Medical brain notes 35

  Voltammetric Measurements Although   early   voltammetric   methods   relied   on   the   use   of   only   two electrodes,   modern   voltammetry   makes   use   of   a   three-electrode   po-tentiostat,   such   as   that   shown   in   Figure   11.4.   A   time-dependent   po- tential   excitation   signal   is   applied   to   the   working   electrode,   chang-  ing   its   potential   relative   to   the   fixed   potential   of   the   reference  electrode.   The   resulting   current   between   the   working   and   auxiliary electrodes   is   measured.   The   auxiliary   electrode   is   generally   a   plat- inum   wire,   and   the   SCE   and   Ag/AgCl   electrode   are   common   refer- ence   electrodes. Several   different   materials   have   been   used   as   working   elec-  trodes,   including   mercury,   platinum,   gold,   silver,   and   carbon.   The earliest voltammetric techniques, including polarography, used mercury for the working   electrode.   Since

  Current in Voltammetry When   an   analyte   is   oxidized   at   the   working   electrode,   a   current   passes   electrons  through   the   external   electric   circuitry   to   the   auxiliary   electrode,   where   reduc-  tion   of   the   solvent   or   other   components   of   the   solution   matrix   occurs.   Reducing  an   analyte   at   the   working   electrode   requires   a   source   of   electrons,   generating   a  current   that   flows   from   the   auxiliary   electrode   to   the   cathode.   In   either   case,   a  current   resulting   from   redox   reactions   at   the   working   and   auxiliary   electrodes  is  called  a  faradaic current.  In  this  section  we  consider  the  factors   affecting  the   magnitude   of   this   faradaic   current,   as   well   as   the   source   of   any   nonfaradaic  currents. Sign   Conventions   Since   the   reaction   of   interest   occurs   at   the   working   electrode, the   classification   of   curre