Oxygen Evolution From Sodium Hypochlorite Solutions

Abstract

The rates of oxygen evolution from carefully purified solutions of sodium hypochlorite have been measured. Methods of purification are described, and it is found that substantially the same rate is observed regardless of the method of purification. The rate of oxygen evolution is proportional to the square of the concentration of hypochlorite ions. The effect of temperature and ionic strength are examined. The rate constant is 7.5X1OP6 (g-rn~l/l.)-~(min)-I a t 60" C and an ionic strength of 3.5; the activation energy is 26.6 kcal/g-mol. These results are compared with the corresponding quantities for the reaction of hypochlorite ions t o form chlorite and chloride ions, and some tentative explanations are offered.In an earlier paper ( I ) , one of the present authors exa~nined the decomposition of aqueous sodium hypochlorite solutions, particularly the rate of deconlpositioil to sodiuin chlorate and chloride. The accompanying reaction giving sodiuin chloride and oxygen was exanlined more superficially. Rates of oxygen evolution were observed on a nu~nber of solutions a t that time, but it was never proved that the oxygen evolutio~l could not be ascribed to the presence of traces of catalyst. The reasons for this uilcertainty were as follows. Firstly, measurements on the catalyzed reaction (2) showed that the observed rates could be accounted for by the presence ol copper in co~lce~ltrations of the order of a pg/l. Secondly, the rates were somewhat variable. Thirdly, it was fouild that the observed rates were r o u g h l~~ proportional to the co~lcentratio~l of hypochlorite, and it is difficult to suggest a simple first-order ~nechailis~n for the u~lcatalyzed reaction. However, relatively low orders of dependence on l~ypocl~lorite concentration are observed for the catalyzed reaction, which suggests that these earlier solutioils contained traces of catalyst. Finally, and perhaps the most convincing reason for believing some catalyst to be present, it was found that the apparent activation energy for oxygen evolution was 21 l;cal/g-11101, less than the 24.8 l;cal/g-mol found for the reaction giving chlorate and chloride; yet the latter is the predominant reaction.Other worlters have also examined this deconiposition, though they usually did not separate the rates of the two reactions. Giordani (3) found a second-order gas evolution; but his conclusio~~s about the reaction to chlorate had not been confirmed, so it seemed worth while to investigate the reaction in inore detail. EXPERIMENTAL P A R TThe main proble~n was to obtain sodiuin hypochlorite free from traces of catalyst. At present-only cobalt, niclcel, and copper are known t o catalyze this deco~~~position, so that the methods of purification were directed towards the removal or coinplexing of these metals. I t seemed much more promising to purify the sodium hypochlorite solution as opposed t o the chlorine and sodium hydroxide solution from which it was made. The following general methods of purification, or a t least of removal of catalyst, were e...

Selectivity between Oxygen and Chlorine Evolution in the Chlor-Alkali and Chlorate Processes

“…As a comparison, the rate constant for the slow first step in the formation of chlorate, found to occur in two steps via ClO 2 – , was 0.95 M –1 s –1 at 40 °C. Two more papers were later published , by the same author and a co-worker. Both of these papers were focused on attempts to completely remove any impurities, such as of copper, that might catalyze the formation of oxygen.…”

“…Both of these papers were focused on attempts to completely remove any impurities, such as of copper, that might catalyze the formation of oxygen. Indeed, in the later paper of Lister and Petterson, it was concluded that the uncatalyzed decomposition of hypochlorite is of second order in OCl – , with a rate constant of 7.5 × 10 –6 M –1 min –1 at an ionic strength of 3.5 and 60 °C. On the basis of an evaluation of activation energies, and comparison with the chlorate formation reaction, it was suggested that the two reactions had activated complexes with the same stoichiometry, Cl 2 O 2 2– , but with different arrangements.…”

Modeling Chlorine Decay in Surface Water

“…The other way for hypochlorite decomposition is the formation of O 2 (Lister and Petterson, 1962). The decomposition may be catalyzed by transition-metal ions such as Ni(II), Cu(II), and Fe(III) (Gordon and Bubnis, 2000).…”

Chlorine Oxides and Chlorine Oxygen Acids

“…Oxides of nickel, cobalt, and copper are very effective; their catalytic activity decreases in that order. The catalytic action of manganese, iron, lead, and tin has been demonstrated, but is at least one order of magnitude smaller [53], [177]. Autocatalytic decomposition is also possible, but its extent is negligible at ambient temperature [177], [178].…”