CONVECTIVE EFFECTS ON CHEMICAL WAVES .3. MULTICOMPONENT CONVECTION IN THE IRON(II) NITRIC-ACID SYSTEM

We consider the effects of convection on chemical waves in which density gradients result from the exothermicity as well as from the isothermal volume change of the reaction. The nitric acid oxidation of Fe(II) supports a constant-velocity traveling front which can be followed by observing the brown ring of the intermediate FeNO2+. The front velocity is extremely sensitive to the width of the tube in which the reaction is performed as well as to the orientation with respect to the gravitational vector. Ascending fronts propagate faster than pure reaction-diffusion fronts. Descending fronts propagate with the greatest velocities, as much as 80 times more rapidly than the reaction-diffusion front. Using the theory of multicomponent (double-diffusive) convection, we can explain this behavior in which there is a thermally induced density decrease in the front as well as an isothermal density increase. We qualitatively account for the observed concentration and orientation dependences.