Acid Barium Sulphates

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.