J. Phys. Chem. 1986, 90, 6865
6865
COMMENTS Reply to Ruoff and Noyes' Comment on Heterogeneities and Stirring Effects In the Beiousov-Zhabotinsky Reaction Sir: Ruoff and Noyes' have challenged our interpretation of the potential fluctuations of a Pt microelectrode in terms of the heterogeneities in the Belousov-Zhabotinsky medium contained in a stirred batch reactor. Based on their occasional observation of similar fluctuations using platinum and silver macroelectrodes, and on the fact that this noise was electrode specific, i.e. it was found only on fresh, untreated metal surfaces and that it would disappear after polishing, Ruoff and Noyes concluded that their findings were artifacts, generated in some unspecified manner by the electrode surface in an otherwise homogeneous medium. They suggest that a similar interpretation applies to our experiments. S t i r r i n g rate w=2.25Hz
directly the spatial distribution of the heterogeneities in question. Two microelectrodes were mounted such that their distance could be accurately adjusted by a micrometer from 0 to 10 mm, and they were connected to the inputs of a differential electrometer. Figure 1 is the record of the difference signal as a function of electrode distance d, obtained for a nonoscillating steady state. The signal increases monotonically with d. It vanishes as d goes to zero (note that at a nominal d = 0 the electrodes do not touch and their actual distance is estimated to be of the order of 0.1 mm) when both electrodes sample the same volume element. The amplitude increases markedly around d = 3-6 mm and becomes constant at greater distances. This behavior is clearly incompatible with Ruoff and Noyes scenario since in a homogeneous medium the difference signal would have a constant amplitude, independent of interelectrode distance. The only obvious interpretation is that there exists a spatially distributed concentration distribution characterized by an average size g of the fluctuations. For d
> g they monitor two volume elements that fluctuate independently of each other, giving a maximal DIMES amplitude. At d = g a plot of the amplitude as a function of d exhibits a turning point.3 The derivative of the fluctuation amplitude as a function of the electrode distance is related to the size distribution of the heterogeneities. The stirring dependence of this function exhibits the expected shrinking of the fluctuations with increased stirring. The full report on this experiment will be published 1ater.j Registry No. Pt, 7440-06-4. (3) Jankowski, P.; Giraudi, A.; Menzinger, M., to be published in J . Phys. Chem.
Department of Chemistry University of Toronto Toronto, M5S 1A1, Canada
Michael Menzinger* Peter Jankowski
Received: August 1 1 , 1986
0022-3654/86/2090-6865$01.50/00 1986 American Chemical Society
