Autoxidation of tartaric acid in air-saturated aqueous
solutions
in the presence of Fe(II) at low pH, 2.5, shows autocatalytic behavior
with distinct initiation, propagation, and termination phases. With
increasing pH, the initiation phase speeds up, while the propagation
phase shortens and reduces to none. We show that the propagation phase
is a chain reaction that occurs via activation of oxygen in the initiation
stage with the production of hydrogen peroxide. The subsequent Fenton
oxidation that regenerates hydrogen peroxide with a positive feedback
is typical of a self-sustained chain reaction. The conditions for
such a chain reaction are shown to be similar to those of a dynamical
system with critical behavior; namely, the system becomes unstable
when the kinetic matrix of pseudo-first-order reaction becomes negatively
defined with a negative eigenvalue giving the rate of exponential
(chain) growth of the reactive species.