give even-electron reduction products; behavior similar to that we have observed by (HP2Mo186") may be exhibited by threeand five-electron blues. In contrast, the corresponding nonprotonated reduced tungstates do not undergo disproportionation. The difference in the disproportionation reactivity between (HP2Mo186-) and (P2W187~) can be attributed to the effect of protonation which diminishes the negative charge on the anions reducing their mutual repulsion.Acknowledgment. The authors wish to thank Drs. I. Taub and M. Simic of the U.S. Army Natick Research and Development Command for the use of the pulse-radiolysis instrumentation and Dr. K. Whitburn for useful discussions and comments concerning technical and scientific aspects of this work. This work was supported in part by the National Science Foundation through Grant No. CHE79-18013. E.P. (visiting professor, Boston University, [1979][1980] acknowledges financial support from the North Atlantic Treaty Organiziation (Sen. Sci. Grant No. B138).
not change from pH 4 up to at least pH 12, when observed by conventional or stopped-flow spectrophotometry. At pH 13 and higher the rate of basic decomposition of NHC12 is fast, and this interferes with the spectral measurements of the initial absorbance. If it is assumed that a 10% change in the concentration of NHC12 could be detected by a change in the initial absorbance, then the protonation constant for NC12-must exceed lOI3 M-I. The protonation constant for NC12-will not exceed that of NHCl-, which is estimated3v3' to be greater than
ConclusionIt is not uncommon for traces of a species to catalyze a reaction, but it is less common for such traces to inhibit a reaction. This work shows that traces of ammonia inhibit the decomposition of dichloramine. This is because ammonia reacts with hypochlorous acid, which is responsible for the instability of dichloramine.Solutions of dichloramine free of other nitrogen compounds can be prepared by cation-exchange chromatography. With use of these solutions, it has been shown that dichloramine is unstable in the presence of even trace concentrations of hypochlorous acid, a compound which is produced in the decomposition of dichloramine. Consequently, the decomposition is an autocatalytic process. Dichloramine produces trichloramine in a termolecular mechanism involving the simultaneous removal of the proton by a general base and chlorination by HOCl. Dichloramine also reacts with trichloramine to produce (37) Anbar, M.; Yagil, G. J. Am. Chem. SOC. 1962,84, 1790-96 nitrogen gas and more hypochlorous acid. This latter reaction may cause trichloramine concentrations to increase when more hypochlorite is added to a system than is necessary to reach breakpoint stoichiometry. All of the dichloramine could be rapidly converted to trichloramine and be unavailable to participate in the subsequent production of NZ. The general-base catalysis observed suggests that the breakpoint chlorination reactions will be speeded by the presence of phosphate or other buffers in the solution.Hypochlorite reacts more rapidly with ammonia than with dichloramine, so that dichloramine solutions are stabilized by the presence of ammonia. The slow decomposition of dichloramine in the presence of ammonia produces chloramine and other compounds.The instability of dichloramine has implications for the analysis of chlorinated water for free and combined chlorine. Several of the standard methods for these analyses, namely the amperometric31b and ~yringaldazine~l" protocols, require that the sample first be buffered at neutral pH. If the sample is initially at a lower pH and does not contain ammonia, then this step will cause some or all of the dichloramine to decompose before analysis. As a result, the values determined for total and combined chlorine will be low and the value for free chlorine may be high.The chromium(I1) reduction of the nitrile-bonded pentaamminecobalt(II1) complexes with 3-and 4-cyanobenzoic acids has been studied. For the 4-cyanobenzoic acid complex, the rate law at 25 "C and I = 1....
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