Tellurium is a rare chemical element with atomic number 52, which, along with oxygen, sulfur, selenium and polonium, belongs to the 16th group of the periodic system, or, according to the old classification, to the VI group (where it is included in the main subgroup) of the periodic table. Being an analog of sulfur and selenium, tellurium forms compounds structurally similar to similar sulfur and selenium oxides, acids, and salts. Among these compounds, telluric acid is also listed, the properties of which are considered in this article.
Composition and structure
Acid is formed from tellurium, which exhibits a high oxidation state (+6). The formula of telluric acid: H 6 TeO 6 , or H 2 TeO 4 * 2H 2 O. By weight, the acid contains: tellurium 55.6%, oxygen 41.8% and hydrogen 2.6%. Compound H 2 TeO 4 is the so-called metatelluric or simple dibasic acid, the salts of which are well known. In an aqueous solution, it gradually turns into a hexahedral ortho acid H 6 TeO 6 .
Spatially and structurally, the molecule is an octahedron, all six vertices of which are formed by OH groups, and the tellurium atom is located in the center, therefore its rational formula, reflecting the structure of the molecule, is as follows: H 6 O 6 Te. You can also write the formula in the form of Te (OH) 6 - this will also be correct.
Physical and chemical properties of telluric acid
The molecular weight of the acid is 229.642 atomic units of mass. The density of this compound reaches 3.07 g / cm 3 . Under ordinary conditions, it is a rather heavy white powdery substance with a crystalline structure. It is quite soluble in hot water. Melting occurs at 136 ° C. When heated in the temperature range from 300 ° C to 500 ° C, the acid decomposes into water and the extremely poorly soluble anhydride is a yellow powder of tellurium oxide (VI) alpha-TeO 3 .
Is telluric acid strong? No, this is a very weak acid, not similar in properties to its analogues - sulfuric and selenium, and a weak oxidizing agent.
Metal atoms can replace hydrogen in full or in part in an acid molecule. Salts - tellurium (sometimes the term "orthotellurate" is used) - as a rule, they are not soluble in water, with the exception of ammonium tellurides and alkali metals, as well as silver salts Ag 6 TeO 6 and mercury Hg 3 TeO 6 . An example of the incomplete substitution of hydrogen by a metal is the sodium salt of Na 2 H 4 TeO 6 .
All tellurium compounds are highly toxic substances (although not as dangerous as selenium compounds) and are harmful to health, so working with them requires strict adherence to safety regulations.
Acid production
Telluric acid can be obtained in several ways:
- From pure tellurium or telluric acid H 2 TeO 3 by oxidation with nitric and chromic acids with further dissolution in water. Then, crystals are precipitated. Usually, nitric acid is used for these purposes.
- Oxidation of crystalline tellurium with an aqueous solution of perchloric acid.
- Oxidation of tellurium dioxide TeO 2 with hydrogen peroxide in an ammonia medium and the subsequent action of concentrated nitric acid on ammonium tellurate.
- During the reaction of tellurium, taken in this case in an amorphous form, with high concentration of hydrogen peroxide in a water bath. After cooling, telluric acid precipitates from the solution in the form of colorless crystals.
- From tellurium dioxide, oxidizing it with potassium permanganate in the presence of nitric acid.
- From a suspension of tellurium dioxide. Hydrogen peroxide is taken to oxidize the reagent. The reaction should proceed in a medium of sodium bicarbonate or carbon dioxide or potassium. Then, the resulting salt is treated on KU-2 cation exchange resin. This method achieves acid production in 93% yield.
Fields of application
Use this acid to produce some salts. In addition, it is often used in chemical methods for the purification of technical tellurium by recrystallization of the acid obtained from it. This method is characterized in that additional recrystallizations are needed to remove chromium, selenium or potassium contaminants. Nevertheless, it is very effective because it gives a high yield of pure tellurium, which is necessary in the production of semiconductors, some alloys, special glasses and other high-tech materials.
Telluric acid is mainly important as one of the products for laboratory analysis, like most inorganic reagents. For these purposes, acid is produced with a purity of not less than 99%. So the greatest use of this compound is in laboratory practice.