Voltammetry at the Thin-Film Mercury Electrode (TFME) Anodic and Cathodic Stripping Voltammetries and Simple Potentiostat Construction
R. S. Pomemy, M. B. Denton, and N. R. Armshung' University of Arizona, Tucson. AZ 85721
Voltammetw at a stationarv electrode is an electroanalvtical method t h i t should be implemented into the curricul&n of advanced undermaduate or maduate courses. The advantages of electrochemistry, which include high sensitivity, good selectivitv for manv elements. a large linear dvnamic range, inexpensive (and easily asse&led)instrumekation, and a wide ranee of indicator electrode sizes and geometries that allow for analysis in unusual environments, are easily demonstrated using this technique. Voltammetry at a stationary electrode involves monitoring of the current flowing at the indicator electrode while its potential versus a reference electrode is varied a t a constant rate (1).When the analyte undergoes a mass transport limited ;eduction or oxidation at the indicator electrode, the peak current seen is directly proportional to the bulk concentration of the analyte. The sensitivity of this approach is limited. If it is possible to preconcentrate the analyte through an electrodeposition process, followed by voltammetrically "stripping" the analyte back into solution, several decades improvement in detection limit can be achieved. This process is demonstrated in a technique called anodic stripping voltammetry (ASV), reviewed previously by Wang (2) for the determination of transition metal cations (3) and more recently by cathodic stripping voltammetry (CSV) for anions and molecular species (4). A hanging mercury drop indicator electrode (HMDE) is often used for these experiments. An electrodeposited thin-film mercury electrode (TFME) (5) is more conveniently used for the teaching laboratory. The TFME is easily constructed and eliminates some of the problems encountered with the HMDE. The TFME can he prepared prior to the ASV or CSV experiment, or in situ by electrodepositing a tbin-mercury film on a glassy carbon electrode. The TFME exhibits good stability and allows for hieh sensitivitv in ASV and has the additional advantage over the HMDE in that (1)problems with reproducible drip ~ off during or between analvsize and (2) nremature d r o fall ses are eliminated. We reAew here the use of the TFME fbr ASV and simple voltammetry of solution cations such as Pb2+ and CdZ+,and the use of CSV for the determination of an environmentally important molecule, thiourea, TU, (4) along with the constru&on of a simple potentiostat, readily adaptable to an advanced undergraduate or first-year eraduate student laboratory course. T h e potentiostat circuit is based on a previous design (6) and is constructed by either the student or the instructor. The comnleted ootentiostat has many of the features of more sophisticated systems such as variable scan rate.. adiustable initial ootential. and variable gain in the current-to-voltage converter. Voltammetry There are m a w articles in this Journal (1,3, 7, 8 ) and others (9,101 thai cover the theory of voltammetry. Linear sweep voltammetry (LSV) involves the polarization of the TFMEIsolution interface in the nresence of the reducible or oxidizable analyte and the subsequent measurement of the ~~
~~~
~
~~~~~~~~
-.I5
n
-3 -.55
-3 -.I5
72
n
Time and Corresponding Potential Figure 1. Voltammogam for Um Pb2+/Pboredox couple at Um TFME taken using a sbip dart records7 (curtem us. time). At time TI, the potential was scanned hom an initial potsotial of -0.05 V at a rate of 10 mV/s. When a potential of -0.55 V was reached, time T2, Um scan direction was reversed A: kinetlcaiivcornroiled urnii me initial wmntial was reached. time T3. Realon " electron nanslsr llmns the cunent at each potential.Rsglon B: mass wansport control IdlHusm) llmlts b e reductive cunent. Reglon C: the dlreclion of potential scan is reversed and the Pbo is reoxidized into solution with a symmetric stripping peak cemered at me
faradaic and nonfaradaic (i.e., double-layer charging) currents that result. The TFME "indicator" or "working" electrode is immersed in an unstirred solution. The r&dtihg current-voltage curves show regions where the current response is limited primarily by the rate of electron transfer (Region A, Fig. I), followed by a peak in the current response, followed by a decay in the current, which is now limited by mass transport, semi-infinite linear diffusion (Re-
' Author to whom correspondence should be addressed. Volume 66 Number 10 October 1989
877
gion B, Fig. 1). The important parameters in LSV are the cathodic peak current (for a reduction process, e.g., Pb2++ 2e-= PhO), ipc,,and the cathodic peak potential, Epc. For a chem~callyreversible, diffusion-controlled (rapid electron transfer), electrochemical process, like the reduction of a metal cation i;
= (2.69 X
105)n3/2~C&'nu1n amp
(1)
where n is the number electrons involved in the reduction oer mole. A is the electrode area in cm2, COis the concentration of the analyte in the hulk solution in moles per ~ m - u~is, the scan rate in volts per second, and Do is the diffusion coefficient of the analyte in the bulk solution in em2 per second (6). For metal cations, such as Pb2+,which are reduced at Hg with rapid electron transfer rates, the peak potential, EP,
