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Anodic Response of Cystine at Preanodized Au and Au‐Ag Electrodes in Perchloric Acid Media
Author(s) -
Johll Matthew E.,
Johnson Dennis C.
Publication year - 1999
Publication title -
electroanalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.574
H-Index - 128
eISSN - 1521-4109
pISSN - 1040-0397
DOI - 10.1002/(sici)1521-4109(199906)11:8<534::aid-elan534>3.0.co;2-a
Subject(s) - cystine , chemistry , electrode , overpotential , anode , electrolysis , analytical chemistry (journal) , inorganic chemistry , electrochemistry , cysteine , organic chemistry , electrolyte , enzyme
This study involves a comparison of the anodic response of the disulfide moiety of cystine at preanodized Au and Au‐Ag rotated disk electrodes (RDEs). The goal of the study was the determination of beneficial effects that might result from the introduction of Ag‐sites into Au surfaces. An anodic wave is observed for cystine at oxide covered Au and Au‐Ag electrodes appearing as a shoulder to the wave for anodic discharge of H 2 O with evolution of O 2 . Both the half‐wave potential for the cystine wave ( E 12,RSSR ) and the overpotential for anodic evolution of O 2 (η O 2 ↑) are shifted by –3.6 mV per atomic percent Ag. These negative shifts are attributed to the higher rate of the anodic discharge of H 2 O at Ag sites in comparison to Au sites. Values for the apparent number of electrons ( n app ) transferred during oxidation of 1.0 mM cystine at 1.50 and 1.55 V (vs. SCE) are approximately 5 eq mol –1 for Au and 6eq mol –1 for Au 80 ‐Ag 20 , as calculated from the slopes of Koutecky‐Levich plots. The larger value of n app at the Au‐Ag electrode is attributed to the higher rate for H 2 O discharge at the Ag sites. Cysteine sulfonic acid is identified as the product of extensive electrolysis ( n =10eq mol –1 ) at a Au electrode. A preanodized Auelectrode in a flow‐through cell is demonstrated for cystine detection using a flow‐injection system. A 6% loss in peak response observed over a 30‐min period is attributed to the slow conversion of an active surface oxide, perhaps corresponding to the equivalent of a monolayer,to an inert oxide, probably corresponding to oxide coverage exceeding a monolayer.