MONOBASIC BARIUM PHOSPHATE

Description

This salt has the formula, Ba(H2PO4)2 and the molecular weight of 137.0293 g/mol. Its CAS number is 10048- 98-3. It is easily prepared by dissolving barium carbonate in an excess of H3PO4. A ratio of at least 3:1 mol of acid is required:
BaCO3 + 2H3PO4?Ba(H2PO4)2 + CO2 +H2O
If the pH exceeds about 3.8, the dibasic salt begins to precipitate. The monobasic salt is soluble so that the solution must be evaporated, usually at high temperature, >80–90°C. An anhydrate, Ba(H2PO4)2, readily forms acicular crystals.
Monobasic barium phosphate forms two related crystalline dimorphs. One is monoclinic and the other orthorhombic. The monoclinic structure has the cell parameters of: a = 7.0335?, b = 8.043 ?, c = 7.2158 ?, a = b = 90°, γ = 96.02°, Z = 2, density = 2.99 g/cm³. The orthorhombic cell has the parameters of a = 7.7926 ?, b = 10.2574 ?, c = 8.5653 ?, a = b = γ = 90°. The space group is Pccn and density is 3.03 g/cm³. In both of these polymorphs, a corrugated network of hydrogen-bonded phosphate groups lies parallel to the a–c plane with Ba.