CHEMICAL WAVES IN THE IODIDE-NITRIC ACID SYSTEM

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 tubes with different diameters and at varying orientations to the gravitational vector. The activation energy of the reaction diffusion front was determined. For the homogeneous reaction, the isothermal density change(ap,) was measured 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 were observed. 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.