Chemistry:Magnesium selenate
| Identifiers | |
|---|---|
| Properties | |
| MgSeO4 | |
| Molar mass | 167.26 g/mol |
| Appearance | Colorless crystals |
| Density | 3.60 g/cm3 (anhydrous) |
| Soluble in water | |
| Structure | |
| Orthorhombic (anhydrous) | |
a = 0.4925 nm, b = 0.9026 nm, c = 0.6816 nm
| |
Formula units (Z)
|
4 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
| Infobox references | |
Magnesium selenate is an inorganic compound of magnesium and the selenate ion, with the chemical formula MgSeO4. It is a colorless, water-soluble salt and forms a large number of crystalline hydrates.
Preparation
Magnesium selenate can be prepared by reaction of magnesium carbonate with selenic acid:
- MgCO3 + H2SeO4 → MgSeO4 + CO2↑ + H2O
The reaction may be carried out at approximately 60–70 °C, followed by crystallization of the hydrated salt from solution.
Hydrates and solubility
Magnesium selenate forms an unusually large number of hydrates. Reported phases include the monohydrate, dihydrate, tetrahydrate, pentahydrate, hexahydrate, heptahydrate, enneahydrate and a hydrate of approximate composition MgSeO4·4.5H2O.[1]
The stable hydrate depends strongly on temperature. The heptahydrate is stable below approximately 276.8 K, the hexahydrate between about 276.8 and 362.7 K, and the 4.5-hydrate above 362.7 K.[1]
At 25 °C, the equilibrium solid phase is MgSeO4·6H2O. Its selected solubility in water is approximately 3.29 mol of MgSeO4 per kilogram of water.[1]
Crystal structures
Anhydrous MgSeO4 crystallizes in the orthorhombic crystal system, with lattice parameters approximately a = 4.925 Å, b = 9.026 Å and c = 6.816 Å.[2]
The common hexahydrate, MgSeO4·6H2O, is monoclinic and crystallizes in space group C2/c (No. 15), with lattice parameters a = 10.224 Å, b = 7.370 Å, c = 24.866 Å and β = 98.41°, with eight formula units per unit cell.[3]
Its structure consists of alternating layers of [Mg(H2O)6]2+ octahedra and SeO42− tetrahedra, linked by hydrogen bonds.[3]
The heptahydrate, MgSeO4·7H2O, is orthorhombic, space group P212121. Its structure contains [Mg(H2O)6]2+, SeO42− and one additional lattice water molecule.[4]
An enneahydrate, MgSeO4·9H2O, has also been characterized. It crystallizes in the monoclinic space group P21/c and contains isolated [Mg(H2O)6]2+ octahedra, SeO42− tetrahedra and three lattice water molecules.[5]
Thermal dehydration
The hexahydrate loses water stepwise on heating. Intermediate products include the pentahydrate, tetrahydrate, dihydrate and monohydrate before anhydrous MgSeO4 is formed.[6]
Typical dehydration stages occur near 78 °C for formation of the pentahydrate, 110 °C for the tetrahydrate, about 205 °C for the dihydrate and around 235 °C for the monohydrate.[6]
X-ray diffraction measurements show that heating MgSeO4·6H2O through approximately 20–450 °C produces a sequence of crystalline and amorphous phases before crystalline anhydrous magnesium selenate is recovered.[2]
References
- ↑ 1.0 1.1 1.2 Chemical Thermodynamics of Selenium. Chemical Thermodynamics. 7. OECD Nuclear Energy Agency. 2005.
- ↑ 2.0 2.1 Stoilova, Donka; Koleva, Violeta (1995). "X-ray Diffraction Study on MgSeO4·6H2O at Elevated Temperatures". Crystal Research and Technology 30 (4): 547–551. doi:10.1002/crat.2170300421.
- ↑ 3.0 3.1 Kolitsch, Uwe (2002). "Magnesium selenate hexahydrate, MgSeO4·6H2O". Acta Crystallographica Section E 58: i3–i5. doi:10.1107/S1600536801020864.
- ↑ Fortes, A. D. (2014). "Crystal structure of magnesium selenate heptahydrate, MgSeO4·7H2O, from neutron time-of-flight data". Acta Crystallographica Section E 70: 113–118.
- ↑ Fortes, A. D. (2015). "Structure of magnesium selenate enneahydrate, MgSeO4·9H2O, from 5 to 250 K using neutron time-of-flight Laue diffraction". Acta Crystallographica Section B.
- ↑ 6.0 6.1 Stoilova, D.; Koleva, V. (1995). "Thermal dehydration of magnesium selenate hydrates". Thermochimica Acta 255: 33–38.
 |  |
