Practical Problems in Voltammetry: 4. Preparation …

1 The fundamental process in electro-chemical reactions is the transfer ofelectrons between the electrode sur-face and molecules in the inter-facial region (either in solution orimmobilized at the electrode sur-face). The kinetics of this heteroge-neous process can be significantlyaffected by the microstructure androughness of the electrode surface,the blocking of active sites on theelectrode surface by adsorbed mate-rials, and the nature of the func-tional groups ( , oxides) presenton the surface (1, 2). Therefore,there has been considerable effortdevoted to finding methods that re-move adsorbed species from theelectrode and produce an electrodesurface that generates reproducibleresults.

2 Some of these methodshave also resulted in the activationof the electrode surface (as judgedby an increase in the rate of elec-tron transfer). These methods arethe subject of this paper, and in-clude mechanical polishing, heatpretreatment, and most common method forsurface Preparation is mechanicalpolishing. The protocol used forpolishing depends on the applica-tion for which the electrode is beingused and the state of the electrodesurface. There are a variety of dif-ferent materials available ( , dia-mond, alumina, silicon carbide),with different particle sizes sus-pended in solution (BAS m alumina polish and 1, 3, 6,and 15 m diamond polishes).

3 Thepad used for polishing also dependson the material being used for pol-ishing Texmet pads are used withalumina polish, and nylon padsshould be used with diamond pol-ish. Working electrodes supplied byBAS have first been lapped to pro-duce a flat surface, and have thenbeenextensivelypolishedtoasmooth, mirror-like finish at thefactory. Therefore, they typicallyonly require repolishing with mor1 m diamond polish by theuser in between experiments. Mate-rials that have a rougher surface( , electrodes which have beenscratched) must first be polishedusing a larger-particle polish in or-der to remove the surface the defects have been re-moved, the polishing should con-tinue with successively smaller-par-ticle-size polish ( , 15 m, then 6 m, then 3 m,andthen1 m).

4 Once polishing has been com-pleted (this can require from 30 s toseveral minutes, depending uponthe state of the electrode), the elec-trode surface must be rinsed thor-oughly with an appropriate solventto remove all traces of the polishingmaterial (since its presence can af-fect the electron transfer kinetics).Alumina polishes should be rinsedwith distilled water and diamondpolishes with methanol or rinsing solution should besprayed directly onto the electrodesurface. After the surface has beenrinsed, electrodes polished withalumina should also be sonicated indistilled water for a few minutes toensure complete removal of the alu-mina particles.

5 If more than onetype of polish is used, then the elec-trode surface should be thoroughlyrinsed between the different discussed above, the effectof any surface pretreatment can bedetermined by its effect on the rateof electron transfer. This can bejudged qualitatively by examiningthe separation of the peak potentialsin a cyclic voltammogram of amolecule whose electron transferkinetics are known to be sensitiveto the state of the surface; a morequantitative determination can bemade by calculating the value of ksfrom this peak potential example, ksfor potassium ferri-cyanide at glassy carbon surfacefollowing a simple polishing proto-col was found to lie in the - cm s-1(3,4) (thisshould be compared with the valuesmeasured for ksfor a platinum elec-trode, which are at least one orderof magnitude larger).

6 The strongdependence of the electron transferkinetics of ferricyanide on the stateof the electrode surface means thatPractical Problems in Voltammetry: 4. Preparation of Working ElectrodesThe condition of the surface of the working electrode can have asignificant effect on the current response in both voltammetric andamperometric experiments. The methods most commonly used for thepreparation of working electrodes (polishing, electrochemicalpretreatment, and heat pretreatment) are discussed in this W. Bott, Systems, Lafayette, can be significant variationsin the peak potential separation af-ter each polishing. Polishing altersthe microstructure, roughness, andfunctional groups of the electrodesurface in addition to removing ad-sorbed species (for example, it hasbeen shown that the oxygen-to-carbon ratio is increased by polish-ing (5)).

7 It has also been reportedthat materials used for the polishingcan affect the value of ks(1,4,6).For example, the electrode surfacecan be contaminated by the ag-glomerating agents required to keepthe alumina particles suspended insolution and by the components ofthe polishing pad. The presence ofthese species can have a deleteriouseffect on the electron transfer kinet-ics by blocking the active sites forthe electron transfer reaction. Forthe most exacting studies, it wassuggested that the alumina suspen-sion be freshly made with ultrapurewater and that the electrode shouldbe polished on glass (a ksvalue cm s-1for ferricyanide was re-ported following polishing underthese stringent conditions (4)).

8 However, it should be noted thatsuch pronounced dependence onthe state of the electrode surface isonly observed for certain systems(the most well characterized exam-ples are the reduction of ferricy-anide, the oxidation of ascorbate,and the adsorption of dopamine).For such systems, polishing is oftenused in combination with anotherpretreatment ( , heat or electro-chemical). However, for manyother systems, the simple polishingdescribed above is adequate (for ex-ample, when using non-aqueouselectrolytes, since blocking of ac-tive sites by adsorbed species is lesscommon in such electrolytes thanin aqueous solutions).

9 Another method for prepara-tion of the electrode surface that isbecoming more widely used is elec-trochemical pretreatment (ECP),particularly for electrodes whichcannot readily be polished ( ,carbon fiber cylinder electrodes).ECP consists of applying condi-tioning potentials to the electrodesurface before the experiment. Asfor polishing, this has the effect ofremoving adsorbed species and al-tering the microstructure, rough-ness, and functional groups of theelectrode surface. The precise ECPprotocol depends upon the applica-tion and varies considerably. Thepotential waveforms typically areheld at, or cycle to, a large positiveor negative potential, either usingsteps or sweeps (constant potential(6), potential scan (7,8), triangularwave (9-15) and square wave (16,17)).

10 Although the development ofthe preconditioning protocols hasbeen largely empirical, the pre-treated electrode surface has beencharacterizatized in order to eluci-date the reasons for the activationof the electrode surface (6,7,17,18).For glassy carbon electrodes, in ad-dition to the removal of adsorbedspecies, the preconditioning poten-tial leads to the formation of anoxygen-rich layer on the carbonsurface. This layer contains oxidesas well as other oxygen-containingfunctional groups which may cata-lyze electron transfer reactions (thecomposition of the functionalgroups in this layer is sensitive tothe pretreatment conditions and de-pends on the solution pH as well asthe potentials used for the pretreat-ment (19)).