New studies of base-hydrolysed aluminium(iii) solutions were made using high-field 'H and n.m.r. spectroscopy. The increased sensitivity of the modern equipment has allowed data t o be obtained at concentrations as l o w as 1 x 1 0-3 rnol dm-3, some of it quantitative, and the 'H n.m.r. spectra of solutes transferred to non-aqueous solvents has provided new structural data. The results enable it t o be stated quite unequivocally that the tridecameric cation [AIO,AI,,(OH),,( H,0),,I7+ is the principal species formed at lower concentrations, and is virtually the only one formed below 20 x 1 0-3 rnol dm-3. A t concentrations higher than this, and at intermediate levels of hydrolysis, an oligomeric mixture is formed with a stoicheiometry close t o {[AI(OH),~,]~f}, and which could be composed of species such as {AI,(OH),] +, [AI,(OH),] +, [A1,(0H),,]'+, and [AI,(OH),,I2+. Partial deuteriation produces isotopic splitting of the proton resonances of bound water which allows these t o be distinguished from the resonances of OH-bridge protons. A n indication of fine structure due t o hydrogen-deuterium substitution o n AIO, fragments is also obtained which favours the more condensed structures and this is in accord with the ease with which the oligomer is converted into the tridecamer o n simple dilution. It is now certain that the dihydroxo-bridged dimer found in the solid state is not a significant constituent of these solutions. Oxygen-I 7 n.m.r. data are also briefly reported.

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The hydrolysis of aluminium salt solutions has been studied using 27AI and 'H n.m.r. and this has led to reliable values for the concentrations of the species of AI(H,O)z+, A12(OH)2(H,0)~84+, and A11304(OH)24(H20),,7+ at different degrees of hydrolysis. These ions do not account for all the aluminium in solution and another species, probably A18(OH),,(H,0),4+ is thought to be present. It is shown that previous controversy over the constitution of these solutions arose because of the different methods used for preparing the solutions : only the first two species exist when hydrated aluminium(lll) chloride is dissolved in water whereas the highly polymeric species appear only after considerable hydrolysis has occurred at higher pH. The cations, particularly the dimer, may also be more hydrolysed than the above formulae indicate and this has probably confused the previous interpretation of the potentiornetric results.

Hydrolysis with sodium carbonate carried out in the shortest possible time and without subsequent ageing produces solutions whose composition depends upon the degree of hydrolysis achieved and contain varying proportions of [Al (OH,),] 3+, [ ( H20)4AI (OH),AI ( 0H2)4]4+, and [AIO,AI,,( OH),,( OH2) ' + and apparently no other species, although many new ions can form quite quickly as the solutions age. The fundamental hydrolysis reaction of AP+ seems to involve only these three species and so can be described in surprisingly simple terms. As soon as any ageing is allowed then a variety of reactions can occur leading in general to the production of larger polymers, some of which may have definable structures.

LS2 9JTAluminium-27 n.m.r. spectra are presented of solutions prepared by the relatively slow hydrolysis of AICI, solutions with aluminium metal in the presence of mercury. The ways in which the spectra change with temperature or concentration are described and changes are also noted to occur as a function of magnetic field strength. It is not possible to give an unequivocal answer as to what is the nature of the species present in these solutions but there seem to be a variety of ions present which are quite easily interconvertible, contain octahedrally co-ordinated At, and are associated in some way with fragments which resemble [A104AI,2(OH)2,(H20)12]7+ (AII3) since some tetrahedrally co-ordinated Al always seems to be present. Thus it is suggested that structures may consist of partial All, units with octahedral units disposed as flexible chains and/or forming cross links.