Rate Measurements by the Pulsed-Accelerated-Flow Method

A pulsed-accelerated-flow spectrophotometer with UV−visible capability is described that permits measurement of pseudo-first-order rate constants as large as 500 000 s-1 (t 1/2 = 1.4 μs). Chemical rate processes are resolved from physical mixing rate processes by variation of flow velocities under conditions of turbulent flow. Two mixing processes are found in the mixing/observation tube. One mixing rate constant, valid for the full length of the tube, is directly proportional to the flow velocity. The other mixing behavior, proportional to the square of the flow velocity, is found only in the immediate vicinity of the 10 inlet reactant streams that collide with one another in the middle of the observation tube. Contributions from the latter mixing become more important for very fast reactions that take place close to the inlet jets. These mixing models and improved signal/noise permit the measurement of rate constants for very fast reactions. Applications of the PAF method to electron-transfer, proton-transfer, hydrolysis, and non-metal redox reactions are reported for pseudo-first-order and second-order reactions.