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The reduction of 2,6-dichlorophenolindophenol (DCPI) by sulphides and sulphites has been studied kinetically by the stopped-flow technique. The reaction is first-order with respect to each of the reactants. From the distribution diagrams for the species DH
+
2, DH and D
− for DCPI and H
2Q, HQ
− and Q
2− for sulphides or sulphites, a mechanism is proposed which suggests partial reactions of all possible combinations of the reacting species at any pH. An equation for calculation of the second-order reaction rate constants
k at any pH is derived, which gives
k as a function of [H
+], the partial reaction rate constants and the dissociation constants of DCPI and H
2S or H
2SO
3. Values of the overall reaction rate constants over a wide pH-range have been determined, together with values of
k for all possible partial reactions. For particular pH-values the second-order reaction rate constant was determined by four different methods. Mean values of
k = 251 ± 1 and 240 ± 1 l.mole
−1.sec
−1 were obtained for pH 3.15 and 4.17, respectively, for the DCPI—Na
2S reaction and
k = 137 ± 1, 127 ± 1 and 136 ± 1 l.mole
−1.sec
−1 for pH 2.02, 4.25 and 5.10, respectively, for the DCPI—Na
2SO
3 reaction. From the slopes of the linear Arrhenius plots activation energies of 6.6 ± 0.2 and 4.0 ± 0.1 kcal/mole for the DCPI—Na
2S and DCPI—Na
2SO
3 reactions, respectively were calculated. The effect of ionic strength on the reactions supports the proposed mechanism.
Sprache
Englisch
Identifikatoren
ISSN: 0039-9140
eISSN: 1873-3573
DOI: 10.1016/0039-9140(91)80319-U
Titel-ID: cdi_proquest_miscellaneous_733825621
Format
–
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