Cation divalent, trivalent

The rare earth (RE) ions most commonly used for applications as phosphors, lasers, and amplifiers are the so-called lanthanide ions. Lanthanide ions are formed by ionization of a nnmber of atoms located in periodic table after lanthanum from the cerium atom (atomic number 58), which has an onter electronic configuration 5s 5p 5d 4f 6s, to the ytterbium atom (atomic number 70), with an outer electronic configuration 5s 5p 4f " 6s. These atoms are nsnally incorporated in crystals as divalent or trivalent cations. In trivalent ions 5d, 6s, and some 4f electrons are removed and so (RE) + ions deal with transitions between electronic energy sublevels of the 4f" electroiuc configuration. Divalent lanthanide ions contain one more f electron (for instance, the Eu + ion has the same electronic configuration as the Gd + ion, the next element in the periodic table) but, at variance with trivalent ions, they tand use to show f d interconfigurational optical transitions. This aspect leads to quite different spectroscopic properties between divalent and trivalent ions, and so we will discuss them separately. 

In the 1930s, Michaelis compared radical cations with trivalent-carbon or divalent-nitrogen intermediates using potentiometric methods. He rationalized their unusual stability as follows The fact that such radicals are capable of existence at all, can be attributed to a particular symmetry of structure resulting in resonance a... 

Figure 6.2 Octahedral ionic radii of first-series transition metal cations, (a) Divalent cations (b) trivalent cations. Filled circles high-spin cations open squares low-spin cations (data from Shannon, 1976).

Exchange Reactions. Exchange reactions in soils commonly involve monovalent-monovalent (e.g., IC-NKQ, monovalent-divalent (e.g., K+-Ca, Na+-Ca2+), movova-lent-trivalent (e.g., K+-A13+), or divalent-trivalent (e.g., Ca2+-Al3+) cations. The... 

Mercury electrode can be used for the potentiometric determination of 29 divalent, trivalent, and tetravalent cations with EDTA. ... 

Figure 5.3. Side view (along the a-axis) of ideal structures of two common configurations of layer-lattice aluminosilieates. A. 1 1 layer lattice, consisting of alternative octahedral and tetrahedral sheets. B. 2 1 layer lattice, consisting of two tetrahedral sheets sandwiching the octahedral sheet. Where the octahedral cations are trivalent, only two out of three octahedral sites are occupied and the mineral is dioctahedral. Where the octahedral cations are divalent, all octahedral sites are occupied and the mineral is trioctahedral.

For all preceding chalcogenide compounds prepared in supercritical amines, the metal centers are always monovalent. This includes both the alkali metal and the transition metal. In attempts to make more electronically interesting compounds, the inclusion of polyvdent metal ions with open shells in the chalcogenide framework was attempted. This was done by replacing the alkali metal cations with divalent cations or trivalent rare earth cations. However,... 

In the case of inverse ferrites (or in any case where more than one type of cation occupies the same set of crystallographic sites), ordering phenomena can be expected (Gorter, 1954). In inverse ferrites, octahedral site occupancy by two kinds of cations (divalent and trivalent) can lead to long-range order, where successive (001) layers of octahedral sites are occupied alternately by D and T cations. In this case, there are two ionic sublattices (Fuentes, Aburto Valenzuela, 1987) on octahedral sites. Fig. 2.4. 

Cerium oxide, ceria, has a fluorite structure and shows oxide anion conducting behavior differ from other rare earth oxides. However, the O ionic conductivity of pure ceria is low because of a lack of oxide anion vacancies. For ion conduction, especially for anion, it is important to have such an enough vacancy in the crystal lattice for ion conduction. Therefore, the substitution of tetravalent Ce" by a lower valent cation is applied in order to introduce the anion vacancies. For the dopant cation, divalent alkaline earth metal ions and some rare earth ions which stably hold trivalent state are usually selected. Figure 9-28 shows the dopant ionic radius dependencies of the oxide ionic conductivity for the doped ceria at 800°C. In the case of rare earth doped Ce02, the highest O ion conductivity was obtained for... 

Cationization by means of divalent and trivalent metal salts has been demonstrated in MALDI experiments for different polymers, namely cyclic polyethylene oxide and -cyclodextrin. In all these cases analyte singly chaig quasi-molecular ions are generated. The latter can be formed by adduction of the corresponding monovalent metal ion, by adduction of the divalent/ trivalent metal ion with simultaneous deprotonation of the analyte molecules, or by simultaneous adduction of the divalent/trivalent metal ion and an appropriate number of deprotonated matrix moieties. 

All divalent, trivalent, and tetravalent chlorides, bromides, and iodides are hygroscopic and an appreciable solution chemistry has been characterized, but only aspects of solution chemistry that relate to hydrates are considered here. The solid-state chemistry of the lower oxidation states (to IV) of cations in combination with the F, Cl, Br, and I atoms, and combinations of these with main-group cations is considered. The numerous reactions some of these halides undergo with organic reagents and solvents are considered only to the extent they relate to solid-state chemistry. 

In a-YbPs the cation is trivalent. One might expect to find divalent Yb in the second modification, /S-YbPs. The P sublattice therein has indeed changed, as it contains partly 4-membered rings (v. Schnering i976). However, the cation coordination is reduced to 7 and the volume is even smaller than that of a-YbPs, so that the cation is certainly trivalent too. We may therefore expect the /S-YbPs structure to be appropriate also for LuPs (and perhaps for LaAss,...). 

Sketch a full Brouwer diagram (log defect concentrations vs log poz) for an oxide ABO3 dominated by fully ionised oxygen and metal vacancies at imder- and overstoichiometiy, respectively. Assume that Schottky defects predominate close to stoichiometric conditions. You may assume that both cations are trivalent, but discuss also the effect it would have if A was divalent and B tetravalent. (Hint the main goal is in any case to obtain and illustrate the po2-dependencies. Use the rules we listed for such constmctions of Brouwer diagrams.)... 

The AE value obtained from the deviation of the g z value from the free spin value increases in order monovalent cations (M ) < divalent cations (M ) < trivalent cations (M )." The AE value also increases with decreasing ion radius when the oxidation state of the metal ion is the same. The same trend has been reported for 02 adsorbed on the surface of various metal oxides, which act as Lewis acids as well. " The scandium ion, which has the smallest ion radius among the trivalent metal cations, gives the largest AE value." ... 

Answer The ratio of the divalent cations to trivalent cations in the ferrite is always one to two. Hence the ferric oxide dissolves in it with a ratio of Fe +r Fe + which is, almost identical to that occurring with Fc304. This comes from the conversion of an equivalent number of oxygen ions into oxygen molecules, which are deposited at the ferrite-ferric oxide interface. The oxygen is recovered at the ferrite-monovalent oxide interface. I believe this is what Kooy had in mind. 

Vanadium has divalent, trivalent, tetravalent and pentavalent cation states and hydrolysis species form with respect to all of these four states. The tetravalent state exists as the oxoanion, and similarly the pentavalent state as V02 . ... 

Description of the Method. The operational definition of water hardness is the total concentration of cations in a sample capable of forming insoluble complexes with soap. Although most divalent and trivalent metal ions contribute to hardness, the most important are Ca + and Mg +. Hardness is determined by titrating with EDTA at a buffered pH of 10. Eriochrome Black T or calmagite is used as a visual indicator. Hardness is reported in parts per million CaCOs. 

The site preference of several transition-metal ions is discussed in References 4 and 24. The occupation of the sites is usually denoted by placing the cations on B-sites in stmcture formulas between brackets. There are three types of spinels normal spinels where the A-sites have all divalent cations and the B-sites all trivalent cations, eg, Zn-ferrite, [Fe ]04j inverse spinels where all the divalent cations are in B-sites and trivalent ions are distributed over A- and B-sites, eg, Ni-ferrite, Fe Fe ]04 and mixed spinels where both divalent and trivalent cations are distributed over both types of sites,... 

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

In aqueous solutions, trivalent lanthanides ate very stable whereas only a limited number of lanthanides exhibit a stable divalent or tetravalent state. Practically, only Ce and Eu " exist in aqueous solutions. The properties of these cations ate very different from the properties of the trivalent lanthanides. For example, Ce" " is mote acidic and cetium(IV) hydroxide precipitates at pH 1. Eu " is less acidic and eutopium(II) hydroxide does not precipitate at pH 7—8.5, whereas trivalent lanthanide hydroxides do. Some industrial separations ate based on these phenomena. 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

Electroanalytical chemistry is one of the areas where advantage of the unique properties of SAMs is clear, and where excellent advanced analytical strategies can be utilized, especially when coupled with more complex SAM architectures. There are a number of examples where redox reactions are used to detect biomaterials (357,358), and where guest—host chemistry has been used to exploit specific interactions (356,359). Ion-selective electrodes are an apphcation where SAMs may provide new technologies. Selectivity to divalent cations such as Cu " but not to trivalent ions such as Fe " has been demonstrated (360). 

For vanadium solvent extraction, Hon powder can be added to reduce pentavalent vanadium to quadrivalent and trivalent Hon to divalent at a redox potential of —150 mV. The pH is adjusted to 2 by addition of NH, and an oxyvanadium cation is extracted in four countercurrent stages of mixer—settlers by a diesel oil solution of EHPA. Vanadium is stripped from the organic solvent with a 15 wt % sulfuric acid solution in four countercurrent stages. Addition of NH, steam, and sodium chlorate to the strip Hquor results in the precipitation of vanadium oxides, which are filtered, dried, fused, and flaked (22). Vanadium can also be extracted from oxidized uranium raffinate by solvent extraction with a tertiary amine, and ammonium metavanadate is produced from the soda-ash strip Hquor. Fused and flaked pentoxide is made from the ammonium metavanadate (23). 

Competitive Extraetion of Anions. The successful extraction of the necessary anion into the organic phase is cmcial for PTC. Often three anions compete for the catalyst cation the one that is to react, the one formed in the reaction, and the one brought in originally with the catalyst. Table 1 hsts the widely differing values of tetra-rr-butylammonium salts. The big difference in the halide series is noteworthy and preparatively important. Hydroxide is 10 times mote difficult to extract than chloride (11) and the divalent and trivalent anions and PO " are stiU more hydrophilic. Thus... 

Monovalent cations are compatible with CMC and have Httle effect on solution properties when added in moderate amounts. An exception is sUver ion, which precipitates CMC. Divalent cations show borderline behavior and trivalent cations form insoluble salts or gels. The effects vary with the specific cation and counterion, pH, DS, and manner in which the CMC and salt are brought into contact. High DS (0.9—1.2) CMCs are more tolerant of monovalent salts than lower DS types, and CMC in solution tolerates higher quantities of added salt than dry CMC added to a brine solution. 

---