Cerium , oxidation oxalate

Cerium sulfide pigments are produced from hydrated cerium oxide or oxalate and calcined in an oxygen-free, sulfide environment. They are silica-encapsulated to minimize water-reactivity and to improve heat stability and chemical resistance properties. Because of their low relative value-in-use, they are used primarily in engineering plastics and in particular the polyamides where high-performance organic colorant alternatives and other inorganic pigment alternatives are few. 

Cerium-oxalate, -carbonate and -hydroxide are considered to be the most important precursors for cerium-derivatives on a commercial scale. The cerium derivatives are yielded from these compounds by additional chemical and/or physical treatment. For example, cerium oxide may be formed easily by calcining cerium carbonate or/and cerium oxalate respectively. 

Ceda-based oxides can be obtained by the decomposition of some compound precursor, such as hydroxide, nitrate, halides, sulfates, carbonates, formates, oxalates, acetates, and citrates.For example, nanosize or porous cerium oxide particles have been prepared at low temperatures by pyrolysis of amorphous citrate," which is prepared by the evaporation of the solvent from the aqueous solution containing cerium nitrate (or oxalate) and citric acid. In the case of mixed oxides, the precursor containing some cations in the same solid salts is prepared. In the same manner of ceria particles, the precursors complexing some cations with citrates are useful to synthsize ceria-zirconia mixed oxides and their derivatives. Also. Ce02-Ln203 solid solutions, where Ln = La. Pr, Sm. Gd. and Tb, have been synthesized from the precursors obtained by the evaporation of nitrate solutions at 353 K in air from an intimate mixture of their respective metal nitrates. The precursors are dried and then heated at 673 K to remove niU ates, followed by calcination at 1073 K for 12h. 

One of the characteristic chemical properties of the group, and certainly the most useful in connection with analysis and isolation, is the ease and completeness of precipitation of the hydrated oxalates from acidic solutions. When the oxalates are ignited at 900°, the oxides (R.E.)203, are obtained, except in the case of cerium(III) oxalate, which gives the dioxide. 

Cerium oxide is extracted from bastnasite by roasting the ore with concentrated sulfuric acid, or with sodium carbonate [262]. Since, in the first process, HFforms as a byproduct, roasting with sodium carbonate is the preferred method. The rare earth elements contained in calcine are extracted by leaching with hydrochloric acid, and recovered from the leachate by precipitation with oxalic acid. During the course of roasting, cerium(III) is oxidized to cerium(IV). Ce02 is insoluble in dilute hydrochloric acid this is in contrast to other trivalent rare earth elements, which can be easily leached out, as can the impurities such as Fe, Ca, and Mg. 

Nanosized cerium oxide also shows excellent properties. Oxalic and oxamic acid, aniline, and reactive dyes (C.I. Reactive Blue 5) could be effectively converted. The use of the catalyst significantly enhances the reaction rate and a synergetic effect between cerium oxide and carbon (used as the support) was noted. In addition, carbon is also active in the catalytic ozonation, and multiwall carbon nanotubes were shown to have high efficiency in the catalytic ozonation of oxalic and oxamic acids. 

Standardized solutions of cerium(IV) perchlorate can be prepared starting from ammonium hexanitratocerate(IV) (Daugherty and Taylor, 1972). Under this firm, cerium(IV) is reduced to cerium(III) by hydrogen peroxide, and cerium(ni) is precipitated by oxalic acid. Cerium(in) oxalate is ignited to form cerium dioxide, which is subsequently dissolved in hydrogen peroxide, and the latter is destroyed by boiling. Finally, cerium(III) is electrolytically oxidized to cerium(IV). 

Redox titrants (mainly in acetic acid) are bromine, iodine monochloride, chlorine dioxide, iodine (for Karl Fischer reagent based on a methanolic solution of iodine and S02 with pyridine, and the alternatives, methyl-Cellosolve instead of methanol, or sodium acetate instead of pyridine (see pp. 204-205), and other oxidants, mostly compounds of metals of high valency such as potassium permanganate, chromic acid, lead(IV) or mercury(II) acetate or cerium(IV) salts reductants include sodium dithionate, pyrocatechol and oxalic acid, and compounds of metals at low valency such as iron(II) perchlorate, tin(II) chloride, vanadyl acetate, arsenic(IV) or titanium(III) chloride and chromium(II) chloride. 

Cerium(IV) oxide may be obtained by heating cerium oxalate, carbonate or other salts at elevated temperatures ... 

The oxalates obtained above, alternatively, are digested with sodium hydroxide converting the rare earth metals to hydroxides. Cerium forms a tetravalent hydroxide, Ce(OH)4, which is insoluble in dilute nitric acid. When dilute nitric acid is added to this rare earth hydroxide mixture, cerium(lV) hydroxide forms an insoluble basic nitrate, which is filtered out from the solution. Cerium also may be removed by several other procedures. One such method involves calcining rare earth hydroxides at 500°C in air. Cerium converts to tetravalent oxide, Ce02, while other lanthanides are oxidized to triva-lent oxides. The oxides are dissolved in moderately concentrated nitric acid. Ceric nitrate so formed and any remaining thorium nitrate present is now removed from the nitrate solution hy contact with tributyl pbospbate in a countercurrent. 

In one acid digestion process, monazite sand is heated with 93% sulfuric acid at 210°C. The solution is diluted with water and filtered. Filtrate containing thorium and rare earths is treated with ammonia and pH is adjusted to 1.0. Thorium is precipitated as sulfate and phosphate along with a small fraction of rare earths. The precipitate is washed and dissolved in nitric acid. The solution is treated with sodium oxalate. Thorium and rare earths are precipitated from this nitric acid solution as oxalates. The oxalates are filtered, washed, and calcined to form oxides. The oxides are redissolved in nitric acid and the acid solution is extracted with aqueous tributyl phosphate. Thorium and cerium (IV) separate into the organic phase from which cerium (IV) is reduced to metalhc cerium and removed by filtration. Thorium then is recovered from solution. 

After separation from other rare earths, ytterbium is usually obtained as its oxide, Yb203. If separated as oxalate, oxalate is converted into oxide by high temperature. Ytterbium oxide is reduced to metallic ytterbium by heating with lanthanum metal in high vacuum. The metal is purified by sublimation and collected over a condenser plate. Aluminum, zirconium, and cerium also are effective reducing agents and may be used instead of lanthanum. 

Two polarographic methods have been developed for the determination of cohalt(II) at concentrations ranging from approximately 1 to 80 mM in an aqueous sample. For the first method [15], which is suitable for samples containing large amounts of nickel]11), the cobalt(II) is oxidized to Co(NH3)6 in an ammoniacal medium with the aid of sodium perborate, after which the cobalt(III) species is determined. A second procedure [16] entails the use of lead dioxide in an acetic acid-acetate buffer containing oxalate to convert cobalt(II) to the 0(0204)3 ion, which can be subjected to polarographic reduction. This latter approach is well suited to the determination of cobalt in the presence of copper(II), iron(III), nickel(II), tin(IV), and zinc(II), whereas the chief interferences are cerium, chromium, manganese, and vanadium. 

Secondly, we studied the oxidation of oxalate by cerium(IV) in sulfuric acid. This reaction is analagous to the classical manganese (111) oxalate reaction studied by H. Taube and also by F. Duke. Indeed, it proceeds through an intermediate containing one or more oxalates per cerium. This decomposes to give cerium(III) and other products. The point I want to raise is that we have been able to detect free radicals in this reaction, using EPR experiments and a flow system. So far, it has not been possible to say whether the radical is the oxalate or CO2 radical, both of which have been proposed in other studies. 

K bicarbonate, K chloride, K iodide, K metaphosphate, K perchlorate, K phosphate, K silico-fluoride, K urea oxalate, sodium ammonium sulfate, sulfur, zinc sulfate and Zr oxide It was claimed that methylene urea reduced the flash to a far greater extent than any of the organic compounds used, ft was also stated that cerium salts were much more effective than any other metallic salts investigated (Ref 4)... 

An analogous reaction, but one in which one ligand supplies two electrons, has been observed for the oxidation of pentammineoxalatocobalt(III) by cerium-(IV). Saffir and Taube (10) have shown the oxalate is oxidized to carbon dioxide in a two-electron transfer, producing one equivalent of cobalt (II) and one equivalent of cerium (III). 

It is apparent that Cr(C2C>4)3 3, m-Cr(OH2)2(0204)2 , and Cr(0H2)4C2044 react smoothly with cerium(IV) in acidic-sulfate media, 1 mole of oxalate being oxidized for each 2 moles of cerium(IV) consumed. The observations made are consistent with the view that the three reactions proceed, at least initially, according to the stoichiometries represented by the respective Reactions 3,1, and 2. The initial absorbances of reactant solutions (values obtained by extrapolation of measured absorbances to zero time) agree well with the values calculated, on the basis of Beer s law, from the absorptivity coefficients of the components. Further, as the reactions in 1.83Af sulfuric acid proceed, the absorbances of the solutions move toward the values expected for the assumed products at rates which demonstrate the reactions are first order in cerium(IV) and complex—see, for example, Figure 2. We thus find no indication that reaction intermediates contribute measurably to the absorbances of reactant solutions, or that reaction conditions cause the rapid equilibration of any of the oxalato complexes with other species... 

Oxidizing reagents such as tetravalent cerium or potassium permanganate solutions may be standardized by oxalic acid, sodium oxalate, or potassium iodide. The reactions of Ce4+ and permanganate ions with oxalic acid in acid medium are given below ... 

Cerous oxalate, Ce2(C2O4)3-10H2O, is formed in the Welsbach treatment of Monazite sands in the production of thorium nitrate, and is the starting point in the preparation of nearly all cerium salts. Since ceric oxide is more easily acted upon by common reagents, it is often prepared as the first step in making other salts from the oxalate. 

In the quantitative determination of cerium, use is made of the fact that cerous oxalate is insoluble in neutral and acid solution. Since a quantitative yield is not necessary in this preparation, some of the cerium is sacrificed to insure complete removal of any iron that may be present. In order to have a neutral solution for the hydrolysis of the iron, a slight excess of ceric oxide is used. 

The oxidation of [Ir(ox)3]3- by cerium(IV) in aqueous acidic sulfate or perchlorate media has given [Ir(ox)3]2. That the product is indeed [Ir(ox)3]2- and not [Irm(ox)2(C207 )]2 finds support from the slow reaction observed in the presence of excess cerium(IV) free or coordinated oxalate radical anion would be expected to react rapidly with CeIv. In solution, [Ir(ox)3]2- undergoes slow pseudo-first-order reaction to yield the highly reactive intermediate [Ir I,(ox)2(C2Oi )]2. ... 

In general, lanthanides can be separated from mixtures with other elements by precipitation as oxalates or fluorides. Cerium and europium can conveniently be removed from the others, the former by oxidation to CeIV and precipitation as the iodate, and the latter by reduction to Eu2+, which can be precipitated as EuS04. 

Cerium (IV) in solution is obtained by treatment of Ce111 solutions with very powerful oxidizing agents, for example, peroxodisulfate or bismuthate in nitric acid. The aqueous chemistry of CeIV is similar to that of Zr, Hf, and, particularly, tetravalent actinides. Thus Ce gives a phosphate insoluble in 4 M HN03 and an iodate insoluble in 6 M HN03, as well as an insoluble oxalate. The phosphate and iodate precipitations can be used to separate Ce from the trivalent lanthanides. Ce is also much more readily extracted into organic solvents by tributyl phosphate and similar extractants than are the Lnm lanthanide ions. 

It maj- also be separated from solutions of uranium salts bj the addition of a little cerium salt and precipitation with aqueous hydrofluoric acid, or with oxalic acid or bj adding a small quantity of thorium nitrate and precipitating with t-nitrobenzoic acid. It may also be absorbed bj- charcoal,iDasic ferric acetate, and by various oxides, sulphides, sulphates, and gelatinous silica. In the case of charcoal the uranium X is completely removed from a solution of a... 

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