The _sulphate_, Ti₂(SO₄)₃, is obtained as a green crystalline powder by heating with sulphuric acid the violet solution obtained by reduction of a solution of the dioxide in sulphuric acid. It dissolves in dilute acids, forming violet solutions. With alkali sulphates it forms _titanium alums_, which can be recrystallised from dilute sulphuric acid, and have the general formulae, properties, and crystal form of the other alums. An _acid sulphate_, 3Ti₂(SO₄)₃,H₂SO₄,25H₂O, is obtained by electrolytic reduction of a strongly acid solution of the dioxide in sulphuric acid, or by treating the chloride repeatedly with hot dilute sulphuric acid. It forms a crystalline violet powder, with silky lustre, insoluble in alcohol, ether, and 60 per cent, sulphuric acid; it dissolves slowly in water, forming a violet solution. When the aqueous solution is treated with excess (2¹⁄₂ molecules) of alkali sulphate, it forms sparingly soluble _double sulphates_, which separate in bright blue crystals; the compounds Ti₃(NH₄)(SO₄)₅,9H₂O, and Ti₃Rb(SO₄)₅,12H₂O, have been obtained in this way. _The Use of Salts of Trivalent Titanium in Volumetric Analysis._--Owing to their powerful reducing properties, these salts have been proposed as very convenient reagents in volumetric analysis,[452] the chloride being most useful in this respect. The estimations must be carried out in absence of air, to avoid atmospheric oxidation; generally the apparatus is filled with carbon dioxide. The titanium solutions for use must also be preserved from the oxidising action of the air. [452] See Knecht, _Ber._ 1903, ~36~, 166; Knecht and Hibbert, _ibid._ 1903, ~36~, 1549; 1905, ~38~, 3318; 1907, ~40~, 3819.