Oxoacids of selenium and tellurium

    Selenous acid, H₂SeO₃, may be crystallized from aqueous solutions of SeO₂ and gives rise to two series of salts containing the [HSeO₃]^- and [SeO₃] ^2- ions. In aqueous solution, it behaves as a weak acid: pK_a(1) ≈ 2.46, pK_a(2( ≈ 7:31. Heating salts of [HSeO₃]^- generates diselenites containing ion 1.1. Tellurous acid, H₂TeO₃, is not as stable as H₂SeO₃ and is usually prepared in aqueous solution where it acts as a weak acid: pK_a(1) ≈ 2.48, pK_a(2) ≈ 7.70. Most tellurite salts contain the [TeO₃] ^2- ion.

(1.1)

   Oxidation of H₂SeO₃ with 30% aqueous H₂O₂ yields selenic acid, H₂SeO₄, which may be crystallized from the solution. In some ways it resembles H₂SO₄, being fully dissociated in aqueous solution with respect to loss of the first proton. For the second step, pKa =1.92. It is a more powerful oxidant than H₂SO₄, e.g. it liberates Cl₂ from concentrated HCl. Reaction in the solid state between Na₂SeO₄ and Na₂O (2 :1 molar equivalents) leads to Na₆Se₂O₉. This formula is more usefully written as Na₁₂(SeO₆)(SeO₄)₃, showing the presence of the octahedral [SeO₆]^6- ion which is stabilized in the crystalline lattice by interaction with eight Na‏ ions. The [SeO₅]^4- ion has been established in Li₄SeO₅ and Na₄SeO₅. The formula, H₆TeO₆ or Te(OH)₆, and properties of telluric acid contrast with those of selenic acid. In the solid, octahedral molecules (1.2) are present and in solution, it behaves as a weak acid: pK_a)1( = 7.68, pK_a)2( = 11.29. Typical salts include those containing [Te)O((OH)₅]^- and [Te(O)₂(OH)₄]^2- and the presence of the [TeO₄]^2- ion has been confirmed in the solid state structure of Rb₆[TeO₅][TeO₄].

(1.2)