|MadSci Network: Chemistry|
Hundreds of different methods have probably been used for back-titration of Al3+ ions in solution using ethylenediaminetetracetic acid (EDTA). A major problem with this as an analytical technique, however, is that EDTA reacts indiscriminately with essentially all 2+, 3+ and 4+ metal ions. Unless you are sure that there are no other metal ions in your sample, it is best to try another technique!
At high pH, EDTA exists as the 4- ethylenediaminetetraceto anion. (The pH values for dissociation of the first, second, third and fourth hydrogen from EDTA to give this species are 1.99, 2.67, 6.16, and 10.22 respectively.)
If you were titrating Ca2+ or Mg2+, you would carry out the reaction at a pH above 10.2, and if sufficient EDTA was present all of your metal would be converted to the Ca(EDTA)2- or Mg(EDTA)2- complex ions.
For aluminium, however, the situation is more complicated, since at high pH it will preferentially form an insoluble Al(OH)3 complex with the hydroxide ion. It is necessary to carry out the titration at a more moderate pH, where EDTA is present chiefly as the 3- ion.
Because of this strong dependence of behaviour on pH, it does not make sense to speak of a single stability constant for a metal-EDTA complex. Generally, the higher the charge on the metal and the higher the charge on the EDTA, the greater the stability constant, complicated by the competition with hydroxide formation that occurs for the more positive ions like Al3+ and Fe3+ . For a good table showing how the stability constants of a range of metal-EDTA complexes vary with pH, see Quantitative Chemical Analysis, I. M. Kolthoff et al. (4th Edition, 1969).
The basic procedure for all back-titrations with EDTA is to add a sufficient amount of EDTA so that the metal ions you are interested in are converted to the complex, then add an indicator which forms a distinctively-coloured complex with a metal ion (e.g., xylenol orange). This second species (e.g., Zn2+ or Ni2+ is then carefully titrated in; since EDTA is such a good binding group, the metal ions will complex with it first, and only when all EDTA is consumed will the brightly coloured complex be formed.
For a detailed procedure for determination of Al3+, see D. J. Prtichard, Anal. Chem. Acta., 32 (1965) 184-186. This uses a compound similar to EDTA where the ethyl group is replaced with cyclohexane in order to avoid some of the other limitations of EDTA.
There is no proven connection between aluminium and Alzheimer's disease - the beta-amyloid plaques found in the brains of Alzheimer's sufferers contain an abundance of carboxylic acid groups (like those in EDTA) which can concentrate all sorts of metals. These metals can then get up to all manner of chemistry (generating free radicals, etc.) with bad consequences for mental function. It is unlikely that aluminium is the nastiest of these metals, and the evidence seems to suggest that any connection is related to the effect of Alzheimer's disease, not its original cause.
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