KINETIC AND THERMODYNAMIC CONSIDERATIONS IN THE DETERMINATION OF ALUMINUM USING PYROCATECHOL VIOLET - IMPLICATIONS FOR THE USE OF KINETIC-BASED DETERMINATIONS OF METAL-IONS IN NATURAL SYSTEMS

Kinetic and thermodynamic factors associated with the use of pyrocatec hol violet (PCV) for the determination of total reactive Al or 'free' Al[Al3++Al(OH)(2+)+Al(OH)(2)(+)] have been investigated. The rate of r eaction of Al with PCV (in MES buffer, pH 6.2) was strongly influenced by the presence of competing ligands. The rate of formation of AI(PCV )(2) on the addition of Al3+ to a PCV-competing ligand mixture was: ox alate approximate to F(-)approximate to malonate>salicylate much great er than no competing ligand>citrate. A similar increase in the reactio n rate relative to standards (i.e. no competing ligand) was observed f or Al pre-equilibrated in humic waters and soil solution (at concentra tions above or below the Al-complexation capacity). The discrepancy in reaction rates may be ascribed to the inhibition through pH-induced h ydrolysis of Al3+ in the absence of ligands (i.e. in standards) or to acceleration in the presence of naturally occurring ligands. It has se rious implications for the use of kinetic-based FIA protocols for the determination of Al fractions or total Al in natural waters. Specifica lly the (usually) slower reaction for Al3+ standards implies that meas urements on systems containing organic ligands may overestimate the co ncentration of 'free' or total Al. Quantitative studies on the thermod ynamics of the citrate-Al3+-PCV system established that the attainment of equilibrium in the pH range 5.0 to 6.6 required approximate to 300 min. Thus, determination of total Al by FIA in systems containing thi s or closely related ligands is not feasible. (C) 1998 Elsevier Scienc e B.V.