Oxidation states in solution

Studies of the oxidation-state behaviour of plutonium in environmental waters illustrate this situation very clearly. 

In 1973, the United States Atomic Energy Commission (USAEC) conducted an intensive investigation into its research efforts relating to the development of nuclear power. The conclusion reached by the environmental team was that the environmental behaviour and long-term fate of the transuranium elements had not been addressed in an effective way. This conclusion prompted the USAEC to develop an investigative programme. Some important landmarks in the environmental research which led to the eventual separation and determination of the plutonium oxidation states in the aquatic environment were  

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An alternative procedure for the study of neptunium oxidation states at trace concentrations has been described by Inoue and Tochiyama (1977). They showed that, in the pH range 6-7, Nplv may be quantitatively absorbed on silica gel whilst Npv remains in solution. In acid solution, however, a precipitate of barium sulfate selectively absorbs Nplv leaving the higher oxidation states in solution. The authors gave no environmental data for neptunium in their publication but Nelson and Orlandini (1979) subsequendy adapted the procedure to demonstrate that the dominant oxidised plutonium species in natural waters is Puv and not Puvl. 

Scheme I further indicates the tendency of the Ln(III) cations to form the mofe unusual oxidation states in solution [73]. Hitherto, organometallic compounds of Ce(IV), Eu(II), Yb(II) and Sm(II) have been isolated. Charge-dependent properties, such as cation radii and Lewis acidity, significantly differ from those of the trivalent species (Table 4). Ln(II) and Ce(IV) ions show very intense and ligand-dependent colors which is attributed to Laporte-allowed 4/-+ 5d transitions [65b]. Complexes of Ce(IV) and Sm(II) have acquired considerable importance in organic synthesis due to their strong oxidizing and reducing behavior, respectively their reaction patterns have been reviewed in detail [40, 44-47, 74], Catalytic amounts of compounds containing the hot oxidation states also initiate substrate transformations as a rule this implies switch to the more stable, catalytically-acting Ln(III) species [75],...

Plutonium chemists use reaction (1) as the net reaction for reactions (2), (3), and (4). This is clearly documented in the Plutonium Handbook edited by Wick, and in Cleveland s book. The Chemistry of Plutonium. Reaction (1) is an accurate representation of an equilibrium and the equilibrium concentration quotient is the product of the quotients for reactions (2), (3), and (4). Therefore, it is correct to discuss equilibrium concentrations of Pu(IV), Pu(VI), and Pu(III), without Pu(V). Circumstances where the net reaction has not been properly considered are those where concentrations of oxidation states in solutions with low acidity are calculated without consideration of Pu(IV) hydrolysis and polymerization. The distributions of Pu oxidation states (including Pu(V) and Pu(IV) polymer in nitric acid systems) were reported in JINC 609 (1973), and Silver has not included... 

The dominant oxidation state in solution is Cu(II), while the dominant hydrolysis product is the dimer CuilOH) . A. series of mononuclear species, CuOH to Cu(OH)4 ", is formed as well, the first three only in very dilute solutions between pH 8 and 12 and the last only in more alkaline solutions. 

Altogether, C q displays a very rich electrochemistry, with eight oxidation states in solution, comprising six negative states, one neutral state, and one positive state. 

The data in Figures 1 and 2 are consistent with the existence of Pu(V) as the dominant oxidation state in solution in seawater. It is not possible to state whether this represents thermodynamic equilibrium or some balance of opposing redox conditions which results in a steady state concentration of Pu(v). Pu(Vl) can be rapidly reduced to Pu(V). However, in the presence of humic materials a significant fraction of the Pu(Vl) is reduced directly to Pu(lV) which hydrolyzes and sorbs to the walls and to particulate matter. Apparently there is competition between reduction of Pu(Vl) to Pu(V) by seawater and complexatlon of Pu(Vl) by humic acid (16). The latter results in rapid reduction of Pu(Vl) to Pu(lV) with subsequent hydrolysis. Complexatlon of Pu(V) by humic acid should be much weaker and may account for the slow reduction of Pu(V) in the dark. Photolysis of the humic material apparently results in some oxidation of Pu(lV) to Pu(V), to provide a metastable Pu(V) concentration. 

Knowledge of the functional role of zinc as a component of metalloen-zymes is much less complete. Zinc differs from iron in several aspects. The known zinc enzymes are colorless, and the doubly ionized atom, having no unpaired electrons, has no paramagnetic moment. The metal has but one stable oxidation state in solution, is tightly bound to the apoenzyme, and can be removed only with severe chemical treatment. In no instance, so far, has the group binding the metal to enzyme been established with certainty. 

There is no evidence for the existence of any low oxidation state in solution, and little, if any, for its existence in the solid state. 

FIG. 22.5. Concentration of free plutonium ions at different oxidation states in solutions of different pH, showing the effect of hydrolysis. 

This result shows that the fraction of Cu ions at equilibrium is generally very small. When, in the absence of complexing agents, a salt of monovalent copper is dissolved in water, the Cu" " ions therefore disproportionate according to reaction (2.75). In a general way, we may conclude that if a metal can form two ions of different oxidation state in solution, the lower valence ion is thermodynamically unstable if its equilibrium potential is more noble than that of the higher valence ion. 

There is no evidence for an oxidation state in solution less than ii. A fluoride complex of tetravalent curium was obtained in aqueous solution when separately prepared CmF4 was dissolved in concentrated (15 m) MF solution (M = alkali-metal ion) [21,159]. Even under these conditions, and using the self-... 

Cerium is the most abundant element of the rare earths. On average the Earth s crast contains 66 ppm of cerium (=66 g per ton), a value that is very comparable with the abundance of copper (68 ppm) (Emsley, 1991). Eew people know that there are on Earth larger resources of cerium than of other more popular elements like cobalt (29 ppm), lead (13 ppm), tin (2.1 ppm), silver (0.08 ppm) or gold (0.004 ppm). A special property of cerium is that it has a stable tetravalent oxidation state besides the trivalent state which is so common for the rare earths. Although the tetravalent oxidation state is also known for solid state compounds of praseodymium and terbium, cerium is the only rare-earth element that has a stable tetravalent oxidation state in solution. Many of the applications of cerium are based on the one-electron Ce +/Ce + redox couple. 

Standard electrode potentials and stability of different oxidation states of transition metal ions in aqueous solutions The stability of a particular oxidation state in solution can be explained in terms of its electrode potential which in turn depends upon enthalpy of sublimation, ionization energy and hydration energy. 

Spectroelectrochemical techniques have been relatively widely used both to confirm chemical reversibility of fullerene reduction reactions and to record the spectra of the fiillerene anions. Although the fullerenes and their anions generally exhibit different colors, their spectra in the visible region are not sufficiently different [29,41,62] to provide a route for unambiguously determining their oxidation state in solution. However, unique features, different for each oxidation state were found spectroelectrochemically in the near IR region of spectra of 50 1 ) [53,98] (Figure 7.14) and... 

Production of unusual oxidation states in solution by pulse radiolysis... 

Vanadium(II) is unstable in water (Baes and Mesmer, 1976) being oxidised to vanadium(III), the reaction being accompanied by the production of hydrogen gas. However, hydrogen can be utilised to maintain the vanadium(II) oxidation state in solution. Data are only available for the first monomeric hydrolysis species ofvana-dium(II), VOH", which forms according to reaction (2.5) (M = p = l, q=l). 

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