ANALYSIS OF LEAD IN SEAWATER BY DIFFERENTIAL PULSE POLAROGRAPHY Introduction Electrochemical methods of analysis can be used for the quantitative analysis of any electroactive species – any species that can be easily oxidized or reduced at an electrode (the “working” electrode). A powerful set of electroanalytical methods are based on voltammetry, in which the current generated by the oxidation or reduction of an analyte is measured as a function of the voltage at the working electrode. Polarography, using a mercury drop as the working electrode, is the oldest form of voltammetry. In this experiment, you will analyze a synthetic seawater sample for its lead content using a form of voltammetry called differential pulse polarography (DPP). The purpose of the experiment is to evaluate the suitability of DPP for the analysis of trace amounts of lead in seawater, a sample matrix that can pose considerable difficulty for many analytical techniques. Background The principles of voltammetry are summarized here; for more detail, please see the following references: • Harris 17.1-17.3; 18.1-18.5 • Skoog 25A-25B, 25E-25F Basis of Voltammetry Voltammetry occurs in an electrolytic cell: a potential is applied between two electrodes until a redox reaction is forced to occur. At the anode, the electrode potential is low enough to “pull” electrons from the solution (oxidation occurs) while at the cathode the potential is high enough to “force” the solution to accept them (reduction). If the redox reaction kinetics are infinitely fast, then the slope of the i vs E plot would be determined by the solution resistance: Page 1 Polarographic Analysis of Lead in Seawater Background reaction begins slope determined by solution resistance current 0 E applied Figure 1. The current passing through an electrolytic cell with an infinitely fast redox reaction is controlled by the solution resistance. Little current flows until the redox reaction begins, and then the current increases with a slope of 1/Rsoln. In actual fact, the current will be controlled by the rate of redox reaction at one (or perhaps both) of the electrodes. As more voltage is applied to the cell, the current reaches a maximum value at which the reaction is occurring as fast as possible. This phenomenon – in which the current through an electrochemical cell is limited by the rate of redox reaction at one of the electrodes – is called electrode polarization. Voltammetry is based on measuring the current limited by the rate of analyte reaction at an electrode; under certain conditions, the current will depend on the concentration of the analyte in the electrolytic cell solution. A plot of measured current as a function of applied potential is called a voltammogram. The following figure shows the characteristics of a typical voltammogram. Page 2 Polarographic Analysis of Lead in Seawater Background Figure 2. Typical voltammogram obtained during analyte reduction. As the voltage at the working electrode becomes more negative, analyte reduction begins and soon reaches its limiting rate; the resulting current is the limiting current (il) and its value is dependent on analyte concentration. The potential that yields have the maximum current is called the half-wave potential, E1/2; its value is characteristic of the analyte. Source: Skoog. As the figure depicts, a voltammogram takes on a characteristic “wave” appearance. As mentioned previously, under certain conditions the limiting current, il, will be linearly proportional to analyte concentration. The location of the wave is specific for a particular analyte; the half-wave potential, E1/2, is the potential at one-half the limiting current. The half-wave potential is approximately equal to the standard thermodynamic potential; thus, the more easily-reduced analytes will have the more positive half-wave potentials. In voltammetry, the limiting current is usually due to electrode polarization at the working electrode. Electrode polarization at the working electrode occurs when the analyte reaction rate is controlled by one of the following phenomena: 1. The rate of mass transfer to or from the electrode. This type of polarization is sometimes called concentration polarization. 2. The rate of electron transfer at the surface of the electrode; this is kinetics polarization. Quantitative analysis using voltammetry works best when the current is mass-transfer limited. At the mass transfer limit, the analyte reacts as quickly as it can get to the electrode surface. Thus, the best analytes are those that undergo rapid electron transfer at the electrode. In voltammetry, an inert electrolyte, the supporting electrolyte, is added to the solution so that the current is carried by ions other than the analyte. In the presence of supporting electrolyte, the mass-transfer limited current will be determined by the rate of analyte diffusion to/from the electrode surface. Fick’s First Law of diffusion requires that the rate of analyte diffusion to the electrode surface will be linearly proportional to the analyte concentration in the solution: the more concentrated the analyte, the Page 3 Polarographic Analysis of Lead in Seawater Background faster it diffuses to the working electrode and reacts. The diffusion-limited current for a planar electrode is described by the Cottrell equation: = DA = Cottrell eqn. id nFA !t CA kCA where n is the number of electrons transferred, F is Faraday’s constant, A is the area of the electrode surface, DA is the analyte diffusion coefficient, and CA is the analyte concentration. At any given time, the diffusion-limited current id is proportional to the analyte concentration. Let’s summarize the situation, then. A sample solution is placed in an electrolytic cell; this solution contains a relatively high concentration of inert electrolyte. A voltage is applied and the analyte reacts at one of the electrodes (the working electrode); the resulting current is measured as the voltage is changed, giving a voltammogram. Since the analyte undergoes rapid electron transfer at the working electrode, and the current is carried through the solution by the supporting electrolyte, the limiting current is determined by the rate of analyte diffusion to/from the electrode surface. According to Fick’s Law, the diffusion rate is linearly proportional to the concentration of analyte, so that doubling the analyte concentration in the sample solution would double the rate of analyte diffusion at the working electrode. Since analyte diffusion is the rate-determining step in the whole process, doubling the diffusion rate would double the rate of reaction, and double the limiting current. The relationship between the diffusion-limited current (at a planar electrode) and the analyte concentration is described by the Cottrell equation. The reaction of the analyte at the working electrode is controlled by the potential (i.e., the electron energy) at the working electrode, EWE. A more negative value of EWE means a higher electron energy, which means that we are trying to “force” the analyte to accept the electron: the conditions are more reducing. In order to control the redox conditions at the working electrode, it is essential to control the value of EWE; a three-electrode system, such as the one in the next figure, is the best way to control EWE. Page 4 Polarographic Analysis of Lead in Seawater Background i electrolytic cell WE CE FEEDBACK RE V Figure 3: simple schematic of a 3-electrode potentiostatic cell, consisting of a Working Electrode (WE), a Counter Electrode (CE) and a Reference Electrode (RE). The voltage applied to the electrolytic cell is controlled by a feedback loop from the potential difference between the working and reference electrodes. The three electrodes are: • the working electrode (WE). The analyte is reduced or oxidized here, depending on the value of EWE. • the counter electrode (CE). This is where the other half of the redox reaction occurs. If the analyte is being reduced at WE, some other species must be oxidized at CE. The vast majority of current flows through the solution between WE and CE; the electron transfer rate at WE and CE are equivalent. • the reference electrode (RE). This is the reference point for the measurement of EWE. Together, the three electrodes comprise a potentiostatic electrolysis system. Potential is applied between the working and counter electrodes to force a redox reaction. The electron energy at the working electrode is monitored by measuring the potential difference between WE and RE; the applied potential is modified by a feedback loop to obtain the desired value of EWE. The current flow along the “upper” circuit in fig 3 is measured as a function of EWE. Polarography The most common working electrode material is mercury. The analyte reaction at the working electrode is very sensitive to changes in the electrode surface; one important advantage of using mercury as the electrode is that a clean, reproducible electrode surface can always be obtained. Voltammetry using a mercury drop as the working electrode is given the special name of polarography. The following figure shows some common mercury drop electrodes. Page 5 Polarographic Analysis of Lead in Seawater Background (a) (b) (c) Figure 4. Mercury drop electrodes used for polarography: (a) Hanging Mercury Drop Electrode (HMDE); (b) Dropping Merucy Electrode (DME); and (c) Static Mercury Drop Electrode (SMDE). See text for description of the electrodes. Source: Skoog. The simplest mercury drop electrode uses a micrometer to force a reproducible volume of mercury into the drop; this is the hanging mercury drop electrode (HMDE). After a drop is formed, the experiment is performed and the drop is discarded. The dropping mercury electrode (DME) is a little more complicated. The DME consists of a narrow capillary through which mercury is constantly flowing. Drops grow at the end of the capillary; every 2-4 seconds, the drop falls off and the process begins again. A mechanical knocker may be employed to dislodge the drop at reproducible intervals.