Kinetic and thermodynamic considerations in the determination of aluminium using pyrocatechol violet: implications for the use of `kinetic-basedʹ determinations of metal ions in natural systems

Kinetic and thermodynamic factors associated with the use of pyrocatechol violet (PCV) for the determination of total reactive Al or `freeʹ Al[Al3++Al(OH)2++Al(OH)+2] have been investigated. The rate of reaction of Al with PCV (in MES buffer, pH 6.2) was strongly influenced by the presence of competing ligands. The rate of formation of Al(PCV)2 on the addition of Al3+ to a PCV–competing ligand mixture was: oxalate≈F−≈malonate>salicylate>>no competing ligand>citrate. A similar increase in the reaction rate relative to standards (i.e. no competing ligand) was observed for Al pre-equilibrated in humic waters and soil solution (at concentrations above or below the Al-complexation capacity). The discrepancy in reaction rates may be ascribed to the inhibition through pH-induced hydrolysis of Al3+ in the absence of ligands (i.e. in standards) or to acceleration in the presence of naturally occurring ligands. It has serious implications for the use of kinetic-based FIA protocols for the determination of Al fractions or total Al in natural waters. Specifically, the (usually) slower reaction for Al3+ standards implies that measurements on systems containing organic ligands may overestimate the concentration of `freeʹ or total Al. Quantitative studies on the thermodynamics of the citrate–Al3+–PCV system established that the attainment of equilibrium in the pH range 5.0 to 6.6 required ≈300 min. Thus, determination of total Al by FIA in systems containing this or closely related ligands is not feasible.