Evanescent wave cavity ring-down spectroscopy (EW-CRDS) has been used to investigate the adsorption of crystal violet (CV + ) to a charged silica-water interface as a function of bulk pH by the direct measurement of the absorbance of the CV + chromophore. Absolute absorbances of order 10 -4 have been routinely detected, showing significant variation in the structure of the silica-water interface. At low ionic strength, the interfacial absorbance of CV + shows a monotonic increase with increasing pH. A simple competitive Langmuir adsorption model, which provides values for the silica surface parameters that are in broad agreement with the existing literature values, has been fit to the data. In addition, interfacial absorbance has been monitored as a function of pH for CV + solutions maintained at high ionic strength with NaCl, KCl, and CaCl 2 . As pH increases, the CV + interfacial absorbance exhibits a pronounced maximum, which occurs at pH 8.7 for Na + and K + and at pH 7.9 for Ca 2+ , followed by a sharp decrease. This trend is attributed to competitive binding between the metal cations and CV + to the silica surface binding site, and it has not been observed in previous measurements using second-harmonic generation. The simple Langmuir model, however, does not accurately describe the high ionic strength behavior.

Evanescent-wave cavity ring-down spectroscopy (EW-CRDS) is used to measure interfacial phenomena when methanol or water is placed in contact with a film of poly(dimethylsiloxane) (PDMS), which is attached to the face of a fused-silica prism that constitutes part of a ring cavity. In the first few minutes after contact, the uptake of methanol is slower than that of water, but after this initial period the methanol diffuses more rapidly in the film than water does. Bulk weight-gain measurements confirm this result and yield diffusion coefficients of (25.1 +/- 0.7) x 10(-7) cm(2)/s for methanol in PDMS and (7 +/- 2) x 10(-7) cm(2)/s for water in PDMS. The interfacial optical losses found in the EW-CRDS measurements result primarily from scatter. In particular, we find that delamination of the film from the fused-silica substrate dominates the optical losses in the case of methanol. This conclusion is confirmed by separate surface plasmon resonance experiments.

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An experiment using ion chromatography in an undergraduate analytical chemistry laboratory is described. The students, working in small groups, are responsible for identifying a real-world system of interest and for developing a strategy to investigate that system. The students gain specific skills and knowledge about chromatography as well as more general analytical skills including sample preparation, system calibration, determination of detection limits, and data analysis. Two specific examples of experiments done by students in the class are presented: the determination of lead in hair dye and the determination of anions in orange juice.

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