Kinetics and mechanism of sulphite oxidation on a rotating platinum disc electrode in an alkaline solution

doi:10.1016/j.electacta.2006.10.038

Electrochimica Acta

Volume 52, Issue 11, 1 March 2007, Pages 3510-3517

https://doi.org/10.1016/j.electacta.2006.10.038 Get rights and content

Abstract

The reaction of sulphite at high pH and room temperature has been examined by cyclic voltammetric methods on a rotating platinum disc electrode. The mechanism of the oxidation reaction is studied by varying the sweep rate, the rotation rate and the concentration. The oxidation of sulphite on a platinum electrode under alkaline conditions appears to follow a catalytic reaction mechanism where weakly adsorbed sulphite is first oxidised to a strongly adsorbed sulphite radical in the rate-determining step. In the next step two sulphite radicals combine and form dithionate, which disproportionate into sulphite and sulphate.

Introduction

In a district heating system fresh water is normally used to distribute the heat through a network of pipelines. The internal corrosion of steel pipes that carry water with low salinity is related to the oxygen content and the hydrogen ion concentration in the water. Therefore, in order to reduce the corrosion rate, hydroxide is added to the water to raise the pH to around 9.5 and most of the dissolved oxygen is removed by heating the water [1], [2]. In a closed district heating system oxygen scavengers are added to fresh water to remove any remaining oxygen from the raw feed water. Sulphite and dithionite can be used for this purpose. The sulphur compounds are very reactive and may exist in many oxidation states. Often several oxidation states coexist in a solution. Some of the oxidation stages of sulphur are shown in Table 1. An overview of sulphur reactions is given elsewhere [3], [4]. The reaction mechanisms of sulphur compounds in alkaline solutions are not well understood since the number of studies under these conditions is limited [5], [6], [7], [8], [9], [10], [11], [12].

If an excess of sulphite or dithionite is added to the water, other sulphur components will be formed, including sulphide, which will increase the corrosion rate of the pipelines. It is therefore important to be able to control the amount of sulphite and dithionite added to the water in order to minimize the risk of an unpredicted increase of corrosion. Several methods to determine the concentration of these compounds are described in the literature [13], [14], [15], [16], [17], but none of these methods is suitable for industrial use because the methods demand periodic sampling and/or addition of analytical reagents often combined with time-consuming procedures [12]. It is important that the response time of a measurement is short. This may be achieved, as already shown for dithionite, by using an electrochemical method [18]. A platinum electrode is used as the sensor for determining the dithionite concentration under steady-state conditions. In this paper the mechanism of the oxidation of sulphite is studied to achieve better insight in factors that might affect the reproducibility of the electrochemical data produced at the platinum electrode surface.

For multi-step redox reactions the relationship between the transfer coefficient and the symmetry factor can give useful information about the reaction mechanism.

The symmetry factor, β, which has a value between 0 and 1, is a fraction which shows the ratio between the activation energy, ΔE^#, and the input electric energy (Fη) in the electrode–electrolyte interphase, where η is the change in electrical potential [19]: $β = \frac{Δ E^{#}}{F η}$ A multi-step reaction A + ne⁻ → Z can be divided into n electron transfer steps. If one of these steps is the rate-determining step (rds) in the overall reaction, it can be described by the general formula shown in Eq. (2): $ν (R + r e^{-} \to S)$ The rds can be preceded by γ steps and followed by (n − γ − rν) steps, where n is the total number of electrons transferred in the overall reaction, γ the number of electrons transferred before the rate-determining step (rds) and ν is the number of times the rds occurs in the overall reaction. If the rds is an electrochemical reaction, the r-value is 1, and if the rds is a chemical reaction, the r-value is 0. If more than one R particle is formed by the preceding γ steps or more than one particle S is required for the following (n − γ − rν) charge transfer steps, the rate-determining step R + e → S would have to be repeated ν times.

An equation for the oxidation current of a multi-step reaction has been derived by Bockris and Reddy and is shown in Eq. (3) [19]: $i = i_{0} exp ((\frac{n - γ}{ν} - r β) \frac{F η}{R T})$ The transfer coefficient, α, is expressed in Eq. (4): $α = \frac{n - γ}{ν} - r β$ It is often simple to obtain experimental values of the transfer coefficient. Eq. (4) is therefore useful as a starting point for determination of reaction mechanisms.

Section snippets

Experimental

Sodium sulphite and sodium hydroxide of analytical grade were added to nitrogen-purged deionised water. Fresh solutions were always made at the start of each experiment to minimize decomposition. The solution was purged with nitrogen (99.999%) to remove oxygen and adjusted to pH 11 by adding 1 M NaOH. The pH was measured before and after each experiment to control that it did not change during the experiment. Continuous pH-measurements during the experiments showed that the pH was stable within

Results and discussion

The polarisation measurements were carried out on a platinum disc electrode from −0.85 to 1.20 V SCE. The low vertex potential is within the potential area for the hydrogen evolution reaction and the high vertex potential is within the potential area for the oxygen evolution reaction. Earlier studies have shown that when the potential of a platinum electrode is cycled between these two potential areas, the electrode surface will consist of an oxide and/or hydroxide layer [20]. Gasana et al. have

Conclusions

•
The oxidation of sulphite on a platinum electrode under alkaline conditions appears to follow a catalytic reaction mechanism where weakly adsorbed sulphite is first oxidised to a strongly adsorbed sulphite radical in the rate-determining step. In the next step two sulphite radicals combine and form dithionate, which disproportionates into sulphite and sulphate.
•
Cyclic voltammetry experiments with a stationary electrode show two oxidation peaks. Other experimental data indicates that sulphite is

Acknowledgment

Support from the Stavanger University Fond for financing the PhD study for Skavås is highly appreciated.

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Kinetics and mechanism of sulphite oxidation on a rotating platinum disc electrode in an alkaline solution

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgment

Electrochim. Acta

Corros. Sci.

Corros. Sci.

Electrochim. Acta

Electrochem. Commun.

J. Electroanal. Chem.

Talanta

Talanta

Anal. Chim. Acta

Electrochim. Acta

Electroanal. Chem. Interf. Electrochem.

Electroanal. Chem. Interf. Electrochem.

Electrochim. Acta