Ironic ions

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

Tetrapotassium peroxodiphosphate is produced by electrolysis of a solution containing dipotassium phosphate and potassium fluoride (52). Alkalinity favors the formation of the P20 g anion, whereas the PO anion is produced in larger yields in acidic solution. It is therefore possible to obtain an 80% yield of K4P20g by choosing the proper conditions. The tetrapotassium peroxodiphosphate can be crysta11i2ed from solution by evaporation of water to form a slurry. The crystals can be separated from the slurry and dried. The material is noncorrosive and cannot be catalyticaHy decomposed by iron ions. 

The most significant chemical property of zinc is its high reduction potential. Zinc, which is above iron in the electromotive series, displaces iron ions from solution and prevents dissolution of the iron. For this reason, zinc is used extensively in coating steel, eg, by galvanizing and in zinc dust paints, and as a sacrificial anode in protecting pipelines, ship hulls, etc. 

Electrolytic corrosion occurs in regions I and IV with the formation of soluble iron ions. Solid corrosion products which can have a protective effect are formed in region II. This is the region of surface film passivity. Certain corrosive sub-... 

Iron fouling is caused by both forms of iron ions the insoluble form will coat the resin bead surface and the soluble form can exchange and attach to exchange sites on the resin bead. These exchanged ions can be oxidized by subsequent cycles and precipitate ferric oxide within the bead interior. 

FIGURE 15.25 The six liganding positions of an iron ion. Four ligands lie in the same plane the remaining two are, respectively, above and below this plane. In myoglobin. His F8 is the fifth ligand in oxymyoglobin, O9 becomes the sixth. 

In deoxyhemoglobin, histidine F8 is liganded to the heme iron ion, but steric constraints force the Fe His-N bond to be tilted about 8° from the perpendicular to the plane of the heme. Steric repulsion between histidine F8 and the nitrogen atoms of the porphyrin ring system, combined with electrostatic repulsions between the electrons of Fe and the porphyrin 77-electrons, forces the iron atom to lie out of the porphyrin plane by about 0.06 nm. Changes in... 

The presence of sulphonic and carboxylic groups enables the iron ions to be in the vicinity of the cellulose backbone chain. In this case, the radicals formed can easily attack the cellulose chain leading to the formation of a cellulose macroradical. Grafting of methyl methacrylate on tertiary aminized cotton using the bi-sulphite-hydrogen peroxide redox system was also investigated [58]. 

The effect of Fe(II) on grafting of 2-hydroxyethyl methacrylate onto polyester fibers in the presence of benzoyl peroxide was investigated [59]. It was found that increasing the iron ion concentration decreases the graft yield. This suggest that excess Fe(ll) ions participate in the generation of free radical species and the iron ions seem to contribute to the termination and, consequently, decrease the graft yield. 

The anodic reaction consists of the passage of iron ions from the metallic lattice into solution, with the liberation of electrons, which are consumed at the cathode by reaction with water and oxygen. 

If the electrode potential of iron is made sufficiently negative, positively charged iron ions will not be able to leave the metallic lattice, i.e. cathodic protection. 

The mechanism of inhibition by the salts of the long chain fatty acids has been examined . It was concluded that, in the case of the lead salts, metallic lead was first deposited at certain points and that at these points oxygen reduction proceeded more easily, consequently the current density was kept sufficiently high to maintain ferric film formation in addition, any hydrogen peroxide present may assist in keeping the iron ions in the oxide film in the ferric condition, consequently the air-formed film is thickened until it becomes impervious to iron ions. The zinc, calcium and sodium salts are not as efficient inhibitors as the lead salts and recent work has indicated that inhibition is due to the formation of ferric azelate, which repairs weak spots in the air-formed film. This conclusion has been confirmed by the use of C labelled azelaic acid, which was found to be distributed over the surface of the mild steel in a very heterogeneous manner. ... 

The iron ions impart a deep red color to the clay, which is not desirable for the manufacture of fine china a white base is aesthetically more pleasing. 

The two iron ions of the Rieske cluster are antiferromagnetically coupled therefore, the ground state has a spin S = while excited states of the spin ladder S = I, i, I, and, are at energies -3J, 8J,... 

When a system contains nonequivalent iron ions, the values relative to each site are reported in different lines. 

Fig. 3. Theoretically expected cysteine Hj8 chemical shifts (ppm) for iron-sulfur proteins, together with associated temperature dependences (arrows). The arrows indicate the direction where the signals move when the temperature is rsiised. The signals Eiris-ing from systems containing nonequivEilent iron ions are labeled according to the ion to which the cysteine is bound. The case of reduced HiPIP is ansdogous to that of oxidized Fd.

The simplest iron-sulfur centers, which were first discovered in ru-bredoxins, consist of one iron ion coordinated by a distorted tetrahedron of cysteinyl sulfur atoms. This environment provides a weak ligand field giving a spin equal to and 2 when the ion is Fe(III) and Fe(II), respectively. It also determines the splitting of the ground spin manifold, and consequently the characteristics of the EPR spectrum. This splitting is generally described in the framework of the spin Hamiltonian ... 

The applications of EPR spectroscopy reviewed in the present chapter are based on the sensitivity of the spectrum displayed by iron-sulfur centers to various characteristics, such as the redox state of the center, the distribution of the valences on the iron ions, the nature and the geometry of the ligands, and the presence of nearby paramagnetic species. Although considerable progress has been made during the past few years in the quantitative analysis of these various effects in the case of the conventional iron-sulfur centers described in Section II, the discovery of centers exhibiting unusual EPR properties as... 

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