Oxidation of iodide

Ha.logen Compounds. Fluorine is unreactive toward ozone at ordinary temperatures. Chlorine is oxidized to Cl20 and Cl20y, bromine to Br Og, and iodine to I2O2 and I4O2. Oxidation of haUde ions by ozone increases with the atomic number of haUde. Fluoride is unreactive chloride reacts slowly, ultimately forming chlorate and bromide is readily oxidized to hypobromite (38). Oxidation of iodide is extremely rapid, initially yielding hypoiodite the estimated rate constant is 2 x 10 (39). HypohaUte ions are oxidized to haUtes hypobromite reacts faster than hypochlorite (40). 

One of the most required methods of determination of iodide-ions in praetiee of ehemieal analysis is photometrie determination of produets of iodination of organie eompounds. The oxidation of iodide to iodine ean be earned out suffieiently seleetively. But in ease of presenee of great abundanee of bromide-ions the seleetive oxidation of iodide-ions is problematie. The variants of determination of iodide-ions with different organie reagents are known, but the absenee of bromide-ions in a system is supposed in most of them. In natural objeets these halides are present simultaneously. 

They form a monolayer that is rich in defects, but no second monolayer is observed. The interpretation of these results is not straightforward from a chemical point of view both the electrodeposition of low-valent Ge Iy species and the formation of Au-Ge or even Au Ge h compounds are possible. A similar result is obtained if the electrodeposition is performed from GeGl4. There, 250 20 pm high islands are also observed on the electrode surface. They can be oxidized reversibly and disappear completely from the surface. With Gel4 the oxidation is more complicated, because the electrode potential for the gold step oxidation is too close to that of the island electrodissolution, so that the two processes can hardly be distinguished. The gold step oxidation already occurs at -i-lO mV vs. the former open circuit potential, at h-485 mV the oxidation of iodide to iodine starts. 

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

Schiemann reaction) are not on sound ground due to the difference in ease of oxidation of iodide and tetrafluoroborate ions. 

The induced oxidation of iodide reveals that some intermediate is more kinetically competent toward iodide oxidation than either HCr04 or VOJ. The stoichiometric coefficients in Eq. (5-2) suggest that chromium(V) is the species responsible. 

If the reagent that is added does not remain in the initial form in the reaction mixture, the investigator must carefully identify the actual species present. For example, the oxidation of iodide ions by chlorite ions in acidic solution has a rate given by... 

We can study the dependence of the rate on the concentration of one substance by using one experiment even when two or more substances are involved. To see how this is done, consider the rate law for the overall second-order oxidation of iodide ions by persulfate ions ... 

The mechanism proposed for the oxidation of iodide ion by the hypochlorite ion in aqueous solution is as follows ... 

Fujishima A, Sugiyama E, Honda K (1971) Photosensitized electrolytic oxidation of iodide ions on cadmium sulfide single crystal electrode. Bull Chem Soc Japan 44 304 Inoue T, Watanabe T, Fujishima A, Honda K, Kohayakawa K (1977) Suppression of surface dissolution of CdS photoanode by reducing agents. J Electrochem Soc 124 719-722 Elhs AB, Kaiser SW, Wrighton MS (1976) Visible light to electrical energy conversion. Stable cadmium sulfide and cadmium selenide photoelectrodes in aqueous electrolytes. J Am Chem Soc 98 1635-1637... 

That is, the activated complex contains one Cr(V) atom and one Fe(II) atom. Espenson has shown, from a consideration of the induced oxidation of iodide ion, that the reaction between Cr(VI) and Fe(Il) requires one added proton. Consequently, the [H ] -dependence of the rate can be viewed as the addition of two protons in a pre-equilibrium followed by the addition of a further proton in the slow step, viz. 

The oxidation of iodide ion by aqueous chromic acid at low acidity is very slow but is subject to marked enhancement in rate on addition of ferrous ion. ... 

Westheimer ° has reviewed other inductions of the chromic acid oxidation of iodide, indicating how these reactions afford insight into the mechanism of the simple oxidation. 

The permanganate oxidation of iodide has been the subject of a recent detailed study by a rapid mixing technique . At 35 °C and at an ionic strength of 0.9 M over the pH range 3-6 the rate expression is... 

Bi(V) in aqueous perchloric acid is very strongly oxidising but kinetic studies have been confined to a few stopped-flow measurements on oxidation of iodide, bromide and chloride ions. The appearance of Bi(III)-halide complexes was first-order with respect to Bi(III) and in all cases the first-order rate coefficient,, was the same, i.e. 161 + 8 sec at 25 °C ([H30 ] = 0.5 M, p. = 2.0 A/), irrespective of the nature or concentration of the halide. A preliminary attack on solvent is compatible with these interesting results, viz. 

The rate of oxidation with Ce(IV) perchlorate depends on the method of preparation . The material from certain preparations gives a deep red complex, containing two equivalents of Ce(IV) to one molecule of H2O2, which decomposes in second order fashion-presumably by means of two concerted one-equivalent oxidations of the substrate. Other preparations give no complex and decompose peroxide much faster. The difference is thought to lie in the degree of association of the oxidant cf. the Ce(IV) oxidation of iodide ion, p. 359). 

Swain and Hedberg have shown that the tertiary alcohol is not an intermediate, for the dehydration process is slower than the rate of formation of dye. Instead it is proposed that the tertiary hydrogen is removed to give a radical-cation which is further oxidised to the carbonium ion. The oxidation involves two steps one fast and one very much slower. This parallels the Ce(IV) oxidation of iodide ion and is therefore probably a function of the oxidant. 

Identical kinetics have been noted for the oxidation of iodide by ferricyanide... 

Specific catalysis by sodium ions is also found in the oxidation of iodide ion by octacyanomolybdate(V) which otherwise shows simple, second-order kinet-ics with E = 5.68 2.57 kcal.mole" and A5 = —39 8.5 eu and /c2 = 3.52+0.13 l.mole sec , at 25.7 °C and fi = 0.1 M. Mo(IV), which is produced stoichiometrically exerts slight retardation upon the reaction. An outer-sphere one-electron transfer is proposed. 

The oxidation of iodide ion by Np(V), which exists in solution mainly as Np02, viz. 

The reduction of iodine by ferrocyanide is simple second-order with Aij (25 °C) = (1.3 + 0.3)x 10 l.mole sec This is the reverse of the oxidation of iodide by ferricyanide (p. 409), but the ratio k(forward)/k(back) does not agree well with the equilibrium constant determined potentiometrically. Addition of 1 strongly retards the reduction and 13 was discounted as a reactant, the mechanism suggested being... 

ARSENITE-INDUCED OXIDATION OF IODIDE BY DICHROMATE ACCORDING TO... 

It was found by DeLury that the overall rate of reduction of chromate is practically unaffected by the concentration of iodide, i.e. the sum of the rates of formation of iodine and aresenic(V) is constant and just equal to the rate at which chromate is reduced in a raction mixture containing no iodide (Fig. 1). The rate of oxidation of arsenite at a sufficiently high concentration of iodide decreases to i of its original value this is in accordance with the value of ci = 2 found. Fig. 1 well illustrates the general feature of coupled reactions, that the reaction of the inductor is always inhibited by the acceptor. The induced oxidation of iodide can... 

Fig. 1. Arsenic(III)-induced oxidation of iodide by dichromate. Data of DeLury .

Luther and Rutter have observed the induced oxidation of iodide during the reactions between chromic acid and vanadium(IV), vanadium(ri[), and vana-dium(II) ions. In all the three systems ci = 2, therefore it is probable that the coupling intermediates are chromium(V) species, these being, especially the two latter systems, too complicated for a detailed kinetic treatment to be given. 

Induced oxidation of iodide caused by vanadium(IV) and vanadium(ri) presumably involves steps analogous to those in the iron(II)-chromium(VI)-iodide system. Some data obtained by Luther and Rutter are summarized in Tables 9 and 10. 

This scheme accounts for both the induced oxidation of iodide (where ci = 2) and that of manganese(ri) (where ci = 0.5) without including step (19). Furthermore it can be seen that in the presence of iodide the rate of disappearence of chromate will not be altered, whilst the rate of oxidation of arsenic(III) will be reduced to one-third of its original value, as found experimentally. 

Inasmuch as the soil used was slightly alkaline (pH 7.70) it was believed that some of the triiodide produced in the soil distillate by Reaction 1 would be lost. The addition of a small quantity of acetic acid to this distillate considerably increased the percentage of ethylene dibromide recovered in the analysis (Table I). However, the addition of acid aids the oxidation of iodide ion to triiodide ion by oxygen of the air according to Equation 3. 

How the oxidation of iodide ions to form molecular iodine by the sonochemi-cally generated radicals can be monitored by the blue colour generated in the presence of freshly prepared starch solution. 

Carbon impregnated with potassium iodide was used as an ozone-scrubbing filter in a chemiluminescence NO analyser. When the level of iodide was inadvertently increased to the high level of 40%, the filter exploded violently dining replacement after use. This was attributed to oxidation of iodide to iodate by ozone, and frictional initiation of the iodate-carbon mixture when the filter was dismantled. 

The interception of the oxidized dye by the electron donor in the electrolyte (i.e., iodide) occurs within 10ns. The rate of the reaction leading to the regeneration of the dye ground state was found to depend strongly on the nature and the concentration of the cations present in the solution. Small cations such as Li+, Mg2+, and La3+ can intercalate into the oxide surface, thereby favoring fast oxidation of iodide by the oxidized state of the sensitizer. 

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