Aqueous solution chemistry and salts of oxoacids of germanium, tin and lead

    When GeO₂ is dissolved in basic aqueous solution, the solution species formed is [Ge(OH)₆]^2-. With hydrochloric acid, GeO₂ forms [GeCl₆]^2-. Although GeO₂ is reduced by H₃PO₂ in aqueous HCl solution and forms the insoluble Ge(OH)₂ when the solution pH is increased, it is possible to retain Ge(II) in aqueous solution under controlled conditions. Thus, 6M aqueous HCl solutions that contain 0.2–0.4 moldm^-3 of Ge(II) generated in situ are stable for several weeks.

Table 13.1 lists standard reduction potentials for theM^4+‏/ M²⁺‏ and M²⁺‏/M⁺ (M=Sn, Pb) couples. The value of E^o(Sn⁴⁺ Sn^2‏+) = ‏0.15V shows that Sn(II) salts in aqueous solution are readily oxidized by O₂. In addition, hydrolysis of Sn²⁺‏ to species such as [Sn₂O(OH)₄]^2- and [Sn₃(OH)₄]²‏⁺ is extensive. Aqueous solutions of Sn(II) salts are therefore usually acidified and complex ions are then likely to be present, e.g. if SnCl₂ is dissolved in dilute hydrochloric acid, [SnCl₃] ^- forms. In alkaline solutions, the dominant species is [Sn(OH)₃] ^-. Extensive hydrolysis of Sn(IV) species in aqueous solution also occurs unless sufficient acid is present to complex the Sn(IV); thus, in aqueous HCl, Sn(IV) is present as [SnCl₆]^2-. In alkaline solution at high pH, [Sn(OH)₆]^2- is the main species and salts of this octahedral ion, e.g. K₂[Sn(OH)₆], can be isolated.  In comparison with their Sn(II) analogues, Pb(II) salts are much more stable in aqueous solution with respect to hydrolysis and oxidation. The most important soluble oxosalts are Pb(NO₃)₂ and Pb(CH₃CO₂)₂. The fact that many water-insoluble Pb(II) salts dissolve in a mixture of [NH₄][CH₃CO₂] and CH₃CO₂H reveals that Pb(II) is strongly complexed by acetate. Most Pb(II) oxo-salts are, like the halides, sparingly soluble in water; PbSO₄ (K_sp = 1.8 × 10^-8) dissolves in concentrated H₂SO₄. For half-reaction as showen in below equation, the fourth-power dependence of the half-cell potential upon [H‏] immediately explains why the relative stabilities of Pb(II) and Pb(IV) depend upon the pH of the solution.

   Thus, for example, PbO₂ oxidizes concentrated HCl to Cl₂, but Cl₂ oxidizes Pb(II) in alkaline solution to PbO₂. It may be noted that thermodynamically, PbO₂ should oxidize water at pH= 0, and the usefulness of the lead–acid battery depends on there being a high overpotential for O₂ evolution. Yellow crystals of Pb(SO₄)₂ may be obtained by electrolysis of fairly concentrated H₂SO₄ using a Pb anode; however, in cold water, it is hydrolysed to PbO₂, as are Pb(IV) acetate and [NH₄]₂[PbCl₆]. The complex ion [Pb(OH)₆]^2- forms when PbO₂ dissolves in concentrated KOH solution, but on dilution of the solution, PbO₂ is reprecipitated.