A second species of polynuclear hydroxyaluminum cation, its formation and some of its properties

Abstract

When a solution of Al(ClO4)3 or AlCl3 with total Al concentration between 5 × 10−4 and 6 × 10−2 M is partially neutralized very slowly with a base, it is possible to produce a solution of polynuclear hydroxyaluminum cations with no solid phases present. When such a solution is aged, the polynuclear hydroxyaluminum cations, AlP1, disappear very slowly at 25 °C and a second type of polynuclear hydroxyaluminum cations, AlP2, is formed. The second type, AlP2, has some properties that are the same as those of AlP1, e.g. they both have an OH/Al mole ratio equal to or very nearly equal to 2.5, diffuse readily across a dialysis membrane, and are removed from solution by the clay mineral bentonite in an exchangeable form. Some of their properties, however, are entirely different, e.g. AlP1 reacts with 8-hydroxyquinoline about 250 times faster with a much lower energy of activation than does AlP2, is much less resistant to attack by HCl, and is precipitated at a pH about 0.5 pH units lower than AlP2. The transformation of AlP1 to AlP2 is a first order process with an energy of activation of 20.8 kcal per mol. The specific rate constant seems to be affected somewhat by the extraneous anion present and the degree of neutralization; experimental values varied from 0.57 × 10−3 to 3.5 × 10−3 day−1 at 25 °C.