Scandium selenates

As discussed in Appendix A, only y6 ((V.152), 298.15 K) will be accepted due to the large change in the medium and the accompanying change in the Na concentration, which will affect the evaluation method. The value accepted is  

A correction to the standard state cannot be made with confidence. 

No other thermodynamic data have been identified for scandium selenates. The data for ScSeO and Sc SeO )2 in [82WAG/EVA] most likely derive from [67KOL/1VA] and are therefore subject to the same criticism as the original data. 

On heating the compound decomposes at 600 to 1100X to SC2O3. The solution in H2O is clear [2]. 

Concentrated H2Se04 is added to a 10% aqueous solution of 802(8004)3 (weight ratio 1 10) and the whole is mixed with concentrated acetic acid until a slight turbidity appears. 

After a few days at room temperature, the crystalline mass formed is filtered, washed with acetic acid, alcohol, and ether, and dried in the air. The isolated white, shiny crystals consist of hexagonal prisms with poorly developed pyramidal bases. Their density is 2.21 g/cm. A salt of the same composition is obtained by evaporation of a very concentrated aqueous solution of 802(8004)3 at 0 to 3°C in a desiccator over H2SO4. After a long time the decahydrate is formed as a glassy mass which passes into the pentahydrate in finely ground state, whereas the crystalline form is stable in air. On heating, the compound is dehydrated in several steps. At 1100°C the oxide is formed. In water a clear solution is easily formed, Trousil [2, p. 300]. 

The octahydrate is mentioned in the older literature see for instance Pascal [14]. 

The IR spectrum was studied in the range from 250 to 2000 cm on KBr pellets and the following assignments are proposed for the observed absorption bands (in cm )  

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Structural Studies.—The structures of some compounds of scandium and yttrium, in addition to those of some of the lanthanides and actinides, have been reviewed. The crystal structure of scandium selenate pentahydrate, Sc(Se04)3,5H20, has been determined by single-crystal JT-ray diffraction. The scandium atoms were shown to be octahedrally co-ordinated by the oxygen atoms of the selenate groups and water molecules. The mean Sc—O distances in the scandium co-ordination polyhedra vary from 2.08 to 2.10 A and the mean Sc—O bond lengths in the selenate groups vary from 1.61 to 1.64 A. Two formula units were found in each cell. 

Sodium Rare Earth Selenates Sodium Scandium Selenates Na3Sc(Se04)3... 

Potassium Rare Earth Selenates Potassium Scandium Selenate K3Sc(Se04)3... 

The monoammonium scandium selenate is stable only up to 185°C and therefore decomposes partially. The second effect corresponds to further decomposition of (NH4)2Se04. The formation of red elemental Se in the solid phase is observed. At 462°C, amorphous 02(8003)3 is formed, which decomposes at 648°C. The bands in the IR spectra of the decomposition products at temperatures between 260 and 750°C are given in the paper, together with the X-ray line diagrams at the same temperatures [1]. (NH4)3Sc(Se04)3 is readily soluble in H2O. Its behavior in aqueous solution was studied by measuring the molar conductance A (in cm -Q -mor ) and pH at 25°C as follows at dilution V in L/mol" ... 

Rubidium Rare Earth Selenates Rubidium Scandium Selenates... 

Simultaneous TG/DTA data are also available for the scandium selenate pentahydrate, which is completely dehydrated by 130°C (Tetsu et al., 1971). The dehydration temperatures obtained by various authors are not directly comparable because different experimental conditions have been used. Under similar conditions, however, there appears to be a linear relationship between the DTA peak temperatures and the atomic number of the rare earth in the series R2(Se04)3 8H2O (R = Rb Lu) (Nabar and Paralkar, 1976b). 

The only ternary rare earth selenate which has been fully characterized by a single-crystal X-ray diffraction study appears to be the triammonium scandium selenate (NH4)3[Sc(Se04)3], which corresponds to the complex anion [Sc(Se04)3] (Valkonen and Niinisto, 1978 table 15). The crystal structure... 

The complex formation between Sc and SeOj" was studied by measurement of the distribution of Sc between the aqueous phase and a cation exchanger at selenate concentrations in the range 0 to 0.167 M. Sodium perchlorate was added to keep the ionic strength at / = 0.5 M. The radioactive nuclide Sc was used to follow the distribution ratio at a total scandium concentration of 2 x 10 M. Measurements were carried out at 298.15, 308.15, and 318.15 K and the pH was adjusted to 4.0 to 4.5. No disturbance from the hydrolysis of Sc occurs at this pH according to the paper although this would be expected from the data in [76BAE/MES]. 

KOL/IVA] Kolosov, I. V., Ivanov-Emin, B. N., Korotaeva, L. G., Tetsu, K., Study of the complex formation of scandium with selenate and sulfate ions, Radiokhimiya, 9, (1967), 473-478, in Russian, English translation in [67KOL/IVA2], Cited on pages 355, 493. 

Lu E, Chu J, Chen Y, Borzov MV, Li G. Scandium terminal imido complex induced C-H bond selenation and formation of an Sc-Se bond. Chem Commun. 2011 47 743-745. 

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