Barium sulphate dissolves in hot concentrated sulphuric acid, forming a golden-yellow solution which deposits doubly refracting, needle-like crystals of BaH2(SO4)2 on cooling. It is readily decomposed by excess of water, but in presence of a small quantity of water it forms BaH2(SO4)2.2H2O, which sets to a hard mass if kept in a desiccator. The same hydrate is obtained in crystalline needles by exposing the solution to the air. A second hydrate, BaH2(SO4)2.H2O, has also been isolated. The anhydrous compound is decomposed at about 160° C., giving barium sulphate.
When examining the solubility of barium sulphate in sulphuric acid at low temperatures, Kendall and Davidson isolated the compound BaSO4.3H2SO4.
Solutions containing 8 – 8.6 molecular percentage of barium sulphate deposited, on long standing, crystals which did not redissolve at 25° C., and which may have been BaH2(SO4)2. The compound BaSO4.3H2SO4 must, therefore, be considered as a metastable phase in this region.
By passing an electric current through a solution of barium sulphate in sulphuric acid it is found that barium migrates to the anode, indicating that barium hydrogen sulphate is probably to be regarded as a complex acid of the formula H2[Ba(SO4)2].
The existence of a complex acid, H2Ba5(SO4)6, forming barium, strontium, calcium, and potassium salts also appears to be possible. |