An electrochemical quartz crystal impedance study on cystine precipitationonto an Au electrode surface during cysteine oxidation in aqueous solution

Electrochemical quartz crystal impedance studies on L-cysteine oxidation in aqueous solutions were conducted. The precipitation of electrogenerated sp ecies on the Au electrode surface was found in acetic and phosphate buffers . Changes in the equivalent circuit parameters, resonant frequencies and ha lf peak width of the conductance spectra (Delta f(G1/2)) Obtained simultane ously during electrochemical experiments were discussed. Significant change s in the motional resistance and Delta f(G1/2) revealed that the adhering p recipitate acted as a non-rigid mass loading to the piezoelectric quartz cr ystal resonance. The resonant frequency shift found in solution was approxi mately twice that in air. Lots of loose and separate precipitate agglomerat es formed and stood like islands on the electrode surface, and for a freque ncy shift of - 10 000 Hz in the acetic buffer, the height of the precipitat e agglomerate could be greater than 5 mu m, as found by SEM. In addition, a white precipitate and similar responses of the equivalent circuit paramete rs were obtained during ferricyanide titration of L-cysteine in acetic, pho sphate and ammonia buffers. IR analysis revealed that the white precipitate is cystine. Electrode collection efficiencies of cystine were evaluated. M oreover, the frequency-point-selection fitting routine has been discussed a nd taken for analyses of the admittance data, and an equation of Delta F-G1 /2 as a function of the equivalent circuit parameters was given. (C) 2000 E lsevier Science S.A. All rights reserved.