A straightforward extension of the common quinine sulfate fluorescence quenching by NaCl experiment is described. After a brief visual demonstration of the quenching of tonic water fluorescence by NaCl, students were asked to suggest a method for determining which species is responsible for the quenching. An appropriate quinine concentration range was experimentally established and the Stern-Volmer constants for quenching by NaCl, KCl, NaBr, KBr, NaI and KI were determined. After pooling data, Stern-Volmer constants for each sodium-cation-potassium-cation halide pair were found to be statistically indistinguishable which led the students to conclude that the anion was responsible for quenching. Mean values for chloride, bromide and iodide quenching of quinine sulfate fluorescence were 182 ± 13 M-1, 317 ± 13 M-1 and 428 ± 64 M-1 respectively (95% confidence limits). This experiment effectively links kinetics, quantum mechanics and spectroscopy and the extension can be applied to both quenching studies employing steady state fluorescence intensity measurements and studies employing time-dependent fluorescence measurements.

Quinine fluorescence quenching by multiple halides salts: An extension of a common physical chemistry laboratory experiment

250th National Meeting (2015)

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