The decomposition of aqueous dithionite III. Stabilization of dithionite by cations

Abstract

At the same conditions of pH (3.5 to 5.0), temperature (23 "C), and concentration (1 to 10 x M), ZnSZO4 is more stable in aqueous solution than Na2SZ04, and the rate of decomposition is much less sensitive to [H+]. At pH 4.5 and 5.0, the decomposition of the Zn salt is essentially linear, whereas the Na2S20, decomposes slowly at first, during an induction period, and then rapidly by an autocatalytic reaction. The rate of decomposition of dilute aqueous Na2S204 at pH about 4 is slowed greatly by the addition of small quantities of In3+, CdZ+, or ZnZ+. The effect of the stabilizing ions is attributed to anti-catalytic action.Canadian Journal of Chemistry, 48, 2778Chemistry, 48, (1970 IntroductionExperiments and Results ZnS204 is used industrially as a reducing agent or for bleaching, especially by the pulp and paper industry. It is unstable in aqueous solution, especially under acidic conditions, but the kinetics and mechanism of its decomposition have not been investigated. Qualitatively it is known to be more stable than Na2S,04 under similar conditions of pH (1). Following our studies of the kinetics and mechanism of the decomposition of Na2S20, (2, 3), aqueous ZnS204 was studied in the same manner. The dependence of its rate of decomposition on p H was determined. Methods of stabilizing dithionite by cations such as Zn2+ were tried since, if successful, they could be of commercial significance.'Present address: Imperial Oil Enterprises Ltd., Sarnia, Ontario. (a) Decomposition of ZnS204Several concentrations of ZnS204 from 0.8 to 8.0 x M were studied, some at fixed p H values in the same way as aqueous buffered Na2S204 described in Part 1 (2), and some in unbuffered solution, with p H measurement (3). The continuous polarographic method previously described (4) was used to determine the concentration of ZnS204 against time continuously; the experimental arrangements are shown in Fig. 2 48, 1970 solution in this range of p H is characterized by a slow "induction" period followed by a much more rgpid autocatalytic reaction. The decomposition of ZnS204 in the same p H region does not follow the same course. Figure 1 shows the decomposition of4 x MZnS204 at pH4.0. It is seen that there is a gradual increase in the rate of decomposition, but the division into an induction period and an autocatalytic period is by no means obvious. This figure may be compared with Fig. 5 of ref. 2 showing the decomposition of 5 x M Na2S204 at p H 4.0. In the case of the sodium salt the curve is sharply S-shaped: an induction period of about 10 min is followed by a sharp decrease in concentration, decomposition is half complete at 14 min and essentially complete at 17 min. The initial rate of decomposition of the zinc salt is in fact somewhat more rapid, but the change in slope is less sharp, although there appears to be a distinct change at about 25 min, and decomposition, which is half complete at 30 min, is essentially complete at 60 min. At higher p H the rate of decomposition is slower and nearly linear; decomposition is h...

Large Amplitude pH Oscillations in the Hydrogen Peroxide−Dithionite Reaction in a Flow Reactor

“…Furthermore, the hypothesis of an autocatalytic kinetics was put forward by Meyer who found an S-shaped curve: An early induction period is followed by a rapid autocatalytic stage to completion. This observation was confirmed by others. − Moreover, with an excess of sulfite ion, so as to minimize the effect of decomposition products, neither the induction period nor the fast reaction are detectable.…”

“…This observation was confirmed by others. [2][3][4][5][6][7] Moreover, with an excess of sulfite ion, so as to minimize the effect of decomposition products, neither the induction period nor the fast reaction are detectable.…”

“…However, we point out that other laboratories have never confirmed these oscillations. 6,7 Our own efforts repeatedly failed to reproduce pH oscillations of any kind in a closed reactor when we studied the disproportionation of S 2 O 4 2-in a concentration range 0.001-0.05 M and in a temperature interval 25-70°C excluding the air oxygen. Only a minimum, followed by a single maximum, appeared on the experimental pH-time curves.…”

Mechanism of the oscillatory decomposition of the dithionite ion in a flow reactor

“…The reaction has autocatalytic kinetics: an early induction period is followed by a fast decomposition. [1][2][3][4][5][6][7] Moreover, the addition of the products of the reaction to fresh dithionite solution decreases the length of the induction period of the decomposition. HSO 3 2 is thought to be the autocatalyst, but H + plays the similar role in an unbuffered solution.…”

“…8 However, this observation has never been confirmed by other laboratories, though many laboratories have studied the kinetics of S 2 O 4 22 decomposition under different conditions using different methods. [1][2][3][4][5][6][7] Our own efforts repeatedly failed to reproduce pH or redox potential oscillations in a closed reactor when we studied the disproportionation of S 2 O 4 22 in the concentration range 0.001-0.05 M and in the temperature interval 20-60 °C excluding the air oxygen. 9 Only a minimum followed by a single maximum appeared on the pH-time curves obtained in a closed reactor.…”