A Fluorometric Approach to Studying the Effects of Ionic Strength on Reaction Rates: An Undergraduate Steady-State Fluorescence Laboratory Experiment

The photophysical properties of the quinine dication are well documented in the scientific literature (1‐4). In the educational literature, this species has been used as the fluorescent compound in an experiment designed to illustrate the principles of fluorimetry to undergraduate students (5). The quenching of quinine fluorescence, particularly by halide ions, has also been widely studied (6‐11) and has been utilized in the educational literature to illustrate the application of the Stern‐Volmer equation (5). This simple, steady-state fluorescence experiment for undergraduate students involves studying the quenching of quinine fluorescence by Cl − and Br − in solutions of different ionic strength. The experiment enhances the student’s understanding and experience in the technique of fluorimetry and illustrates how the Debye‐Huckel law can be used in conjunction with the Stern‐Volmer equation to study how the ionic activity of a solution influences the rate of a reaction. The experiment has particular merit as a seemingly unique situation where the Debye‐Huckel law can be applied without the use of simplifying assumptions that are usually needed in the calculation of ionic activity coefficients. Such an example has not been presented in the educational literature to date. A sequence of postlaboratory tasks to facilitate a stepwise approach to the manipulation of experimental data is given in the Supplemental Material. W Theory A complete kinetic scheme for the excitation of the quinine dication followed by the excited-state quenching of its fluorescence by a halide ion is given in the Supplemental Material. W Students use this scheme in conjunction with the Debye‐Huckel law to derive eq 1, which describes the dependence of the observed, second-order rate constant for the quenching process, kq, on the ionic strength, I, of the solution,