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Limiting supersaturations for dissolved gases manifested by gas evolution oscillators and by direct experiments cannot be accounted for by the application of classical nucleation theory (CNT). The theory predicts bubbles containing 104-105 molecules at nucleation, with Helmholtz energies of ca. 104kT per bubble, much too high for homogeneous nucleation to occur spontaneously in a finite time. We investigate alternative unstable structures ('blobs') which do not have well-defined interfaces, which may exist transiently at {he point of nucleation as the precursors of true bubbles and which circumvent the need for a large Helmholtz energy for their formation. Effects due to global or local depletion of the solution concentration at nucleation are also considered.t This paper is Part 100 in the series Chemical Oscillations and

The oxidation of malonic acid by ceric ions has been investigated in sulfuric acid solution under a variety of conditions. The initial rate a t low ceric ion concentrations is first order in each of the two reactants and has an activation energy of 11.6 kcal/mol; the instantaneous rate constant increases somewhat with time during a single run. At higher concentrations of ceric ion, semilogarithmic plots are sigmoid with a reduced rate constant a t long times. The rate decreases slightly with increasing sulfuric acid concentration. Rates of carbon dioxide evolution may be much less than rates of ceric ion reduction because of supersaturation effects. The observations can be explained if dissolved oxygen reacts with organic radicals to catalyze the rate of initial attack on malonic acid, but oxygen must also be consumed irreversibly during these reactions. Computations with plausible rate constants have simulated the experimental observations. These oxygen effects can rationalize peculiar almost discontinuous changes in rate when bromomalonic acid is oxidized by ceric ion. These effects may also explain the previously puzzling observation that some Belousov-Zhabotinsky solutions are oscillatory in bulk but become quiescent but excitable when spread in a thin film in contact with air.