Anomalous potentiometric response of the platinum electrode in the oscillatory processes with hydrogen peroxide

Katarzyna Pękala ,  Rafał Jurczakowski ,  Barbara J. Palys ,  Marek Orlik 

University of Warsaw, Department of Chemistry, Pasteura 1, Warsaw, Warszawa 02-093, Poland

Abstract

The oscillatory oxidation of thiocyanate ions with hydrogen peroxide in alkaline media and in the presence of Cu2+ as a catalyst was observed both in the batch reactor and in the flow, continuously stirred tank reactors (CSTR) [1,2]. Luminescent traveling waves were recently reported by us [3]. The main reactions between SCN- - H2O2 - OH- - Cu2+ are:
4H2O2 + SCN- = HSO4- + HOCN + 3H2O (1)
HOCN + 2H2O = NH4+ + HCO3- (2)
The complicated mechanism of this reaction involves 30 steps with 23 intermediates. The oscillatory course of this process and other oscillators involving H2O2 can be monitored potentiometrically using Pt electrode. We tested also the response of the gold and the glassy carbon electrodes in the SCN- - H2O2 - OH-- Cu2+ system. To our surprise we have found that the oscillatory variations of the Pt electrode potential could not be interpreted as reflecting the changes of the redox potential of the solution, contrary to the responses of the glassy carbon and Au electrodes which both appeared to behave as nearly inert electrodes (Fig. 1).

Figure1_2_1.JPG

Fig. 1. Comparison of the oscillatory changes of the potential of the Pt, Au and GC electrodes in the NaSCN - NaOH- - H2O2 - CuSO4 system.

The difference between the responses of the various theoretically inert electrodes in the NaSCN - NaOH- - H2O2- CuSO4 system is a novel phenomenon which shows that one has to be very careful with the interpretation of the potential of a given theoretically inert electrode. It is important also because platinum is very often used for the monitoring of the course of various oscillatory processes. If the response of Pt electrode does not reflect directly the bulk redox potential of the solution, it will not be concordant with the theoretical changes of the redox potential predicted by the model of the given process, even if this model is essentially correct. We have found that an anomalous potentiometric response of the platinum electrode is a case also for the Na2S2O3 - H2O2 - H2SO4 - Cu2+ system. Since both systems contain hydrogen peroxide, one can suppose that it is the specific interaction of H2O2 with platinum surface which makes the response of Pt electrode anomalous. In order to study in more detail this specific state of Pt electrode in both systems we used Raman and SERS spectroscopy. The atypical behavior of the Pt electrode implies further studies of its specific interaction with the studied system which may also reveal unknown details of the mechanism of the investigated oscillatory process.

[1] M. Orbán, J. Am. Chem. Soc. 108 (1986) 6893

[2] Y. Luo, M. Orbán, K. Kustin, I. R. Epstein, J. Am. Chem. Soc. 111 (1989) 4541

[3] K. Pekala, R. Jurczakowski, A. Lewera, M. Orlik, J. Phys. Chem. A, Letters, 111 (2007) 3439

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Presentation: Poster at SMCBS'2007 International Workshop, by Katarzyna Pękala
See On-line Journal of SMCBS'2007 International Workshop

Submitted: 2007-09-21 16:26
Revised:   2009-06-07 00:48
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