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This means the loss of a proton (H+). We see this reaction with aquo complex ions. Some ions lose H+ spontaneously giving an acidic aqueous solution. This occurs particularly where the central ion is small and highly positively charged (the positively charged ion repels the positively charged proton). Consequently, aqueous solutions of aluminium (Al3+) and iron(III) (Fe3+) salts are particularly acidic:
|
[Fe(H2O)6]3+(aq) |
These equilibria can be driven to the right and further protons may be lost by the addition of an alkali like sodium hydroxide solution. This results in the precipitation of a neutral hydroxide, the colour of which can help to identify the original cation:
|
[Fe(H2O)6]3+(aq) + 3OH-(aq) ⇒ Fe(H2O)3(OH)3(s) + 3H2O(l) |
Iron(III) hydroxide is a rusty brown colour.
The addition of excess alkali may have no effect or may cause further deprotonation to give a negatively charged ion. Being charged it is soluble in water, so the original hydroxide precipitate re-dissolves. Aluminium behaves in this way:
|
Al(H2O)3(OH)3(s) + OH-(aq) ⇒ [Al(H2O)2(OH)4]-(aq) + H2O(l) |
The negative aluminium ion is known as an aluminate.
