Platinum electrode rest potentials

Potentiodynamic Technique. Adsorption of methanol on Pt in acid solution was studied by Breiter and Gilman (3) using a potentiostatic technique. The anodic sweep, with a sweep rate of 800 V/s, was started at rest potential and extended to 2.0 V with respect to a hydrogen reference electrode in the same solution. As shown in Figure 10.8, the current was recorded as a function of potential (time) in the absence (curve A) and in the presence (curve B) of methanol. The increase in current in curve B is due to oxidation of the adsorbed methanol on the platinum electrode. Thus, shaded area 2 minus shaded area 1 (Fig. 10.8) yields the change 2m (C/cm ) required for oxidation of the adsorbed methanol ... 

Figure 9.1 Electrochemical behavior of dioxygen (1 atm) in aqueous solutions (1 M NaC104) at a platinum electrode (area 0.458 cm2) A) the cyclic voltammogram was initiated at the rest potential with a scan rate of 0.1 V s-1 (B) the rotated-disk (400-rpm) voltammogram was obtained with a scan rate of 0.5 V min-1. 

Experiments showed that coagulation increases for applied potential differences greater than +0.2 V vs. NHE below this value, clot formation is very small. The rest potential of various materials used for vascular prostheses and cardiac valves was determined. In Table 17.2 some of the materials tested are mentioned. It was concluded that metallic electrodes with a negative potential vs. NHE in the blood are anticoagulant while those with positive potential are coagulant. Unfortunately, the metals most useful for prostheses are the most easily corroded those of platinum and gold, not corroded, are unsuitable because of their positive rest potentials. Attempts to resolve the problem have utilized prostheses of plastic materials compatible in terms of their qualities of physical resistance, durability, etc. with their end use. 

The behavior of the platinized platinum electrodes is entirely different from that of the Ag/Ag halide electrodes, both under flow conditions and at rest. Whereas the nonflow potential for the nonpolarizable electrodes remained constant with time, the rest potential for two platinized platinum electrodes increased with time. Also, the flow of liquid produced a more pronounced effect on the platinum electrodes than on the Ag/Ag halide electrodes. The erratic behavior of the platinum electrodes appears to be due to polarization effects which are difficult to eliminate. In Figure 3 the effect of flow on both Ag/AgCl and platinized electrodes can be assessed. The effect of flow on the platinum electrodes was known to Helmholtz (19) and still appears to remain unexplained. 

The Eh values are too high to be explained by the formation of either PtS or PtS2. The rest potentials of the platinum electrode are dependent upon the concentration of dissolved hydrogen sulfide (z[H2S]]. This can be explained by selective adsorption of H2S species on the platinum followed by the discharge of the proton mediated via chemisorbed H2S. The corresponding reactions are (11] ... 

The Tafel equation also describes the evolution of oxygen at a platinum anode. Bockris and Huq found that, with solutions carefully purified by preelectrolysis, the oxygen electrode exhibits reversible behavior (E = 1.24 V, compared with the theoretical 1.23 V). The exchange current density, however, is only of the order of 10" to 10" °A/cm in dilute sulfuric acid so polarization occurs readily, and relatively large overpotentials are observed at moderate current densities. In solutions of ordinary chemical purity the Nemst relation fails for the oxygen electrode because of mixed-potential behavior. Criddle, using platinum electrodes in highly purified 1 M KOH, obtained a rest potential of 1.59 V. The potential is reduced by peroxide, which may be formed with impurities such as metals, protein, or carbon. 

The O2/H2O system is very slow so that the exchange current a I equilibrium is extremely low (10 /10 A cm 2) as a consequence, any other reaction at the electrode will hamper its study and that could be the reaction of impurities or other redox reactions involving the electrode itself. The so-called noble metals are not really inert and do interact with oxygen a platinum surface in contact with an O saturated solution adsorbs oxygeti as an electronically conducting monolayer but can be further oxidized to PIO, PtO . A detailed analysis of these phenomena, which falls outside the scope of the present review, can be found elsewhere [311. A platinum electrode, when a complete electronically conducting monolayer of I l—O is formed at the surface of the metal, behaves as an ideally inert electrode in such conditions, rest potentials dependent on pO2 and pH can be measured during a few hours, close to... 

The four-electron reduction of oxygen [reaction (I)] is very irreversible and therefore experimental verification of the thermodynamic reversible potential of this reaction is very difficult. The exchange current densities for reactions (I) and (II) are typically 10" -10" A/cm of real surface area for Pt and other noble metals at room temperatures. Any other side reaction, even if slow and otherwise difficult to detect, may compete with reaction (I) or (II) in establishing the rest potential. Indeed, unless special experimental procedures are used, the thermodynamic potential cannot be obtained at ambient temperature in aqueous electrolytes. Even on the most active platinum electrode in pure acid or alkaline aqueous solution under ordinary conditions, the rest potential in the presence of oxygen at 1 atm and ambient temperature usually does not exceed 1.1 V vs. the NHE and most often has a value close to 1.0 V. In early work on O2 electrochemistry, before reliable thermodynamic data were available, the potential 1.08 V vs. RHE was considered as the reversible value for reactions (I) and (II). 

The question remains open as to what is the nature of irreversible rest potential of most ordinary platinum and other noble metal electrodes in the presence of O2 in the solution. The importance of this potential lies in the fact that it is the starting point for the cathodic polarization of practical... 

Some experimental features of this rest potential on platinum which are valid also for other noble metal electrode materials are the following ... 

According to Bockris and his coworkers/ the anodic current from the oxidation of impurities is under most laboratory conditions sufficient to account for the mixed potential of oxygen electrode. This does not, however, explain the surprising reproducibility of this rest potential independent of the nature and concentrations of impurities in various laboratories. Further, the transport of impurities in the solution phase to the electrode surface should be at least partially rate controlling and hence the rest potential should be sensitive to stirring. The rest potential of platinum, however, usually exhibits little dependence on stirring. 

The rest potentials of platinum and other electrodes have also been examined in hydrogen peroxide solutions. On surfaces such as carbon, graphite, and lithiated nickel oxide, the open-current values are the thermodynamically reversible value for the 02-H02 couple in alkaline electrolytes or close to it. On these surfaces, the further reduction of peroxide [reaction (V)] and the overall four-electron reaction are both very inhibited. These surfaces also do not have much catalytic activity for the heterogeneous decomposition of the peroxide. On the other hand, with platinum the overall four-electron reduction is far less inhibited and the platinum surface is a reasonably effective catalyst for peroxide decomposition. Even so, the open-... 

FIG. 5—Profiles of electrode potential, referred to the rest potential of zinc, at the surface of a rod shaped zinc platinum/ zinc short-circuited cell in acidic chloride media. The dashed lines represent platinum electrodes of varying length [9]. 

Irreversibly established rest potential, also called open-circuit potential, experimentally obtained in O2 saturated solutions varies for different electrode materials. On the most active platinum in pure acid solution saturated with O2... 

Rates of corrosion can also be measured using an electrochemical technique known as potentiodynamic polarization. The potential of the test metal electrode relative to a reference electrode (commonly the saturated calomel electrode SCE) is varied at a controlled rate using a potentiostat. The resultant current density which flows in the cell via an auxiliary electrode, typically platinum, is recorded as a function of potential. The schematic curve in fig. 2 is typical of data obtained from such a test. These data can provide various parameters in addition to corrosion rate, all of which are suitable for describing corrosion resistance. The corrosion potential F corr is nominally the open circuit or rest potential of the metal in solution. At this potential, the anodic and cathodic processes occurring on the surface are in equilibrium. When the sample is polarized to potentials more positive than Scon the anodic processes, such as metal dissolution, dominate (Anodic Polarization Curve). With polarization to potentials more negative than Scorr the cathodic processes involved in the corrosion reaction such as oxygen reduction and hydrogen evolution dominate (Cathodic Polarization Curve). These separate halves of the total polarization curve may provide information about the rates of anodic and cathodic processes. The current density at any particular potential is a measure of the... 

The preconcentration of trace metals by electrodeposition is an integral part of anodic-stripping voltammetry. The method consists of the preelectrolysis of the stirred solution with a small mercury drop or solid electrode as the cathode (112-114). The metals, which are deposited and dissolve in the mercury, are then stripped from the amalgam after a suitable rest period by a reversal of the electrode potential. The resulting current-polarization curve is characteristic of the metal and its concentration. Concentrations as low as 10 M of metal ions require a preelectrolysis of about 60 min or longer. Other electrodes such as mercury films, platinum, gold, silver, and various forms of carbon have been used (77 ). 

It is the most commonly used reference electrode. It has a constant and reproducible potential. The electrode basically consists of a platinum wire dipped into pure mercury which rests in a paste of mercurous chloride and mercury. The paste is in contact with a solution of potassium chloride which acts as a salt bridge to the other half of the cell (Fig. 2.13). 

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