Divalent ions alkaline earth metals

Most divalent and trivalent ions, with the exception of the alkaline-earth metals, are effectively chelated by the hydroxycarboxylates citric and tartaric acid, and citric acid will also sequester iron in the presence of ammonia. Another hydroxycarboxylate, gluconic acid, is especially useful in caustic soda solution and as a general-purpose sequestering agent. 

These considerations go far to explain the selectivity behavior of a series of macroheterobicyclic ligands (Fig. 9). Table 4 shows that an increase in the number of coordination sites from 6 to 8 or 9 leads to an increased preference for large cations. Ligand VI, with 11 coordination sites is no longer suited for the complexation of even the largest alkali and alkaline earth metal cations. Furthermore, it is clear that a relatively large coordination number (5 6) is required to produce selectivity for divalent ions. 

Rao, T. S., A. L. Page, and N. T. Colemam. 1968. The influence of ionic strength and ion-par formation between alkaline-earth metals and sulfate on Na-divalent cation-exchange equilibria. Soil Sci. Soc. Proc. 32 543-639. 

If we consider the fluorides, for example, which form pure coordination lattices (p. 33), then those from the alkaline earth metals with the exception of magnesium and beryllium crystallize in the fluorite structure, in which the cation is surrounded by eight fluorine ions for CaF2 and CdF2, which have the same structure, r+/r is 0.71 and 0.69 respectively just at the limit V 3— 1 — 0.73. The fluorides of other divalent ions, such as Mn, Fe, Co and Ni and also Mg, crystallize in a structure with coordination number six (rutile type). It is only for BeF2 that the ratio r+/r = 0.23 lies below the limit of this coordination number and it has a structure similar to that of cristobalite (Si02) with four neighbours (see also p. 66). 

The standard electrode potentials for all the rare earths have similar values and are comparable with the redox potentials of alkaline earth metals [144], Thus the lanthanides are strong reducing agents, and form trivalent ions easily. Both europium and samarium can exist in both trivalent and divalent states and the divalent states are not stable in aqueous solutions. Cerium can exist in both tetravalent and trivalent states in solution but Ce(III) is the most stable. 

The zinc reduction of Eu + to Eu +, followed by its precipitation as the sulfate, is a traditional step in the separation of europium from other lanthanides. In general, the solubilities of the inorganic compounds of the Ln + ions resemble those of the corresponding compounds of the alkaline earth metals (insoluble sulfate, carbonate, hydroxide, oxalate). Both europium and the Sm + and Yb + ions can also be prepared by other methods (e.g. electrolysis), although these solutions of the latter two metals tend to be short-lived and oxygen-sensitive in particular. Eu + is the only divalent aqua ion with any real stability in solution. Several divalent lanthanides can, however, be stabilized by the use of nonaqueous solvents such as HMPA and THE, in which they have characteristic colors, quite distinct from those for the isoelectronic trivalent ions on account of the decreased term separations. 

It is generally accepted that aluminum is tetrahedrally coordinated in aluminosilicate glasses and melts, provided that cations such as alkali metals or alkaline earth metals are present in sufficient amounts to charge compensate the replacement of Si by AF+ [i.e., Si + —> AP" -t- (1/ )M" +, where monovalent or divalent M ions occur in nonframework sites associated with the tetrahedral aluminosilicate framework]. Experimental data for aluminosilicate glasses derived from both low-angle x-ray-scattering experiments and EXAFS studies show that as aluminum is substituted for silicon, the average T (tetrahedrally coordinated ion) -O bond... 

A preferential uptake of the Ba " ion (due to having an ionic radius close to that of the ammonium ion) among the alkaline earth metals also confirms this hypothesis. At the same time, the affinity for other divalent elements is at least one order of magnitude higher than that for alkali or alkaline earth metal cations. The Kd values for most transition metals and lead are in the range from 20000 to more than 100000. However, the absolute values of their uptake in 0.1 M metal nitrate solutions are relatively low. The lEC values are higher than 1.0 meq g" for only two ions, Cu and Hg. ... 

In addition, all of the lanthanides have now been obtained in the divalent state in dilute solution in a CaF2 matrix by y-irradiation 74), fused salt electrolysis (52), or alkaline earth metal reduction 68). Attempts to reduce americium, the analog of europium, to this lower oxidation state have also been successful under similar conditions 51) the Am " ion, which occupies Ca " sites in the crystal, was identified by its ESR spectrum. Similar attempts to obtain uranium (II) have been unsuccessful 51). 

In the fused state, the straightforward method is to allow either the lanthanide metal or the alkaline earth metal to react with a melt of the alkaline earth halide and the lanthanide trihalide. This approach yields the desired divalent ions if inert containers are used. Satisfactory reduction has been obtained in molybdenum, tungsten, and tantalum. The... 

For the solid-state reduction techniques, crystals are grown containing trivalent ions, as in the photoreduction process, but the divalents produced are now stabilized. One of these techniques involves heating the trivalent crystal with the appropriate alkaline earth metal. A convenient way of doing this is to heat the crystal with the alkaline earth metal in a vacuum so that the crystal is baked in metal vapor (11). The reduction can also be carried out by solid-state electrolysis 4,7). An electric field is applied to the crystal at an elevated temperature such as above 600°C. for calcium fluoride or above 400°C. for strontium chloride. If the reduction is carried out below these temperatures, nonuniform results are obtained. 

Through our studies of alkaline earth, lanthanide, and yttrium ions, and their interaction with the polyuronide al c acid, we would like to predict the environmental behavior of the actinides. We chose to begin our study by investigating the interaction of soil constituents with the divalent alkaline earth metal ions and the trivalent lanthanides, which serve as convenient trivalent actinide analogs. Soil is primarily composed of polyelectrolytes. By choosing a simple polyelectrolyte such as the polyuronide alginic acid, we can begin to establish possible pathways of metal ion transport. 

In contrast to the transition metals, divalent alkaline earth metal ions typically sorb at pH values above the pHp/c. For these cations, sorption is believed to occur primarily through relatively weak ion pair formation (outer sphere complexes). The relative affinity to form complexes with surface hydroxyls can be correlated with... 

This chapter deals with the interactions between DNA and divalent metal ions present in the organism ( biometals ) as bioconstituents or biochemical effectors (activa-tors/inhibitors) such as alkaline-earth metals (e.g., Ca, Mg) and the biologically active transition metals (e.g., Zn, Fe, Mn,... 

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