Addition of hydrogen peroxide to a neutral or acid solution of a titanium compound gives a yellow colour, due to the formation of a peroxide, TiO₃,aq. Such solutions have the same oxidising powers as hydrogen peroxide, but do not give the blue colouration with chromium salts. By treatment of the solution with dilute alkalies, an hydrated peroxide is thrown down, which, when dried over phosphoric anhydride, has the formula TiO₃,3H₂O, and forms a yellow, horny mass. The freshly precipitated peroxide dissolves in acids and alkalies; from the latter solutions, by addition of hydrogen peroxide and alcohol, pertitanates of various composition can be obtained, of which the following are examples : Na₂O₂,TiO₃,3H₂O; (NH₄)₂O₂,TiO₃,H₂O₂; BaO₂,TiO₃,5H₂O; K₂O₄,K₂O₂,TiO₃,10H₂O, etc. These salts lose hydrogen peroxide when treated with dilute acids, and their constitutions are unknown. An interesting series of fluoroxypertitanates has been prepared by oxidation of the solution of titanium dioxide in hydrofluoric acid with hydrogen peroxide, and addition of metallic fluorides. The ammonium compound, (NH₄)₃TiO₂F₅, crystallises in yellow octahedra, isomorphous with the salts ZrF₄,3NH₄F and CbOF₃,3NH₄F. The potassium salt, K₂TiO₂F₄, crystallises well from water, and is easily obtained in the pure state; various barium salts are known. Similar compounds with oxalic acid have also been prepared. ~Atomic Weight of Titanium.~--The first reliable determinations of this constant were carried out by H. Rose in 1829. He determined the ratio TiCl₄ : 4AgCl, by dissolving the pure tetrachloride, weighed in sealed glass bulbs, in water in closed flasks, precipitating the dioxide by ammonia, and weighing the silver chloride obtained by adding silver nitrate to the filtered and acidified solution. He obtained the values 48·27 and 48·13, which agree very well with the accepted value, 48·1. In the same year, Mosander, using a method not specified, obtained the value 47·15. Determinations carried out by Pierre (1847) and Demoly (1849) led to widely varying results. A series of determinations carried out by Thorpe during the years 1883-1885 gave consistent results. The method used was the precipitation of silver halide from the tetrachloride and tetrabromide, and the mean value of seven series of determinations gave the number 48·08. The International Committee have adopted this result as the basis of the accepted value, 48·1. ~Detection.~--The specific reactions for the element are frequently masked by the presence of other metals, especially of iron, columbium, tantalum, and vanadium, which most frequently accompany it in nature, and from which a quantitative separation is frequently very difficult (see p. 338). The most characteristic reactions are the following: