Kinetic model of the H(2)O(2)-SCN(-)-OH(-)-Cu(2+) oscillator and its application to the interpretation of the potentiometric responses of various inert electrodes monitoring the reaction course.
The high complexity of the kinetic mechanism of the H(2)O(2)-SCN(-)-OH(-)-Cu(2+) oscillator, involving numerous intermediates, causes the oscillations monitored potentiometrically with gold or glassy carbon electrodes to exhibit opposite phases compared with the oscillations recorded with palladium or platinum electrodes. Following our previous work on the outline explanation of these phenomena, involving the concept of the mixed potential, in this paper, we present their more detailed and advanced study. For that purpose, we built up a simplified but realistic kinetic model of the studied oscillator, involving nine intermediates. Of those nine species, Cu(OH)(3)(-), Cu(OH)(2)(-), and HO(2)(*) were found to be crucial for the explanation of the potentiometric responses of various electrodes, under an additional assumption that the interfacial exchange current density of the HO(2)(*)/HO(2)(-) couple increases in the series GC < Au < Pt. Calculated oscillatory variations of the mixed potential for various model electrodes, compared with experimental results, allowed us to conclude that the potentiometric oscillations are caused largely by the oscillations of the [Cu(OH)(3)(-)]/[Cu(OH)(2)(-)] concentration ratio, irrespective of the electrode material used as a potentiometric sensor. For the Au electrode, the increase of the potential within every oscillatory peak largely reflects the increase in the [Cu(OH)(3)(-)]/[Cu(OH)(2)(-)] ratio. The simultaneous shift of the relatively high Pt electrode potential toward more negative equilibrium potential of the Cu(OH)(3)(-)/Cu(OH)(2)(-) couple is caused by the increase of the exchange current density of the latter couple. Thus, even the opposite phases of the potentiometric oscillations are explainable in terms of the oscillatory behavior of the same redox couple. Understanding of such phenomena is crucial for the proper interpretation of potentiometric data in complex chemical systems.
Wiśniewski
Albin
A
Laboratory of Electroanalytical Chemistry, Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland.
Pekala
Katarzyna
K
Orlik
Marek
M
D002244:Carbon / Q000737:chemistry; D003300:Copper / Q000737:chemistry*; D004566:Electrodes; D006046:Gold / Q000737:chemistry; D006861:Hydrogen Peroxide / Q000737:chemistry*; D017665:Hydroxyl Radical / Q000737:chemistry*; D007700:Kinetics; D008956:Models, Chemical; D009991:Oscillometry; D010165:Palladium / Q000737:chemistry; D010984:Platinum / Q000737:chemistry; D011199:Potentiometry; D013861:Thiocyanates / Q000737:chemistry*
D013861:Thiocyanates; D017665:Hydroxyl Radical; D010984:Platinum; D010165:Palladium; D002244:Carbon; D006046:Gold; D003300:Copper; D006861:Hydrogen Peroxide; C031760:thiocyanate