ELECTROANALYTICAL TECHNIQUES

CHAPTER 1 INTRODUCTION

2.2 ELECTROANALYTICAL TECHNIQUES

Electroanalytical techniques are based on the transformation of chemical information into an analytically useful signal (Rahman et al. 2008). Electroanalytical techniques use two or three electrode electrochemical cell systems (working electrode (WE), auxiliary electrode (AE) and reference electrode (RE) (Figure 2.6). The use of the three electrodes along with the potentiostat allows accurate application of potential functions and the measurement of the resultant current.

The WE is used to monitor the response of the analyte. They are constructed from a wide range of conduction materials; these include mercury, silver, gold, gold, platinum, graphite, carbon paste and glassy carbon. The AE is used to control the potential applied to the working electrode and to complete the circuit for carrying the current generated at the WE. The most commonly used AE are platinum wire or sometimes graphite. The reference electrode is an electrode whose potential is known and is constant. The potential of the RE electrode is taken as the reference against which the potentials the other electrodes are measured. The most commonly used RE are saturated calomel electrode (SCE) and silver/silver chloride (Ag/AgCl) electrode (Schollz 2002).

A B

Figure 2.6. Photographs taken of the instruments used in this work: showing an electrochemical cell consisting of (A) two Metrohm electrodes; and (B) three electrodes of BASi voltammetric cell.

Electroanalytical techniques are capable of determining trace concentrations of an electroactive analyte such as environmental pollutants. They can also provide useful information regarding physical and chemical properties of an electroactive analyte. These methods include cyclic voltammetry, stripping voltammetry, differential pulse voltammetry, and amperometry (discussed in Section 2.1).

2.2.2 Cyclic Voltammetry

Cyclic voltammetry (CV) is a commonly used electrochemical technique. In this work, CV will be used for electrochemical synthesis and characterization of polyaniline and for investigation of the potential at which hydrogen peroxide is being reduced. It utilizes the three electrodes along with the potentiostat which allows accurate application of potential functions and the measurement of the resultant current. Cyclic voltammetry is widely used in the study of redox behavior of electrochemically active species, electrochemical properties of the analyte, and the kinetics of electrode reactions. Cyclic voltammetry offers the possibility of identifying reactive intermediates or subsequent products (multi-oxidation state electroactive species like

Working electrode

Reference electrode

polyaniline) (Gunzler and Williams 2001). It also serves as method for teaching the concept of electrochemistry. Figure 2.7 is the typical cyclic voltammogram for a reversible redox process.

Figure 2.7. Typical cyclic voltamogram (www.cartage.org.lb/.../CyclicVoltammetry.htm accessed 27 January 2010).

The important parameters in a CV are peak currents (i^c_pand i^a_p) and peak potentials (E^c_pandE^a_p) of the cathodic and anodic peaks. For the reversible electrochemical reaction, the CV must have the following characteristics: the separation potential (∆E_P) should be equal to 59 mV. The separation potential can be calculated from equation (2.2.1):

mV

E n E

E_p  ^a_p  _p^c  59

 (2.2.1)

The second characteristic is that the peak positions are not affected by the change of the scan rate. Thirdly, the ratio of peak currents is equal to 1 ( _c 1

p a p

i

i ). Lastly, the peak currents are

proportional to square root of the scan rate. In some cases electrochemical reactions show irreversible or quasi-reversible processes. For quasi-reversible process, ∆E_Pbecomes greater than 59 mV/n and although current intensity increases with the scan rate; peak current is not proportional to the square root of scan rate. For irreversibility on the other hand, the reduction product cannot be reoxidized, meaning that the anodic peak is not observed (Gunzler and Williams 2001).

2.2.3 Differential Pulse Voltammetry (DPV)

The principle of DPV is comparable to that of normal pulse voltammetry (NPV) in that potential is also scanned with a series of pulses. Unlike NPV, each potential pulse is fixed to small amplitudes (10 to 100 mV) and is superimposed on a slowly changing base potential. In each pulse the current is measured at two points: the first point is taken just before the application of the pulse while the second is taken at the end of the pulse just before it decreases back to baseline. The difference between the currents is amplitude of the pulse (I_pulse), therefore the DPV results from a plot of I_pulseversus potential (Figure 2.8) (Monk 2002). The DPV is more sensitive compared to CV. Therefore, DPV was used for investigation of the accurate potential at which hydrogen peroxide is being reduced and to get an accurate potential.

Figure 2.8. Typical potential wave form for differential pulse voltammogram (A) and differential pulse voltammogram (B) (Bionalytical Systems manual 2009).

2.2.4 Anodic stripping voltammetry

Anodic stripping voltammetry (ASV) involves two steps, in the first step a negative potential is applied and the metal ion is preconcentrated on the surface of the electrode. In the second step, positive potential is applied and the metal ion is stripped off the electrode. The electrochemical signal is normally observed during the second step. Due to the preconcentration of the analyte, ASV offers high sensitivity, selectivity, low detection limits and a wide linear range (Monk 2002; Gunzeler and Williams 2001). For this reason Environmental Protection Agency (EPA) has recommended ASV to be one of the standard techniques for heavy metal analysis (McGaw and Swain 2006; Berezhetskyy et al. 2008).

Wu et al. (2008) developed a novel sensor for simultaneous detection of Pb, Cd and Zn, based on the differential pulse anodic stripping response at a bismuth/poly(p-aminobenzene sulfonic acid) (Bi/poly(p-ABSA)) film electrode. Several methods based on the determination of heavy metals using ASV has been reported (Legeai and Vittori 2006; Xu et al. 2008; Injang et al.

2010; Kokkinos et al. 2008; Renedoand Martínez 2007).