Characterizing homogeneous chemistry using well-mixed microeddies.

Abstract

Well-mixed reaction volumes are often sought in engineered microchemical devices and can be an important feature of naturally occurring physicochemical processes such as pitting corrosion. Steady streaming eddies can serve as well-mixed, easily controlled microliter chemical reactors for characterizing homogeneous chemical reactions. Here, steady streaming eddies are produced by oscillating a liquid-filled cuvette around a stationary cylindrical electrode (radius 406 microm, length 1.6 cm) at audible frequencies (75 Hz). Oxidant (ferricyanide) electrochemically dosed at small rates (<or=30 nmol/s) from the cylindrical electrode accumulates to millimolar concentrations within the closed streamlines of each eddy, where it mixes and reacts with an antioxidant (vitamin C) present in the bulk solution. The composition in the eddy is controlled by varying the oxidant dosing rate and the bulk antioxidant concentration (<or=10 mM), as well as the cuvette oscillation amplitude. A simple algebraic mole balance is combined with Raman spectroscopy measurements of oxidant concentration in the eddy and bulk to determine the reaction rate law and homogeneous rate constant (45 +/- 9 M(-1) s(-1)) for the antioxidant properties of vitamin C against ferricyanide. Numerical solutions to the full Navier-Stokes equations and species continuity equations illustrate the distribution of species during the reaction and general limitations to the assumption of a well-mixed eddy.