Redox reactions in aqueous solution

The standard reduction potentials for the M³⁺‏/M couples (Table 1.1) show that Al³⁺‏(aq) is much less readily reduced in aqueous solution than are the later M³⁺ions.

This can be attributed, in part, to the more negative Gibbs energy of hydration of the smaller Al³⁺‏ ion, but an important contributing factor  in differentiating between the values of Eo for the Al³⁺‏/Al and Ga³⁺‏/Ga couples is the significant increase in the sum of the first three ionization energies (Table 1.1).

   Although In(I) can be obtained in low concentration by oxidation of an In anode in dilute HClO₄, the solution rapidly evolves H₂ and forms In(III). A value of ^_0.44V has been measured for the In³⁺‏/In‏ couple (equation 12.69).

   For the Ga³⁺‏(aq)/Ga‏(aq) couple, a value of E^o = ^_0:75V has been determined and, therefore, studies of aqueous Ga‏ are rare because of the ease of oxidation of Ga‏ to Ga³⁺‏. The compound Ga‏[GaCl₄]^- can be used as a source of Ga‏ in aqueous solution, but it is very unstable and rapidly reduces [I₃]^-, aqueous Br₂, [Fe(CN)₆]^3-  and [Fe(bpy)₃]³⁺‏.

E^o for the reduction of Tl(III) to Tl(I) in molar HClO₄ is 1.25 V, and under these conditions, Tl(III) is a powerful oxidizing agent. The value of Eo is, however, dependent on the anion present and complex formed (see Section 7.3); Tl(I) (like the alkali metal ions) forms few stable complexes in aqueous solution, whereas Tl(III) is strongly complexed by a variety of anions. For example, consider the presence of Cl^- in solution. Whereas TlCl is fairly insoluble, Tl(III) formsthesolublecomplex [TlCl₄]^-and, at [Cl]^- =  1mol dm^_3, E^o(Tl³⁺‏=Tl⁺‏( ‏ = 0.9 V. Thallium(III) forms a more stable complex with I^- than Cl^-, and at high [I]^-, [TlI₄]^- is produced in solution even though E^o(Tl³⁺‏=Tl⁺‏)  is more positive than E^o(I₂=2I^-) (‏0.54 V) and TlI is sparingly soluble. Thus, while tabulated reduction potentials for the Tl³⁺‏=Tl⁺‏ and I₂=2I^-  couples might suggest that aqueous I^- will reduce Tl(III) to Tl(I) , in the presence of high concentrations of I^- ,Tl(III) is stabilized.

In alkaline media, Tl(I) is also easily oxidized, since TlOH is soluble in water and hydrated Tl₂O₃ (which is in equilibrium with Tl³⁺and [OH]^- ions in solution) is very sparingly soluble in water (Ksp ≈ 10 ^-45).