Iron(III) selenate is an inorganic compound of iron and the selenate ion, with the chemical formula Fe2(SeO4)3. Both anhydrous and hydrated forms are known, including a well-characterized nonahydrate, Fe2(SeO4)3·9H2O.
Preparation Anhydrous iron(III) selenate can be prepared hydrothermally by reaction of iron(II) oxalate with selenic acid at about 450 K.
The nonahydrate can be crystallized at room temperature from aqueous solutions acidified with selenic acid.Giester, G.; Miletich, R. "Crystal structure and thermal decomposition of the coquimbite-type compound Fe2(SeO4)3·9H2O." Neues Jahrbuch für Mineralogie, Monatshefte. 1995. 5: 211–223.
Iron(III) selenate can also be obtained by reaction of iron(III) oxide or hydroxide with selenic acid.
Structure Anhydrous Fe2(SeO4)3 crystallizes in the monoclinic crystal system, in space group P21/n (No. 14). Its lattice parameters are a = 8.530(2) Å, b = 8.888(2) Å, c = 11.952(2) Å and β = 91.13(1)°, with four formula units per unit cell.
The structure is built from FeO6 octahedra and SeO4 tetrahedra linked by shared oxygen atoms.
The nonahydrate is isostructural with the sulfate mineral coquimbite. It crystallizes in the trigonal space group P1c and contains Fe-centred octahedra, SeO4 tetrahedra and both coordinated and interstitial water molecules linked by an extensive hydrogen-bonding network. The hydrogen positions in Fe2(SeO4)3·9H2O have been used as a structural model for the corresponding coquimbite sulfate structure.
Thermal decomposition Iron(III) selenate is less thermally stable than the corresponding iron(III) sulfate.Samplavskaya, K. K.; Selivanova, N. M.; Mazepova, V. I. "Thermal stability of iron selenate". Izvestiya Vysshikh Uchebnykh Zavedenii, Khimiya i Khimicheskaya Tekhnologiya. 1964. 7 (4): 540–543.
On heating, the nonahydrate loses water between approximately 45 and 200 °C, passing through lower hydrates including the tetrahydrate, dihydrate and monohydrate. Near 230 °C, anhydrous Fe2(SeO4)3 is formed, accompanied by partial reduction of Se(VI) to Se(IV).
Further heating decomposes the anhydrous selenate. Intermediate mixed selenite–selenate phases form before iron(III) oxide becomes the final iron-containing product at higher temperature.
Double salts Iron(III) selenate forms double salts with alkali-metal selenates. One example is the alum-like compound CsFe(SeO4)2·12H2O, whose structure has been determined by low-temperature neutron diffraction.
The caesium iron selenate double salt crystallizes in the cubic space group Pa and adopts the α-alum structure.
References