Iodate reduction

The Landolt reaction (iodate + reductant) is prototypical of an autocatalytic clock reaction. During the induction period, the absence of the feedback species (Irere iodide ion, assumed to have virtually zero initial concentration and fomred from the reactant iodate only via very slow initiation steps) causes the reaction mixture to become kinetically frozen . There is reaction, but the intemiediate species evolve on concentration scales many orders of magnitude less than those of the reactant. The induction period depends on the initial concentrations of the major reactants in a maimer predicted by integrating the overall rate cubic autocatalytic rate law, given in section A3.14.1.1. 

First wave at pH 0 (shifts to — 0.08 at pH 12) second wave corresponds to iodate reduction... 

Iodine is present in the environment predominantly in the oxidation states —1 (1, iodide) and - -5 (lOs", iodate). Reduction of lOs" to 1 occurs at pe = 13.3 at pH 5 and pe° = 11.3 at pH 7. Hence 1 is expected to predominate in the soil solution except in oxic alkaline soils (Whitehead, 1984). However Yuita (1992) found predominantly IO3 in acid Japanese soils contaminated with iodine the concentrations in solution were some 20 times those of 1 and I2. On flooding the soils, the total concentration of 1 in solution increased 10- to 50-fold, predominantly as I. The concentrations of sorbed 1 were not measured, but both lOs and 1 are expected to be bound to organic matter and oxides and hence their concentrations in solution are expected to increase with reductive dissolution reactions. Further, for a given concentration in solution, 1 is more rapidly absorbed by plants than IO3 (Mackowiak and Grossl, 1999). Hence flooding is expected to increase accumulation in plants both through increased solubility and increased absorption. 

C ) Chlorite-iodate-reductant. Oscillators 2, 7, 8 a, 9 a, 10 a, 11 and 12 of Table 8 are of this type. The requirements for a successful substrate appear to be that (M8) be thermodynamically favorable and relatively rapid while (M10) is slow. 

Different iodine species exhibit very different sorption and transport behavior in geologic samples. Sorption of iodate is consistently greater than that of iodide, while the transport of organoiodine (exemplified by 4-iodoanUine in this study) is quite limited, and related to the amount of organic matter in the sample. The physical and chemical processes affecting iodine transport include iodate reduction, irreversible retention or mass loss, and rate-limited and nonlinear sorption of iodine. 

The halogens may be determined polarographically by reduction of their halate form (except for chlorate). Bromide, for example, may be determined by DPP after oxidation to the bromate species yielding a cathodic current at 1.53 V versus SCE. The detection limit is 1 X 10 moll . Possible interferences would arise from iodate reduction. 

The chlorite-iodide reaction is clearly the minimal oscillator of the subfamily of chlorite-iodide xidant oscillating reactions, and, since iodate and iodine appear to be produced in all of them, it can be considered to be the progenitor of a still larger family that includes the chlorite-iodate-reductant and cWorite-iodine-reductant groups listed in Table 4.1. Clearly, the chlorite-iodide reaction cannot be minimal for the entire family of chlorite-based oscillators since many of them contain no iodine species. It is not at all obvious that a minimal oscillator for the entire chlorite family exists. 

In Landolt -type reactions, iodate ion is reduced to iodide tlirough a sequence of steps involving a reductant species such as bisulfde ion or arsenous acid (H AsO ). The reaction proceeds through two overall... 

The iodate is a poison potassium iodide, however, is used in foodstuffs. Thus the iodate must be completely removed frequently by a final reduction with carbon. After re-solution in water, further purification is carried out before recrystallization. Iron, barium, carbonate, and hydrogen sulfide are used to effect precipitation of sulfates and heavy metals. 

Bromide ndIodide. The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaHy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colorless leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to cataly2e ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidi2ed to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

The modes of thermal decomposition of the halates and their complex oxidation-reduction chemistry reflect the interplay of both thermodynamic and kinetic factors. On the one hand, thermodynamically feasible reactions may be sluggish, whilst, on the other, traces of catalyst may radically alter the course of the reaction. In general, for a given cation, thermal stability decreases in the sequence iodate > chlorate > bromate, but the mode and ease of decomposition can be substantially modified. For example, alkali metal chlorates decompose by disproportionation when fused ... 

The oxidizing power of the halate ions in aqueous solution, as measured by their standard reduction potentials (p. 854), decreases in the sequence bromate > chlorate > iodate but the rates of reaction follow the sequence iodate > bromate > chlorate. In addition, both the thermodynamic oxidizing power and the rate of reaction depend markedly on the hydrogen-ion concentration of the solution, being substantially greater in acid than in alkaline conditions (p, 855). 

The appearance of free iodine during the periodate oxidation of compounds having an active hydrogen atom (27) or an ene-diol structure (1,39) has frequently been observed, and this implies that further reduction of iodate, formed from periodate during the main reaction, takes place. It has, in fact, been shown that, in acid solution, iodate is fairly readily reduced by such compounds as triose reductone (27), dihydfoxy-fumaric (39), and tartronic (32) acids. 

Obviously, simultaneous reduction of both periodate and iodate occurs. 

Iodates by ene-diols, reduction of. . Isomerization of tetra-O-acetyl-2-... 

Reduction of iodates by ene-diols. . 100 Reduction of unsaturated acetates. 153 Replacement of a sulfonate ester... 

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 ... 

With a more powerful reductant, e.g. titanium(III) chloride, the iodate is reduced to iodide ... 

The reduction potential is 1.23 volts hence under these conditions potassium iodate acts as a very powerful oxidising agent. 

Discussion. These anions are both determined as silver bromide, AgBr, by precipitation with silver nitrate solution in the presence of dilute nitric acid. With the bromate, initial reduction to the bromide is achieved by the procedures described for the chlorate (Section 11.56) and the iodate (Section 11.63). Silver bromide is less soluble in water than is the chloride. The solubility of the former is 0.11 mg L 1 at 21 °C as compared with 1.54 mg L 1 for the latter hence the procedure for the determination of bromide is practically the same as that for chloride. Protection from light is even more essential with the bromide than with the chloride because of its greater sensitivity (see Section 11.57). 

The polarographic method is applicable to the determination of inorganic anions such as bromate, iodate, dichromate, vanadate, etc. Hydrogen ions are involved in many of these reduction processes, and the supporting electrolyte must therefore be adequately buffered. 

The "iodine clock" reaction Rate of reduction of iodate by sulfite was increased... 

The removal of inorganic salts from reaction mixtures afforded by polymeric materials may be simply and effectively accomplished by dialysis,166 178 after decomposition of remaining periodate with ethylene glycol130 131 or butylene glycol. 161 170 Alternatively, the iodate and periodate ions may be removed as such, or after reduction to free iodine. The iodate and periodate ions have been effectively precipitated by means of sodium carbonate plus manganous sulfate,6 or by lead dithionate,191 barium chloride,24 192 193 strontium hydroxide194 202 or barium hydroxide,203 204 lead... 

Calculations derived from the measurement of final periodate consumption indicate the number of reactive groups and can often be interpreted to reveal the extent of overoxidation. Chemically, this determination involves the use of one of two general reactions. These are (a) the reduction of periodate and iodate to free iodine in acid solution, and (b) the reduction of periodate to iodate in neutral solution. 

The most widely used of the methods involving a reduction of periodate, only, to iodate employs the arsenite ion4 as reductant, in a solution maintained at neutrality with sodium bicarbonate, with iodide ion as catalyst. 

Iodine water is added to an acetic acid sodium acetate buffered sample to reoxidise to iodate any iodine-containing substances produced by reduction of iodate by naturally occurring reducing substances present in the sample. Total iodate (i.e., iodate present in the original sample as iodate plus additional iodate produced by iodine water treatment) is then reacted with phenol solution at... 

The oxidation-reduction methods with potassium iodate invariably based on the formation of iodine monochloride (ICl) in a medium of strong hydrochloric acid solution. 

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