O-Si-C CHsCH -SOiH Ion exchange Separates cations, with divalent ions more strongly retained than monovalent ions phosphate buffer systems are often used, sometimes with low concentrations of polar nonaqueous modifiers added the presence of the benzene group on the benzenesulfonic acid moiety gives this phase a dual nature, and the ability to separate based upon nonpolar interactions... 

Polycations are strongly retained by the bare silica capillary wall, since they can ion-pair at multiple sites on the anionic wall. Since all electrostatic bonds must be broken to free the solute, this problem becomes increasingly severe as the number of positive charges increases. Polycations may include small molecules such as tetrazoles and even small peptides. Either permanently coated or charged-reversed capillaries are usually required to separate polyvalent cations. Monovalent cations show some wall effects, but these do not preclude the bare silica capillary from being used. Polyanions and monovalent anions can usually be separated on bare silica. 

The alkali metals and to some extent the alkaline earth metals can be separated from each other through the use of cation-exchange resin and elution with HCl. Sodium ions are less strongly retained than either potassium ions or heavier alkali metal ions. 

In contrast to conventional cation exchangers, a reversed elution order is observed with crown ether phases, which is mainly determined by the size ratio between crown ether ring and alkali metal ion. Due to the high affinity of poly(benzo-15-crown-5) toward potassium and rubidium ions, these are more strongly retained than lithium, sodium, and cesium ions, respectively. However, the complexing properties of crown ethers also depend on the counter ion being employed. Thus, in potassium salts, for example, an increase in retention in the order KC1 < KBr < KI is observed with an increasing size of the counter ion. 

When metals dissolved in 0.5 M HCl are passed through a strongly acidic cation exchanger, Cd and Sn(lV) are eluted, whereas Zn, Cu, Mn, Ni, Co, U, and Ti are retained on the column. Cd, Sn(IV), Bi, and Hg(II) have been separated from most other metals by elution from a cation exchanger with 0.4 M HBr [21]. The effect of various media on the separation of Cd from other metals on Dowex 50 cation exchange resin has been investigated [22]. 

Silver may be separated from Ce, Zr, Th, Be, and Fe(III), on strongly acidic cation-exchangers, by converting these metals into anionic complexes, or separated from Cu, U, Al, and Zn by selective elution with nitric acid [14]. After retention of Pb, Ag, and Hg on Dowex 50, lead is eluted first with 0.25 M ammonium acetate, then silver with 0.5 M ammonia solution. Silver has been separated on a cation-exchanger from Hg, Co, Ni, and Zn on the basis of the differing stabilities of their EDTA complexes at pH 4.6 [15]. Silver retained in a column with a macroporous cation-exchange resin bed has been eluted with 2 M HNO3 or 0.5 M HBr in aqueous acetone solution [16]. 

Mixtures of alkaline-earth metals are separated on strongly acidic cation-exchangers. The cations are retained on a cationite column, and then they are eluted selectively with appropriate complexants, based on the differences in stability of complexes formed by the alkaline-earth metals with suitable complexing eluents. 

Strongly acid cation-exchangers retain Te from a 0.3 M HCl medium, whereas Se is not sorbed. Mixtures of Te and Se were separated on Dowex 50W-X8 cation-exchanger from a mixed medium containing formic acid, methyl ethyl ketone, and 50% methanol [24]. 

A strongly acidic cation-exchanger retains T1(I), but not Bi, Cu, Fe, Pb, or Zn, from an EDTA solution at pH 4. The Tl may be eluted with 2 M HCl. Thallium(I) is also retained from tartrate, citrate, or pyrophosphate solutions (pH 3-5), while Fe, Cu, Zn, Cd, Pb, and Sb pass to the eluate. In this case, thallium can be eluted with 6 M HCl. Mixed HCl-acetone... 

Strongly acid cation-exchangers have been used for the separation of Th from rare-earth elements and other metals [8,9]. From the metal cations retained on the column 3-5 A/ HCl elutes rare-earth- and most other metals except thorium. Thorium is eluted with 10 M HCI or 3 M H2SO4, as well as with 5 MHNO3 [10], ammonium oxalate [11], ammonium carbonate [1] or ammonium sulphate solutions. Cation-exchange chromatography has also been used to separate thorium with the use of media such as HBr [12], formic acid -I- dimethyl sulphoxide [13], and nitric acid -1- methanol -i-TOPO [14]. 

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