Chemical Waves in the Iodide-Nitric Acid System

Abstract

Chemical waves can be observed during the oxidation of iodide by 3-4.5 M nitric acid. The rate of the wave propagation and the wave shape under convection-free and convective conditions were studied using tub with differentdiametersand atvaryingorientations to thegravitationalvector. Theactivationenergyofthereactiondiffusion front wasdetermined. For the homogeneous reaction, the isothermal density change (ApJ wasmeasured and found to be negative. Because the reaction is exothermic, the Pojman and Epstein analysis of convective stability predicted only simple convection could occur. However, double-diffusive convection and fingering wereobserved. With special convection cells fed through permeable membranes we proved that double-diffusive convection could occur by having two solutal species (iodide and triiodide-starch complex) with significantly different diffusion coefficients and not only from competing thermal and solutal density changes. A mechanism for the angular dependence of the gravitational anisotropy in front velocities is proposed in terms of the Boycott effect