ANALYSIS OF LEAD IN SEAWATER

Transcription

1 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. The principles of voltammetry are summarized here; for more detail, please see the following references: Harris ; 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

2 0current reaction begins slope determined by solution resistance 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/R soln. 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

3 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 (i l) and its value is dependent on analyte concentration. The potential that yields have the maximum current is called the half-wave potential, E 1/2; 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, i l, will be linearly proportional to analyte concentration. The location of the wave is specific for a particular analyte; the half-wave potential, E 1/2, 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

4 faster it diffuses to the working electrode and reacts. The diffusion-limited current for a planar electrode is described by the Cottrell equation: Cottrell eqn. i d = nfa D A!t C A = kc A where n is the number of electrons transferred, F is Faraday s constant, A is the area of the electrode surface, D A is the analyte diffusion coefficient, and C A is the analyte concentration. At any given time, the diffusion-limited current i d 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, E WE. A more negative value of E WE 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 E WE ; a three-electrode system, such as the one in the next figure, is the best way to control E WE. Page 4

5 i electrolytic cell FEEDBACK WE CE 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 E WE. 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 E WE. 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 E WE. The current flow along the upper circuit in fig 3 is measured as a function of E WE. 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

6 (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. A scan using the DME will thus involve many mercury drops. As the drop grows, the diffusion-limited current increases since, according to the Cottrell equation, current is proportional to electrode surface area. A polarographic scan will thus consist of large current fluctuations, as shown in the left scan in figure 5. The static mercury drop electrode (SMDE) has a number of important advantages over the DME; we will discuss these in lecture. When a drop is needed, a plunger is quickly raised and lowered, allowing a reproducible volume of mercury into the capillary. The mercury drop is thus formed rapidly; during most of its lifetime, unlike with the DME, the drop volume is constant. A mechanical knocker discards the drop at the desired intervals. In the technique of DC Polarography (DCP), the current is monitored as a function of potential at the mercury drop electrode. Drop formation and growth in both the DME and the SMDE during the DCP scan result in large current fluctuations, as shown in figure 5. One way to minimize these fluctuations is to only measure the current once during the lifetime of any single drop; this technique is called Page 6

7 applied voltage about 20 mv/sec applied voltage measure current at indicated times time (a) linear sweep: DCP time (b) linear sweep: sampled-dcp (width of pulses is not to scale) (width of pulses is not to scale) applied voltage time time (c) pulsed: NPP (d) sweep + pulses: DPP Figure 6: variation of applied voltage during scans (excitation profile) for selected polarographic methods. The arrows indicate current sampling. (a) in direct current polarography, the working electrode voltage is sweeped linearly while the current is monitored; (b) in sampled direct current polarogoraphy, the current is sampled at the end of the drop lifetime; (c) in normal pulse polarography, the voltage is pulsed (50 ms) at the end of the drop life and the current is sampled at the end of the pulse; (d) in differential pulse polarography, the voltage is swept linearly, and a pulse is also applied at the end of the drop life. The current is sampled before and after the pulse, and the difference is ploted in the polarogram. sampled DC Polarography (or Tast Polarography). If the current is measured at the same instance during the lifetime of the drop, the surface area will be the same for each measurement; thus, the fluctuations associated with drop growth disappear, and the familiar voltammetric wave is apparent. Page 7

8 Figure 5. Polarograms for DC polarography (left) and sampled DC polarography (right) using a dropping mercury electrode. For the polarogram on the right, the current is sampled at the end of the drop lifetime; this procedure eliminates the current fluctuations associated with changing drop size. Source: Skoog. DCP and sampled-dcp can be considered linear sweep voltammetric methods, because the applied voltage is changed linearly with time. By contrast, in the pulsed methods, the applied voltage is pulsed during the course of a scan. The two most common pulse polarography methods are normal pulse polarogoraphy (NPP) and differential pulse polarography (DPP). Figure 6 shows how the working electrode potential changes during time for the four methods. The pulsed polarography methods are more sensitive than the sweep methods. The output of differential pulse polarography appears as peaks rather than waves, so that it is easier to analyze mixtures of analytes whose half-wave potentials may be similar, as shown in figure 7. The most sensitive technique in voltammetry is stripping voltammetry. This method consists of two distinct steps: deposition. A voltage is applied so that the analyte reacts and is subsequently absorbed into (or onto) the working electrode. For example, in the analysis of lead using a mercury drop electrode, Pb 2+ cations are reduced and dissolve into the mercury drop. The deposition step may last several minutes. stripping. Deposition is followed by the stripping scan. In the analysis of lead, E WE will be scanned from negative to more positive values, and the Pb dissolved in the mercury drop will be oxidized during the scan. During the scan, the analyte reacts and disperses back into the solution it is stripped from the electrode. The voltage may be pulsed during this step, just as in normal voltammetric methods. Stripping voltammetry is usually used for the analysis of trace (ppb) concentrations of metal cations. During the deposition, the metal is reduced into (or onto) the electrode; during the subsequent scan, the metal is oxidized. This technique is commonly called anodic stripping voltammetry (ASV), since the working electrode acts as an anode during the actual scan. Page 8

9 Why is ASV so much more sensitive than the other polarographic methods? The reason is this: the deposition step results in a much higher concentration of analyte near the surface of the working electrode. Thus, the during the scan the rate of diffusion of analyte to the electrode surface is much greater than that observed in the other methods, giving a larger limiting current. Figure 7. Analysis of a mixture of metal cations using normal pulse polarography (top scan) and differential pulse polarography (bottom scan). Multicomponent analysis is much easier using the bottom scan. Source: Vogel (?). Page 9

10 POLAROGRAPHIC DETERMINATION OF LEAD IN SEAWATER: PROCEDURE In this experiment, you will determine the lead concentration in a sample of artificial seawater using both calibration curve and standard addition methods. The sample matrix contains the ions commonly found in seawater, at their typical concentration levels. Although seawater is a fairly complicated matrix, none of the major ions interfere with the polarographic determination of lead. In addition, the high ionic strength of the sample is actually an advantage, since it is unnecessary to add additional supporting electrolyte. The polarographic instrument you will be using can be somewhat temperamental. It operates best after it has been used for some time. Therefore, we will warm up the instrument while demonstrating the different polarographic methods described in the BACKGROUND section. Once the measurements are reproducible, you will analyze your artificial seawater sample. Instrument Settings Purge Time: 4 min (initially) Equil. Time: 15 sec Filter: off Initial E: V Deposition: 30 sec Drop Enable: ON Purge Time: 4 min Potentiostat Settings Replicates: 1 Pulse Height: 50 mv Offset: off Final E: 1.20 V SMDE Settings Drop Size: small Have your instructor double-check your settings. Mode: SMDE Chart Recorder Settings X Scale: 50 mv/cm Y Scale: 500 mv/cm Demonstration: Mixture Analysis by Polarography Cycles: 1 Drop Time: 0.5 sec Current Range: 10 µα Scan Rate: 20 mv/sec You instructor will demonstrate how to set up the electrochemical cell. Pipet 5.00 ml of the acetate buffer solution into the cell and add 100 µl of the four-component mixture containing Zn, Cu, Pb, and Cd. You initial scan should be preceded by a 4 min purge cycle to eliminate dissolved oxygen; every scan thereafter needs only 30 sec of purging. The working electrode voltage is to be scanned from +0.15V to 1.20V at 20 mv/s. Collect a DCP scan, and you will notice the great current fluctuations. Offset the recorder pen in the Y direction and collect a sampled DCP scan. Without moving the pen, collect a NPP and then a DPP scan in succession. Which of these would you prefer to use for mixture analysis? In all of these scans, you should see four peaks, one for each component. The standard reduction potentials for the cation solutes are as follows: Page 10

11 Procedure Cd e! Cd Cu e! Cu Pb e! Pb Zn e! Zn E = 0.403V E = V E = 0.126V E = 0.763V Based on these values, which peak corresponds to which cation? Are the half-wave potentials about what you expect? [Why not?] To verify the identity of the lead peak, add a 25 µl spike of 1000 ppm lead standard and run another DPP scan; the lead peak height will increase. Turn the chart paper over and set the Current Range to 50 µa. Collect another DPP scan; the peaks will be smaller than previously due to the change in scale. Now switch the initial and final potentials, so that we will be scanning from 1.20V to +0.25V, and change the scan type to Stripping DPP. Press Start to perform anodic stripping voltammetry following a 30-sec deposition. You should notice a great increase in sensitivity. Do you notice anything else about the scan? Note that longer deposition times are more common; stripping voltammetry is typically times more sensitive than conventional voltammetry. Analysis of Lead in Seawater Ask your instructor to show you how to clean the electrochemical cell for this next part. Then pipet 5.00 ml of your sample into the cell. Change the Current Range back to 10 µa and set the initial and final voltages to 0.10V and 0.80V. Get a fresh sheet of chart paper and run a DPP scan of your sample; you should see a peak corresponding to the lead cation. Spike the solution with 25 µl of 1000ppm lead standard and repeat the scan; you do not need to move the recorder pen between scan. Repeat this procedure for two more standard additions. Now empty out the electrochemical cell, move the recorder pen to the right and repeat the entire procedure (sample analysis + 3 standard additions) two more times. You should have three sets of standard addition measurements, where each set of standard addition measurements contains four DPP peaks. Finally, collect a calibration curve for a comparison between the calibration curve and standard addition methods. Empty the cell, add 5 ml of acetate buffer and 25 µl of 1000 ppm Pb standard. Run a DPP scan. Add another 25 µl and run another scan; repeat this procedure two more times. Your calibration curve data should consist of a single set of four DPP peaks. Page 11

12 POLAROGRAPHIC DETERMINATION OF LEAD IN SEAWATER: DATA SHEET Name: unknown #: standard 1 standard 2 standard 3 standard 4 sample Calibration Curve Method 95% CI: [Pb] in sample solution Results sample addition 1 addition 2 addition 3 Standard Addition Method Trial 1 Trial 2 Trial 3 Results 95% CI: [Pb] in sample solution

13 POLAROGRAPHIC DETERMINATION OF LEAD IN SEAWATER: DATA TREATMENT You should have three sets of standard addition data and a calibration curve. In order to obtain an estimate using the calibration curve, you should average the peak heights from the three measurements of your sample and treat the average as if it were a single measurement. Your data should be pretty linear, and you can use the usual expressions to obtain a point estimate and the standard error of that estimate with the calibration curve method: x u = y u b 0 b 1 s(x u )= s res b n + (x u x) 2 S xx The equation for the standard error relies on an assumption of homogeneous variance. However, one of the sources of error in your measurements will be pipeting error and, in this experiment, this will lead to a cumulative error in the solution volume. So in this instance, the confidence interval obtained from the calibration curve is questionable, because the measurement errors will not be independent of one another (they won t be homogeneous, either). Your standard addition data will also have this problem. However, since you collected three sets of standard addition data, you can obtain three independent estimates of the analyte concentration using the standard additions method. You should use these three estimates to calculate a confidence interval for the true lead content of the seawater. This confidence interval should be accurate, since it is not affected by the cumulative or nonhomogeneous measurement errors. After the above data analysis, you should have two estimates of the lead concentration in the artificial seawater sample. The purpose of this experiment was to determine whether DPP was a suitable method for the analysis of lead in seawater. Seawater is a complicated matrix, which can potentially lead to severe additive and multiplicative effects. Do you see evidence of these effects? Ask yourself the following questions: what would I expect to see in the DPP scans if additive interferences were present in the sample matrix? how would my results compare (calibration curve and standard addition) if multiplicative interferences were present in the sample matrix? Based on your results, you should assess the suitability of DPP for trace lead analysis in seawater. Page 13