Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

Phosphoric acid is a necessary component of permanganate chemical oscillators. When Mn(II) ions are present in a stoichiometric excess, the reaction is of first order with respect to the concentration of MnO₄^- ions and of fractional order with respect to the concentration of Mn(II). When MnO₄^- ions are in a stoichiometric excess, the reaction is autocatalytic in nature and the inflexion time increases linearly with the concentration of H₃PO₄; analysis of the sigmoidal curve gave the rate constant of the non-catalytic step, k₁, and that of the catalytic step, k₂, which is indirectly proportional to the concentration of H₃PO₄. The probable mechanism of the Guyard reaction involves a reversible equilibrium between Mn(IV) and colloidal MnO₂ and subsequent coagulation to form a MnO₂ suspension. Similarly to permanganate chemical oscillators, phosphoric acid or its ions play the key role, since they stabilize the colloidal MnO₂ which serves as a reservoir of Mn(IV) ions.