Interference in the Kinetic Determination of Ascorbic-Acid by Sulfide and Sulfite (Journal article)

Konidari, C. N./ Karayannis, M. I.


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 DH2+, DH and D- for DCPI and H2Q, HQ- and Q2- 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 H2S or H2SO3. 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 1.mole-1.sec-1 were obtained for pH 3.15 and 4.17, respectively, for the DCPI-Na2S 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-Na2SO3 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-Na2S and DCPI-Na2SO3 reactions, respectively were calculated. The effect of ionic strength on the reactions supports the proposed mechanism.
Institution and School/Department of submitter: Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας
Keywords: 2,6-dichlorophenolindophenol
URI: http://olympias.lib.uoi.gr/jspui/handle/123456789/8269
ISSN: 0039-9140
Link: <Go to ISI>://A1991GJ40900012
http://ac.els-cdn.com/003991409180319U/1-s2.0-003991409180319U-main.pdf?_tid=70b2933b40deebcd165ba123ac5f08af&acdnat=1333030799_aa0733fe0cb7b20adf05c5c5f92e54ef
Publisher: Elsevier
Appears in Collections:Άρθρα σε επιστημονικά περιοδικά ( Ανοικτά)

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