TELLUROUS ACID (Te0,). It has been already mentioned that when tellurium is heated in contact with air, it burns, and a white vapour is formed : this is oxide of tellurium, or tellurous acid. It may also be obtained by the action of nitric acid on the metal ; by adding water to the solution, part of the oxide is precipitated, and the remainder is obtained by evaporation to dryness. It is a white granular anhydrous powder, which slowly reddens moist litmuiepaper. and is insoluble in water and adds. It is dissolved by a solution of potash or soda, and by fusing with their carbonates crystallisable salts are formed : when these are decomposed b' adds, hydrated tellurous acid is precipitated, which, if washed with very cold water and dried at a temperature not above 53' Fehr., may be preserved without suffering change. The hydrate is soluble in water, acids, ammonia, and the alkaline carbonates, which last it decomposes : the aqueous solution reddens litmus-paper : when zinc, tin, and some other metals are left in a solution of this acid, they deoxidise it, and metallic tellurium is precipitated in the state of a black powder. Its salts are called tellurites.
Telluric Acid (Te0.).—This compound is obtained by fusing tellurous acid with nitrate of potash, which oxidises it completely, and the result is tollurate of potash. When chloride of barium is added to the last named salt, tellurate of baryta is precipitated, which being decomposed by sulphuric acid, yields a solution of telluric acid, and this solution furnishes hexagonal crystals of the acid. It acts but feebly as an acid, the dilute solution reddening litmus-paper with difficulty, and its taste is rather metallic' than sour : the crystals contain water, two-thirds of which they lose at about 212'; the remainder below a red heat becoming a mass of a fine orange colour, which is completely insoluble in water, either cold or boiling, or hot hydrochloric or nitric acids, or solution of potash. It is decomposed at a high temperature, and converted into a white powder, which is tellurous acid. Its salts are called tellasrates.
Hydrogen and Tdlurium form telluretted hydrogen (TeH).—When
tellurium is alloyed by fusion with tin or zinc, and the compound is acted upon by hydrochloric acid, the hydrogen of the decomposed acid dissolves tellurium, and telluretted hydrogen gas is obtained. This gas has a smell resembling that of hydrosulphuric acid : it is soluble in water, forming a colourless solution, which becomes brown by exposure owing to separation of tellurium. As it possesses acid properties, though to a slight extent only, it has been called hydro telluric acid. It decomposes many metallic salts, yielding an alloy of tellurium with the other metal. Chlorine, nitric acid, and the oxygen of the air, all take the hydrogen from the tellurium.
Chlorine and Tellurium form two compounds. When a feeble current of chlorine gas is passed over tellurium at a high temperature, the chloride formed passes over as a violet-coloured vapour, which con denses at first into a black liquid, and eventually into a solid of the same colour. It is decomposed by the action of water into metallic tellurium, which is precipitated, and bichloride of tellurium, which remains in solution.
It is composed of one equivalent of chlorine, and one equivalent of tellurium (TeC1).
The Dichloride of Tellurium is obtained, as above stated, by the action of water on the bichloride, but is better procured by passing a larger quantity of chlorine over tellurium at a lower temperature than in forming the chloride. It is volatile, and any excess of chlorine being separated by agitation with mercury and rectification, it is obtained as a white crystalline solid.
Sulphur and Tellurium combine in two proportions : the bisulphidc is obtained when hydrosulphuric acid gas is passed through a solution of tellurous acid, or of a soluble tcllurite. It is of a dark brown colour, and is soluble in a solution of potash.
To-sulphide of Tellurium (To%) is obtained by mixing a solution of persulphide of potassium with one of a salt of telluric acid. It is of a deep yellow colour; but it is a very unstable compound, for it speedily becomes black, and is converted into biaulphide.