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.
