Hydrolysis - Types - Metal Aqua Ions

Metal Aqua Ions

Metal ions are Lewis acids, and in aqueous solution they form aqua ions of the general formula M(H₂O)_nm+. The aqua ions undergo hydrolysis, to a greater or lesser extent. The first hydrolysis step is given generically as

M(H₂O)_nm+ + H₂O M(H₂O)_n-1(OH)(m-1)+ + H₃O+

Thus the aqua cations behave as acids in terms of Brønsted-Lowry acid-base theory. This effect is easily explained by considering the inductive effect of the positively charged metal ion, which weakens the O-H bond of an attached water molecule, making the liberation of a proton relatively easy.

The dissociation constant, pK_a, for this reaction is more or less linearly related to the charge-to-size ratio of the metal ion. Ions with low charges, such as Na+ are very weak acids with almost imperceptible hydrolysis. Large divalent ions such as Ca2+, Zn2+, Sn2+ and Pb2+ have a pK_a of 6 or more and would not normally be classed as acids, but small divalent ions such as Be2+ undergo extensive hydrolysis. Trivalent ions like Al3+ and Fe3+ are weak acids whose pK_a is comparable to that of acetic acid. Solutions of salts such as BeCl₂ or Al(NO₃)₃ in water are noticeably acidic; the hydrolysis can be suppressed by adding an acid such as nitric acid, making the solution more acidic.

Hydrolysis may proceed beyond the first step, often with the formation of polynuclear species via the process of olation. Some "exotic" species such as Sn₃(OH)₄2+ are well characterized. The rate of hydrolysis tends to increase as pH rises leading, in many cases, to the precipitation of a hydroxide such as Al(OH)₃ or AlO(OH). These substances, major constituents of bauxite, are known as laterites and are formed by leeching from rocks of most of the ions other than aluminium and iron and subsequent hydrolysis of the remaining aluminium and iron.