Telluric acid
Telluric acid is a chemical compound with the formula . It is a white solid made up of octahedral molecules which persist in aqueous solution. There are two forms, rhombohedral and monoclinic, and both contain octahedral molecules. Telluric acid is a weak acid which is dibasic, forming tellurate salts with strong bases and hydrogen tellurate salts with weaker bases or upon hydrolysis of tellurates in water. It is used as tellurium-source in the synthesis of oxidation catalysts. Preparation. Telluric acid is formed by the oxidation of tellurium or tellurium dioxide with a powerful oxidising agent such as hydrogen  peroxide, chromium trioxide or sodium peroxide. Crystallization of telluric acid solutions below 10 °C gives . It is oxidizing, as shown by the electrode potential for the reaction below, although it is kinetically slow in its oxidations. Chlorine, by comparison, is +1.36 V and selenous acid is +0.74 V in oxidizing conditions. Properties and reactions. The anhydrous acid is stable in air at 100 °C but above this it dehydrates to form polymetatelluric acid, a white hygroscopic powder (approximate composition ), and allotelluric acid, an acid syrup of unknown structure (approximate composition ). Typical salts of the acid contains the anions  and . The presence of the tellurate ion has been confirmed in the solid state structure of . Strong heating at over 300 °C produces the α crystalline modification of tellurium trioxide, α-. Telluric acid and its salts mostly contain hexacoordinate tellurium. This is true even for salts such as magnesium tellurate, , which is isostructural with magnesium molybdate and contains octahedra. Other forms of telluric acid. Metatelluric acid, , the tellurium analogue of sulfuric acid, , is unknown. Allotelluric acid of approximate composition , is not well characterised and may be a mixture of and . Other tellurium acids. Tellurous acid , containing tellurium in its  +4 oxidation state, is known but not well characterised. Hydrogen telluride is an unstable gas that forms hydrotelluric acid upon addition to water.