Iron, standard reduction potential

Table 25.2 Standard reduction potentials for iron, ruthenium and osmium in acidic aqueous solution...

The overall voltage generated by a standard galvanic cell is always obtained by subtracting one standard reduction potential from the other in the way that gives a positive value for E (.gH Example applies this reasoning to zinc and iron. 

In damp air, materials with standard reduction potentials less than 0.88 V oxidize spontaneously. Atmospheric O2 easily oxidizes iron and aluminum, the most important structural metals ... 

C19-0031. Tin cans are made of iron, coated with a thin film of tin. After a break occurs in the film, a tin can corrodes much more rapidly than zinc-coated iron. Using standard reduction potentials, explain this... 

Tin plating is a common procedure to protect iron and its alloys from rusting. Tin is less easily oxidized than iron, as revealed by the relative magnitudes of the standard reduction potentials of the two metals ... 

For a particular iron(III) oxidant, the rate constant (log kpe) for electron transfer is strongly correlated with the ionization potential Ip of the various alkylmetal donors in Figure 4 (left) (6). The same correlation extends to the oxidation of alkyl radicals, as shown in Figure 4 (right) (2). [The cause of the bend (curvature) in the correlation is described in a subsequent section.] Similarly, for a particular alkylmetal donor, the rate constant (log kpe) for electron transfer in eq 1 varies linearly with the standard reduction potentials E° of the series of iron(III) complexes FeL33+, with L = substituted phenanthroline ligands (6). 

Standard reduction potentials of iron and zinc are as follows ... 

The diagram for the states of iron shows the linkage of the + 3 and zero states and the + 3/0 standard reduction potential. 

The heme cofactors of a and b cytochromes are tightly, but not covalently, bound to their associated proteins the hemes of c-type cytochromes are covalently attached through Cys residues (Fig. 19-3). As with the flavoproteins, the standard reduction potential of the heme iron atom of a cytochrome depends on its interaction with protein side chains and is therefore different for each cytochrome. The cytochromes of type a and b and some of type c are integral proteins of the inner mitochondrial membrane. One striking exception is the cytochrome c of mitochondria, a soluble protein that associates through electrostatic interactions with the outer surface of the inner membrane. We encountered cytochrome c in earlier discussions of protein structure (see Fig. 4-18). 

FIGURE 19-5 Iron-sulfur centers. The Fe-S centers of iron-sulfur proteins may be as simple as (a), with a single Fe ion surrounded by the S atoms of four Cys residues. Other centers include both inorganic and Cys S atoms, as in (b) 2Fe-2S or (c) 4Fe-4S centers, (d) The ferredoxin of the cyanobacterium Anabaena 7120 has one 2Fe-2S center (PDB ID 1 FRD) Fe is red, inorganic S2 is yellow, and the S of Cys is orange. (Note that in these designations only the inorganic S atoms are counted. For example, in the 2Fe-2S center (b), each Fe ion is actually surrounded by four S atoms.) The exact standard reduction potential of the iron in these centers depends on the type of center and its interaction with the associated protein. 

C. E. Ophardt, "Redox Demonstrations and Descriptive Chemistry Part I. Metals/ ]. Chem. Educ., Vol. 64,1987, 716. Redox reactions of iron(III) with thiosulfate, iron(II) with permanganate, and tin(II) with mercury(I) are used to show how an abbreviated table of standard reduction potentials is used to predict the products of these reactions from the relative positions of the oxidizing agents and reducing agents in the table. 

Figure 5 Redox predominance diagrams for iron (a) and manganese (b) boundaries represent the standard reduction potential for reduction of the (thermodynamically-stable) species above the boundary to the (thermodynamically-stable) species below the boundary. If the redox predominance regions of two species (e.g., the gray regions of Fe + and Mn04 ) do not overlap along the y-axis when the two diagrams are superimposed, reaction between the two species is thermodynamically favored...

A different way of preventing iron corrosion is to use a sacrificial anode. A comparison of the standard reduction potentials of iron and magnesium... 

If it is shown that titanium can be used as a sacrificial anode to protect iron, what conclusion can be drawn about the standard reduction potential of its half-reaction ... 

It is clear that pH plays a role in maintaining iron (II) in solution. Solubility of iron (il) at low pH is obviously a most important factor but the possible effect of the standard reduction potential of the Fe+3 redox couple at different pH values should not be overlooked. This may explain the results of Leichter and Joslyn (, Lee and Clydesdale (32J and others who found that regardless of the source the iron found in bread and non-yeast leavened baked goods (high pH foods) respectively, was mainly in the iron (III) state and/or insoluble. 

In the presence of ascorbate which has a standard reduction potential of +l l 0mv the formation of Fe+2 will take place spontaneously (1 5., 1 8) and reaction 1 (Figure 6) will go to the right. However, at the same time, at a low pH. both a Fe+2 -ascorbate and a Fe+3 -ascorbate complex may form ( uh, 1 8), represented by reactions 2 and 1 (Figure 6). Upon addition of ferric iron to an ascorbic acid solution reaction 2 will probably take place more rapidly than reaction 1, but in time reaction 1 will predominant if the pH is maintained at a low level, as observed by Nojeim and Clydesdale (1 3), with the overall effect being reduction. Thus ascorbate, due to its reduction potential relative to iron at a low pH, and... 

An iron container can be covered with a layer of another metal such as tin or zinc. A tin can is made by applying a thin layer of tin over iron. Rust formation is prevented as long as the tin layer remains intact. However, once the surface has been scratched, rusting occurs rapidly. If we look up the standard reduction potentials, we find that iron acts as the anode and tin as the cathode in the corrosion process ... 

Well-known phenomena as corrosion of e.g. cobber roofs are closely related to the electrochemistry. Because corroded metal as e.g. iron looses its strength corrosion chemistiy has great practical importance. Metals are especially exposed to corrosion as metals are easily oxidised. From tables of standard reduction potentials one will see that with the exception of noble metals as e.g. gold the standard reduction potentials of common metals are lower than for oxygen. This means that the oxidation of most metals is a spontaneous reaction even though not metals are equally oxidised in the presence of oxygen. 

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