Reduction potentials iodine

The direct iodometric titration method (sometimes termed iodimetry) refers to titrations with a standard solution of iodine. The indirect iodometric titration method (sometimes termed iodometry) deals with the titration of iodine liberated in chemical reactions. The normal reduction potential of the reversible system ... 

The normal reduction potential of the iodine-iodide system is independent of the pH of the solution so long as the latter is less than about 8 at higher values iodine reacts with hydroxide ions to form iodide and the extremely unstable hypoiodite, the latter being transformed rapidly into iodate and iodide by self-oxidation and reduction ... 

Figure 5.16. Plot of data for the external heavy-atom quenching of pyrene fluorescence in benzene at 20°C. Polaro-graphic half-wave reduction potentials Ein are used as a measure of the electron affinity of the quencher containing chlorine (O), bromine ( ), or iodine (3). From Thomaz and Stevens<148) with permission of W. A. Benjamin, New York.

The concept of reduction potential is introduced in Chapter 6. When the reduction potentials of two species differ by 0.1 V or more, the resulting redox reaction will proceed rapidly and stoichiometrically so that it may be used as the basis for a titrimetric procedure. The end point of a redox titration may be observed by following the potential of the titrand with an indicator electrode or with a visual indicator. In two special cases, the reagent (potassium permanganate and iodine) is self-indicating (vide infra). 

C. E. Ophardt, "Redox Demon-f strations and Descriptive Chemistry Part 2. Halogens/ /. Chem. Educ., Vol. 64,1987,807. Using an abbreviated table of reduction potentials as a predictive tool, reactions of bromine and iodine in various oxidation states are demonstrated. 

Schuchmann H-P, von Sonntag C (1988) The oxidation of methanol and 2-propanol by potassium peroxodisulphate in aqueous solution free-radical chain mechanisms elucidated by radiation-chemical techniques. Radiat Phys Chem 32 149-156 Schwarz HA, Bielski BHJ (1986) Reactions of H02 and 02 with iodine and bromine and l2 and I atom reduction potentials. J Phys Chem 90 1445-1448... 

Polyhalide radical anions have recently been reviewed I4- and I6-have been observed in terf-butanol solution, but they are unknown in aqueous solution (127). The equilibrium constant for formation of I2-[reaction (31)] is the link between the reduction potentials of the iodine atom, the diiodine radical anion, and diiodine. Numerous measurements of this equilibrium constant have been made over the years. There are even two reports of the enthalpy of the reaction, obtained from the temperature dependence of the equilibrium constant (35). Published values for the formation constant of I2- are listed in Table IV (32, 36, 128, 129, 149, 314, 318). As noted in Fornier de Violet s review (127) and in Elliot and Sopchyshyn s paper (109), there is a systematic discrepancy between the flash photolysis results and the pulse radiolysis results. Fornier de Violet suggested that the pulse radiolysis results might be in error because of unrecognized adduct formation... 

There is nothing in the foregoing discussion that restricts it to reactions at the cathode or to ions it holds, in fact, for any electrode process, either anodic, i.e., oxidation, or cathodic, i.e., reduction, using the terms oxidation and reduction in their most general sense, in which the concentration of the reactant is decreased by the electrode process, provided the potential-determining equilibrium is attained rapidly. The fundamental equation (10) is applicable, for example, to cases of reversible oxidation of ions, e.g., ferrous to ferric, ferrocyanide to ferricyanide, iodide to iodine, as well as to their reduction, and also to the oxidation and reduction of non-ionized substances, such as hydroquinone and qui-none, respectively, that give definite oxidation-reduction potentials. 

Note that reduction of iodine has the higher reduction potential. This half-reaction will proceed in the forward direction as a reduction. The iron half-reaction will proceed in the reverse direction as an oxidation. Rewrite the half-reactions in the correct direction. l2(s) -> 21 (aq) (reduction half-cell reaction)... 

The half-cell potential for the reduction of iodine is less positive than the reduction potential of Ozlg) in water at pH = 7(0.815 V), so the oxidation of 0.10 M 1 occurs in preference to the oxidation of water. The anode reaction is therefore... 

It must be kept in mind that the interference with redox properties alluded to above may be thermodynamic and not kinetic that is, the decreased reducibility when residues 67 or 59 are derivatized may arise not from modifications in the direct electron pathway but from a decrease in the reduction potential of the protein. Margalit and Schejter (230) have observed that the alkaline state IV of cytochrome c, with methionine-80 replaced by another ligand, has a reduction potential which is lowered from the normal - -260 mV to less than 90 mV at pH 11.2 or above. If similar factors are at work in the derivatives iodinated or nitrated at tyrosine-67, where the III to IV transition is lowered from pH... 

The effectiveness of the purification by the halogens depends on their oxidation-reduction potentials and decreases from chlorine to iodine. In addition, the latter can disproportionate ... 

To illustrate the use of these diagrams, the stability of I2 in 1 Af base will be considered. The diagram for iodine-containing species in base shows I2 will disproportionate to 1 and IO [Eq. (15)]. IO should, however, also be unstable, undergoing disproportionation to 1 and lO [Eq. (16)]. Eurther disproportionation of lOj is not expected because the reduction potential involving formation of IO4 is more positive than any reduction potential to the left of 10. It should be noted that these diagrams only predict what reactions are allowed to happen they say nothing about how fast the reactions will occur (see Section I.G.). 

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