Oxide-Retained Cations

The relative retention of divalent ions by amorphous Fe hydroxyoxides is 

Although differing in detail, the retention by these three major soil components is rather similar. The measurements were done in the absence of organic matter or sulfide ions, so only the attraction to 02--dominated surfaces was determined. 

Conversely, when the concentrations of these cations are too high—A1 toxicity in acid soils or Fe and Mil toxicity in rice paddies—raising the soil pH by liming is ef- 

The retention of this group increases, that is, mobility and plant availability decreases, rapidly at first and the rate slows with time. This behavior is similar to diffusion and suggests that the mechanism is the slow transfer of surface ions into the weathered layer on soil particles. The initial rate is rapid because the surface concentration is relatively high and die diffusion path length is short. As diffusion inward proceeds, the surface concentration decreases and the diffusion path length increases. The shallower concentration gradient slows the rate of further cadon diffusion. 

One school of thought maintains that certain sites on soil surfaces can retain these cations strongly radiographs show the cations are both bunched and spread out on soil surfaces. Another school suggests that these adsorption sites are where the cations can mix as solid solutions with the other ions on the surfaces. The free energy of mixing on the surfaces (Appendix 3.2) is responsible for the strong retention rather than any uniquely favorable adsorption spots on soil particle surfaces. In any case, soil retention can reduce the aqueous solubility of these ions to well below that in equilibrium with their pure hydroxyoxides. 

---

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. 

There are also reactive intermediates known as radical cations. The geometries of such species can also be understood using our notions of bonding. One example is the one-electron oxidation of an alkene, where the electron is removed from the tt orbital. For all al-kenes besides ethylene (see Chapter 2 for a discussion of the radical cation of ethylene), oxidation retains a planar structure in the alkene. However, the mixing of the alkyl group s IT-like group orbitals with the now singly occupied tt orbital becomes even more pro-... 

Oxidizing nature of the fission process. The fission of a mole of UF.1 would yield more equivalents of cation than of anion if the noble gas isotopes of half-life greater than 10 min were lost and if all other elements formed fluorides of their lowe.st reported valence state. If this were the case the system would, presumably, retain cation-anion equivalence by reduction of fluorides of the most noble fission products to metal and perhaps by reduction of some U + to U +. If, however, all the elements of uncertain valence state listed in Article 12-6.2 deposit as metals, the balance would be in the opposite direction. Only about 3.2 equivalents of coml)iiicd cations result, and since the number of active anion equivalents is a minimum of 4 (from the four fluorines of UF4), the deficiency must 1)0 alleviated by oxidation of the container. The evidence from the Aircraft Reactor Experiment, the in-pile loops, and the in-pile capsules has not shown the fission process to cause serious oxidation of the container it is possible that these experiments burned too little uranium to yield significant results. If fission of UF4 is shown to be oxidizing, the detrimental effect could be overcome by deliberate and occasional addition of a reducing agent to create a small and stable concentration of soluble UF3 in the fuel mixture. 

Antimonic acid has been used as an ion-exchange material for a number of cations in acidic solution. Most interesting is the selective retention of Na" in 12 Af HQ, the retention being 99.9% (24). At lower acidities other cations are retained, even K". Many oxidation and polymerization catalysts are listed as containing Sb203. 

In all of these oxide phases it is possible that departures from the simple stoichiometric composition occur dirough variation of the charges of some of the cationic species. Furthermore, if a cation is raised to a higher oxidation state, by the addition of oxygen to tire lattice, a conesponding number of vacant cation sites must be formed to compensate tire structure. Thus in nickel oxide NiO, which at stoichiomen ic composition has only Ni + cations, oxidation leads to Ni + ion formation to counterbalance the addition of extra oxide ions. At the same time vacant sites must be added to the cation lattice to retain dre NaCl sUmcture. This balanced process can be described by a normal chemical equation thus... 

In part the parabolic law may also apply to multilayer oxide systems where the cation diffusion coefficient is much higher in the lower oxide tlran in the higher oxide, which, growing as a thin layer, undergoes plastic deformation at high temperatures, thus retaining the overall oxide layer as impervious to enuy of tire gas. 

Generally the configuration is retained on oxidation, though a certain amount of isomerization can take place. (Br2 added trans to trans-Pt(PEt3)2Cl2 in the dark, but scrambling was found in the light [175].) Anionic and cationic complexes can be made ... 

Figure 2. Radical cations (polarons) and dications (bipolarons) obtained by oxidation of the neutral chain. The rotated angles and counter-ions needed to retain electroneutrality are not shown.

To avoid this phase change, zirconia is stabilized in the cubic phase by the addition of a small amount of a divalent or trivalent oxide of cubic symmetry, such as MgO, CaO, or Y2O3. The additive oxide cation enters the crystal lattice and increases the ionic character of the metal-oxygen bonds. The cubic phase is not thermodynamically stable below approximately 1400°C for MgO additions, 1140°C for CaO additions, and below 750°C for Y2O3 additions. However, the diffusion rates for the cations are so low at Xhtstsubsolidus temperatures that the cubic phase can easily be quenched and retained as a metastable phase. Zirconia is commercially applied by thermal spray. It is also readily produced by CVD, mostly on an experimental basis. Its characteristics and properties are summarized in Table 11.8. 

Powder XR diffraction spectra confirm that all materials are single phase solid solutions with a cubic fluorite structure. Even when 10 mol% of the cations is substituted with dopant the original structure is retained. We used Kim s formula (28) and the corresponding ion radii (29) to estimate the concentration of dopant in the cerium oxide lattice. The calculated lattice parameters show that less dopant is present in the bulk than expected. As no other phases are present in the spectrum, we expect dopant-enriched crystal surfaces, and possibly some interstitial dopant cations. However, this kind of surface enrichment cannot be determined by XR diffraction owing to the lower ordering at the surface. 

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

In most cases oxidation of uncharged borabenzene complexes produces cations which can only be observed electrochemically. The iron compounds 62 and 63 may serve as an example. Oxidation is fully reversible in rigorously dried CH2C12 but irreversible in more basic solvents such as THF and acetonitrile (62). Preparative oxidation with Ce(IV) salts cleanly produces monosubstituted ferricenium cations 64 (Scheme 10) (66). In contrast to the above mentioned boranediyl extrusions, the substituent at boron is retained here in the newly formed cyclopentadienyl ring. 

Uptake measurements were made [16] at several oxide/solution ratios, reported as surface loading (SL) or m2 oxide surface/liter of solution, as PdCl, 2 concentration was increased and pH was held constant at the optimal value (Figure 6.10a). Each SL indeed indicated a plateau near the steric value [16], For Pt and Pd ammine cations, the maximum surface density over many oxides appears to be a close-packed layer, which retains two hydration sheaths representative results for PTA uptake over silica from a recent paper [19] are shown in Figure 6.10b. The physical limit of cationic ammine surface density thus appears to be 0.84 pmol/m2, or about 1 cationic complex/2 nm2. Cationic uptake, therefore, is inherently half of anion uptake in many cases. 

An additional important feature of this class of polymers lies in the fact that their polymerisation and doping processes may be driven by a single electrochemical operation which, starting from the monomer, first forms the polymeric chain and then induces its oxidation and deposition in the doped form as a conductive film on a suitable substrate. The polymerisation reaction may be basically described as an electrophilic substitution which retains the aromatic structure and proceeds via a radical cation intermediate ... 

---