MAGNESIUM SELENITE

Description

Magnesium selenite has the molecular formula of MgSeO3 and the molecular weight of 151.2658 g/mol. Its melting point is 557°C. Its CAS number is 15593- 61-0. It is relatively insoluble in water at 0.05454 g/ 100 ml. However, the most common form is the hexahydrate obtained by adding sodium selenite to a solution of magnesium chloride:
MgCl2(aq)+ Na2SeO3(aq) ?MgSeO3·6H2O(solid)
It can also be prepared by treating a solution of magnesium chloride with selenous acid and adding sodium carbonate to start precipitation. The hexahydrate is a colorless crystal with a density of 2.09 g/cm³ that is insoluble in water. It is soluble in dilute acids. It is orthorhombic in crystal structure and when heated in a sealed tube to 150°C forms monoclinic prisms of the dihydrate, when heated to 100°C in air, it loses 5.0 mol of water to form the monohydrate. The monohydrate then loses the last water of hydration around 190°C to form the anhydrate.
The dihydrate has been the subject of further investigation. Hydrothermally prepared MgSeO3·2H2O consists of pairs of edge-sharing MgO6 (4O, 2H2O) octahedra [d_av(Mg–O) = 2.095 ?]. These are linked by pyramidal SeO3 units [dav(Se–O)=1.697?].
A 7.5 hydrate has also been discovered. The crystal structure of magnesium selenite as the 7.5-hydrate, Mg(SeO3)·7.5H2O (space group=P6₃/mmc), is characterized by two crystallographically distinct [Mg(H2O)6]²⁺ octahedra, one of which is disordered over two different orientations. The selenite groups and water molecules (with partially disordered H atoms) bridge the octahedra via hydrogen bonds.

Chemical properties

colorless ortho-rhomb crystal(s) [LID94] [MER06]

The Uses of MAGNESIUM SELENITE

The selenites, in general, have several versatile and practical applications. The Mg and Ca salts are used as microfertilizers and repellants in agriculture. They are also used as catalysts in organic synthesis and they are precursors for the preparation of pure selenides of stoichiometric proportion. They are also important intermediate products in the production and purification of selenium metal itself.