Chemical Properties of Tellurium

In spite of its definite metallic tendencies, Chemical Properties of Tellurium are considerable similar to sulphur and selenium. Its activity is, however, much less marked, and in general the compounds formed are less stable than their sulphur and selenium analogues.

At high temperatures tellurium combines directly, but only to a small extent, with hydrogen, forming hydrogen telluride.

In a fine state of division the element is not acted on by atmospheric oxygen, but above its melting-point it burns with a green-bordered blue flame, giving the dioxide, even in the driest oxygen.

Water is without effect on the element even at a red heat, but the combined action of water and ozone produces telluric acid at the ordinary temperature. The action of hydrogen peroxide upon tellurium is influenced considerably by the physical state of the element; colloidal tellurium is readily oxidised, but crystalline tellurium is not readily attacked and has first to be dissolved in an aqueous solution of alkali hydroxide, when oxidation becomes possible with formation of tellurate. Hydrogen peroxide of 60 per cent, strength reacts very slowly with tellurium at a temperature of 100° C., but with increasing amount of telluric acid formed, the rate of dissolution increases. Amorphous tellurium as ordinarily prepared behaves in a similar manner to the crystalline variety, but if it is dried by treatment with alcohol and ether instead of by heating at 105° C. it will dissolve readily in a concentrated solution of hydrogen peroxide.

Nitric acid oxidises tellurium only to the stage of the dioxide. Under certain conditions, using large quantities of material and a high column of liquid, tellurium nitrite may be obtained as a flesh-coloured precipitate which can be dried at 100° C. without decomposition; at higher temperatures decomposition sets in, leaving a yellow residue of tellurium dioxide.

Sulphuric acid dissolves the element to form a red solution, the colour being due possibly to sulphur-tellurium sesquioxide, STeO₃, which has not been isolated. On warming, tellurium dioxide and sulphur dioxide are produced. If the sulphuric acid is hot and concentrated, a red solution is obtained from which tellurium may be separated by the addition of water. This solution is similar to that obtained with pyrosulphuric acid, in which Auerbach has shown that the tellurium is present in monatomic form. If the red solution is boiled for some time, white crystals of pyrotelluryl sulphate, 2TeO₂.SO₃, separate. These crystals are soluble in hydrochloric acid and decompose on heating with water. A red solution is also obtained with cold anhydrous selenic acid and presumably contains the corresponding selenium-tellurium sesquioxide, SeTeO₃.

Chemical Properties of Tellurium and the halogen elements are related with good combination of these elements with tellurium. The powdered crystalline form is inflamed by fluorine in the cold and by warm chlorine, the product in the latter case being the tetrachloride. With bromine the product is the dibromide, whilst iodine reacts only at a higher temperature, giving a tetra-iodide. Hydrogen chloride does not affect the element.

Pyrosulphuryl chloride, sulphuryl chloride, thionyl chloride and sulphur monochloride all convert tellurium into the tetrachloride, the reaction proceeding most readily with the last named. In the case of thionyl chloride and sulphur monochloride the product is tellurium dichloride if the tellurium is present in excess. With pyrosulphuryl chloride the compound formed is TeCl₄.SO₃; when this is acted upon by dry ammonia the product contains tellurium nitride.

Combination takes place between many metals and molten tellurium. By examination of the freezing-point curve for mixtures of the metal and tellurium in varying proportions, the existence and composition of the compound formed has been indicated. The compounds AuTe₂, Bi₂Te₃, As₂Te₃, and many others, have thus been detected. The freezing-point curves of mixtures of tellurium with sulphur and with selenium give no indication of the formation of compounds, only solid solutions being formed. The red Japanese sulphur is a solid solution containing the three elements, sulphur, selenium and tellurium.

Sodium and potassium combine with tellurium with the evolution of much heat, an atmosphere of hydrogen being advisable. The reaction may be moderated by dissolving the alkali metal in liquid ammonia; the resulting normal tellurides, Na₂Te, K₂Te, and the polytelluride, Na₄Te₃, are easily oxidised and should be protected from the action of atmospheric oxygen. Tellurium also dissolves in fairly concentrated solutions of the alkali hydroxides, giving red solutions containing a mixture of telluride and tellurite, but if sodium hyposulphite is also present, crystalline sodium telluride can be obtained as the sole product. Fusion with potassium carbonate has a similar effect to treatment with aqueous alkali hydroxide. On the addition of water, the mixture of tellurite and telluride obtained with concentrated alkali or with fused potassium carbonate, undergoes decomposition, with liberation of tellurium.

Tellurium displaces some of the nobler metals, such as gold and silver and to a less extent copper, from solutions of their salts, thus showing some resemblance to the metals, but its true position as a metalloid is well seen from its behaviour in the form of electrodes in alkaline solution, when, under the influence of the current, it dissolves at both electrodes, as positive Te^•••• ions at the anode and as negative Te'' ions at the cathode. Both these ions are themselves colourless but have a tendency to complex formation, the negative ions dissolving tellurium to form deeply coloured polytelluride ions, Te_x'', whilst the positive ions react with hydroxyl ions thus:

Te^•••• + 6OH' → TeO'' + 3H₂O,

a reaction which explains the formation of tellurous acid by the action of water on tellurium tetrachloride. If either of the tellurium electrodes is replaced by one of platinum, finely divided tellurium separates at that electrode. The element can be deposited at a lead cathode in smooth, thick layers, with theoretical current efficiency, from a bath containing a solution of the following composition per litre: TeO₂, 300 grams; HF (48 per cent.), 500 grams; H₂SO₄, 200 grams. A current of 1.6 amps./dm². is passed at the ordinary temperature, and with a tellurium anode containing selenium, the latter element remains entirely in the slimes, so that such a bath may be used for refining tellurium. If hydrochloric acid is used instead of hydrofluoric acid, the result is less satisfactory.

Corresponding with the increase in metallic tendency, tellurium appears to enter less readily than selenium and sulphur into the composition of organic compounds; in organic combination it can exert both bi- and quadri-valency.