Analysis and improvement of rate constant determination of reactions involving charged reactants

Kinetics of tris(2,2′-bipyridine)ruthenium(II), Ru(bpy)32+, luminescence quenching by copper(II) (in the form of chloride, nitrate, sulfate and perchlorate salt) was studied using pulse laser photolysis technique. The pseudo-first order rate constant versus quencher concentration plots obtained were found to be nonlinear, bending upward. The ionic strength effect contribution was evaluated by applying the Debye–Hückel extended law and was found to be as important as other effects such as cation-counter anion complex and ion-pairing complex formation which were all found to be dependent on the counter anion. It is shown that the slope of the tangent line to the pseudo-first order curve at zero quencher concentration is equal to the quenching rate constants at zero ionic strength. Also, this value corresponds to quenching solely by Cu2+ and is free from contributions from other species that are present at higher concentrations. This method produced a value, (1.6 ± 0.2) × 107 M−1 s−1, (lower than previously published ones) which is in agreement with the quenching rate constant measurement analysis presented. Comparison between Stern–Volmer plots obtained using steady-state fluorimetry data and laser photolysis data showed that in 50 mM CuCl2 and CuSO4 aqueous solutions about 5% of Ru(bpy)32+ is in the form of ion-pairing complexes. Our method was also applied to quenching by another divalent cation, methyl viologen, where it was found that charge transfer complexation effect contribution was about 50% of that of ionic strength effect, while ion-pairing complexation was not significant in the concentration range used. The quenching rate constant at zero ionic strength was found to be (2.3 ± 0.2) × 108 M−1 s−1. The method proposed is also applicable to pulse radiolysis and stopped flow measurements.