Chlorine dioxide disproportionation

A secondary competing reaction can occur where chlorine dioxide disproportionates in the alkaline solution, producing sodium chlorite and chlorate ... 

In basic solution chlorine dioxide disproportionates with the formation of chlorate and chlontc, and the latter is used to form the free acid ... 

Liquid chlorine dioxide, ClOj, boils at 284 K to give an orange-yellow gas. A very reactive compound, it decomposes readily and violently into its constituents. It is a powerful oxidising agent which has recently found favour as a commercial oxidising agent and as a bleach for wood pulp and flour. In addition, it is used in water sterilisation where, unlike chlorine, it does not produce an unpleasant taste. It is produced when potassium chlorate(V) is treated with concentrated sulphuric acid, the reaction being essentially a disproportionation of chloric(V) acid ... 

Equation 22 gives the maximum theoretical obtainable chlorine dioxide from the disproportionation of HCIO,. Experimentally, differences in the stoichiometry have been reported. This is because the chloride formed in equation 21 can catalyze the reaction to form more chlorine dioxide as in equation 22. Proposed mechanisms for these reactions and the kinetics under various conditions have been described (16,108). 

Acid—Sodium Chlorite System. The addition of a strong inorganic acid into an aqueous sodium chlorite solution produces chlorous acid, which rapidly disproportionates into chlorine dioxide. One proposed set of reactions using hydrochloric acid is (110) ... 

Hypochlorous Acid—Sodium Chlorite System. In this method, chlorine gas is educted into water forming a hypochlorous acid solution which then reacts with aqueous sodium chlorite to produce chlorine dioxide (114—116). Hypochlorous acid, formed from the disproportionation of chlorine gas in water ... 

The reaction chemistry changes when the initial reactant concentrations are low or there is excess hypochlorous acid present. The [CI2O2] intermediate disproportionation route to chlorine dioxide becomes less important (eq. 48), and the route to chlorite formation by hydrolysis predominates as does the reaction with any available excess HOCl to form chlorate and chlorine ... 

Attempts to measure the rates of exchange of chlorous acid with water and with chlorate both proved unsuccessful because the rates are considerably less than those of disproportionation and decomposition in the same systems. There is rapid exchange with chlorine dioxide k is approximately 200 l.mole . ... 

Reaction (11), the disproportionation of the C102 ion to CIO2 and Cl , affords chlorine-free chlorine dioxide. Reaction (12) is less useful when CIO2 is being used to combat taste and odor, since the product is inevitably contaminated with hypochlorous acid, thus defeating the objective of using a chlorine substitute. The precursor in either case is sodium chlorite, NaC102, which is a powerful oxidizer, and has to be stored carefully. 

Chlorine dioxide is a bent (O-Cl-0 bond angle 117°), relatively stable free-radical species with a solubility in water about an order of magnitude greater than chlorine gas. Both the O and Cl atoms have some radical density and may enter into one-electron redox reactions. The redox potential for addition of an electron to aqueous CIO2 is 0.936 V, very close to that for OCl (Merenyi et al., 1988), but lower than that of HOCl. The compound is stable for long periods in acidic solution, but decomposes in alkali by disproportionation to chlorite (CIO2 ) and chlorate (CIO3 ) ... 

Peroxidase, Oxidase, and Superoxide Dismutase Horseradish Peroxidase.—Chlorine dioxide, CIO a, is formed by the disproportionation of chlorite ClOa" catalysed by HRP,... 

Chlorite (C102 ) is a disinfection by-product that is formed when chlorine dioxide (CIO2) is used for disinfecting drinking water, while chlorate (CIOs ) is formed when chlorine dioxide or chloramine is used. Chlorination using hypochlorite acid solutions, which contain some CIOs a product of HCIO disproportionation, may also contribute to CIOs contamination in disinfected water. Even at low levels, chlorite may lead to hemolytic anemia... 

The acidification of 1.3W sodium chlorite (220) with 10% acetic acid yielded almost entirely chlorine dioxide as the major product of the disproportionation. A small amount of oxygen was detected but no chlorine or perchlorate ions were found. 

Kieffer and Gordon (120) used a variety of conditions to study the stoichiometry of the disproportionation of chlorous acid as a function of time at an ionic strength of 2.0. They reported that at the beginning of the reaction, between 0.7 and 2.0M hydrogen ion, less chlorine dioxide than the amount predicted by Eq. 13 is formed, and, as the reaction proceeds, the relative amount of chlorine dioxide... 

The rate of disproportionation of acidified chlorine (III) solutions varies with the pH. The reaction is very slow at a pH greater than 4 less than 10 W chlorine dioxide is formed in 2 hr if the initial sodium chlorite concentration (28) is 3 x 10 Only if the pH is less than 3 will more than 1% of the sodium chlorite decompose within 10 min. Kieffer and Gordon (120) found that the rate of decomposition of chlorous acid did not vary appreciably with hydrogen ion concentration in the 2.0 to 0.2M range in the absence of initial chloride ion. The half-life for the decoitpos-ition was approximately 3 hr for 1.2M hydrogen ion,... 

In addition to the concentration of hydrogen ion and chloride ion, the relative amounts of reactants, chlorine(III), and chlorine (or hypochlorous acid), also affect the stoichiometry. For the chlorine(III)-hypochlorous acid reaction, Emmenegger and Gordon (48) noted that with chlorine(III) in excess there was a marked improvement in the production of chlorine dioxide. Actually, the theoretical amount preducted by Eq. 22 is exceeded, but chlorine or hypochlorous acid catalysis of the disproportionation of chlorous acid might explain this discrepsuicy. Whereas eui increase in the chlorine(III) concentration increases the amount of chlorine dioxide produced (40, 48, 162), an increase in the hypochlorous acid concentration favors the formation of chlorate ion (40, 99, 226). White and co-workers (226) suggest that this is consistent with the inclusion of the reaction... 

Solid chlorine dioxide polyhydrate cem be handled safely at low temperatures, usually in the form of blocks encased in ice coatings (227), and it is frequently shipped cold and regenerated as a gas (228) for bleaching flour. Stable liquid mixtures of chlorine dioxide with chlorine at low temperatures are also reported (212). It has been alleged that pyridine and chlorine dioxide form a stable solid complex that can be used to furnish free chlorine dioxide on addition of water (216). Chlorine dioxide is reported (2) to form a complex of low volatility in solution with neutral or basic perborate, which supposedly can be again dissociated by acidification of the solution with release of chlorine dioxide. In fact, the stabilized solutions have all the properties of neutral or slightly basic solutions of chlorite ion, which disproportionate upon the addition of acid. 

E. Disproportionation Reactions The reactions of chlorine dioxide have, in the main. 

Carbonate and probably other general bases also catalyze the disproportionation (82, 84). Chlorine dioxide slowly disappears in the presence of hypo-chlorous acid, as mentioned earlier. Chlorine dioxide is oxidized by ozone to Cl20g (77). The thermodynamics of the reduction of chlorine dioxide by hydrogen peroxide in basic solution, in which hydroperoxy ion is presumed to be the reactive species, has been determined at three temperatures (58). The overall reaction may be formulated as... 

Thus at equilibrium in a neutral solution, chlorine dioxide would be almost completely disporportionated, and even in fairly acid solution the disproportionation would be largely complete however, the chcuige is slow except in an alkaline solution (28, 217). In the dark at 0 C, an aqueous solution was made up to contain 0.155W chlorine dioxide and O.OOllW hydrogen ion with chloride ion present as a catalyst, and only 1% of the chlorine dioxide decomposed in seven weeks. 

Taube and Dodgen (217) did not observe any difference in the rate of decomposition of chlorous acid for two solutions with widely different concentrations of chlorate ion. Hong (99) found that chlorate had only a very small, but noticeable, effect on the rate of formation of chlorine dioxide from a chlorous acid solution. At (4-6) x 10 M chlorate ion concentrations, it has been obsejrved (120) that chlorate ion has at most only a very small effect on the rate of disproportionation of chlorous acid. This evidence indicates that the reaction between chlorous acid and chloric acid must be very slow. This hypothesis is consistent with the slow rate of hydrolysis of chlorine dioxide... 

Apart from the analysis of the main components - chloride, nitrate, and sulfate - a peculiarity of swimming pool water analysis is the determination of the two chlorine species - chlorite, CIO2, and chlorate, CIO3. These are disproportionation products of chlorine dioxide, which is used in some countries to fumigate swimming pool water for disinfection purposes [24,25]. Chlorine dioxide easily dissolves in water, but decomposes slowly by the formation of chlorous acid and chloric acid ... 

The kinetics of this last reaction have been studied as a function of pH. Another reaction in the complex series of interactions among the oxoacids of chlorine, the disproportionation of chlorine dioxide to chlorate and chlorite, follows the rate law below, with E - 10.8 kcal mol ... 

After all the iodide has been consumed, the iodine reacts rapidly with chlorite until all the chlorite has disappeared as well. In this second stage, iodine(I) intermediates (HOI, ICl), chloride ions and, under some conditions, chlorine dioxide are produced. The iodine(I) intermediates disproportionate comparatively slowly to produce iodine and iodate in the third stage of the reaction. After the iodide is consumed, the stoichiometry becomes very complex it depends on the ratio and the absolute concentrations of the reactants iodine and chlorite [23] it cannot be described by a single stoichiometric equation. Production of chlorine dioxide [23-25] becomes important at higher (> 10 M) initial reactant concentrations. 

Technetium oxide trichloride, TCOCI3, was obtained by reaction of technetium dioxide in a chlorine stream at 30f)-35() °C. The brown, slightly volatile product can be sublimed at about. 500 "C in vacuum. TcOCls shows a strong band in the IR at 1017 cm, which can be attributed to the metal-oxygen stretehing mode. Tlie compound is readily hydrolyzed and disproportionates [.30.98.99] ... 

Moissan and Lebeau (1901) produced sulfuryl fluoride by the combination of sulfur dioxide with fluorine (217). Other processes which have been used to produce the gas are (a) the thermal decomposition of barium fluorosulfonate or certain other fluorosulfonates (188, 221, 808), (b) the reaction of sulfur dioxide with chlorine and hydrogen fluoride in the presence of activated charcoal at 400° (11), (c) the reaction of sulfur dioxide and chlorine with potassium or sodium fluoride at 400° (328), (d) the disproportionation of sulfuryl chlorofluoride at 300-400° (328), (e) the reaction of sulfuryl chloride with a mixture of antimony trifluoride and antimony pentachloride at about 250° (86), (f) the reaction of sulfur dioxide with silver difluoride (86), (g) the reaction of thionyl fluoride with oxygen in an electrical discharge (314), (h) electrolysis of a solution of fluorosulfonic acid in hydrogen fluoride (264), ( ) the reaction of fluorine with sodium sulfate, sodium sulfite or sodium thiosulfate (229, 239), (j) the reaction of hydrogen fluoride with sulfuryl chloride (820). 

Other alkali-metal chlorates are produced by analogous technology while sodium and potassium bromate are produced electrolytically starting both from bromide ion and bromine solutions. The production of bromate is, however, a very small-scale process and the cells have not been optimized to any extent for example while cells with lead dioxide and platinized titanium have been described, some plants still use solid platinum electrodes The mechanism of bromate formation is identical to that described for chlorate by reactions (5.10)—(5.13) the kinetics are, however, different. The hydrolysis of bromine is slower than chlorine but the disproportionation step is much faster (by a factor of 100) and it is therefore advisable to use a more alkaline electrolyte, about pH 11. 

The first reaction predominates if the product contains a large amount of water (-18%). This reaction is analogous to the disproportionation of aqueous hypochlorite. However, disproportionation is much slower in solid calcium hypochlorite than in solution. Under dry conditions, the second reaction predominates. It is catalyzed by transition metals including iron and manganese. It may occur explosively 150°C. Thus, calcium hypochlorite products usually contain some water or an additive such as magnesium sulfate heptahydrate. The third reaction is the reverse of chlorination. The fourth reaction is due to the adsorption of carbon dioxide from air or the release of carbon dioxide from carbonate salt impurities. It is accelerated by water and temperature. The first reaction accounts for -70%, and the second reaction -30%, of the decomposition of solid calcium hypochlorite made in the United States and stored in sealed containers at 25°C. ... 

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