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Scandium sulphate, Sc2(SO4)3

Scandium sulphate, Sc2(SO4)3, is obtained by dissolving the oxide, hydroxide, or carbonate in sulphuric acid, and gently heating until water and excess of sulphuric acid are eliminated. It is a white powder of density 2.579, and specific heat 0.1639 between 0° and 100°. The sulphate dissolves readily in water; at 12° there are 44.5 parts of anhydrous sulphate in 100 of the saturated solution (Crookes). Unlike the rare earth sulphates, the solubility does not diminish with rise of temperature. Scandium sulphate is not deliquescent and is insoluble in alcohol.

From a concentrated aqueous solution the hexahydrate, Sc2(SO4)3.6H2O, crystallises out in small globular aggregates. This hydrate effloresces in dry air and leaves the pentahydrate, Sc2(SO4)3.5H2O. The pentahydrate is the stable phase in contact with the solution at 25°, at which temperature 100 grams of solution contain 28.5 grams of anhydrous sulphate. The solubility alters with the addition of sulphuric acid, as shown by the following data: -

Grams of H2SO4 per litre0.024.549.0121.5243.3
Normality of H2SO40.00.51.04.869.73
Grams Sc2(SO4)3 per 100 of solution28.5229.2919.878.361.32


When dried over sulphuric acid the pentahydrate changes into the tetra- hydrate, and this at 100° becomes converted into the dihydrate. The latter may be dehydrated at 250°.

Scandium sulphate solution is only slowly and incompletely precipitated by oxalic acid and by sodium thiosulphate. Moreover, the equivalent conductivity is abnormal, in that (λ1024 - λ32) is much smaller than would be anticipated. This is shown by the following data: -

Temperature, 25° C.
v=32641282565121024λ1024 - λ32
Sc2(SO4)328.333.840.147.757.672.143.8
Ce2(SO4)335.142.049.960.272.787.152.0
Al2(SO4)351.160.671.283.195.3107.256.1
Cr2(SO4)367.478.390.7105.0119.3128.160.7


The explanation of these anomalous results is that scandium sulphate is really the scandium salt of a complex scandium-sulphuric acid, H3[Sc(SO4)3]; thus, Sc[Sc(SO4)3]. In confirmation of this view it is found that whereas in migration experiments with scandium nitrate and chloride solutions nothing abnormal is observed, with scandium sulphate a considerable quantity of scandium migrates to the anode (Meyer and Bodlander).

A basic sulphate, Sc2O3.2SO3, is produced when scandium sulphate is heated to dull redness (Crookes). When scandium sulphate pentahydrate is dissolved in sulphuric acid of density 1.6 acid scandium sulphate (or scandium-sulphuric acid), Sc2(SO4)3.3H2SO4 (or H3Sc(SO4)3), crystallises from the solution.

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