Ferrous oxide reduction

Iron Vanadate is, metallurgically, the most important vanadate. Precipitation of a solution of a vanadate with ferrous sulphate gives rise to a precipitate of indefinite composition, ortlio-, pyro-, meta-, and perhaps a poly-vanadate being present, as well as ferric or ferrous oxide. Reduction of the vanadate to a vanadyl salt may also ensue. The precipitate is usually colloidal and carries down with it some sodium vanadate. The dried powder may be either green, yellow, brown, or red the more nearly the precipitate approximates to a red colour the lower is its vanadium content. An iron vanadate has also been prepared by electrolysis of a solution of sodium vanadate between iron poles.1... 

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). 

The reaction between ferric ion, Fe+3, and cuprous ion, Cu+, to produce ferrous ion, Fe+2, and cupric ion, Cu+2, is plainly an oxidation-reduction reaction ... 

Oxidation- reduction An inert metal dips into a solution containing ions in two different oxidation states. An example consists of a platinum wire dipping into a solution containing ferrous and ferric ions. Such a cell is described by Pt Fe2 (c,). Fe3 (c2). The comma is used to separate the two chemical species which are in the same solution. These electrodes are similar to the gas electrodes, except that the two species involved in the electrode reaction are ions. The electrode reaction in the example is Fe3 + e Fe2, and there is the possibility of the electrode either donating or accepting electrons. 

The nickel-chromium plating process includes the steps in which a ferrous base material is electroplated with nickel and chromium. The electroplating operations for plating the two metals are basically oxidation-reduction reactions. Typically, the part to be plated is the cathode, and the plating metal is the anode. 

Most commonly, iron is discussed as being in either the ferrous (Fe2+) or ferric (Fe3+) state. Changes between these two depend on the soil s pH and Eh (where Eh is a measure of the oxidation-reduction potential of soil) as discussed in Chapter 9. Add conditions and low Eh values tend to lead to the production of ferrous ion, while high pH and high Eh values result in the predominance of ferric ion. It should be noted that the ferrous ion is more soluble than the ferric ion and, thus, it will be more available to plants. 

In well-aerated soil, it is expected that all species will be in their highest oxidation states. However, this does not happen for reasons elucidated in previous chapters. In well-aerated soil, both ferrous and ferric iron can exist along with elemental iron.3 Zinc, copper, and especially manganese can apparently exist in a mixture of oxidation states simultaneously in soil. Add to this a multitude of organic species that are also capable of oxidation-reduction reactions and the result is truly a complex voltammetric system [12,13],... 

Although oxidation and reduction must occur together, it is convenient when describing electron transfers to consider the two halves of an oxidation-reduction reaction separately. For example, the oxidation of ferrous ion by cupric ion,... 

A well-known example of an oxidation-reduction reaction leading to the formation of radicals is the decomposition of hydrogen peroxide by ferrous ions. Haber and Weiss (36) formulated this decomposition as follows ... 

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes I, Respiration consumes oxygen. 2, Microbial reduction of reactive metal Oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by Os. Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the hiofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides.

OXIDATION-REDUCTION INDICATOR. A substance that has a color in the oxidized form different from that of the reduced form and can be reversibly oxidized and reduced. Thus, if diphenylamine is present in a ferrous sulfate solution to which potassium dichromate is being added, a violet color appears with the first drop of excess dichromate. 

The Fe atoms of the cytochromes undergo oxidation and reduction during respiration, cycling between the ferrous (Fe2+) and ferric (Fe3+) oxidation states. The absorption spectra of the oxidized and reduced forms differ (fig. 14.4). In the 1930s, David Keilin used this property to measure the oxidation-reduction states of cytochromes in living cells. Under anaerobic conditions, the cytochromes rapidly became reduced in the presence of 02, they became oxidized. Certain molecules that inhibited respiration (CO, N3, or CN ) blocked the oxidation other inhibitors (amy-tal, rotenone, and malonate) blocked the reduction. Keilin found that the transfer of electrons from cytochrome c to 02... 

The first report of the polymerization of tetrafluoroethylene was by Plunkett in 1941, who had a cylinder of tetrafluoroethylene cut open to see why the expected amount of gas was not released when the valve was opened. His perspicacity led to the discovery of an inert, white, opaque solid with a waxy feel. Various methods of polymerization were tried after the adventitious discovery and the preferred methods for polymerization now involve aqueous media and super-atmospheric pressures. Suitable initiators (Hanford and Joyce) include ammonium, sodium, or potassium persulfate, hydrogen peroxide, oxygen, and some organic peroxy compounds. Oxidation-reduction initiation systems involving the use of persulfate with either ferrous ion or bisulfite or the use of bisulfite with ferric ion are also useful and have been discussed by Berry and Peterson. 

At high fields the NMR spectrum of cyanoferricytochrome c contains resolved hyperfine-shifted fines at +1.2 and +4.4 ppm (Fig. 29). Upon addition of dithionite these fines, as well as the resonances between —10 and —30 ppm, disappear immediately, indicating fast reduction to the diamagnetic ferrous oxidation state. The primary reaction product has no proton resonances at higher field than +1 ppm. However, with time elapsing after the reduction the NMR spectrum of ferrocytochrome c appears slowly. On the other hand the entire ferrocytochrome c spectrum was observed immediately after reduction of ferricytochrome c. 

A third chemical weathering mechanism that is of importance is oxidation/ reduction that involves mainly the elements carbon, iron, manganese and, of course, oxygen. An equilibrium reaction between dissolved C02 and bicarbonate ions can lead to the precipitation of ferrous iron, giving a hematite (ferric oxide) precipitate ... 

Because any potentiometric electrode system ultimately must have a redox couple (or an ion-exchange process in the case of membrane electrodes) for a meaningful response, the most common form of potentiometric electrode systems involves oxidation-reduction processes. Hence, to monitor the activity of ferric ion [iron(III)], an excess of ferrous iron [iron(II)] is added such that the concentration of this species remains constant to give a direct Nemstian response for the activity of iron(III). For such redox couples the most common electrode system has been the platinum electrode. This tradition has come about primarily because of the historic belief that the platinum electrode is totally inert and involves only the pure metal as a surface. However, during the past decade it has become evident that platinum electrodes are not as inert as long believed and that their potentiometric response is frequently dependent on the history of the surface and the extent of its activation. The evidence is convincing that platinum electrodes, and in all probability all metal electrodes, are covered with an oxide film that changes its characteristics with time. Nonetheless, the platinum electrode continues to enjoy wide popularity as an inert indicator of redox reactions and of the activities of the ions involved in such reactions. 

Conroy CW, Tyma P, Daum PH et al (1978) Oxidation-reduction potential measurements of cytochrome c peroxidase and pH dependent spectral transitions in the ferrous enzyme. Biochim Biophys Acta 537 62-69... 

Consider now the dissolution of iron by replacing the Pt WE with an Fe wire and adding 1 M Fe2+ to the solution in chamber B (via dissolution of FeS04, for example), as shown in Fig. 22. Calculation of the reversible potential for iron dissolution indicates that it would be the same as the standard reversible potential, -0.44 V (NHE), as the ferrous ion is at unit activity. We will assume that only iron oxidation/reduction can occur in this cell. Changing the polarity of the variable voltage supply allows removal of electrons from the WE, forcing net oxidation to occur there and net reduction to occur on the Pt CE in chamber C. Figure... 

It has been generally assumed that iron is transported across biological membranes in the ferrous form and that ferric iron would have to be reduced before it can be used by the organism. Thus, based on nutritional studies it has long been recognized that Fe(II) is1 more effectively absorbed than Fe(III), and this has been attributed to differences in the thermodynamic and kinetic stability of the complexes and chelates formed by these cations (for review, see Ref. 2). The experimental proof of a transport in the ferrous form has, however, not been given until quite recently in studies of iron transport in isolated mitochondria (23) as well as in enterobacteria (33). In rat liver mitochondria we have found that Fe(III) donated from a metabolically inert water soluble complex of sucrose interacts with the respiratory chain at the level of cytochrome c (and possibly cytochrome a) (23, 32) (Figure 1 B), which has a oxidation-reduction potential of around +250 mV (34) and is localized to the outer phase of the mitochondrial inner membrane (35). 

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