DRL10008-Anodic-Stripping-Analysis

Home , Bulk electrolysis, Linear sweep voltammetry, Reference electrode, Voltammetry

Document #: DRL10008 (REV005 | NOV 2019)

Highly Sensitive Electrochemical Determination of Lead in Tap Water Anodic Stripping Voltammetry with Disposable Screen Printed Carbon Electrodes

This experiment demonstrates the use of anodic stripping voltammetry to determine lead (Pb) in tap water. Inexpensive and disposable screen-printed carbon electrodes are first coated with mercury micro-droplets (microns in diameter) by reduction of mercury acetate (Hg2+) at a constant electrode potential. Then, lead ions (Pb2+) in a sample are reduced and collected into the mercury droplets. This pre-concentration step allows determination of very low concentrations (ppb) of lead when anodic differential pulse voltammetry (DPV) is used as the method of detection and standard addition as the method of calibration. 1. Introduction 2. Background

According to the United States Environmental Several operational steps are involved in typical ASV Protection Agency (EPA), lead, a common element analysis (see: Figure 1). These steps include associated with many health concerns, is a significant pretreatment, preconcentration, and oxidative pollutant in the environment.1 Trace levels (ppm) of stripping. Use of small and easy to operate lead can have serious health effects in many animals instrumentation is well suited to the teaching lab, and in humans; therefore, a highly sensitive detection however this type of analysis could be performed method is important for environmental monitoring directly in the environment. and clinical testing. In this lab, lead in public drinking First, a water sample is pre-treated to convert any water drinking will be determined using an solid lead into its ionic form (푃푏2+). Chelating ligands, electrochemical technique called anodic stripping such as acetate, are often added to facilitate voltammetry (ASV). extraction, dissolution, and stabilization of lead. ASV is well suited for determining trace lead in liquids. Other salts may also be added as ions of supporting The advantages of ASV include sensitivity (ppb), electrolyte, or as buffer components for pH control. speed (tens of minutes), and low cost of Second, 푃푏2+ is converted to 푃푏 and deposited at a measurement equipment, i.e., the potentiostat. In mercury (퐻푔) electrode under a reduction potential addition, the portability of some potentiostats makes as shown in Equation 1. it possible to do field work in remote places where access to standard AC power is prohibited. 2+ 푃푏 + 2푒 ⇌ 푃푏(푠) ( 1 )

Figure 2-1. Bare Carbon Screen Printed Electrode Figure 1-1. Anodic Stripping Voltammogram (ASV) and Mercury Plated Screen Printed Electrode of lead

Document #: DRL10008 (REV005 | NOV 2019)

There are two types of 퐻푔 electrodes commonly used to recover. Consequently, a larger peak current is in ASV analysis: hanging mercury drop (HMD) type detected in DPV when compared to LSV. and mercury film (MF) type. An HMD electrode is formed at the end of a capillary tube that is filled with 퐻푔 and the other end of the tube is connected to an 퐻푔 reservoir. HMD electrodes can be regenerated by discarding an old drop and forming a new one. This desirable characteristic makes it easy to obtain clean, reproducible Hg surface. In comparison, the surface of an MF type electrode cannot be regenerated. However, such an electrode is simple in construction, and consists of many microscopic 퐻푔 droplets (휇푚 in diameter) adhering on a carbon substrate surface. The high surface area to volume ratio of the Hg droplets enhances mass transfer rate. Thus, during the same enrichment time, an MF electrode can electrolyze more 푃푏 into the Hg than at an HMD electrode. Finally, the 푃푏 enriched in 퐻푔 is oxidized and converted back to 푃푏2+ as the electrode potential is swept toward a positive limit (see: Equation 2).

2+ 푃푏(푠) ⇌ 푃푏 + 2푒 ( 2 )

In a very short time (as compared to the time of enrichment), all 푃푏 that was preconcentrated into the Hg droplets is oxidized to 푃푏2+ and released back into the electrolyte solution. The name anodic Figure 3-1. Differential Pulse Waveform (top) and stripping refers to this step, where 푃푏 is stripped out of Current Sampling (bottom) 퐻푔 via electrochemical oxidation.

During anodic stripping, current is recorded while the electrode potential is swept linearly. If the linear sweep is a pure monotonic ramp, then the technique is called linear sweep voltammetry (LSV). Sometimes, a pulse train is superimposed onto the linear sweep to enhance the sensitivity. In this lab, we will use such a pulse technique named differential pulse voltammetry (DPV). 3. Differential Pulse Voltammetry

Differential Pulse Voltammetry measures the difference in current before and after a potential pulse (see: Figure 3-1). Compared to LSV, this technique avoids measurement of the current due to double layer charging (capacitive current), which contains little analytical information of interest for stripping analysis. In addition, the time delay (at constant potential) before each pulse allows the depletion zone just adjacent to the electrode surface

Document #: DRL10008 (REV005 | NOV 2019)

4. Solution Preparation In a 100 푚퐿 volumetric flask, add 20 푚퐿 of 1 푀 퐻퐶푙 Solution to approximately 50 푚퐿 DI water. Dissolve 0.13 푔 퐻푔(퐶퐻 퐶푂푂) into the solution. Dilute to the INFO: 3 2 mark with DI water. Label this solution as ퟒ 풎푴 Due to the sensitivity of ASV, 푯품(푪푯ퟑ푪푶푶)ퟐ in ퟎ. ퟐ 푴 푯푪풍 solution. all glassware should be soaked in ퟑ푴 푯푵푶ퟑ for at 4.4 Mercury Acetate (ퟎ. ퟒ 풎푴) in least 24 hours. ퟐퟎ 풎푴 Hydrochloric Acid Solution

4.1 Nitric Acid Solution (ퟑ. ퟎ 푴) Prepare 100 푚퐿 of 0.4 푚푀 퐻푔(퐶퐻3퐶푂푂)2 in 20 푀 퐻퐶푙 acid solution by dilution of the Prepare 1 퐿 of 3.0 푀 nitric acid solution. For this 4 푚푀 퐻푔(퐶퐻3퐶푂푂)2 in 0.2 푀 퐻퐶푙 Solution. solution, you will need concentrated nitric acid In a 100 푚퐿 volumetric flask, add 10 푚퐿 of the (퐻푁푂3, 68.9%). 4 푚푀 퐻푔(퐶퐻3퐶푂푂)2 in 0.2 푀 퐻퐶푙 Solution and dilute to the mark with DI water. Label this solution as ퟎ. ퟒ 풎푴 CAUTION: 푯품(푪푯ퟑ푪푶푶)ퟐ in ퟎ. ퟐ 풎푴 푯푪풍 solution. Nitric acid is highly corrosive. Avoid contact with skin. If 4.5 Sodium Acetate (푵풂푶푨풄) Buffer any contacts the skin, rinse (ퟏ. ퟎ 푴) profusely with copious amounts of water. Prepare 100 푚퐿 of 1.0 푀 Sodium Acetate Buffer. (푁푎(퐶퐻 퐶푂푂), 푀푊 = 82.03 푔/푚표푙). Sodium acetate is Into a 1 퐿 volumetric flask and under constant stirring, 3 abbreviated 푁푎푂퐴푐. slowly add 190 푚퐿 concentrated 퐻푁푂3 to approximately 700 푚퐿 water. Dilute to the mark with In a 100 푚퐿 volumetric flask, dissolve 8.20 푔 푁푎푂퐴푐 in distilled water. Label the solution as 3 푀 퐻푁푂3 50 푚퐿 water. Add 4.2 푚퐿 of concentrated 퐻퐶푙 and Solution. The nitric acid solution is for cleaning all dilute to the mark with DI water. Label this solution as glassware. ퟏ. ퟎ 푴 푵풂푶푨풄 buffer. 4.2 Hydrochloric Acid Solution (ퟏ. ퟎ 푴) 4.6 Sodium Acetate (푵풂푶푨풄) Buffer (ퟐퟎ 풎푴) Prepare 100 푚퐿 of 1.0 푀 hydrochloric acid solution. For this solution, you will need high purity, Prepare 1 퐿 of 20 푚푀 Sodium Acetate Buffer. Prepare concentrated hydrochloric acid (퐻퐶푙, 37%). this solution by dilution of the 1.0 푀 푁푎푂퐴푐 buffer previously prepared. CAUTION: In a 1 퐿 volumetric flask, add 20 푚퐿 of the 1.0 푀 푁푎푂퐴푐 Hydrochloric acid is highly buffer to approximately 700 푚퐿 DI water and dilute to corrosive. Avoid contact with the mark. Label this solution as ퟐퟎ 풎푴 푵풂푶푨풄 buffer. skin. If any contacts the skin, rinse profusely with copious 4.7 1000 ppm Lead Acetate amounts of water.

(ퟏퟎퟎퟎ 풑풑풎) in ퟐퟎ 풎푴 푵풂푶푨풄 Buffer Add 8.3 푚퐿 of concentrated 퐻퐶푙 to approximately 70 푚퐿. Mix gently and dilute to the mark with distilled Prepare 100 푚퐿 of 1000 푝푝푚 (4.83 푚푀) lead acetate water. Label this solution as 1 푀 퐻퐶푙 Solution. (푃푏(퐶퐻3퐶푂푂)2, 푀푊 = 325.29 푔/푚표푙) in 20 푚푀 푁푎푂퐴푐 buffer. 4.3 Mercury Acetate (ퟒ 풎푴) in In a 100 푚퐿 volumetric flask, dissolve 0.18 푔 of ퟎ. ퟐ 푴 Hydrochloric Acid Solution 푃푏(퐶퐻3퐶푂푂)2 in 70 푚퐿 of 20 푚푀 푁푎푂퐴푐 buffer, then, dilute to the mark with 20 푚푀 푁푎푂퐴푐 buffer. Label this Prepare 100 푚퐿 of 4.0 푚푀 mercury acetate solution as ퟏퟎퟎퟎ 풑풑풎 푷풃ퟐ+ in ퟐퟎ 풎푴 푵풂푶푨풄 buffer. (퐻푔(퐶퐻 퐶푂푂) , 푀푊 = 318.7 푔/푚표푙) in 0.2 푀 퐻퐶푙 acid 3 2 solution.

Document #: DRL10008 (REV005 | NOV 2019)

4.8 Lead Acetate (ퟏ 풑풑풎) in oxidized from the carbon surface, it will remain stable and intact during DPV stripping. ퟐퟎ 풎푴 푵풂푶푨풄 Buffer

Prepare 100 푚퐿 of 1 푝푝푚 (4.83 휇푀) 푃푏(퐶퐻3퐶푂푂)2 in 20 푚푀 푁푎푂퐴푐 buffer by dilution of the 1000 푝푝푚 푃푏(퐶퐻3퐶푂푂)2 푖푛 푁푎푂퐴푐 buffer solution previously prepared. In a 100 푚퐿 volumetric flask, add 100 휇퐿 of the 1000 푝푝푚 푃푏2+ 푖푛 푁푎푂퐴푐 buffer to ~70 푚퐿 of the 20 푚푀 푁푎푂퐴푐 buffer. Mix gently then dilute to the mark with 20 푚푀 푁푎푂퐴푐 buffer. Label this solution ퟏ 풑풑풎 푷풃ퟐ+ in ퟐퟎ 풎푴 푵풂푶푨풄 buffer.

5. Setup and Preparation Figure 5-1. 5x4 mm Carbon Screen Printed Electrode A. Physical Setup Connect a 5 × 4 푚푚 carbon screen printed electrode to the compact voltammetry hand grip as shown in For this lab, obtain a stirrer, magnetic stir bar, Pine Figure 5-2. You should have one or two spacers WaveNow USB potentiostat, compact voltammetry inserted into the grip, behind the screen printed kit, large volume compact voltammetry cell electrode, otherwise the electrode will not stay in (AKSPEJAR), and 5푥4 푚푚 carbon screen printed place. The grip and electrode fit into the tan cap, electrodes. which fits into the large volume compact voltammetry electrochemical cell. There are two main components of the electrochemical measurement system used in this experiment: the potentiostat and the electrodes. Often, three electrodes are used with a potentiostat and are called the working, counter, and reference electrodes. The electrochemical reaction of interest occurs at the working electrode. The working electrode is an inert material that is a sink for electron transfer. The reference electrode has its own standard and reversible electrochemical potential and serves as a Figure 5-2. Connection between Screen Printed standard reference for cell potential. Current should Electrode and Hand Grip never flow through a reference electrode, thus the counter electrode (or auxiliary electrode) is the third The tan cell cap has two small holes for purge lines. electrode in the system. Charge flows (as current) Insert one length of the purge tubing into solution, through the counter electrode to balance a change touching the bottom of the cell (called the purge in cell potential, due to the reaction at the working line). Insert another length of the purge tubing into electrode. For this experiment, the screen printed the cell, but above the solution level (called the electrode contains all three electrodes (see: Figure blanket line). 4). Add 15 푚퐿 of the 0.4 푚푀 퐻푔(퐶퐻3퐶푂푂)2 in 0.2 푚푀 퐻퐶푙 solution into the electrochemical cell. Add a 5.1 Mercury Deposition magnetic stir bar to the cell. Set the constant stir rate appropriate for the cell dimensions, around 500 푅푃푀. The first experimental step in this laboratory is to The stirring should be significant but ensure that deposit a layer of mercury micro droplets onto the solution stays in contact with the screen printed surface of a carbon working electrode. 퐻푔 is electrode elements at all times while stirring. Place reduced to the working electrode by application of the cell on a stirrer and connect to the WaveNow a reductive (negative) potential. 퐻푔 deposition to potentiostat as shown (see: Figure 5-4). the carbon electrode need only be done one time for this laboratory. As long as the 퐻푔 layer is not

Document #: DRL10008 (REV005 | NOV 2019)

Figure 5-3. Cell Setup for Mercury Deposition onto Figure 5-4. All Components for this Laboratory, Carbon Electrode. including Potentiostat and Stirrer 퐻푔 microdroplets are formed on the screen printed electrode carbon surface by reducing 퐻푔2+ to 퐻푔 at −1 푉 푣푠. 퐴푔/퐴푔퐶푙 for 15 푚푖푛 in 0.4 푚푀 퐻푔(퐶퐻3퐶푂푂)2 in 0.2 푚푀 퐻퐶푙 solution. Within AfterMath, start a new Bulk Electrolysis (BE) Experiment (see: Figure 5-5). Ensure the solution is stirring, then click “Perform” in AfterMath to start the deposition program. The total time for this part of the experiment can be determined from all steps of the bulk electrolysis and is 17 minutes. When the experiment has finished, rinse the electrode thoroughly with DI water.

Store the electrode under 푁2 sparged water to avoid oxidation of the 퐻푔 microdroplets on the electrode surface.

Figure 5-5. Mercury Deposition Parameters

Document #: DRL10008 (REV005 | NOV 2019)

5.2 General DPSV Analysis Protocol

The following instructions are a general protocol to perform trace metals determination by DPSV. You should have already completed the previous step to reduce mercury to the surface of carbon screen printed electrode. The method used in this experiment is standard addition.

Add a stir bar and 50 푚퐿 of test solution to the large volume compact voltammetry electrochemical cell. Insert the screen printed electrode (plated with퐻푔) into the hand grip (see: Figure 5-2) and assemble the hand grip and cap into the cell.

Degas the solution by sparging with 푁2 for at least 10 푚푖푛. Turn the three way valve to flow 푁2 through the purge line. Continuously bubble 푁2 through solution while constantly stirring at a fixed speed (about 500 푟푝푚). Do not change the stir bar rate for future steps in the experiment. An easy way to accomplish this is to keep the stirring Figure 5-6. DPSV Parameters rate knob fixed in position and simply turn the After the experiment has completed, rename or stirrer off when not in use. reorganize the DPSV experiment accordingly in After the solution has been purged for 10 푚푖푛, the AfterMath to indicate the contents of the sample user will complete an anodic stripping (e.g. 0 푝푝푚 푃푏2+, pure water). voltammetry with DPV experiment (DPSV) (see: After measuring the initial sample solution, add Figure 5-6 for suggested DPSV parameters). an aliquot of 100 휇퐿 1 푝푝푚 푃푏2+ (equivalent to The first step in DPSV is to concentrate lead ion 2 푝푝푏 increment in 푃푏2+ concentration; see: Table into the mercury film (deposition period in 1) to the electrochemical cell and repeat steps 4 AfterMath, also called preconcentration). During and 5. You do not need to repeat the initial this step, maintain both stirring and 푁2 bubbling. purging of solution for 10 minutes, as doing so is Monitor the experimental progress in AfterMath. only necessary for the initial 50 푚퐿 of solution. After the deposition period (300 푠 in Figure 5-6), Follow the analysis procedure for all solutions the DPV stripping waveform will start listed in Table 1. Data sufficient for calibration immediately. Turn off the stirrer and adjust the curve construction will be complete when the three way purge valve so 푵 enters the cell ퟐ total added 푃푏2+ concentration is 10 푝푝푏. through the blanket line. The gas from the blanket line should fill the cell headspace, not agitate solution. INFO: Be sure to use AfterMath to its INFO: fullest capabilities. This includes carefully naming When you discontinue entries, using folders to stirring, turn the power off on organize data, and adding the stirrer. DO NOT adjust the notes as appropriate. RPM knob.

Document #: DRL10008 (REV005 | NOV 2019)

5.3 Standard Addition Method INFO: Standard addition is used to minimize any matrix Always analyze solutions with effects. Matrix effects are unwanted side reactions the lower concentration of that can occur simultaneously with the intended analyte FIRST. Then, add reaction and affect the analytical signal of interest. aliquots of standard so The species in the matrix can be those that are known concentration of analyte is as well as unknown. With standard addition, a series increasing with sample of samples are tested with a constant volume of number. known standard. One sample in a series does not contain the standard (or spike). The analytical signal, ퟐ+ DPV peak current, is plotted against concentration. Sample 1 ppm 푷풃 in Known Standard addition is valid if the agreement between # Buffer Standard (흁푳) [푷풃ퟐ+] (풑풑풃) signal and concentration is linear. 1 0 0 After a linear regression has been completed across the series of standards and unknown, the line is 2 100 2 extrapolated to when current = 0, where the linear regression intersects the 푥 −axis. The value at this 3 100 4 intersection must them be the concentration of the 4 100 6 unknown (sign is irrelevant). In this part of the lab, the user will perform DPSV as before, but with a water 5 100 8 sample of unknown 푃푏2+ concentration. The goal will 6 100 10 be to determine the unknown concentration of 푃푏2+ in the water sample. Table 1. List of Solution Preparation and Composition 6. Experimental 7. Unknown Analysis A. Measurement of Lead in Pure Water Here, the user will analyze six unknown water samples (see: Table 1). Water samples will contain some by Standard Addition unknown concentration of 푃푏2+.

This laboratory encourages the user to first perform an For this experiment, the user should place ퟏ 풎푳 of analysis of pure water with standard additions of 푃푏2+. ퟏ 푴 푵풂푶푨풄 buffer and ퟒퟗ 풎푳 of water analyte sample This aids in familiarization of the instrument, utilization into the large volume compact voltammetry cell of the software, and verification that a linear (step 1 in the General DPSV Protocol). Use of the response can be obtained. more concentration Buffer solution will maintain appropriate pH conditions but allow the bulk of the For this experiment, the user should place test solution to be the unknown water sample. ퟓퟎ 풎푳 풐풇 ퟐퟎ 풎푴 푵풂푶푨풄 buffer into the large volume compact voltammetry cell (step 1 in the General Solutions should be analyzed starting with the lowest 2+ DPSV Protocol). Since an unknown water sample is concentration of 푃푏 . Subsequent samples should 2+ not used, using entirely buffer is appropriate. be analyzed in order of increasing 푃푏 concentration. The first sample does not contain a A total of six solutions will be studied by DPSV (see: spike of 푃푏2+solution. Table 1). The first analysis should be completed with sample #1, where 푃푏2+ is absent. All 푃푏2+ additions INFO: will be to the same initial volume of buffer. Always analyze solutions with the lower concentration of analyte FIRST. Then, add aliquots of standard so concentration of analyte is increasing with sample number.

Document #: DRL10008 (REV005 | NOV 2019)

8. Data Analysis baseline is best drawn through the zero current leading and trailing baseline. After completion of a DPSV experiment by standard The peak current will be displayed with a line addition, the user should have at least six differential drawn to the green dot. pulse voltammograms. The general analysis includes: Record the peak height and known 푃푏2+ Find the DPSV peak height (current) in AfterMath concentration in a spreadsheet program. for each sample where lead was present (five samples for pure water, all six samples for the unknown). Record the peak current for each sample in a spreadsheet program Prepare an overlay plot. In an overlay plot, all six DPSV voltammograms should be place into a single plot (see: Figure 8-2). Perform a standard addition linear calibration analysis. Plot peak current vs. known concentration of 푃푏2+ as points. Perform regression analysis on the data and draw a linear curve of best fit through the data points and record the linear equation of the line. A Figure 8-1. Peak Height Determination in AfterMath. correlational coefficient > 0.98 is expected. Extrapolate the best fit line to the 푥 −axis, where 8.2 Prepare an Overlay Plot 푦 = 0. From the slope of the regression, determine the An overlay plot can be made very easily in AfterMath 푃푏2+ concentration. This will correspond to the (see examples of overlays in Figure 8-2). It is as easy point on the extrapolated regression curve that as copying/pasting. The directions are: crosses the 푥 −axis. This is the concentration of Right click a node in the archive tree structure 푃푏2+ in your unknown sample. and select Add, then select Plot. 8.1 Determine DPSV Peak Height For each standard addition, including the first where no standard was added, click the “+” sign Determination of a peak height is very easy in next to the DPSV Experiment icon. Click the Aftermath. The user can set the appropriate baseline, “Voltammogram” icon. from which peak height is determined (see: Figure Click the data trace (the voltammogram) and 8-1). The directions for peak height determination in copy it (either press Ctrl-C, or select Copy from AfterMath are: the edit menu, or right click the data and select In AfterMath, right click the data trace that copy). contains a peak of interest. Click the New plot icon you created in step 1, Select Add tool, then select peak height. Three then click the blank plot in the plot window. colors or orbs will appear on the plot. The green Paste the data (either press Ctrl-V, or select Paste orb should be moved onto the peak apex, the from the edit menu, or right click the data and highest point in the peak. You can use the zoom select paste). (+ magnifying glass) in Aftermath to fine tune your Repeat steps 2 – 4 for all DPSV obtained in an selection. The pink orbs are the baseline point(s) experiment. Adjust axes as appropriate. and the black orbs extend the length of your baseline. INFO: Right click the pink orb and select properties. Detailed instructions follow From the menu, choose “free horizontal” as the regarding how to construct method by which the baseline is drawn. Click OK. plots and perform analysis. Drag and drop the pink orb (notice, a horizontal line is drawn through the pink orb) until the

Document #: DRL10008 (REV005 | NOV 2019)

Figure 8-3. Standard Addition Plot for [푷풃ퟐ+] in Tap Figure 8-2. Differential Pulse Stripping Water by Differential Pulse Stripping Voltammetry Voltammograms for Tap Water with Standard Addition of 푷풃ퟐ+ 8.4 Determine Unknown 푷풃ퟐ+ Concentration 8.3 Construct Standard Addition Plot After the standard addition plot has been In a graphing program of your choice (Microsoft® constructed, simply determine 푥 −intercept of the Excel, for example), create a scatter plot for your regression line. This can easily be done from the data obtained for each experiment (see: Figure 8-3). equation of the line found during regression analysis. The general directions are as follows: The overlay example shows data for public sewer Plot peak current (from step A) on the 푦 −axis and water obtained in Pennsylvania (see: Figure 8-2). The 2+ known concentration of 푃푏 on the 푥 −axis. standard addition plot (see: Figure 8-3) was based Display the data as points, no connecting line. upon the voltammograms shown in the overlay example (see: Figure 8-2). The linear equation for the Perform regression analysis on the data points, i.e. regression was add a trend line and view the resulting equation and correlational coefficient. Record the 푦 = 2.437푥 + 12.91, 푅2 = 0.997 regression (linear) equation. The correlational coefficient should be > 0.98. It was determined that the unknown concentration of lead in the public water sampled was 5.30 푝푝푏. The For DPSV in pure water, the intercept will be close action limit for lead in water in Pennsylvania is 15 푝푝푏. to (0, 0) since one would not expect current when no 푃푏2+ was present to strip. For DPSV of unknown samples, the intercept will not be zero. Extrapolate the regression line to the 푥 −axis (when 푦 = 0 for the linear equation).

Document #: DRL10008 (REV005 | NOV 2019)

9. Questions 10. References

What is the unknown concentration of lead in the (1) Bard, A. J.; Faulkner, L. R.; Leddy, J.; Zoski, C. water analyzed? If drinking water was analyzed, G. Electrochemical Methods: Fundamentals how does the concentration compare to the and Applications; 2nd ed.; Wiley: New York, legal limit in public drinking water? 1980.

What is the purpose of purging and blanketing (2) Wang, J.; Tian, B. Screen-Printed Stripping with nitrogen gas? What might you expect would Voltammetric/potentiometric Electrodes for happen if this step was not performed? Decentralized Testing of Trace Lead. Anal. Why is the stir bar used in the preconcentration Chem. 1992, 64, 1706–1709. step of a differential pulse anodic stripping (3) Florence, T. M. Anodic Stripping Voltammetry voltammetry experiment? What do expect with a Glassy Carbon Electrode Mercury- would be the results if the solution was not stirred Plated in Situ. J. Electroanal. Chem. Interfacial during this step? Electrochem. 1970, 27, 273–281. Write the electrochemical half reaction of interest that occurs during the mercury plating 11. Reprints process, the preconditioning step, and the stripping step of DPSV. Reprints of this document are available upon request from:2,3 Stripping voltammetry can detect several metals at the same time, as long as their thermodynamic Pine Research Instrumentation electrode potentials are separated by about 100 2741 Campus Walk Avenue mV. Draw the stripping voltammogram you Building 100 would expect for a mixture of 퐶푑2+, 푃푏2+, and Durham, NC 27705 퐶푢2+ (HINT: look up the appropriate half reaction USA in a textbook). http://www.pineresearch.com

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12. Instructor Resources

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access to the instructor’s guide for this laboratory exercise