Lanthanides iodates

Fig. 104, The occurrence of the lanthanide iodates. Rectangles indicate groups of isostructural compounds and arrows indicate decomposition paths heating in Nj. (Nassau et al., 1975.)...

Fig. 108. The IR absorption spectra of some lanthanide iodate hydrates in the region 4000-100 cm (Nassau et al., 1974).

Solid Compounds. The tripositive actinide ions resemble tripositive lanthanide ions in their precipitation reactions (13,14,17,20,22). Tetrapositive actinide ions are similar in this respect to Ce . Thus the duorides and oxalates are insoluble in acid solution, and the nitrates, sulfates, perchlorates, and sulfides are all soluble. The tetrapositive actinide ions form insoluble iodates and various substituted arsenates even in rather strongly acid solution. The MO2 actinide ions can be precipitated as the potassium salt from strong carbonate solutions. In solutions containing a high concentration of sodium and acetate ions, the actinide ions form the insoluble crystalline salt NaM02(02CCH2)3. The hydroxides of all four ionic types are insoluble ... 

The chlorides, bromides, nitrates, bromates, and perchlorate salts ate soluble in water and, when the aqueous solutions evaporate, precipitate as hydrated crystalline salts. The acetates, iodates, and iodides ate somewhat less soluble. The sulfates ate sparingly soluble and ate unique in that they have a negative solubitity trend with increasing temperature. The oxides, sulfides, fluorides, carbonates, oxalates, and phosphates ate insoluble in water. The oxalate, which is important in the recovery of lanthanides from solutions, can be calcined directly to the oxide. This procedure is used both in analytical and industrial apptications. 

Determination of cerium as cerium(IV) iodate and subsequent ignition to cerium(IV) oxide Discussion. Cerium may be determined as cerium(IV) iodate, Ce(I03)4, which is ignited to and weighed as the oxide, Ce02. Thorium (also titanium and zirconium) must, however, be first removed (see Section 11.44) the method is then applicable in the presence of relatively large quantities of lanthanides. Titrimetric methods (see Section 10.104 to Section 10.109) are generally preferred. 

Bancroft and Gesser [870] conclude that kinetic factors are predominant in determining whether decomposition of a metal bromate yields residual bromide or oxide. The thermal stabilities of the lanthanide bromates [877] and iodates [877,878] decrease with increase in cationic charge density, presumably as a consequence of increased anionic polarization. Other reports in the literature concern the reactions of bromates of Ag, Ni and Zn [870] and iodates of Cd, Co, Mn, Hg, Zn [871], Co and Ni [872], Ag [864], Cu [867], Fe [879], Pb [880] andTl [874]. 

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. 

The solution reactions of Cm3+ closely resemble those of the lanthanide and actinide +3 ions, and the fluoride, oxalate, phosphate, iodate, and hydroxide are insoluble. Complexes appear to be weaker than those of preceding elements. 

A simple and rapid method for the iodometric determination of microgram amounts of chromium(ni) in organic chelates is based on the oxidation of chromium(III) with periodate at pH 3.2, removal of the umeacted periodate by masking with molybdate and subsequent iodometric determination of the liberated iodate . Iodometric titration was also used for determination of the effective isoascorbate (see 2) concentration in fermentation processes . The content of calcium ascorbate can be determined with high sensitivity by complexometric titration with edta, which is superior to iodometry. The purity of /3 -diketonate complexes of Al, Ga, In and Ni was determined by complexometric titration with edta at pH 5.5-3, with RSD < 0.01 for determining 5-30% metal ion. Good analytical results were obtained by a similar procedure for the metal content of 15 lanthanide organic complexes. ... 

Perchlorates and iodates. Thorium perchlorate forms upon dissolution of thorium hydroxide in perchloric acid and crystallizes as Th(C104)4 4H20. The precipitation of tetravalent actinides as iodates has long been used to separate these elements from lanthanides at low pH. One of the earliest forms that Pu was isolated in was that of Pu(I03)4. The structure and most properties of Pu(103)4 are currently unknown, but a remarkable feature is that it is insoluble in 6M HNO3. 

The actinide iodate system is one of considerable interest that has attracted chemists for more than 150 years (vide supra). In fact one of the first forms that was isolated in was as the iodate salt, presumably as 1 0(103)4 [63], The precipitation of iodate compounds of the actinides has been used for decades as a method of separated them from lanthanides and other fission products. The precipitation of thorium iodate is perhaps best known in this regard [64-66], but several patents exist describing selective precipitation of transuranium elements [67-72], Despite the key importance of iodate in actinide chemistry the structures of actinide iodates were not described in detail until approximately 2000. 

The thermal stabilities of the lanthanide bromates and iodates [43] decrease with increase in cationic charge density, presumably as a consequence of increased anionic polarization. Metallic lead reacts [44] with K, Ca and Ba iodates to yield the iodites at about 700 K ... 

In 0.2 M Na2C03, the light lanthanides (cerium group) are precipitated quantitatively, while the remaining lanthanides and scandium are only partly precipitated [18). Separation of Ce(IV) as the hydroxide (pH 1) enables the separation of Ce from other REE. Ti, Zr, or Fe(III) can be used as carriers. Ce(IV) may also be precipitated as iodate. 

The lanthanides are separated from most other elements by precipitation of oxalates or fluorides from nitric acid solution. The elements are separated from each other by ion-exchange, which is carried out commercially on a large scale. Cerium and europium are normally first removed, the former by oxidation to Celv and removal by precipitation of the iodate which is insoluble in 6M HN03 or by solvent extraction, and the latter by reduction to Eu2+ and removal by precipitation as insoluble EuS04. 

Thermal studies of other lanthanide salts have been reviewed by Niinisto and Leskela (1987). For most salts the total dehydration occurs in a single-step reaction, although for a few systems (e.g. the selenate and iodate hydrates) dehydration occurs via a multi-step process. 

Cm(iii) is a class A or hard metal ion and thus complexes far more strongly to oxygen and fluoride donors than to more polarizable donors such as chloride or sulfur. Solution reactions of Cm resemble those of the trivalent lanthanides and actinides. The fluoride, oxalate, phosphate, iodate, and hydroxide are essentially water-insoluble and the chloride, iodide, perchlorate, nitrate, and sulfate are water-soluble. The first hydrolysis constant for Cm ", i.e. for the reaction ... 

The actinide ions in aqueous solution resemble the tripositive lanthanide ions in their precipitation reactions, allowing for differences in the redox properties of early members of the actinide series. The chloride, bromide, nitrate, bromate, and perchlorate anions form water-soluble salts, which can be isolated as hydrated solids by evaporation. The acetates, iodates, and iodides are somewhat less soluble in water. The sulfates are sparingly soluble in hot solutions, somewhat more soluble in the cold. Insoluble precipitates are formed with hydroxide, fluoride, carbonate, oxalate, and phosphate anions. Precipitates formed from aqueous solution are usually hydrated, and the preparation of anhydrous salts from the hydrates without formation of hydrolyzed species can only be accomplished with difficulty. The actinide(iv) ions resemble Ce(iv) in forming fluorides and oxalates insoluble even in acid solution. The nitrates, sulfates, perchlorates, and sulfides are all water-soluble. The iv state actinide ions form insoluble iodates and arsenates even in rather strong acid solution. The... 

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