APPLICATION OF THERMAL LENS SPECTROMETRY TO KINETIC SPECIATION STUDIES OF METAL-IONS IN NATURAL-WATER MODELS WITH COLLOIDAL LIGANDS

Thermal lensing spectrometry (TLS) has been applied to measurement of kinetics with the aim of extending kinetic speciation methods to the low concentrations of metal ions in the presence of colloidal ligands which are important in many natural systems. Using bromination of acetone as a test reaction, it is shown that 2-3 significant figure rate constants can be obtained from runs with a total absorbance change of < 0.003. Results are presented on TLS kinetic analysis of hydrous ferric oxide colloids hydrolyzing in the presence of sulfosalicylic acid (SSA) to give FeSSA and on the kinetic analysis of Cu(II) bound to hydrous ferric oxides. In both cases, it is seen that lability is increased at the lower concentrations accessed by TLS as compared to behaviour previously reported at concentrations required for analysis by conventional spectrophotometry. That this is a change of the colloids and not a disagreement between TLS and conventional methods is confirmed by demonstrating agreement between the two methods at different concentrations of samples of Cu(II) ion exchanged into zeolite particles of defined size. Noise levels were higher than in bromination of acetone. Studies using colloidal latex particles of known size show that light scattering by nonabsorbing particles causes no interference in TLS absorbance measurements but homodisperse hematite colloids which absorb at the pump laser wavelength cause a significant noise increase.