Lanthanides bromates

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]. 

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 ... 

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. 

RE(N0 )2 NH NO 4H20 for light lanthanide separation (La, Nd, Pr) 2RE(N02)3 3Mg(N03)2 24H20 for middle lanthanide separation (Sm, Eu, Gd). Bromates and ethylsulfates have been found useful. Fractional crystallization is particularly slow and tedious for the medium and heavy rare earths. 

The classical methods used to separate the lanthanides from aqueous solutions depended on (i) differences in basicity, the less-basic hydroxides of the heavy lanthanides precipitating before those of the lighter ones on gradual addition of alkali (ii) differences in solubility of salts such as oxalates, double sulfates, and double nitrates and (iii) conversion, if possible, to an oxidation state other than -1-3, e g. Ce(IV), Eu(II). This latter process provided the cleanest method but was only occasionally applicable. Methods (i) and (ii) required much repetition to be effective, and fractional recrystallizations were sometimes repeated thousands of times. (In 1911 the American C. James performed 15 000 recrystallizations in order to obtain pure thulium bromate). 

It has always been assumed that the hydration numbers for the lanthanides are higher than six, probably between 8 and 10, in analogy with the presence of enneaaquo ion [M(OH2)9 +] in neodymium bromate and ethylsulphate (see later, p. 121). Lanthanide hydration numbers have not been rigorously established, but some attempts have been made to study the problem by NMR technique (13—15). It is rather unfortunate that only low value for the hydration numbers ( 6) have been obtained, except for Er(III) and Yb(III) ions (756), where the hydration number is seven. 

The classical example of nonacoordinated lanthanide ion is, of course, the enneaaquo ions, [M(OH2)9] +, present in bromate, sulphate and ethylsulphate salts. The nine water molecules form a tricapped trigonal prism around the central lanthanide ion. The nine M—0 distances for La and Nd in La2(S04)s-9 H2O and in Nd(Br03)3 9 H2O are virtually the same, being 2.72 and 2.49 A (average) respectively (Table 17) 182—186). The three equatorial M—0 distances for M = Pr, Er and Y, in ethylsulphates are somewhat larger than other six M—0 distances to the oxygens situated on the prism corners (Table 17). 

Classical methods of separation [7] are (1) fractional crystallization, (2) precipitation and (3) thermal reactions. Fractional crystallization is an effective method for lanthanides at the lower end of the series, which differ in cation radius to a large extent. The separation of lanthanum as a double nitrate, La(N03)3-2NH4N03-4H20, from praseodymium and other trivalent lanthanide with prior removal of cerium as Ce4+ is quite a rapid process and is of commercial significance. Other examples are separation of yttrium earths as bromates, RE(Br03>9H20 and use of simple nitrates, sulfates and double sulfate and alkali metal rare earth ethylenediamine tetraacetate complex salts in fractional crystallization separation. 

These are readily prepared by reaction of the lanthanide oxide or carbonate with the acid. Salts of noncoordinating anions most often crystallize as salts [Ln(OH2)9]X3 (X e.g. bromate, triflate, ethyl sulfate, tosylate). These contain the [Ln(OH2)9] + ion (Figure 4.1), even for the later lanthanides, where in aqueous solution the eight-coordinate species [Ln(OH2)8] predominates. 

Separation of the light lanthanides, after removal of the Ce, has been accomplished in many ways, based mainly on solubility differences fractional crystallisation of the double magnesium nitrates, 2Ln (N03)3.3Mg(N03)2.24 H2O, was an early method (James, 1908). Tlie heavy lanthanides from the double sulphate solution (above) and from ores such as xenotime have been separated by fractional crystallisation of the bromates (James, 1908). Prandtl (1938) used double ammonium oxalates. Hartley (1952) obtained a 85% yield of mixed anhydrous lanthanide chlorides by direct chlorination of a mixture of monazite and carbon at 900" most of the impurities are more volatile. 

A noteworthy example of the application of the technique of fractional crystallisation in Inorganic Chemistry is the accomplishment of the challenging job of separation of lanth-anides. Simple salts like nitrates, oxalates, bromates and sulphates as well as double salts like 2 La (N03)3 3 Mg (N03)2 24H20 of these elements are easily crystallisable. Taking advantage of small differences of solubilities of these salts in water, separation of lanthanides has been achieved by repeated fractional crystallisation of these salts. 

This method has been considered the best of the classical separation procedures for producing individual elements in high purity. The most suitable compounds are ammonium nitrates (for La, Pr, and Nd) and double magnesium nitrates (for Sm, Eu, Gd). Manganese nitrates have also been used for separation of lanthanides of the cerium group (La-Nd). Bromates and sulphates have been used in the separation of the yttrium group (being the heavy lanthanides or HREE)... 

Since about 40% of R in minerals of the lighter lanthanides (large ionic radii) is cerium, it early attracted interest to remove the large majority of cerium in a simple process. Bromate oxidizes Ce(III) to a precipitate in solutions buffered by the reactive, but only slightly soluble base calcium carbonate. Ethers can extract ah orange cerium(IV) complex from nitric acid. Concentrates with minor amounts of other R can be recrystallized as (NH4)2Ce(N03). ... 

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