Chlorous acid and the chlorites

Chlorous acid and its salts. As compared with the other acids of chlorine and their salts, chlorous acid and the chlorites are less well known and have no really important large-scale uses. Chlorous acid and chloric acid are formed together by the reaction between chlorine dioxide and water ... 

Chlorous Acid and the Chlorites. When chlorine dioxide, ClOg, is passed into a solution of sodium hydroxide or other alkali a chlorite ion and a chlorate ion are formed ... 

In a similar way, hypochlorous acid yields the hypochlorite ion, chlorous acid yields the chlorite ion, chloric acid yields the chlorate ion, and perchloric acid yields the perchlorate ion (Table 2.4). 

Chlorous acid is the least stable of the oxoacids of chlorine it cannot be isolated but is known in dilute aqueous solution. HOBrO and HOIO are even less stable, and, if they exist at all, have only a fleeting presence in aqueous solutions. Several chlorites have been isolated and NaC102 is sufficiently stable to be manufactured as an article of commerce on the kilotonne pa scale. Little reliable information is available on bromites and still less is established for iodites which are essentially non-existent. 

G. Gordon, R. G. Kieffer and D. H. Rosenblatt, Progr. Inorg. Chem. 15, 201-86 (1972). The first half of this review deals with the aqueous solution chemistry of chlorous acid and chlorites. 

According to Scott (Ref 5, p 262), hypochlorites decolorize indigo, but do not decolorize KMn04 solutions. If arsenous acid is present, indigo is not decolorized until all the arsenous acid has been oxidized to the arsenic form (distinction from chlorous acid and chlorites). 

The reaction is symbolized 2C102+H20=HC102+HC10S, so that the aq. soln. of N. A. E. Millon s chlorine trioxide will contain a mixture of chloric and chlorous acids and chlorine, and a soln. of the same gas in alkali lye will contain a mixture of the alkali hypochlorite, chlorite, chlorate, chloride. The chlorites are accordingly formed when soln. of chloric acid, HC103, or chlorine dioxide, C102, are partially reduced by the methods employed for the preparation of chlorine di- and tri-oxides. J. E. de Vrij 3 prepared the acid by reducing chloric acid with tartaric acid. G. Bruni and G. Levi used oxalic acid ... 

An attempt was made by Jeanes and Isbell to evaluate their experimental data (Table XII) and to determine the mechanism of the reaction of chlorites with aldoses. Two of the experiments were carried out in acid solution at 0° and 20°, and the third in neutral media, as with the experiments in Table XI. Reaction 41 was believed to express the course of the oxidation in highly acid solution. The deviations, as expressed in higher consumption of chlorous acid and in higher yields of chloride, are... 

With chlorous acid as the main oxidizing agent, the reaction is slow in neutral solution and becomes faster under acidic conditions. A solution containing equimolar amounts of sodium chlorite and acetic acid affords a slightly faster rate of oxidation for a-n-glucopyranose than for the 8-d anomer. Thus, the reaction appears to follow a different path from that of the bromine-water and the hypoiodite oxidations, and to be similar to that of cyanohydrin formation. ... 

We only differentiate between chlorite ion and chlorous acid when the mechcmistic implications are clear. When both species may be present, we use the... 

The absorption spectra of chlorous acid eind sodium chlorite are given in publications by Buser etnd Hanisch (28), Leonesi and Piantoni (136), and Stitt, et al. 

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 rate of chlorite bleaching increases as pH decreases, but only between pH 2 and 9 is the rate proportional to the concentration of chlorous acid present in solution. At low pH values, evolution of the noxious and corrosive gas chlorine dioxide increases. In practice it is necessary to keep the pH above 3 in order to minimise the formation of chlorine dioxide. It is necessary to monitor the pH during chlorite bleaching because acid is liberated by the... 

Chlorine dioxide can be formed out of the chlorite by either of the reactions described by Equation 25.3 or 25.8. Equation 25.1 describes the chlorite formation. The chlorine concentration in the liquid is normally very low as the reaction with caustic (Equation 25.4) is very fast. The concentration of chlorine and/or hypo-chlorous acid HOCl can increase on depletion of hydroxide ions in the liquid. As in step two of the hypochlorite unit, the caustic concentration is in the order of 4-10 g l-1 and it is possible to conclude that at the gas-liquid interface the concentration might be quite low owing to the fast reaction with chlorine. 

N. A. E. Millon4 prepared soln. of potassium chlorite and 8odium chlorite by adding chlorous acid slowly and gradually to a soln. of the alkali hydroxide until the product has a yellow colour. By rapidly evaporating the soln. to complete dryness, a deliquescent salt was obtained which, at 160°, decomposes into chlorate and chloride. This decomposition occurs if the soln. be slowly evaporated in vacuo. If an excess of chlorous acid is used with the potash-lye, a red liquid is obtained which gives off the acid on evaporation, and leaves a residue of the neutral salt. It has been suggested that the red liquid is a soln. of acid potassium chlorite. 

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