Lanthanides solubility

Different forms of lanthanide differ in their toxicity. There are three forms of lanthanides soluble (chlorides, nitrates, acetates), insoluble (oxides, carbonates), and chelated compounds (DTPA). Most of the available information on lanthanide absorption and toxicity comes from the soluble lanthanide salts. In one study, rats given DTPA (chelating agent) 1 or 2 days after oral administration of cerium chloride were found to have significantly reduced whole body retention of soluble cerium (from 40% to 2%). 

No water quality objectives or other water quality standards were found for the lanthanide metals. Aquatic toxicology data were only found for the lanthanide soluble salts. These soluble salts are known to have high chronic toxicity in fish, moderate chronic toxicity in green algae, and low acute toxicity in daphnids based on exposures in moderately hard water in terms of lanthanide per liter. 

Lanthanide solubility in seawater 520 7.1. Anoxic marine basins 576... 

Another plausible control on lanthanide solubility in seawater includes the formation of lanthanide carbonates (Choppin 1986, 1989). Previous determinations of lanthanide carbonate solubility products include the works of Jordanov and Havezov (1966) and Firsching and Mohammadzadel (1986). In the former study lanthanide solubility products expressed as Ai p(M)= at 25 C and zero ionic strength, were found to... 

Originally, general methods of separation were based on small differences in the solubilities of their salts, for examples the nitrates, and a laborious series of fractional crystallisations had to be carried out to obtain the pure salts. In a few cases, individual lanthanides could be separated because they yielded oxidation states other than three. Thus the commonest lanthanide, cerium, exhibits oxidation states of h-3 and -t-4 hence oxidation of a mixture of lanthanide salts in alkaline solution with chlorine yields the soluble chlorates(I) of all the -1-3 lanthanides (which are not oxidised) but gives a precipitate of cerium(IV) hydroxide, Ce(OH)4, since this is too weak a base to form a chlorate(I). In some cases also, preferential reduction to the metal by sodium amalgam could be used to separate out individual lanthanides. 

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. 

The lanthanides form many compounds with organic ligands. Some of these compounds ate water-soluble, others oil-soluble. Water-soluble compounds have been used extensively for rare-earth separation by ion exchange (qv), for example, complexes form with citric acid, ethylenediaminetetraacetic acid (EDTA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA) (see Chelating agents). The complex formation is pH-dependent. Oil-soluble compounds ate used extensively in the industrial separation of rate earths by tiquid—tiquid extraction. The preferred extractants ate catboxyhc acids, otganophosphoms acids and esters, and tetraaLkylammonium salts. 

Fra.ctiona.1 Precipituition. A preliminary enrichment of certain lanthanides can be carried out by selective precipitation of the hydroxides or double salts. The lighter lanthanides (La, Ce, Pr, Nd, Sm) do not easily form soluble double sulfates, whereas those of the heavier lanthanides (Ho, Er, Tm, Yb, Lu) and yttrium are soluble. Generally, the use of this method has been confined to cmde separation of the rare-earth mixture into three groups light, medium, and heavy. 

A soluble sodium tripolyphosphate is produced as are iasoluble lanthanide and thorium hydroxides (hydrated oxides). 

Oxides (Ln Oj), fluorides (LnF ), sulfides (Ln S, LnS), sulfofluorides (LnSF) of lanthanides are bases of different functional materials. Analytical control of such materials must include non-destructive methods for the identification of compound s chemical forms and quantitative detenuination methods which does not require analytical standards. The main difficulties of this analysis by chemical methods are that it is necessary to transform weakly soluble samples in solution. 

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). 

However, solubility, depending as it does on the rather small difference between solvation energy and lattice energy (both large quantities which themselves increase as cation size decreases) and on entropy effects, cannot be simply related to cation radius. No consistent trends are apparent in aqueous, or for that matter nonaqueous, solutions but an empirical distinction can often be made between the lighter cerium lanthanides and the heavier yttrium lanthanides. Thus oxalates, double sulfates and double nitrates of the former are rather less soluble and basic nitrates more soluble than those of the latter. The differences are by no means sharp, but classical separation procedures depended on them. 

Lanthanide sulfates solubility, 6, 922 Lanthanite structure, 6, 848 Lanthanum, hexanitrato-structure, 1, 101... 

Two practical advantages of luminescence species engulfed in antenna dendrimer scaffolds are apparent, namely their miscibility with organic media (solvents or/and resins) and their ability to form thin films. For example the lanthanide-cored dendrimer complexes described in this chapter can be regarded as organic-soluble inorganic luminescers. 

The PBE dendron has a glass transition at about 40 °C and is soluble in various organic solvents (e.g., THF, acetone, toluene). It is therefore a moldable, thermoplastic, film-forming material. This practical feature is maintained for the lanthanide-cored dendrimer complexes. The complexes are partially miscible with poly(methyl methacrylate), affording transparent luminescence compositions by mixing in solvent. 

The simple hydrocarbon substrates included ethene, 1,2-propa-diene, propene and cyclopropane (22). Their reactivity with Sm, Yb and Er was surveyed. In contrast to the reactions discussed above, lanthanide metal vapor reactions with these smaller hydrocarbons did not provide soluble products (with the exception of the erbium propene product, Er(C H ) ). Information on reaction pathways had to be obtained primarily by analyzing the products of hydrolysis of the metal vapor reaction product. 

As described in Section 9.1.2.2.3, several lanthanocene alkyls are known to be ethylene polymerization catalysts.221,226-229 Both (188) and (190) have been reported to catalyze the block copolymerization of ethylene with MMA (as well as with other polar monomers including MA, EA and lactones).229 The reaction is only successful if the olefin is polymerized first reversing the order of monomer addition, i.e., polymerizing MMA first, then adding ethylene only affords PMMA homopolymer. In order to keep the PE block soluble the Mn of the prepolymer is restricted to <12,000. Several other lanthanide complexes have also been reported to catalyze the preparation of PE-b-PMMA,474 76 as well as the copolymer of MMA with higher olefins such as 1-hexene.477... 

Table 2—Solubility of lanthanide chlorides, sulfates, and hydroxides and the pH at which the hydroxides precipitate from solutions ...

Most lanthanide compounds are sparingly soluble. Among those that are analytically important are the hydroxides, oxides, fluorides, oxalates, phosphates, complex cyanides, 8-hydroxyquinolates, and cup-ferrates. The solubility of the lanthanide hydroxides, their solubility products, and the pH at which they precipitate, are given in Table 2. As the atomic number increases (and ionic radius decreases), the lanthanide hydroxides become progressively less soluble and precipitate from more acidic solutions. The most common water-soluble salts are the lanthanide chlorides, nitrates, acetates, and sulfates. The solubilities of some of the chlorides and sulfates are also given in Table 2. 

Lanthanides form soluble complexes with many inorganic and organic substances however, the nature of the bonding in these complexes has not been completely determined. There is evidence for either ionic or covalent bond formation or a combination of both. Lanthanides are complexed by inorganic ions, but not as readily as are the transition elements. The inorganic complexes are not as important... 

Because of the small solubility products of their hydroxides, the lanthanides do not exist as ions even in dilute solutions.5 At the pH of... 

Clearance to pulmonary lymph nodes will occur at a fractional rate of 0.0001 per day. Dissolution of the deposited particles and absorption of cerium into the systemic circulation will occur at rates that are between the extremes represented by CeCh in CsCl particles and Ce oxide or Ce in fused aluminosilicate particles as given by the functions included in Figure 9. These rates should not be expected to be constant over the entire clearance period and will depend upon the overall composition of the bulk aerosol particles, which indude particle size, amount of stable lanthanide present, acidity, and the solubility of other components of the particles. The accuracy of predicting respiratory tract clearance and internal organ uptake of radiocerium will depend heavily upon adequate determination of the particle solubility characteristics. 

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