Aluminium, aqua ions

However, it is a quite striking fourth parameter that oxidizing aqua ions under equal circumstances are more acidic (have lower pK). Thus, iron(III) is distinctly more acidic than aluminium(III) and chromium(III), copper(II) more acidic than nickel(II) and zinc(II), and quite excessive acidity is observed for mercury(II)41), palladium(II)42,81) and thallium(III) aqua ions, compared with the much smaller beryllium(II) and aluminium(III). This tendency takes extreme proportions 841 in gold(III) complexes. 

Aluminium species are oxidation state (III). In aqueous solution, the simple ion exists as Al(H20)g . This ion readily dissociates to give other ions such as A1(H20)50H, all of which are colorless. Over a wide pH range under physiological conditions in alkaline solution, the species appear to be Al(OH)2+, Al(OH)3, AKOH), AlalOH) ]-, AlelOHlf, and AlglOH) . Study of the substitutions of aluminum aqua ion by ligands such as SO , citrate, and EDTA has been established by Al NMR spectroscopy [72]. 

Fig. 3.10 Equilibrium concentrations of the hydrolysed aqua-complexes of Al(lll)-ions versus pH calculated according to Baes and Mesmer (1976) for 25 °C the thick solid line is the total amount of dissolved aluminium squares refer to the experiments of Roelofs and Vogelsberger (2006)...

---