An unusual spatiotemporal phenomenon was observed in propagating front
s in the bromate-sulfite system with an initial pH gradient, but not i
n chlorate-sulfite nor iodate-sulfite systems. Periodic acceleration a
nd deceleration of the front were observed, which appeared as a ''jump
ing'' wave. An experimental method was developed to produce the pH gra
dient systematically and reproducibly in a tube. The effect of the gra
dient was studied. With the use of infrared and visible imaging the co
mpetition between chemically and thermally induced convection was obse
rved. A qualitative model is proposed that includes the effect of pH g
radient and the specific mechanism of the bromate oxidation of sulfite
.