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Potential-pH diagrams were constructed from literature data for phenol, 2,4,6-tri-t-butylphenol, and hydroquinone (1,4-di hydroxybenzene). The relationship between chemical and electrochemical phenomena is described, and the significance of the results of chemical experiments discussed in relation to the potential-pH diagrams for the case of phenol oxidations.Potential (E)-pH diagrams, sometimes called Pourbaix diagrams, are convenient ways of summarizing equilibrium information about reactions, particularly oxidation-reduction reactions, which take place in solution. The construction of an E-pH diagram has been described by Pourbaix.' Once constructed, it defines the range of potential and pH over which any species of interest is thermodynamically stable. E-pH diagrams can thus be used to predict whether or not two chemical species will react but of course, give no information whatsoever about the rate of reaction. If each of the two species occupy the same area on an E-pH diagram, they will coexist. On the other hand, if they occupy different areas on the E-pH diagram, they are likely to react. Using this principle, Pourbaix has determined the conditions under which all common metals are likely to corrode in water, It is worth pointing out that the electrochemical potential E is directly related to the free energy AG of the system by AG = -nFE where n is the number of electrons involved in the redox reaction and F is the Faraday constant. It follows, then, that an E-pH diagram can be simply transformed into a plot of the free energy of the system as a function of pH. It is not, however, just a matter of scaling, since allowance must be made for the number of electrons n involved in the various electrode reactions.Considering the convenience of Pourbaix diagrams, it is surprising that relatively little use has been made of them in the field of organic chemistry. One of the few examples in the literature is that due to Waters who used a simple two line E-pH diagram to determine the range of pH within which the oxidation of phenol would proceed via a one-electron step as opposed to a two-electron step. Although Waters' paper has been described as important,3g4 little use has been made of E-pH diagrams and few have been published, although in some cases the basic data for their construction exists in the literature. The construction of an atlas of E-pH diagrams comparable with that devised by Pourbaix for the metals would seem desirable.In this paper, we will briefly describe how a potential-pH diagram is constructed, and discuss the sort of approximations that are commonly made during its construction. A simple three-line diagram for phenol will be presented, and the generalization of this diagram to other substituted phenols described. The influence of the solvent on these diagrams will be briefly examined. Finally, we will describe the construction of two more complex E-pH diagrams for the compounds 2,4,6-tri-t-butylphenol in aqueous ethanol (57% w/w) as solvent and 1,4-dihydroxybenzene in water as solvent.Examp...

The dissolution of gold in acidic thiourea solutions, together with the oxidation of thiourea on gold, platinum, and glassy carbon (GC) electrodes, has been investigated using voltammetric methods. The cyclic voltammograms on platinum and GC electrodes showed a single pair of peaks for the oxidation of thiourea to formamidine disulfide and its subsequent reduction. The mechanism of the reaction is discussed. When a gold electrode was used, gold dissolution took place at a more negative potential than thiourea oxidation, with the latter reaction being shifted to a potential about 300 mV more positive than that on the platinum electrode. It was found that, at 25°C and without formamidine disulfide, gold did not dissolve when the potential was scanned rapidly. This result shows that the process of gold dissolution might include a slow surface step which can be accelerated by formamidine disulfide. It is possible that the slow surface step results from the strong adsorption of thiourea on the gold surface. The adsorption of thiourea on gold was demonstrated using the electrochemical quartz crystal microbalance. © 2001 The Electrochemical Society. All rights reserved.

Results and DiscussionKinetic studies of gold leaching.-The REQCM was used to study the effect of lead on the dissolution of gold in air-saturated 20 mM cyanide solutions. Shown in Fig. 1 are two kinetic plots which show the change in mass of gold as a function of time. It can be seen that the mass of the electrode, m, decreases as the gold is leached, and this decrease can be related to the dissolution rate through Eq. 2

Molecular core binding energies have been measured by ESCA for series of surface‐fluorinated high‐density polyethylene films. Detailed analysis of the results shows that the technique can be used to establish a comprehensive picture of the early stages of the surface fluorination process. Both the depth of fluorination, and stoichiometry and molecular structure as a function of depth, in the range 0–50 Å have been established. In addition, minor features such as oxidation, hydration, and hydrocarbon contamination at the surface are clearly revealed. Detailed consideration of the results allows a discussion of the dynamics of the overall fluorination process.