Details

Autor(en) / Beteiligte

• Konidari, Constantina N.
• Karayannis, Miltiades I.

Titel

Interference in the kinetic determination of ascorbic acid by sulphide and sulphite

Ist Teil von

• Talanta (Oxford), 1991-09, Vol.38 (9), p.1019-1026

Ort / Verlag

Netherlands: Elsevier B.V

Erscheinungsjahr

1991

Link zum Volltext

Quelle

Elsevier Journal Backfiles on ScienceDirect (DFG Nationallizenzen)

Beschreibungen/Notizen

• 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.