Reactions Involving Chlorine and Other Halogens

The resulting unstable molecular ion Oj) rapidly adds another electron and protons to yield hydrogen peroxide. In alkaline solutions the same pathway is followed, but owing to the much lower polarization, the reaction becomes practically reversible (b = 0.03 V) its rate then is determined by oxygen transport to the surface, and polarization is of the concentration type (Bagotsky and Yablokova, 1953). 

Polarization is much higher for the electrochemical reduction of hydrogen peroxide. The slope has the unusually high value of about 0.3 V. At a given current density the electrode potential in this reaction is again independent of solution pH. These and certain other data indicate that addition of the first electron to the peroxide molecule and simultaneous peroxide decomposition is the rate-determining step  

At the platinum electrode the individual steps of the four-electron reaction cannot be studied separately. Slope b has its usual value of about 0.12 V, but in contrast to what is seen at the mercury electrode, the polarization is practically independent of solution pH (i.e., the potential at a given current density shifts by 0.06 V in the negative direction when the pH is raised by a unit). It follows that the reaction rate depends on hydrogen ion concentration. The step in which an electron and a proton are transferred while the 0-0 bond is broken is probably the ratedetermining step. 

The standard electrode potential of this reaction is 1.358 V vs. SHE the equilibrium potential is independent of solution pH. 

Anodic chlorine evolution by electrolysis of concentrated chloride solutions is used for the large-scale industrial production of chlorine. The cathodic reaction, which is the ionization of molecular chlorine, is used in certain types of batteries. 

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