Ligand Concentration Effects on Aluminum Complexation by a Chestinut Leaf Litter Extract

The quenching of fluorescence at an emission wavelength dependent on the ligand concentration was used to measure organically complexed Al at pH 4.5 across a range of dissolved organic carbon (DOC) concentrations of chestnut (Castanea sativa L.) leaf‐litter extract. Simultaneous measurements of total inorganic Al were made by a 15‐s reaction with 8‐hydroxyquinoline. These data were combined to calculate quasiparticle model complex stability coefficients ( c K T ) for Al binding. For the range of DOC from 26 to 150 g m −3 , the variation in the value of log c K T was not statistically significant. A conditional stability constant for a 1:1 quasiparticle Al/leaf‐litter complex was calculated after correcting c K T for Al hydrolysis and ligand protonation. Its mean value varied from 10 8.9 to 10 8.3 across the range of DOC investigated, but was also not significantly dependent on DOC according to statistical criteria. It was concluded that fluorescence quenching can be used to determine organically bound Al for an environmentally important range of leaf‐litter DOC concentrations.