Hydroxides potential range

The anodic evolution of oxygen takes place at platinum and other noble metal electrodes at high overpotentials. The polarization curve obeys the Tafel equation in the potential range from 1.2 to 2.0 V with a b value between 0.10 and 0.13. Under these conditions, the rate-controlling process is probably the oxidation of hydroxide ions or water molecules on the surface of the electrode covered with surface oxide ... 

Figure 4 Change in volume of Chemlok 220 top coat in IN NaOH solution and in 3.5%(Wt.) solution of NaCl. Volume changes were minimal, illustrating the potential range of responses of adhesive agents to exposure to hydroxide ion.

Using the rotating disk electrode, Seliv-anov et al. [214] have investigated the zinc electrodeposition from zincate electrolyte containing polyethylene polyamine. The limiting current density of [Zn(OH)4] ion diffusion through a film of zinc oxides and hydroxides is shown to be responsible for the formation of dark zinc deposits in the potential range from —1.33 to —1.47 V. 

At highly negative potentials, the doublelayer capacity has been found to depend on the kind of cations and to remain independent of their concentration. Strong adsorption of hydroxide anions was observed in wide concentration and potential ranges. 

If the experimental observations are summarised, a constant finding is the value of 0.5 for the transfer coefficient of the oxidation reaction, with all the combinations of hydrogen peroxide concentration and pH. Obviously, this is valid only in the potential range in which this transfer coefficient was experimentally determined. With potentials outside this range, the transfer coefficient cannot be used as a criterion. This value of the transfer coefficient is a primary requirement which every postulated reaction mechanism should meet theoretically. Furthermore, it is certain that hydroxide ions interfere in the oxidation reaction. 

Ni is a frequent component for alloys as e.g. for stainless steels. Polarization curves of Fe53Ni and FelONi still show features known for pure Ni (Fig. 5). The current increase and the peaks in the transpassive range are suppressed to a large extent in acidic and alkaline solutions due to the influence of Fe [15, 48], Angular resolved XPS measurements indicate a bilayer structure of the passive film with an outer hydroxide and an inner oxide part. Circa 1 nm hydroxide is found with no change with the electrode potential. The oxide part increases linearly with the potential up to 5 nm and levels off to a constant value for the transpassive potential range at 0.70 V in 1 M NaOH and at 1.40 V for pH 2.9 [15, 48], At 0.70 V in 1 M NaOH one observes... 

In the reaction of phenyl vinyl ketone with hydroxide ions in alkaline media two types of polarographio behaviour were observed, according to the pH-value. At pH below about 11 -5 phenyl vinyl ketone is reduced in two steps of equal height, the more negative corresponding to the reduction of propiophenone formed at the surface of the electrode in the electrolytic process. )3-Hydroxypropiophenone which is formed in the reaction of phenyl vinyl ketone with hydroxide ions is reduced in one two-electron step in the same potential range as the second wave of... 

The anodic passivation of semiconductors in aqueous solution occurs in much the same way as that of metals and produces a passive oxide film on the semiconductor electrodes. Figure 22.25 shows the anodic dissolution current and the thickness of the passive film as a function of electrode potential for p-type and n-type silicon electrodes in basic sodium hydroxide solution [32,33], As mentioned earlier, silicon dissolves in the active state as divalent silicon ions and in the passive state a film of quadravalent insoluble silicon dioxide is formed on the silicon electrode. The passive film is in the order of 0.2-1.0 nm thick with an electric field of 106 107 V cm 1 in the film within the potential range where water is stable. 

Fig. 7.78 Stress corrosion potential ranges of pipeline steel in hydroxide, carbonate-bicarbonate, and nitrate solutions in slow strain-rate test. Strain rate 2.5 x 10 6 s 1. Arrows indicate open circuit corrosion potentials for each environment. Redrawn from Ref 68...

Oxidation of peptides and amino acids at copper oxide electrodes is achieved at very high pH (typically in 0.1 M hydroxide) and potentials in the range of +0.40 to +0.60 V versus Ag/AgCl [112-114], In this potential range, a copper electrode is covered with a Cu(Il) oxide film (CuO) containing hydroxyl radicals that can described as CuO( OH) [113-115]. Amino acids that have been oxidized include Ala, Arg, Asn, Asp, Cys, Gin, Gly, His, Leu, Lys, Met, Phe, Pro, Ser, Thr, Trp, Tyr and Val [112,107], and it has been suggested that oxidation occurs according to [113] ... 

The ORR proceeds at fte catalyst surface, where electrons are readily available, at a concentration that is determined by the electronic density of states of the metal. Protons are supplied from the electrolyte, with a concentration determined by the composition of the electrolyte and by the distribution of the electrolyte potential. The ORR involves, at least, three surface-adsorbed intermediate species. They are, surface oxide, Oad, hydroxide, OHad, and superoxide, OOHad- Surface processes that transform these species into one another and ultimately into water involve kinetic barriers that determine the net rate of the overall reaction. In the electrode potential range of interest, from 0.6-1.0 V versus SHE at the cathode, the formation of surface oxides as intermediates of the ORR interferes with the formation of surface oxides from the splitting of water. 

A further example of the corrosion behavior of unalloyed steel in sodium hydroxide with the interaction of other factors is explained in Fig. 1-14 (Prakins et al., 1972). On one hand the SCC resistance of unalloyed steel in sodium hydroxide depends on the strain, on the other hand it depends on the electrode potential. Only at a low strain rate of 1.5 x 10 or 1.6 x 10 s in the potential range C/she = -0.8 to -0.5 can the material suffer SCC in the boiling solution. 

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