Hydrogen, redox potential

When either hydrogen ions or hydroxide ions participate in a redox half-reaction, then clearly the redox potential is alTected by change of pH. Manganate(Vir) ions are usually used in well-acidified solution, where (as we shall see in detail later) they oxidise chlorine ions. If the pH is increased to make the solution only mildly acidic (pH = 3-6), the redox potential changes from 1.52 V to about 1.1 V, and chloride is not oxidised. This fact is of practical use in a mixture of iodide and chloride ions in mildly acid solution. manganate(VII) oxidises only iodide addition of acid causes oxidation of chloride to proceed. 

As the above redox potentials indicate, only in the presence of very powerful oxidising agents does hydrogen peroxide behave as a reducing agent. For example ... 

Many of the reactions of halogens can be considered as either oxidation or displacement reactions the redox potentials (Table 11.2) give a clear indication of their relative oxidising power in aqueous solution. Fluorine, chlorine and bromine have the ability to displace hydrogen from hydrocarbons, but in addition each halogen is able to displace other elements which are less electronegative than itself. Thus fluorine can displace all the other halogens from both ionic and covalent compounds, for example... 

A second example is that of an Ala-to-Cys mutation, which causes the fonnation of a rare SH S hydrogen bond between the cysteine and a redox site sulfur and a 50 mV decrease in redox potential (and vice versa) in the bacterial ferredoxins [73]. Here, the side chain contribution of the cysteine is significant however, a backbone shift can also contribute depending on whether the nearby residues allow it to happen. Site-specific mutants have confirmed the redox potential shift [76,77] and the side chain conformation of cysteine but not the backbone shift in the case with crystal structures of both the native and mutant species [78] the latter can be attributed to the specific sequence of the ferre-doxin studied [73]. 

BW Beck, Q Xie, T Ichiye. Computational study of S—H S hydrogen bonds m [4Ee-4S]-type ferredoxm x-ray and NMR structures Characterization and implications for redox potentials. Protein Sci, submitted. 

Dissolved oxygen has a higher redox potential than the hydrogen ion at all values of pH, i.e. it is a more powerful oxidant. 

The effects of concentration, velocity and temperature are complex and it will become evident that these factors can frequently outweigh the thermodynamic and kinetic considerations detailed in Section 1.4. Thus it has been demonstrated in Chapter 1 that an increase in hydrogen ion concentration will raise the redox potential of the aqueous solution with a consequent increase in rate. On the other hand, an increase in the rate of the cathodic process may cause a decrease in rate when the metal shows an active/passive transition. However, in complex environmental situations these considerations do not always apply, particularly when the metals are subjected to certain conditions of high velocity and temperature. 

The redox potential is determined with a probe consisting of a platinum electrode and a Hg/Hg2Cl2 Cl or Ag/Ag Cl—Cl reference electrode. If Ep is the potential of the platinum probe, Ej the potential of the reference electrode and tne redox potential of the soil (in mV on the hydrogen scale) then... 

Cr(VI)] increase the rate by raising the potential of the alloy into the transpassive region, the converse applies in the acid (Fe2(S04)j test, since reduction of Fe to Fe during the test will result in a decrease in the redox potential and the whole sample will corrode with hydrogen evolution. 

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

Ultraviolet spectroscopy metal in water complexes, 2, 309 redox potentials and, 1,498 Ultraviolet-visible spectroscopy, 1, 243 Umbellicomplexone metallofluorescent indicator, 1. 558 Undecametallic complexes, 1, 167 Uni thiol chelating agent heavy metal poisoning, 6. 767 Unsaturated compounds hydrogenation... 

With its oxygen functionality, graphite oxide has chemical properties more akin to those of layered disulfides or sheet silicates than to those of graphite (Gi, T1,A2). Many studies have been of an extremely applied nature the possibility of fluorination (LI, N1), redox potentials in the presence of hydrogen peroxide (V2), the apparent density (L2), the adsorption isotherms with nitrogen (L3), and the diffusion of Cs in graphite oxide (R2). 

The importance of hydrogen bonds for the redox potential of the Rieske cluster has been demonstrated by site-directed mutagenesis of... 

The lower redox potential in menaquinone-oxidizing be complexes can be attributed to the absence of the hydrogen bond from the Oy of Ser 163 (ISF)/Ser 130 (RFS) to the bridging sulfur S-1 (see Section III,B,2). This serine is completely conserved in Rieske proteins of bci and bef complexes, but in all sequences of Rieske proteins from menaquinone-oxidizing be complexes, alanine or glycine is found in... 

Fig. 4. Representation of the ligand sphere of the [2Fe-2S] cluster of the Rieske protein from spinach and the attribution of g-tensor to moleculEir axes as discussed in the text. Ser 130 has been observed to influence the redox potentiEd of the cluster via hydrogen interactions with the acid-labile bridging sulfur.

The potential of the cluster units described here to participate in intermolecular chalcogen-chalcogen interactions combined with the easy modification of their outer coordination sphere with ligands of different nature, i.e., redox active, hydrogen donors, bi-functional, etc., make these systems useful blocks for the construction of supramolecular materials with multi-physical properties. 

The standard redox potential E° for these reactions in solutions at pH 7 (when referred to the potential of the hydrogen electrode in the same solution) varies from -0.218V for reaction (15.71) to +0.170V (for reaction (15.70) (i.e., to a rough approximation are close to zero). 

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