Copper(II) selenate is an inorganic compound of copper and the selenate ion, with the chemical formula CuSeO4. The best-known form is the pale-blue pentahydrate, CuSeO4·5H2O.
Preparation Copper(II) selenate can be prepared by dissolving copper(II) oxide in selenic acid:
CuO + H2SeO4 → CuSeO4 + H2O
Crystallization from aqueous solution gives the pentahydrate.
Anhydrous CuSeO4 has also been prepared hydrothermally from metallic copper and selenic acid at about 220 °C.
Structure and properties Copper(II) selenate pentahydrate crystallizes in the triclinic crystal system, in space group P, and is isotypic with copper(II) sulfate pentahydrate.
Its lattice parameters are a = 6.083 Å, b = 6.226 Å and c = 10.867 Å, with α = 77.14°, β = 82.20° and γ = 72.91°, and two formula units per unit cell.
The Cu2+ ions are surrounded by six oxygen atoms in strongly distorted octahedral environments as a result of the Jahn–Teller effect. Four coordination positions are occupied by water molecules and two by selenate oxygen atoms.
Anhydrous CuSeO4 crystallizes in the monoclinic space group P21/n and adopts the MnAsO4 structure type. The Cu2+ ions again show pronounced [4+2] coordination, and their coordination polyhedra are linked into chains along the crystallographic a axis.
Copper(II) selenate is soluble in water but insoluble in ethanol.
Thermal decomposition The pentahydrate undergoes stepwise dehydration on heating. Thermogravimetric and mass-spectrometric studies show that dehydration occurs in three stages and is complete by about 300 °C.
The anhydrous salt decomposes at higher temperatures through several successive reactions between approximately 480 and 900 °C. Oxygen is released first, followed by evolution of selenium dioxide and formation of copper oxides.
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