Monitoring Reaction Kinetics in Solution by Continuous-Flow Methods: The Effects of Convection and Molecular Diffusion under Laminar Flow Conditions

Continuous-flow methods are a simple and efficient tool for monitoring the kinetics of chemical reactions in solution. After a reaction has been initiated by a mixing step, liquid flows down an observation tube while the reaction proceeds. The kinetics can be monitored by suitable detectors that are positioned downstream from the mixing point, assuming that the distance from the mixer is linearly related to the “age” of the reaction mixture. It is widely accepted that kinetic experiments of this kind necessarily require turbulent flow in the observation tube, which implies considerable sample consumption due to high flow velocities and large tube diameters. Reduction of flow velocity and tube diameter leads to laminar flow which is characterized by a maximum velocity in the center of the tube and a zero velocity at the tube walls, therefore resulting in a “blurring” of the time axis. However, a number of recent continuous-flow studies that were carried out under these conditions (Konermann et al. Biochemistry 1997, 36, 5554−5559. Konermann et al. Biochemistry 1997, 36, 6448−6454. Zechel et al. Biochemistry 1998, 37, 7664−7669) have indicated that the extent of the dispersion problem is much less pronounced than might be anticipated. In this work, detailed computer simulations are used to study the effects of laminar flow on continuous-flow experiments. It is shown that the distortion of the measured kinetics under laminar flow conditions is surprisingly small, especially when the reaction occurs on a time scale where molecular diffusion in the tube has notable effects on the age distribution function. The results of this study clearly indicate the feasibility of continuous-flow experiments in the laminar flow regime.