MEASUREMENT OF HYDROLYSIS, POLYMERIZATION AND COMPLEXATION IN DILUTE ALUMINUM SOLUTION

The nature of chemical reactions taking place during the titration of dilute Al solutions in the presence of either chloride or sulfate were examined by refining the apparent Al hydrolysis products assuming the presence of solution species with n (OH/Al mole ratio) equal to 1, 2, 2.5, 3 and 4. A large degree of polynuclear character of Al solutions was found even at Al concentrations as low as 10('-5) M. The n value and size of the polynuclear complexes are affected by Al concentration. The concentration constant, pQ(,13), is found to be at least 17.4-17.7. It is concluded that the mononuclear Al(OH)(,2) species is never significant.;A new polynuclear complexation mechanism for Al is proposed to account for the high concentration of high n value polynuclear species in the titration refinements. The proposed linear double chain structure has a structure consistent with boehmite and diaspore. This structure differs from the linear single chain and ring based polynuclear structures by the presence of rows of three coordinated oxygens in the bond central chain and rows of two and one coordinated oxygens along the plane edges. A rearrangement of internal charge in this structure is proposed in which part of the charge is removed from the three coordinated oxygens to result in an uncharged hydroxyl with the charge shifted to the one coordinated site neutralizing the hydroxyl. This results in a general formula for the polynuclear structure of (Al(OH)(,3))(,x)(Al(OH)(,2))(,2)('2+). This structure results in a higher n value for a lower number of Al than does the other polynuclear complexation schemes and therefore explains the presence of high n value polymers in unaged Al solutions which would have required polymers of greater than a hundred Al cations.;The refinement of Al titration data in K(,2)SO(,4) provide pQ values one to three pQ units lower than those obtained from equivalent KCl solutions. A catalytic mechanism is proposed in which Al polymerization is facilitated by the formation of mononuclear Al hydroxy sulfate complexes which combine together to form nonsulfate containing polynuclear complexes. The increased hydrolyzed concentration and lower ionic charge resulting from these complexes would increase the rate of polymerization in these systems. (Abstract shortened with permission of author.).