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).

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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