Hexavalent ions

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

How can we be sure that the U +(Q2-) complex in a mixed metal oxide is present as the UO octahedron This can be done by studying solid solution series between tungstates (tellurates, etc.) and uranates which are isomorphous and whose crystal structure is known. Illustrative examples are solid solution series with ordered perovskite structure A2BWi aUa 06 and A2BTei-a Ua 06 91). Here A and B are alkahne-earth ions. The hexavalent ions occupy octahedral positions as can be shown by infrared and Raman analysis 92, 93). Usually no accurate determinations of the crystallographic anion parameters are available, because this can only be done by neutron diffraction [see however Ref. (P4)]. Vibrational spectroscopy is then a simple tool to determine the site symmetry of the uranate complex in the lattice, if these groups do not have oxygen ions in common. In the perovskite structure this requirement is fulfilled. 

The behavior of nickel on anodic polarization is matched by the behavior of iron and cobalt on the surface of which oxygen is also liberated at higher current densities. Chromium anode dissolves at low current densities to form bivalent cations. When it becomes passive its potential increases by about 1 Volt. With further inorease of potential chromium enters a state called transpassive state in which instead of bivalent ions hexavalent ions are formed which reaot with the hydroxyl ions present in the electrolyte to form chromate ions according to equation ... 

Stable hexavalent ions of the type An02 (An = U, Np, Pu) can be generated in aqueous media. The uranyl ion (U02 ) is the most stable form of uranium in aqueous media under aerobic conditions. In general, aqueous Np and Pu compounds can be prepared from Np and Pu ... 

Chromium occurs in three basic forms metallic chromium (Cr(0)), trivalent chromium (Cr(III)), and hexavalent chromium (Cr(VI)). Hexavalent chromium can exist as chromium hexavalent ion and as part of a number of compounds including calcium chromate, chromic acid, chromium trioxide, lead chromate, strontium chromate, potassium dichromate, and zinc chromate. 

In certain applications, the quaternary ammonium salts have advantages over tertiary amines for actinide extractions. Quantitative extraction of the transplutonium elements from nitrate media by quaternary ammonium salts can be achieved with a lower aqueous-phase nitrate concentration than is required for tertiary amines thus, aluminum nitrate may be used instead of lithium nitrate. The separation factor between Am and Cm can be as high as three in a quaternary ammonium nitrate system (50). The effect of the length of the alkyl chain on Cm, Am, Bk, Cf, and Es extraction by alkyldioctylammonium nitrates suggests that steric factors substantially influence the extraction selectivity (51). A comparison of the extraction of tetravalent and hexavalent actinides by tetraheptyl ammonium nitrate shows that tetravalent ions are more easily extracted than hexavalent ions e.g.,... 

Organo phosphorus solvents were selected because they are radiation resistant, possess weak reductant properties and that their affinity for hexavalent ion is high. 

Uranium-doped tungstates with ordered perovskite structure having compositions A2BWi xUx06 have been used for a detailed study of the luminescence properties of the octahedral uranate group 10-13,21,22,25) pgj alkaline-earth ions Ca " ", Sr , Ba i have been used the B ions were the alkaline-earth ions, or Cd ". This type of compounds is particularly suited for such a study in view of the high site symmetry of the hexavalent ions in compounds with this structure and because of the fact that only one crystallographic site can be occupied by hexavalent uranium (c.f. Sect. 1). 

Peppard et al. used HgMEHP in various diluents, and tri-n-octyl phosphine oxide to effect a sequential separation of various tri-, tetra-, and hexavalent ions from urine. 

Values of y ranged from 1 to 3 for various di-, tri-, and hexavalent ions. Th(IV) had a y value of 1 as did U02, except in the case of TOPO where a value of 3 was also obtained at high P concentrations. 

Uranium is a strong oxiphile element, occurring as the tetra-valent ion U" " and the hexavalent ion U (uranyl ion). Oxidation of the former is possible over a wide pH range by alteration of the primary sources of uranium, oxygen being supplied by air dissolved in water. Uranium is then separated from all the naturally radioactive products of disintegration. 

Precipitation occurs as a result of the reduction of the hexavalent ion to a tretravalent ion with modification of pH, pressure, temperature, etc. Uranium is fixed by clay minerals, iron and manganese hydroxides, phosphates and organic matter. 

The uranium dioxide is a p-type semiconductor with interstitial oxygen ions O and manium hexavalent ions in the place of tetravalent ions (electronic defect ). 

Chromium has divalent, trivalent and hexavalent ions. The divalent state is unstable in water, producing hydrogen whilst being oxidised to a higher valency state (Baes and Mesmer, 1976). Chromium(lll) has a large range of stability, whereas chromium(VI), unlike vanadium(V), only forms anionic species and, as such, will... 

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