Barium metaphosphate is an inorganic substance with the molecular formula Ba(PO<sub>3</sub>)<sub>2</sub>. It is a colourless solid that is insoluble in water, though is soluble in acidic solutions through "slow dissolution". X-ray crystallography shows that this material is composed of Ba<sup>2+</sup> cations attached to a polyphosphate ((PO<sub>3</sub><sup>âÂÂ</sup>)<sub>n</sub>) anion. A number of hydrated forms are known which are actually cyclic metaphosphates, Ba<sub>2</sub>(P<sub>4</sub>O<sub>12</sub>)÷3.5H<sub>2</sub>O, Ba<sub>3</sub>(P<sub>3</sub>O<sub>9</sub>)<sub>2</sub>÷6H<sub>2</sub>O.
Barium metaphosphate can be prepared by the reaction of barium carbonate with metaphosphoric acid:
or alternatively by the aqueous reaction of barium chloride and sodium metaphosphate: BaCl<sub>2</sub>(aq) + 2NaPO<sub>3</sub>(aq) â Ba(PO<sub>3</sub>)<sub>2</sub> + 2NaCl
The combination of soda and barium polyphosphate forms a low-melting glass with a high coefficient of thermal expansion. The melting point of the glass increases with barium content. This glass makes seals with low melting metals like aluminium (melting point 650 ðC). Normal borosilicate glasses soften above the melting point of aluminium. This mixture is prepared by heating a mixture of diammonium phosphate, sodium carbonate, and barium carbonate.