Colorimetric procedures were developed for the determination of inorganic monomeric aluminium (Al) in the presence of organic ligands including high molecular weight fulvic acids and short chain carboxylic acids. The procedures take advantage of marked differences in reaction rate between the chromogens aluminon and pyrocatechol violet (PCV) and aluminium in either free inorganic forms or organically complexed forms. The aluminon method uses a 30 s reaction and has a working range for total Al concentrations between 20 and 250 PM. The more sensitive PCV method uses a reaction time of 60 s and provides a linear calibration line in the range 0.2-25 VM. Both methods are calibrated by using standard solutions containing known total A1 concentration prepared with varying ratios of inorganic monomeric and organically complexed Al. The determination of A1 in soil solution and its fractionation into inorganic monomeric and organically complexed forms requires the measurement of total Al in filtered soil solutions by inductively coupled plasma atomic emission spectroscopy (ICPAES) and the use of one of the short reaction time colorimetric procedures.Limited testing on soil solutions from acid, Al toxic soils showed better agreement between inorganic monomeric Al concentration determined by the new procedures and relative yield data than that obtained by existing colorimetric methods. * Part I, Aust. J. Soil Res., 1989, 2 7 , 79-90.

Benzo substitution cis to the nitrogen of 2,2'-bipyridyl (bipy), to form 2-(2'-pyridyl)quinoline (pq), transforms the tris-complex with iron(11) from low spin in Fe(bipy)32+ to essentially high spin but near the crossover in Fe(pq)32+. For this complex, the ligand field splitting near the crossover, Δc is estimated from magnetic and spectral data as c. 12000 cm-l. A similar value but 150 cm-l lower is estimated for the analogous complex with the sterically related ligand 2-methyl-1,10-phenanthroline. This apparent difference in Δ values could arise from a direct change in ligand field potential, or from other factors, such as a change in distortion effects. Due to steric hindrance, pq prefers to form bis-complexes Fe(pq)2X2, pseudooctahedral and high spin, with suitable monodentates, X = Cl2 Br, NCS, ClO4. Double benzo substitution of bipy to form 2,2'-biquinolyl (biq) so increases steric hindrance that the tris-complex could not be formed. Instead biq forms pseudotetrahedral complexes Fe(biq)22+ and Fe(biq)X2 (X = Cl, Br).

Colorimetric methods of analysis and separation procedures were evaluated for their ability to selectively measure the inorganic monomeric forms of aluminium (Al) toxic to plant roots. An Al-fulvate complex of natural origin was prepared and characterized by gel filtration chromatography and 2 7Al NMR to act as a model organic complex of Al. All five colorimetric methods tested failed to adequately discriminate between inorganic monomeric and organically complexed forms of Al. Best discrimination was achieved by the 15 s 8-hydroxyquinoline method, with only 46% of the organically complexed Al being measured as inorganic monomeric Al. All methods with longer reaction times measured >80% of the complexed Al as inorganic monomeric Al. Separation procedures by using cation exchange resins to remove inorganic monomeric Al from aluminium fulvate solutions were found to produce a decrease in sample pH of 0.8, even though the resin was used in the potassium form to eliminate H+ ion exchange. The decrease in sample pH caused the Al complex to dissociate and 34% of the Al was retained on the resin. With buffered Chelex 100 in the potassium form, change in sample pH was reduced to 0.2 pH units, but 23.5% of the Al was still retained on the resin. Separation of organically complexed forms of Al from inorganic monomeric forms by gel filtration chromatography (GFC) was shown to be ineffective when sulfate is present due to co-elution of the SO4-Al complex with the fulvic acid-Al complex.

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