Characterization by nuclear magnetic resonance spectroscopy, ferron assay, and acidification of partially neutralized aluminium solutions

Abstract

Aluminium solutions (0.1 mol dm^–3) were hydrolysed at 25 °C by a mild slow procedure involving the addition of NaOH to an aluminium salt. The amount of monomeric and polymeric species deduced from an ²⁷Al n.m.r. analysis of the solutions are in good accordance with results obtained by a spectrophotometric technique using ferron (8-hydroxy-7-iodoquinoline-5-sulphonic acid). It is shown that a minimal number of aluminium species are present, the dominant polymer being of the ‘Al₁₃’ form. A small amount of polymeric species remains unidentified although limited results obtained when solutions are heated above 70 °C seem to indicate that these species contain tetrahedral aluminium. It is also shown that the ‘Al₁₃’ form can be converted into monomeric species by addition of strong acid to the solution. This transformation however follows a complex path.