Optical Investigations of Film Formation and Removal on Gold Anodes in Acidic Oxalate Solutions
RECENT investigations of the oxidation of oxalate ions in acidic solutions using gold anodes have provided electrochemical evidence for the formation of a film on this anode during steady-state electrolysis at potentials well below that for Au2O3 formation, 1.36 V normal hydrogen electrode1. The determination of the nature of the film on gold at potentials below 1.36 V using electrochemical techniques has introduced uncertainties as to whether the film is a ‘three dimensional’ oxide film or a ‘two dimensional’ chemisorbed oxygen film. Some investigators2–5 using charging curves as their basis conclude that the formation of an oxide occurs only above 1.36 V, the formation of a chemisorbed oxygen film being the process occurring below this potential. The formation of this chemisorbed oxygen film begins at potentials as low as 1 15 V (ref. 6). Other work, however, also based on electrochemical evidence, indicates that the formation of an oxide film does occur below 1.36 V (ref. 7).
The kinetics of electrode processes. ─Part III. The behaviour of platinum and gold electrodes in sulphuric acid and alkaline solutions containing oxygen
The first paper of this series contained a general survey of the reaction of hydrogen and oxygen at platinum electrodes in dilute sulphuric acid solution. We have now investigated in greater detail some of the phenomena which were observed, using mainly small polarizing currents, by means of which they can be more conveniently studied, and extending the scope of the experiments to other metals and solutions. In this paper an account is given of further experiments on platinum electrodes in the presence of oxygen, and of an investigation of the behaviour of gold electrodes in sulphuric acid and alkaline soutions. The experimental arrangements were similar to those previously described. Platinum Electrodes in Oxygen Saturated Solutions . Cathodic Depolarization by Oxygen .—If a platinum electrode in sulphuric acid is anodically polarized until oxygen is liberated and the current is then reversed, a marked depolarization process occurs at about eH = + 0·05. Bowden's suggestion that this process is the reduction of a platinum oxide was shown in Part I to be untenable, because the length of the arrest of the potential is much reduced even by gentle stirring. The depolarization much be caused by some substance which is formed in the solution during the anodic polarization. Since it appeared that ordinary oxygen in the solution is not very active as a cathodic depolarizer in this region, it was suggested that the discharged oxygen remained for a time in a particularly active condition, but test of the solution failed to reveal any enhanced oxidizing power.
STABILIZATION OF FREE RADICALS AT LOW TEMPERATURES
Thus far, the lifetimes reported for the majority of unstable chemical species have been, a t best, in the order of milliseconds. Generally these species are formed either under conditions of nonequilibrium or exist for such short periods or in such small concentrations as to make measurement difficult. However, several recent studies have shown that short-lived species can be trapped in solids a t low temperatures and that, if a sufficiently low temperature is maintained, the active species appear to have an indefinitely long life-most certainly greater than hours. Since there is some uncertainty regarding the specific meaning of “unstable chemical species,” I should like to clarify my own use of the expression. By “unstable chemical species’’ I mean atoms, molecules, and ions that exist usually as separate entities for very short periods. At room temperature they react rapidly with other substances and with each other, so that generally they are observed only by their chemical reactions and by means of spectroscopy. I shall exclude such short-lived species as electronically excited atoms and molecules that lose their separate identity by radiation. For convenience I shall use the term “free radical” interchangeably with “unstable chemical substance.” Although some work was done on the observation of free radicals at low temperatures as long as 30 years ago, the major advances have been made within the last 5 years. It is interesting to note that many of the laboratories undertaking these studies did so without knowing that others were attempting similar or related investigations. Moreover, most of the laboratories have used different experimental techniques and have had differing objectives. It is characteristic of this new field that the investigators speak excitedly of beautiful colors, queer phenomena, and explosive events. Several methods are available for the production of free radicals for lowtemperature stabilization. These methods can be grouped conveniently into two categories: (1) those in which the radicals are frozen into a solid after preparation a t high temperature; and (2) those in which the radicals are produced directly in the solid a t low temperature. In the first group, radicals can be formed in a gas by electric discharges, by high temperature, or by chemical reactions, and then solidified on a cold wall before they can react. In the second group, radicals can be formed in the cold solid by irradiation of the proper material with y rays, X rays, or ultraviolet light,