Interference in the Kinetic Determination of Ascorbic-Acid by Sulfide and Sulfite
(EN)
Konidari, C. N.
(EN)
Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας
(EL)
Konidari, C. N.
(EN)
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.
(EN)
