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 un
der convection-free and convective conditions were studied using tubes
with different diameters and at varying orientations to the gravitati
onal vector. The activation energy of the reaction diffusion front was
determined. For the homogeneous reaction, the isothermal density chan
ge(ap,) was measured and found to be negative. Because the reaction is
exothermic, the Pojman and Epstein analysis of convective stability p
redicted 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 conve
ction could occur by having two solutal species (iodide and triiodide-
starch complex) with significantly different diffusion coefficients an
d not only from competing thermal and solutal density changes. A mecha
nism for the angular dependence of the gravitational anisotropy in fro
nt velocities is proposed in terms of the Boycott effect.