Alkaline earth ions separation

Sakamoto [243] determined picomolar levels of cobalt in seawater by flow injection analysis with chemiluminescence detection. In this method flow injection analysis was used to automate the determination of cobalt in seawater by the cobalt-enhanced chemiluminescence oxidation of gallic acid in alkaline hydrogen peroxide. A preconcentration/separation step in the flow injection analysis manifold with an in-line column of immobilised 8-hydroxyquinoline was included to separate the cobalt from alkaline-earth ions. One sample analysis takes 8 min, including the 4-min sample load period. The detection limit is approximately 8 pM. The average standard deviation of replicate analyses at sea of 80 samples was 5%. The method was tested and inter calibrated on samples collected off the California coast. 

The selective cation binding properties ol crown ethers and cryptands have obvious commercial applications in the separation of metal ions and these have recently been reviewed (B-78MI52103.79MI52102, B-81MI52103). Many liquid-liquid extraction systems have been developed for alkali and alkaline earth metal separations. Since the hardness of the counterion is inversely proportional to the extraction coefficient, large, soft anions, such as picrate, are usually used. 

Because the alkaline earth ions behave very similarly to each other in aqueous solutions, it is very difficult to distinguish them and especially to separate them. 

Fig. 1 includes three categories of metal ions. In the first group, the water molecules of the inner coordination sphere are so labile that substitution takes place at almost every encounter. The overall rate for the complex formation is thus diffusion controlled. It is not possible to separate one single substitution step from the overall process. The rate constants in this group (to which most of the alkali and some of the alkaline earth ions belong) are therefore only in a very trivial sense characteristic of the nature of the metal ion. 

Self-diffusion coefficients of polyvalent cations in these perfluorinated ionomer membranes have not been reported. It can be inferred from the use of the sulfonate membranes as Donnan dialysis devices that transport of cations such as CuflT), Mg(II), and Al(III) under a concentration gradient is rapid. Also, column chromatographic separation of the alkaline-earth ion is readily accomplished with a powdered Nafion perfluorosulfonate polymer, which is again an indication of facile diffusion of these cations within the polymer phase. 

Figure 5. Chromatographic separation of alkaline earth ions using 1200 EW Nafion...

Cryptands are cyclic (mainly) polyether molecules with usually three chains linked at nitrogen caps at each end of the molecule (Figure 4.42), much like the sarcophagines but with a different capping atom and different donors. They can, depending on host cavity size, bind metal ions (alkali or alkaline earth ions preferred) or small molecules. A wide range of molecules of this family have been prepared. They can be effective in metal ion selection from a group of ions, useful in both analysis and separation of mixtures. They also help solubilize metal ions in aprotic solvents. 

FIGURE 28-24 Typical applications of ion chromatography. (a) Separation of anions on an anion-exchartge column. Eluent 0.0028 M NaHCOj to 0.0023 M Na2C03. Sample size 50 pL. (b) Separation of alkaline earth ions on a cation-exchange column. EluenI 0,025 M phenylene-diamine dthydrochlohde to 0,0025 M HCI. Sample size 100 mL, (Couriesy of Dionex Corp., Sunnyvale, CA.)... 

Because the alkaline earth ions behave very similarly to each other in aqueous solutions, it is very difficult to distinguish them and especially to separate them. There are, however, differences in the solubilities of some of their salts in non-aqueous media. Thus, lOOg anhydrous ethanol dissolves... 

In 1928 KendalP described experiments involving the separation of ions by means of the ionic migration technique . In 1953 Longsworth successfully separated alkaline earth ions, some amino acids and low-molecular organic acids by moving boimdary technique . He also used the names leading and trailing electrolytes for the first time. 

No attempts, corresponding to those described above for the alkali halides, have been made to separate the relaxation rates for the alkaline earth halides into ion-ion and ion-solvent contributions and to analyze these in terms of the electrostatic and electronic distortion models of ion quadrupole relaxation. To do so, certain presently lacking information would be needed such as halide ion chemical shift data. It can, however, be said from the well-established structure-stabilizing effect on water of the alkaline earth ions and from studies of water translational [281] and rotational motions [79] in aqueous alkaline earth halide solutions that a modification of the ion-solvent contribution to halide ion relaxation due to the cations is of great importance. In line with this, in their analyses based on the macroscopic viscosity, Deverell et al. [52] found the ion-ion contributions to Br relaxation to be small at least at not too high electrolyte concentration. Such an interpretation is also suggested by the... 

Deuding [7] has employed acetic acid-ethanol (6 + 96) and 2N HCl- er. -butanol (6 + 96) as solvents for the separation of alkali and alkaline earth ions. Violuric acid and lithium tetracyanoquinodimethanide were used for detection. 

Subsequent to any decomposition, but also in the case of liquid samples such as water and urine, the analytes of interest are generally present in dilute solution together with a large excess of foreign ions (e.g.. alkali-metal and alkaline-earth cations). Separation and concentration of the analytes may be necessary to improve the limit of detection and exclude interference. Useful techniques in this regard include liquid-liquid extraction. solid-phase extraction, special precipitation reactions, and electrolytic deposition. 

The filtrate obtained after the Group III metal ions have precipitated contains the alkali metal ions and the alkaline earth ions. The Analytical Group IV ions, Ba, Ca, Mg, and Sr, are precipitated as carbonates or phosphates by adding (NH4)2C03 or (NH4)2HP04. The filtrate from this separation contains the Analytical Group V ions, and Na. ... 

The properties of hydrated titanium dioxide as an ion-exchange (qv) medium have been widely studied (51—55). Separations include those of alkaH and alkaline-earth metals, zinc, copper, cobalt, cesium, strontium, and barium. The use of hydrated titanium dioxide to separate uranium from seawater and also for the treatment of radioactive wastes from nuclear-reactor installations has been proposed (56). 

Alkaline-earth metals are often deterruined volumetricaHy by complexometric titration at pH 10, using Eriochrome Black T as indicator. The most suitable complexing titrant for barium ion is a solution of diethylenetriaminepentaacetic acid (DTPA). Other alkaline earths, if present, are simultaneously titrated, and in the favored analytical procedure calcium and strontium are deterruined separately by atomic absorption spectrophotometry, and their values subtracted from the total to obtain the barium value. 

The proportion of hydrochloric acid in the mobile phase was not to exceed 20%, so that complex formation did not occur and zone structure was not adversely affected. An excess of accompanying alkaline earth metal ions did not interfere with the separation but alkali metal cations did. The hthium cation fluoresced blue and lay at the same height as the magnesium cation, ammonium ions interfered with the calcium zone. 

A mercury cathode finds widespread application for separations by constant current electrolysis. The most important use is the separation of the alkali and alkaline-earth metals, Al, Be, Mg, Ta, V, Zr, W, U, and the lanthanides from such elements as Fe, Cr, Ni, Co, Zn, Mo, Cd, Cu, Sn, Bi, Ag, Ge, Pd, Pt, Au, Rh, Ir, and Tl, which can, under suitable conditions, be deposited on a mercury cathode. The method is therefore of particular value for the determination of Al, etc., in steels and alloys it is also applied in the separation of iron from such elements as titanium, vanadium, and uranium. In an uncontrolled constant-current electrolysis in an acid medium the cathode potential is limited by the potential at which hydrogen ion is reduced the overpotential of hydrogen on mercury is high (about 0.8 volt), and consequently more metals are deposited from an acid solution at a mercury cathode than with a platinum cathode.10... 

Figure 4.20 Separation of comon anions and alkaline earth cations by ion chronatography using conductivity detection.

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