Standard Reduction-Oxidation Potential

The peroxodisulfate ion in aqueous solution is one of the strongest oxidising agents known. The standard oxidation—reduction potential for the following reaction is 2.08 V (77,78). 

Table 3.5 Standard oxidation-reduction potentials. (From the CRC Handbook of Chemistry and Physics)... 

Source A. J. Bard, R. Parsons, and J. Jordan (eds.), Standard Potentials in Aqueous Solution (prepared under the auspices of the International Union of Pure and Applied Chemistry), Marcel Dekker, New York, 1985 G. Chariot etal. .Selected Constants Oxidation-Reduction Potentials of Inorganic Substances in Aqueous Solution, Butterworths, London, 1971. 

The oxidation-reduction potential or redox potential ( h) is a measure of the tendency of a solution to be oxidizing or reducing. Oxidation and reduction are basically electrical processes that are readily measiued by an electrode potential. All measurements are referred to die standard hydrogen electrode, the potential of which is taken as 0.00 V at 298 K, the H2 pressure as 101325 N/m (1 atm) and activities of H2 and as unity. When the half-cell reaction is written as an oxidation reaction ... 

Sn + can reduce Pd ions since the standard oxidation-reduction potential of Sn /Sn is 0.15 V and that of Pd +/Pd is 0.987 V. As shown in Section 5.7 and Figure 5.10, the flow of electrons is from a more electronegative couple (here Sn /Sn ) toward a less electronegative (more positive) couple (here Pd +/Pd). 

Find the values of the standard oxidation-reduction potentials of the indicated systems (see Appendix 1, Table 21). Write the Nernst equation. 

The oxidation-reduction potential of a pyridine nucleotide coenzyme system is determined by the standard redox potential for the free coenzyme (Table 6-8) together with the ratio of concentrations of oxidized to reduced coenzyme ([NAD+] / [NADH], Eq. 6-64). If these concentrations are known, a redox... 

The oxidation-reduction potential, E, (or redox potential) of a substance is a measure of its affinity for electrons. The standard redox potential (E0 ) is measured under standard conditions, at pH 7, and is expressed in volts. The standard free energy change of a reaction at pH 7, AG0, can be calculated from the change in redox potential AE0 of the substrates and products. A reaction with a positive AE0 has a negative AG0 (i.e. is exergonic). 

It was demonstrated that water electrolysis generates mixed oxidant systems. Oxidation-reduction potential (Hsu and Kao 2004) is not the best parameter for system characterisation. In general, the online analysis of generated species is an unsolved problem. Probably, not all electrolysis products are known. This can be seen in analysing the active and total chlorine concentration. The standard method uses DPD but the effect of other chemicals on the DPD method is one reason that active chlorine is sometimes incorrectly measured. Amperometric analysis and... 

Oxidation-reduction potential (or redox potential, E) is the potential of compounds to accept electrons and is by convention measured relative to that of hydrogen. Thus E is very negative for NADPH (a strong reductant) but positive for 02 (a strong oxidant). Standard redox potentials (Eo values in volts) refer to standard conditions (1M redox components) at neutral pH (pH 7). The standard free energy change at pH 7 for a particular redox reaction (AGo ) is given by ... 

Table 1.17 Standard oxidation-reduction potentials of simple redox systems at 2S°C...

Table 1.18 Standard oxidation-reduction potentials of combined redox and acid-base systems at 25°C...

Similarly, from the Tables 1.17 and 1.18 we can see, for example, that permanganate ions (in acid medium) can oxidize chloride, bromide, iodide, iron(II), and hexacyanoferrate(II) ions, also that iron(III) ions may oxidize arsenite or iodide ions but never chromium(III) or chloride ions etc. It must be emphasized that the standard potentials are to be used only as a rough guide the direction of a reaction will depend on the actual values of oxidation-reduction potentials. These, if the concentrations of the species are known, can be calculated easily by means of the Nernst equation. 

Using this example we can show the correlation between standard oxidation-reduction potentials without applying the thermodynamical concepts mentioned above. The oxidation-reduction potential of the system... 

From such examples it becomes apparent that the greater the difference between the standard oxidation-reduction potentials, the higher the value of the equilibrium constant, that is the reactions become the more complete. In practice, a difference of 0-3 V for n = 1 secures a value for K greater than 10s, which means that in practical terms the reaction will take place quantitatively. If, on the other hand, the difference of standard potentials, as defined by equations (i) and (v) is negative, the reaction is not feasible in fact it will proceed in the opposite direction. 

Fig. 12. Standard oxidation-reduction potentials for the steps involved in the conversion of O2 to water at 25°C and pH 7.

The requirement for producing the 2-exo-methylenepanam derivatives of Step 1 is that the /3-lactam is reduced with a metal having a standard oxidation-reduction potential of up to — 0.3 v/see in an amount of at least one mole per molecule of halide and with a metal compound having a higher standard oxidation-reduction potential than the metal. 

Partition coefficients in the octanol-pH 7.4-phosphate-buffer system. c Nitrothiazole oxidation-reduction potentials (volts) as calculated from their half-wave potentials, as determined using a Polarecord E 261 polarograph (Metrohm AG, Herisau, Switzerland) and a saturated Ag/AgCl reference electrode. Measurements were performed at 20°C and a drop time of 1 drop/2.8 sec. The compounds were dissolved in 1 ml dimethyl formamide and added to 24 ml of a borax-potassium biphosphate buffer of pH 7.3 [prepared according to J. M. Kolthoff, J. Biol. Chem. (1925) 68, 135]. A pH of 7.4 resulted. Standard error of determination 3 mv. 

The most of chemical reactions accompanied by electron transfer from an atom of one reagent (reducer) to an atom of another reagent (oxidizer). Each element can have some oxidation states. The standard oxidation-reduction potential between two oxidation states of element is bonded with standard thermodynamic free energy of the transition from one state to another by the following equation ... 

In contrast to standard borohydride reductive nanoparticle synthesis, we have developed an alternative strategy to amino acid encapsulated nanoparticles by utilizing a metal nanoparticle (M°-(Ligand))/metal ion (M"+) precursor redox pair with matched oxidation/reduction potentials. Simply, a metal nanoparticle such as Pt°-(Cys) acts as the principal reductant to a complimentary selected metal ion of Au + resulting in a new stabilized metal nanoparticle of Au°-(Cys) and the oxidation product of the original nanoparticle Pt"+. Malow et al. have reported a metathesis/transmetallation type reaction between a platinum colloid and a Au cyanide compound. Similarly, we employed a Pt°-(Cys)/AuCl4 pair and 0.5-2.0 equivalents of Au to Pt -(Cys). XRD analysis of the nanoparticle products revealed differences in crystallinity... 

The oxidation-reduction potential is thus seen to be determined by the ratio of the activities of the oxidized and reduced states, in agreement with the general equation (1). The standard potential is evidently that for a system in which both states are at unit activity. 

When all the species concerned, viz., A, B, , X, Y, etc., are in their standard states, i.e., at unit activity, the potential is equal to the standard oxidation-reduction potential of the system. It is important to remember that in order that a stable reversible potential may be obtained, all the substances involved in the system must be present the actual potential will, according to equation (3), depend on their respective activities. 

Determination of Standard Oxidation-Reduction Potentials.—In principle, the determination of the standard potential of an oxidation-reduction system involves setting up electrodes containing the oxidized and reduced states at known activities and measuring the potential B by combination with a suitable reference electrode insertion of the value of B in the appropriate form of equation (3) then permits B to be calculated. The inert metal employed in the oxidation-reduction electrode is frequently of smooth platinum, clthough platinized platinum, mercury and particularly gold are often used. 

In the actual evaluation of the standard potential from the experimental data a numbe of difficulties arise, and, as a result of the failure to overcome or to make adequate allowance for them, most of the measurements of oxidation-reduction potentials carried out prior to about 1925 must be regarded as lacking in accuracy. In the first case, it is rarely possible to avoid a liquid junction potential in setting up the cell for measuring the oxidation-reduction potential secondly, there is often... 

In recent years care has been taken to eliminate, or reduce, as far as possible the sources of error in the evaluation of standard oxidation-reduction potentials highly dissociated salts, such as perchlorates, are employed wherever possible, and corrections are applied for hydrolysis if it occurs. The cells are made up so as to have liquid junction potentials whose values are small and which can be determined if necessary, and the results are extrapolated to infinite dilution to avoid activity corrections. One type of procedure adopted is illustrated by the case described below. ... 

Another method of evaluating standard oxidation-reduction potentials is to make use of chemical determinations of equilibrium constants. ... 

Standard Oxidation-Reduction Potentials.—Some values of standard oxidation-reduction potentials at 25° are given in Table The sign... 

TABLE LIII. STANDARD OXIDATION-REDUCTION POTENTIALS AT 25 ... 

Variation of Oxidation-Reduction Potential.—From a knowledge of the standard oxidation-reduction potential of a given system it is possible to calculate, with the aid of the appropriate form of equation (3), the potential of any mixture of oxidized and reduced forms. For approximate purposes it is sufficient to substitute concentrations for activities the results are then more strictly applicable to dilute solutions, but they serve to illustrate certain general points. A number of curves, obtained in this manner, for the dependence of the oxidation-reduction potential on the proportion of the system present in the oxidized form, are... 

Ionization in Stages.—When a metal yields two positive ions, and M +, there are three standard potentials of the system these are the potentials of the electrodes M, and M, in addition to the oxidation-reduction potential If the values of these standard... 

Oxidation-Reduction Indicators.—A reversible oxidation-reduction indicator is a substance or, more correctly, an oxidation-reduction system, exhibiting different colors in the oxidized and reduced states, generally colored and colorless, respectively. Mixtures of the two states in different proportions, and hence corresponding to different oxidation-reduction potentials, will have different colors, or depths of color every color thus corresponds to a definite potential which depends on the standard potential of the system, and frequently on the hydrogen ion concentration of the solution. If a small amount of an indicator is placed in another oxidation-reduction system, the former, acting as a potential mediator, will come to an equilibrium in which its oxidation-reduction potential is the same as that of the system under examination. The potential of the given indicator can be estimated from its color in the solution, and hence the potential of the system under examination will have the same value. 

---