Investigations of polyoxometalates in aqueous solutions. I. The formation of Al-13(OH)(32)(7+) cation

The formation of polyhydroxo aluminum(III) complexes has been investigated at 30 degrees C and in a 3 M (K)Cl ionic medium by p[H] measurements. The u ncommon "integral titration" technique employed has enabled measurements of oversaturated solutions up to OH- to Al(III) ratios as large as 2.65. This has allowed the detection of the undescribed species Al-13(OH)(3S)(4+). Th e data can very satisfactorily be explained by assuming the species Al-2(OH )(2)(4+), Al-3(OH)(6)(3+), Al-13(OH)(32)(7+), and Al-13(OH)(35)(4+). The Al (III) concentration has been changed from approximate to 0.0025 to approxim ate to 0.040 M and the spacings of the titration curves at different alumin um levels are a clear and direct evidence for the formation of Al-13(OH)(32 )(7+), which dominates the hydrolysis products. The data presented in this paper are best accounted for if the trimer Al-3(OH)(6)(3+) is substituted f or Al-3(OH)(4)(5+) which is frequently reported. The formation of the "13" cations may result from the reaction of four Al-3(OH)(6)(3+) with a transie nt Al(OH)(4)(-) species which is formed, upon addition of a rather concentr ated basic solution, owing to a local excess of OH-.