Generation of hydroxyl ions

The deposition-precipitation of Ni(II) shows the typical pH versus time behaviour found in high surface area Ni/SiOz systems [8-11]. The pH-curve displays a maximum after this value the nucleation and growth of the solid phase start up rapidly and the rate of generation of hydroxyl ion is lower than its consumption leading to a temporary decrease in the pH value (Fig. 1). 

We have already seen that patch repairing is not usually adequate to stop further deterioration in the presence of chloride attack. If a structure with extensive chloride attack is to be patch repaired then it must be recognized that patching the corroding areas can accelerate corrosion elsewhere. When we stop the anodic reaction (2.1) we stop the generation of hydroxyl ions at the cathode (equation (2.2)). Therefore, areas previously protected from corrosion because they were made cathodic by the proximity of the anode (now repaired) will rise above the critical chloride/hydroxyl ratio and corrosion will be initiated. This often occurs around the new patch as shown schematically in Figure 6.4. This incipient anode problem is avoided by applying an electrochemical rehabilitation technique. 

The generation of hydroxyl ions in equations (7.2) and (7.3) will increase the alkalinity and help to rebuild the passive layer where it has been broken down by the chloride attack. 

The generation of hydroxyl ions in equations 6,2 and 6.3 will increase the alkalinity and help to rebuild the passive layer where it has been broken down by the chloride attack. The chloride ion itself is negative and will be repelled by the negatively charged cathode (reinforcing steel). It will move towards the (new external) anode. With the carbon based anodes it may then combine to form chlorine gas at the anode ... 

However, research shows that the most important aspect of the chloride removal process is the generation of hydroxyl ions, the rebuilding of the protective passive film and the removal of the chlorides immediately around the rebar. SHRP research showed that even with a modest total charge passed and only 50 to 80% chloride removal, and with chloride levels still above the corrosion threshold, treatment will give a very low corrosion rate and very passive half cell potentials which last more than five years without reactivation (Bennett and Schue, 1993), If this is true then we should perhaps call the process electrochemical chloride mitigation and avoid requirements to remove more than 90% of the chloride, as this may not be necessary. 

Nevertheless, there are many instances where electrochemical corrosion mechanisms may play a primary role in affecting the service performance of bonded joints. It should be noted that such mechanisms of attack involve both the presence of (a) anodic sites, where reaction with the metallic substrate occurs and electrons are generated, and (b) cathodic sites, where the electrons are consumed. The major reaction leads to the generation of hydroxyl ions, and the liquid present at these sites will become strongly basic and so possess a relatively high pH. Thus, typically an aqueous (electrolyte) layer needs to be present, since, without such an aqueous film, no electrical current can flow from the anodic to the cathodic sites. These aspects are illustrated, for example, by the schematic electrochemical corrosion mechanism for an organic coating on a steel substrate shown in Fig. 4, which is discussed in detail in Section 2.S.2.2. 

Denitrification is a process in which facultative organisms will reduce nitrate to nitrogen gas in the absence of molecular oxygen. This consequendy results in the removal of BOD. The denitrification process also generates one hydroxyl ion so that alkalinity requirements are reduced to half when both nitrification and denitrification are practiced. 

The degree of dissociation is very small but the diphenylcyanomethyl radical is sufficiently reactive to induce polymerization in styrene. Methyl radicals or hydrogen atoms bring about polymerization of vinyl monomers in the gas phase.Hydrogen peroxide in the presence of ferrous ions initiates polymerization in the aqueous phase or in aqueous emulsions through generation of hydroxyl radicals according to the Haber-Weiss mechanism... 

Thus, antioxidant effects of nitrite in cured meats appear to be due to the formation of NO. Kanner et al. (1991) also demonstrated antioxidant effects of NO in systems where reactive hydroxyl radicals ( OH) are produced by the iron-catalyzed decomposition of hydrogen peroxide (Fenton reaction). Hydroxyl radical formation was measured as the rate of benzoate hydtoxylation to salicylic acid. Benzoate hydtoxylation catalyzed by cysteine-Fe +, ascorbate - EDTA-Fe, or Fe was significantly decreased by flushing of the reaction mixture with NO. They proposed that NO liganded to ferrous complexes reacted with H2O2 to form nitrous acid, hydroxyl ion, and ferric iron complexes, preventing generation of hydroxyl radicals. 

Biaglow JE, Kachur AV (1997) The generation of hydroxyl radicals in the reaction of molecular oxygen with polyphosphate complexes of ferrous ion. Radiat Res 148 181-187 Biaglow JE, Field KD, Manevich Y, Tuttle S, Kachur A, Uckun F (1996) Role of guanosine triphosphate in ferric ion-linked Fenton chemistry. Radiat Res 145 554-562 Bielski BHJ (1991) Studies of hypervalent iron. Free Radical Res Commun 12/13 469-477 Bielski BHJ, Allen AO, Schwarz HA (1981) Mechanism of disproportionation of ascorbate radicals. J Am Chem Soc 103 3516-3518... 

Figure 10.19 Schematic of a single bipolar membrane (not to scale) showing generation of hydroxyl and hydrogen ions by water splitting in the interior of the membrane. Electrolysis takes place in the thin interfacial region between the anodic and cathodic membranes...

Iron being a redox-active metal, it most hkely exerts its toxic effects through the generation of hydroxyl radical or by generation of ferryl ion. In iron loaded condition, there is generation of radical species leading to hpid peroxidation. Lipid peroxidation of cellular membrane would have deleterious effects on their function and hence on the function of the cell. There is also irreversible oxidation of ascorbic acid. Deficiency of ascorbic acid can lead to a reduction in the amount of iron available for erythropoiesis. 

Extensive calculations of the reaction free energy profile for the postulated mechanism of SNase (Aqvist and Warshel, 1989 1990) achieved quantitative agreement between calculated and observed free energy barriers. It was concluded that the general-base mechanism itself is not a sufficiently effective way of generating the hydroxyl ion and the Ca2t ion is essential for stabilisation. 

In the equilibrium constant, the superscript asterisk indicates that the dissolution reaction is written in terms of protons. The subscript following K (zero in this case) is the number of hydroxyl ions associated with AF+ in the reaction. According to Nordstrom et al. (1990), Kq = 10. Reactions similar to (7.35) can be written in which kaolinite is dissolved to form each of the aluminum hydroxy complexes listed in mass-balance equation (7.22) for total aluminum. These reactions can be generated by successively adding the cumulative reactions for the Al-hydroxy complexes—Eqs. (7.23) to (7.26)—to Eq. (7.35). For example, adding Eqs. (7.35) and (7.23) and their log values for 25°C, we obtain ... 

The Fenton procedure involves the addition of Fe + ions to the solution, thus resulting in the generation of hydroxyl radicals ... 

JL Roberts and DT Sawyer. Base-induced generation of superoxide ion and hydroxyl radical from hydrogen peroxide. J. Am. Chem. Soc. 100 329-330, 1978. 

Van Steveninck, J., Van Der Zee, J. and Dubbelman, T.M.A.R. (1985) Site of specific and bulk-phase generation of hydroxyl radicals in the presence of cupric ions and thiol compounds. Biochem. J. 232 309-311. 

The formation of the H2Fe(CO)4 catalyst described above presumably occurs via nucleophilic attack of hydroxyl ion on a CO ligand of Fe(CO)s with generation of the anionic metallocarboxylic acid followed by decarboxylation and protonation (Reaction 6). 

---