Quinine fluorescence quenching at low ionic strength

The effect of ionic activity on the quenching of the quinine dication fluor escence by chloride and bromide ions is examined in solutions of sufficient ly low ionic strength to enable ionic activities to be taken into account b y means of the Debye-Huckel equation. An equation for the dependence of the Stern-Volmer constant on the ionic strength is derived in a novel way and an iterative, linear least-squares computer program is described from which the optimized values of the ionic size parameter for chloride (a(Cl-) = 4. 56 +/- 0.07 Angstrom) and bromide (a(Br-) = 4.98 + 0.23 Angstrom) quenching were obtained. The ratio of these parameters (a(Br-)/a(Cl-) = 1.09 +/- 0.0 7) is found to be similar to the ratio of the ionic radii of the two specie s, r(Br-)/r(Cl-) = 1.08. The experimental results suggest that the quenchin g of quinine fluorescence by bromide ions is about 28% faster than that obs erved for chloride ions. The results are consistent with the notion that th e quenching process is dynamic one involving the formation of an activated complex whose rate of deactivation is dependent on the ionic strength of th e medium. (C) 2000 John Wiley & Sons, Inc.