ANODIC STRIPPING VOLTAMMETRY

September 1997

The object of this experiment is to gain an understanding of anodic stripping voltammetry through the determinations of trace amounts of copper in a macro-sized biological sample

and the amount of copper in micro-sized samples of copper based alloys and to gain an understanding of factors involved in performing quantitative analytical determinations.

REFERENCES

Adrian W. Bott, “Stripping Voltammetry”, Current Separations 1992, 12:3, 141-147. (attached)

Scott R. Peters and Jonathon O. Howell, “A Controlled Growth Mercury Electrode”, Current Separations 1994, 13:1, 12-16. (attached)

Morris Bader, “A Systematic Approach to Standard Addition Methods in Instrumental Analysis”, J. Chem. Ed. 1980, 57, 703. (attached)

General Chemistry Lab Manual, Pomona College, Claremont, California.

S. B. Adeloju, A. M. Bond, and M. H. Briggs, “Multielement Determination in Biological Materials by Differential Pulse Voltammetry”, Anal. Chem. 1985, 57, 1386-1390. (available in the laboratory to check out)

Giles F. Carter and Hossein Rasi, “Chemical Composition of Copper-Based Coins of the Roman Republic”, R. O. Allen, Editor, Advances in Chemistry (Archaeological Chemistry-IV) 220, American Chemical Society: Washington, D.C., 1989, Chap. 11. (available in the laboratory to check out)

BAS CV-50W Version 2 MF-9093 Instruction Manual for the Voltammetric Analyzer, July 1995. (available for use in the laboratory only)

BAS CGME MF-9058 Instruction Manual for the Controlled Growth Mercury Electrode, May 1993. (available for use in the laboratory only)

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INTRODUCTIONIn this experiment, two copper-base alloy samples (a standard alloy sample and a bronze

Roman coin) and a biological sample are analyzed for the amount of copper in the sample by anodic stripping voltammetry (ASV). The ASV is done with a controlled growth mercury electrode (CGME) and a BAS CV-50W Voltammetric Analyzer using the method of standard addition. As part of the experiment, an Eppendorf pipet will be calibrated.

Stripping VoltammetryStripping voltammetry is a sensitive electroanalytical technique for the determination of

trace amounts of metals in solution. The technique consists of three steps. First, metal ions are deposited onto an electrode which is held at a suitable potential. The solution is stirred during this step to maximize the amount of metal deposited. Second, stirring is stopped so that the solution will become quiet. Third, the metal deposits are stripped from the electrode by scanning the potential. The observed current during the stripping step can be related to the amount of the metal in the solution.

The stripping step may consist of a positive or a negative potential scan, creating either an anodic or cathodic current respectively. Hence, Anodic Stripping Voltammetry (ASV) and Cathodic Stripping Voltammetry (CSV) are two specific stripping techniques. The attached paper by Bott discusses stripping voltammetry in detail.

In addition to varying the direction of the scan, the manner in which the potential is scanned may also differ. The simplest technique is Linear Sweep Voltammetry (LSV) where the potential is scanned linearly as a function of time. Another commonly used technique is Differential Pulse Voltammetry (DPV), which has a lower detection limit than LSV. This is due to its pulsed waveform which measures the current in pulses by taking two measurements and recording the difference as the potential is increased. This helps to reduce the background current. The waveforms from each pulse superimpose upon one another to form a staircase waveform since the pulse amplitude is constant while the potential increases in small increments. The potential excitation waveforms of these two methods are shown in the BAS CV-50W Instruction Manual.

ElectrodesMercury is often used as an electrode material because it is chemically inert in most

aqueous solutions and hydrogen is evolved only at negative potentials. Thus, reductions of many species may be studied. It is useful because amalgams are formed with the mercury by the metals produced when the cations are reduced. The metals can also diffuse freely into the electrode because it is liquid. Mercury also forms insoluble salts with cations, which is useful for CSV.

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Predominantly, two types of mercury electrodes have been used for the "working" electrode, which is the electrode where the reaction occurs and the responses are recorded. These are the Hanging Mercury Drop Electrode (HMDE), which may also be called Static Drop Mercury Electrode (SDME), and various types of carbon electrodes on which a thin layer of mercury is plated onto the carbon surface either before using the electrode for a determination or simultaneously with the plating of metal ions. The higher surface to volume ratio of the mercury film carbon electrode results in more metal to be deposited for a given amount of time than with the HMDE. Reproducible results, however, are more difficult to attain with carbon electrodes since the carbon needs to be polished and pretreated and the mercury film may not be identically formed. A reference electrode is used to monitor the potential of the working electrode. To complete the electrochemical cell an auxiliary electrode is necessary. The potentials of the working and auxiliary electrodes are adjusted with respect to the reference electrode to maintain the desired potential at the working electrode. A salt bridge is placed between the analyte and the reference electrode to avoid contamination of the analyte.

Determination of Copper in the SamplesIn this experiment, a HMDE is used with an Ag/AgCl reference electrode and a platinum

auxiliary electrode. These three electrodes are part of the Controlled Growth Mercury Electrode (CGME) unit which is used in conjunction with the BAS CV-50W Voltammetric Analyzer. Differential Pulse Stripping Voltammetry will be performed anodically to detect trace amounts of copper. The attached paper by Peters and Howell further describes the CGME.

Since this is a trace analysis technique, all glassware must be rinsed thoroughly before use with nitric acid and ultra-pure water to minimize contamination. The metal alloy samples are dissolved with acid, the biological sample is digested to destroy organic material, and a standard copper solution is made. After solutions are prepared, the sample in the cell is purged with nitrogen gas to remove oxygen. Then, the sample solution is stirred in a reproducible manner while the working electrode is held at the desired deposition potential for a given length of time. The stirring is stopped and, after a short period of time for the solution to become quiet, the potential of the working electrode is scanned in the positive direction to remove the copper from the electrode. The monitor will display the potential versus the current graph. A typical graph is shown in Figure 1.

To determine the concentration of the copper in the solution, the method of standard addition is used. In this method, as described in the article by Bader, a known amount of a standard solution is added to the sample after the first run and the experiment is rerun. The observed current for the second determination is the sum of the current due to the copper in the sample plus the current due to the added standard. If more than one standard addition is made, then a plot of current versus concentration of added standard solution can be prepared. The unknown concentration can then be found by extrapolating to the x-axis, as seen in Figure 2.

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With an unknown sample, once the concentration of the copper in the solution has been determined, the percent copper in the sample can be calculated.

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Figure 15

Figure 26

EXPERIMENTAL PROCEDURE

BE SURE TO READ ALL OF THESE INSTRUCTIONS THOROUGHLY BEFORE BEGINNING ANY EXPERIMENTAL WORK. PARTNERSHIP

INSTRUCTIONS ARE GIVEN AT THE END OF THIS SECTION.

Ultra-pure water is obtained from the Millipore Water Purification System in Room 13. Follow the instructions listed next to the system. Initially, collect two liters of water and discard this water. Then collect four liters of water for use. Record the amount of water collected in the log book.

Because of the sensitivity of the methods employed, much care is needed to ensure that contamination is avoided. All glassware that will be used must be rinsed with a solution of 1:4 concentrated nitric acid and water and then rinsed in the ultra-pure water. Volumetric pipets and flasks will be used extensively throughout this experiment. Please consult your General Chemistry Lab Manual for their proper use. One of the reasons for poor precision and accuracy in this experiment is the improper use of volumetric pipets and flasks.

Proper Techniques for Using the Eppendorf Pipet and the FinnpipetteTo obtain the most accurate dispensing values, adjust the volume of the pipet by turning

the setting ring so the numbers read a value past the desired volume and then return back to the proper position. Firmly attach a plastic tip to the pipet taking care to keep the instrument and tip as contamination free as possible. For best pipeting results, change the tip with each sample in the experiment. To fill the pipet, press the control button down to the first stop, immerse the pipet into the solution in a vertical fashion, and slowly let the control button return to its initial position. Remove the tip from the solution by sliding it along the inside of the container. If it is necessary, use a Kimwipe to clean off any droplets of solution on the outside of the pipet tip being careful not to touch the end of the tip with the tissue. When emptying the pipet, hold the tip at an angle against the inside of the container (DO NOT immerse in the solution) and slowly press the control button down to the first stop and pause. Then press the button down to the second stop to completely empty the tip. Remove the tip making sure the control button remains completely compressed. During the calibration procedure, it is possible to remove the pipet in the proper manner, by sliding it up the side of the container. However, when pipeting into the cell vial, it is important not to touch the tip to the inside of the white cover since droplets of solution might be deposited and therefore will foul up the concentrations in the vial. Once the tip is moved away from the experimental solutions, the control button can be relaxed (slowly) and the tip removed (using the lateral ejector button) and replaced if desired. If there are any remaining drops of solution inside the pipet tip after it was been emptied, a new tip must be used. In addition, if this occurs, it is necessary for the experiment in process to be restarted

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because you are dealing with such small volumes that any little fraction of error can significantly affect the experimental results. ***Never lay the pipet down when the tip contains liquid since this has the potential to contaminate and damage the pipet mechanism.

Calibration of the Eppendorf PipetMake three separate weighings of 50 microliters of ultra-pure water in a weighing vial to

simulate the standard addition technique. Carefully weigh and record each of the additions making sure to cover the weighing vial for each measurement to minimize evaporation. Determine the average mass, standard deviation, and percent relative standard deviation. Be sure to record the temperature of the water in the stock beaker since the density of water varies with temperature. Determine the calculated volume of water using density (The density of water at different temperatures can be found in your General Chemistry Laboratory Manual) and compare the value with the Eppendorf volume. The expected tolerances for Eppendorf pipets 0.5% for accuracy and 0.2% for precision. If your values do not agree reasonably well with these values, repeat your calibration. The Finnpipette does not need to be calibrated. The calibration certificate is taped to the lid of the pipet box.

Preparation of a 1.0 M Potassium Nitrate Solution and a Blank SolutionTo prepare a solution of 1.0 M potassium nitrate weigh out approximately 10.1 g of

potassium nitrate and place it in a 100 ml flask. Dilute to volume with ultra-pure water and mix well until all of the potassium nitrate dissolves. This solution can be prepared conveniently while the bovine liver sample is digesting as described below. The blank solution is prepared by pipetting 10 ml of the 1.0 M potassium nitrate solution into a 100 ml volumetric flask and diluting to volume with ultra-pure water. Be sure to mix this solution well.

Digestion and Preparation of the Biological Sample This procedure is based upon the procedure in the article by Adeloju, Bond, and Briggs.Weigh to the nearest 0.1 mg approximately 300 mg of oven dried bovine liver, which is

stored in a desiccator, and put the bovine liver into a 125 ml Erlenmeyer flask. Add 10 ml of concentrated nitric acid and 1 ml of concentrated sulfuric acid. Insert a glass funnel and heat on a hot plate under the hood. When the brown-orange nitrogen oxide fumes are just given off, cool the flask for about 2 minutes. Add 10 ml of concentrated nitric acid and heat again until brown fumes are just given off. Repeat this process twice more so that a total of 30 ml has been added.

With the final addition of nitric acid, heat until all of the nitrogen oxide fumes are given off. Rinse the funnel with ultra-pure water and remove. Cool for 2 minutes and then add 3 ml of 1:1 concentrated hydrochloric acid and water. Replace the funnel. Heat this mixture for

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approximately one hour or until the water of the solution has evaporated so that it is less than 25 ml. During this time, the metal samples may be prepared. Cool to room temperature and transfer the solution quantitatively to a 50 ml volumetric flask and carefully dilute to volume.

Pipet 10 ml of this bovine liver digestion into a 100 ml volumetric flask. Be sure to rinse the pipet with the original solution several times before adding the solution to the flask. Add 10 ml of the 1M potassium nitrate solution and then dilute to volume. This is the dilute bovine liver solution which will be used for analysis.

Preparation of the Copper Base Alloy Sample and Standard Copper Solutions.Weigh to the nearest 0.1 mg, approximately 25 mg of the known copper base alloy

sample and place the weighed sample in a 250 ml beaker.Using a file, obtain approximately 25 mg of the Roman coin sample and weigh to the

nearest 0.1 mg. Place the weighed sample in another 250 ml beaker.Cut a piece of the copper foil of approximately 25 mg and weigh to the nearest 0.1mg.

Place this weighed sample in a third 250 ml beaker.Add 4 ml of ultra-pure water, 6 ml of concentrated hydrochloric acid, and 1 ml of

concentrated nitric acid to each of the metal samples. Heat gently on a hot plate under the hood until the samples are dissolved. Do not boil the solution until all of the metal has dissolved.

After the alloy samples and the foil have fully dissolved, dilute the solutions with ultra-pure water to approximately 50 ml and boil briefly to remove oxides of nitrogen and chlorine.

Quantitatively transfer each solution in the beakers to separate 250 ml volumetric flasks. Rinse the beakers with several small portions of ultra-pure water being careful that you do not exceed the capacity of the flask. Dilute the volumetric flasks to volume with ultra-pure water and mix well. The solution made from the copper foil will serve as your standard and will not be diluted further.

Pipet 1 ml of each of your alloy sample solutions with the Finnpipette into 100 ml volumetric flasks and add 10 ml of the 1M potassium nitrate solution. Dilute to volume with the ultra-pure water. These are the dilute solutions that you will be working with and will be referred to as your sample solutions.

Be sure to label clearly all of your prepared solutions using the white tape in the lab.

***This is a good place to stop for the first day. Before returning to lab for the second day, calculate the copper concentration in g/ml in the standard copper foil solution and the concentrations of added copper in 15 ml of sample solution after the addition of 50, 100, and 150 microliters of the standard copper foil solution to the 15 ml of sample solution (25, 50, and 75 microliters for the bovine liver sample). These will be the additions to the cell vial in the standard addition procedure and this data must be entered into the computer before a run can be made. See the first portion of the report form referring to the Standard Solution for assistance

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with these calculations.

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Figure 311

Cleaning and Setting up the CGMEThe CGME must be cleaned before it is used. The diagram in Figure 3 of the CGME

will be referred to with parentheses indicating the number of the part to be adjusted. The CGME can be turned on by the switch on the back of the instrument.

Begin by making sure that the mercury capillary is in the highest position possible. If not, the height may be adjusted loosening knob 22, which is on the stand to which the electrode assembly is mounted, and carefully moving the electrode assembly. Remove the cell vial (which looks like a shot glass) by sliding the stir motor (17) to the right while holding onto the vial. The two nitrogen purge lines (25) may be carefully pulled out of the white cell cover. The cell cover can then be slid out and rinsed with ultra-pure water. Remove the Teflon tape from the mercury capillary. This electrode as well as the reference electrode, which is stored in a 3 M NaCl solution, and the platinum wire auxiliary electrode should also be rinsed with ultra-pure water. It is also important that all of the electrodes, the tubes, and the white cell cover be rinsed with ultra-pure water between samples.

Reinsert the white cell cover so that it slides in easily. If resistance is felt, then the cover may be upside down. Replace the nitrogen tubes in the correct positions in the cover. The left tube should just stick out of the underside of the white cell cover and the right tube should approach the bottom of the vial. Be careful that the right tube does not interfere with the motion of the stir bar. The capillary tube may be lowered through the vial cover by loosening knob 22 and moving the electrode assembly until the top of the assembly is lined up with the designated black line on the electrode stand. Be sure to retighten knob 22. Attach the white wire to the reference electrode, the red wire to the auxiliary electrode, and the black wire to the mercury connection (12).

With a pipet, add 15 ml of the first sample that you wish to analyze to a clean and dry cell vial and mount it under the cell cover by rotating the Stir Motor (17) under the vial. The first run you should make is the blank potassium nitrate solution. Make sure that the motor is pushed as far to the left as possible. Insert a clean stir bar into the vial as well. Be sure that the gas purge flow control knob (10) is in the closed position and then turn on the nitrogen tank and adjust the pressure to approximately 10 psi, turning the regulator control valve to the right. If you are not familiar with the operation of compressed gas tanks, the instructor will go over the proper operation with you.

Setting up the BAS CV50W Voltammetric AnalyzerTurn on the BAS CV50W Voltammetric Analyzer, BAS Controlled Growth Mercury

Electrode unit, CTX color monitor, and HP DeskJet 560C printer by turning on the outlet strip located on top of the CV50W Voltammetric Analyzer. Now, manually turn on the computer by pressing the right most button to the unit. (For proper operation we have found that it is important that the computer be off when the other units are first turned on.) If any of the units

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do not appear to be on, find the individual on-off switch on that/those unit(s). After a short period during which a lot of things appear on the screen, login> appears at the bottom of the screen. To login, type Chem 160lab and press the Enter key; press the Enter key again when prompted. When the "Pomona College Chemistry Department" display appears move the arrow on the screen with the arrow key on the keyboard to 2. - CV50W Version 2. The BAS CV-50W display should appear now. Be sure that there is sufficient paper in the HP DeskJet 560C printer.

The parameters for the Voltammetric Analyzer must be set. Click on Method and Select Mode and when the Mode of Operations Box appears select 5) Stripping Techniques and at the bottom DPSV ( Differential Pulse Stripping Voltammetry) and click on OK. When the General Parameter Box appears, set the Init E (mV) = -800 and the Final E (mV) =150 (the potential range for the copper determination), Sensitivity = 100A/V, and the Deposition Time (sec) = 60. Click on OK after the entries are made. For the Specific Parameters, the default parameters will be used. To verify the parameters entered, select Method and click on List Parameters so that the parameters and data will appear on the screen. Click on Method again and open Controlled Growth. When the Dialog box appears, make sure that the box "Drop Control On" is marked and click OK. If it is not marked, then, click it so that it is “X-ed”. Click OK. Click on Control and open Cell On/Off. When the dialog box appears, make sure that Cell Off is selected. Click on Analysis and open Results Options. When the dialog box appears, make sure that the box “Find Peaks after Run” is marked.

Be sure that the Mode Switch (19) on the CGME unit is in the SDME position. The Drop Size on the cell stand (5) should be adjusted to 15 and the Stir Rate (7) should be placed at 700 Repetitions per minute.

Check the contents in the cell. The shorter nitrogen tube (left) should be inserted just past the cover of the cell so that it can barely be seen beneath the cover. The longer one (right) should be near the bottom of the vial but not impeding the motion of the stir bar. The three electrodes should be submerged beneath the surface of the solution about halfway down the vial. When everything is in the proper position, click on Control and open Immediate Purge/Stir. Type 120 seconds and click both purge and stir in order to stir and bubble the solution for two minutes. Click OK to begin. Adjust the control knob (10) and the right knob (there are two brass knobs side by side) on the nitrogen tank together until there is a reasonable flow of nitrogen bubbles through the solution. A good stream is necessary, but not one which is turbulent and causes splashing on the cell cover. If a mercury drop does not form at the tip of the electrode, consult the instructor.

It is unnecessary to do a complete analysis for your initial run using the potassium nitrate solution as the sample. Instead, simply follow the instructions given to do a immediate stir/purge and then begin the run. To begin a run, click on Control and Start Run. For best results, be careful not to bump or shake the table when a run is in process because the instrument

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in very sensitive. The run is completed when the graph appears on the screen and the computer makes a beeping sound. If the graph displayed is marked by large circles (ie. they are identifiable as circles), go to Graphics and choose Graph Options. Change the Fill Size to 1 and click OK. When the run is complete, click on Print and go to Print Options. Make sure that all five of the boxes are selected. While in Print Options, clearly identify your sample in the box marked Label. You may now choose Print Graph under the Print title. It is unnecessary to use the Save command at any point in the CV50W program. The only time you need to Save anything is while in the WRITE program which will be explained in the following section. After you have obtained a graph for the blank solution of 0.1 M potassium nitrate, add 50 l of the copper standard solution to the cell and do a second run. Print out this second run. The current for the copper peak should increase by approximately 3amp. If the current increase differs significantly from this amount, see the instructor. After you have done the rest of the experiment, estimate the amount of copper in the blank potassium nitrate solution. If the amount is not negligible, the concentrations of your sample solutions should be adjusted accordingly. (See the Report section.)

Determinition of Precision and Accuracy with the Known Alloy SamplePipet 15ml of your known Copper Base Alloy sample into a clean and dry cell vial and

follow the above instructions three separate times to determine the precision and accuracy of the instrument (You do not need to repipet a new sample solution for each run). Once the computer starts printing the graph, obtain a hard copy of the precision data by following the instructions given below.

Hard Copy of Precision DataTo collect a hard copy of the precision data, highlight the desired data. Then

bringthe cursor to Print and select the copy text option. Next, double click on the ProgramManager icon in the bottom left corner of the screen. Double click on Accessories and open Write. Go to Edit and choose paste. Your precision data should now appear on the screen in the WRITE word processing program. Since you will be adding all your other precision data and all your standard addition data to the same file, identify each copied and pasteddata. Choose File, Save As, and save the data under your e-mail name. Return to the CV50W program by double clicking on the BAS Program icon (bottom left) and then double clicking on the CV50W icon.

Repeat this editing process for all three of your precision measurements. Before you can paste the data from your 2nd, etc. runs in WRITE, you must choose File and open your document under your e-mail name. Be sure to save your file after each addition of data before returning to CV50W. The file saved in WRITE should be a cumulative file for your precision data and your standard addition data of all the samples.

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Do not print this file until your standard additions are complete for all your samples.

Once the data has been saved, run this trial two more times in order to determine the reproducibility and precision of the instrument in terms of the peak potentials and the peak currents. It is not necessary to print the second and third runs; just be sure that you edit the data into your e-mail file in WRITE as explained above. Since the Accessories window is already open at this point, click on the open windows behind the CV50W window until the Write icon is visible and continue with the editing.

The copper peak is the peak closest to 0 V. The actual potential will be used in setting up the parameters for the Standard Addition procedure.

Standard Addition Analysis of the Copper Base Alloy SamplesYou are now ready to begin the standard addition analysis of the the known Copper Base

Alloy sample which is already in the cell vial. Click on Analysis and go to Standard Addition. Enter the three concentrations into the “conc 1” column that you calculated for 50, 100, and 150 microliter additions to a cell with 15ml of solution. In the “Peak 1” column enter Ep(mV) = (the potential for the copper as determined above, which should be close to 0 mV) , Tol (mv) = 20, and compound = copper. For Number of peaks = 1, Number of additions = 3, Concentration Unit = g/ml, Base filename = your first initial and four other letters to identify the sample. Under the comments, write down the complete information about your sample. Once the parameters are filled out, click Run to begin.

After the computer runs this sample, a box will appear in the upper left hand corner and you will hear a beeping sound indicating that it is ready for the next run. Make sure not to touch anything until this box appears because the run is still in process. Following the instructions for using the Eppendorf pipet (see earlier section), add 50 microliters of your standard copper solution through the standard addition port (14) into the solution. Be careful to eject all of the solution out of the tip without submerging it into the vial solution. Turn the stirrer on the CGME unit ON manually for approximately one minute to make sure that the addition is well mixed into the solution. Then Click Continue. Repeat this twice more so that there has been a total of three additions of 50 microliters each. If the tip touches the solution or a bubble of the standard solution remains in the pipet tip, you may opt to start the run over by pipetting another 15ml of the sample solution into a clean and dry vial. Click End to end the current additions.

The program will automatically take you back to the main screen after the three additions. Click on Analysis and go to Standard Addition. All of your parameters should still be set as before. Click on Plot, and then OK for Peak Selection = 1. Your standard addition plot should appear. The four points should be in a fairly straight line with a correlation of > 0.99. If you are satisfied with your additions (the graph does not have any scattered points), close the window that displays your parameters by clicking on the box with a horizontal bar in it in the

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upper left hand corner and continue with the procedure. If not, rinse the vial, stir bar, and electrodes and repeat the process with a new 15ml sample. After closing the window, your Standard Addition graph should still be on the screen. Click on Print and choose Print Graph. Once the computer starts printing the graph, obtain a hard copy of the standard addition data.

Hard Copy of Standard Addition DataTo collect a hard copy of the standard addition data, follow the instructions given

previously for the Precision Data. Add your standard addition data to your WRITE file named after your e-mail name.

Repeat this editing process for each of your samples making sure to give each sample a Base Filename and make a comment in the indicated boxes on the standard addition preparation screen so you can differentiate between them.

Click on Graphics and go to Multi Graph. Then you will see a menu of your files. Highlight your addition data. These can be identified by the name you gave starting with your first initial and four other letters. There should be four entries with your given name and 001, 002, 003, 004 afterwards. Then click OK. A multi-curve of each of the four previous standard addition runs should appear. With each successive addition, the copper peak should increase a similar increment. If you are satisfied with these results, choose Print Graph.

A hard copy of the data for the four entries used in the multi-graph should also be recorded for all samples. To obtain this information, go to File and Choose Load Data. Select one of your graphs and press OK. Edit the data for that graph to your e-mail named file in WRITE and then repeat the process for the other three graphs. Do this for all three of your samples. The difference of the currents between each addition should be the same if the system were perfect since the same volume of the copper standard solution is added each time. These data give additional information on the precision of the experiment. Be sure to comment on these data if you write a long report.

Once a hard copy of the standard addition and multi-curve graphs and all the data is obtained, remove the vial and rinse off the electrodes and the stir bar with ultra-pure water. To clean the cell vial pour off most of the liquid into a designated "waste" beaker being careful not to pour off the mercury in the bottom of the vial. Pour the mercury into the provided "dirty" mercury bottle. Repeat the above procedures for the Roman coin copper base alloy sample, making note of the following two exceptions; it is unnecessary to do more than one initial run before proceeding to the standard addition analysis and a multi-graph of the standard additions is not needed. IMPORTANT: Be sure to change the Basefile Name and the Comments for each new sample.

Analysis of the Biological Sample16

Since the copper peak may shift between the alloy samples and the biological samples you should make one run in the manner given in the section on “Determining the Precision of the Instrument with an Alloy Sample”. After you have determined the potential of the copper peak you can proceed with the standard addition analysis of the bovine liver sample in the same manner as with the alloy samples except 25 microliters is used for each addition of the standard instead of the 50 microliters used with the alloy samples. Therefore, take note that the concentrations and potential on the standard addition parameter screen must be changed (in addition to the base filename and comments) before preceding with the standard addition process.

The bovine liver sample differs from the alloy samples for this analysis because the computer is less consistent on where the Results lines are drawn. Therefore, conduct the DPSV procedure as indicated earlier, but in addition, record the data for manually placed Results lines and compare the results of each. This should be done in the following manner after the DPSV graph is printed. Go to Graphics and choose Single Graph. A new graph should appear without any Result lines drawn in. Then go to Analysis and choose Results Options. Select Manual in the Method box at the bottom left of the window. Then click on Find Peaks. Place the arrow point of the cursor at the maximum on one side of the copper peak and press and hold the right button on the mouse down. Drag the cursor to the opposite maximum of the peak and release the button. Press and release the left button on the mouse and then continue to draw whatever Result lines you wish making sure to press the left button when each is completed. With every Result line that you drew, a new set of data should be recorded in the CV50W*DPSV window. Edit this information to your e-mail name file in WRITE as indicated previously. Be sure to label the data clearly. If the current values differ significantly from the auto method, do a standard addition plot using your APL disk and calculate the percent copper using this data. Show this data on the report sheet by listing a second column at the appropriate place. (If you are running late, this part may be skipped; however, for a long report discuss qualitatively the possible error introduced if a manually drawn line was used to calculate the current.)

When the standard addition procedure has been completed for all samples, print your e-mail named WRITE file by going to File and choosing Print while in the WRITE program.

Clean Up When you are through with your last run, be sure to delete all of your experimental data

by going to File on the ruler and click on delete files. Highlight your files and then click on OK. Confirm the number of files you want to delete by clicking on OK.

To shut down the Voltammetric Analyzer when you are through with your last run, close all of the open windows. Then, go to File and choose exit windows. Confirm your intent of closing windows by choosing OK. The "Pomona College Chemistry Department" display

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should appear. It is now safe to turn-off the computer switch and then the master switch on the power strip. Close all nitrogen valves and return the Gas Purge Control Knob (10) to the closed position. Turn off the BAS CV50W Voltammetric Analyzer (the switch is on the back of the instrument). Close the nitrogen tank and close the regulator control valve so that it can spin freely. Finally, open the left knob (of the two brass knobs mentioned earlier) and release the remaining nitrogen from the system.

Clean all the glassware with ultra-pure water. Disconnect all the electrodes (the white, black, and red wires) and remove the reference electrode and the auxiliary electrode from the cell top. Rinse each with ultra-pure water and store the reference electrode in the vial containing 3 M NaCl. Raise the CGME to its top position, rinse with ultra-pure water, wipe dry with a Kimwipe, and wrap the end of the capillary with a piece of Teflon tape (spool provided). Place the storage cell vial in position and replace the apparatus cover.

Partnership Instructions

1. Calibration of the Eppendorf pipet is to be done individually, i.e., each partner must

report his or her own results for the calibration.

2. Prepare only one 1 M potassium nitrate stock solution, one standard copper solution,

one known copper base alloy solution, and one bovine liver solution. Each partner will prepare

a Roman coin sample solution. With the exception of the Roman coin sample, take turns doing

the work of solution preparation and using the BAS CV50W Voltammetric Analyzer.

3. Since each partner is expected to turn in her or his own report (both short and long

reports), make Xerox copies of all printed graphs and data.

REPORT

Short ReportTurn in two copies of the data report sheet. (Either make a Xerox copy of a completed

copy of the report sheet or fill out both copies.), the duplicate pages of your notebook which should show all of your calculations, and all of the printed graphs and printed data.

Printed graphs and data include:

1. The three graphs for the initial runs of the three samples.

2. The three standard addition graphs for the three samples.

3. The multi-curve graph for the standard alloy sample.

4. The hard copy of the precision data, standard addition data, and the data

showing the discrepancy between the automated and manually drawn Results

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lines

(if available).

If there is one obvious bad point in any of your standard addition graphs, do an APL least

squares plot of the data omitting the one bad point. Indicate on the report sheet if your data have

come from an APL plot.

The bottle of potassium nitrate lists the maximum amount of copper that will be present

in the solid potassium nitrate as 0.0005%. Compare your experimental estimated amount of

copper in the blank solution with the calculated maximum amount which could be in your

solutions.

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Long Report Paper

In addition to the usual items to be included in a long report, the following items should

also be included in the long report for this experiment.

1. Include a brief discussion of the theories of stripping voltammetry in order to show

that you understand the nature of the determinations. Discuss the nature of differential pulse

voltammetry to show that you understand how the measurement of the current is performed.

2. In your discussion of the results of the precision of the instrument, comment on both

the precision of the peak current values and of the peak potential values. Why is a relative

standard deviation not calculated for the potential values?

3. Comment on the differences in the voltammograms for the different samples,

especially, between the alloy samples and the bovine liver sample. In this discussion indicate

what metals might be responsible for other peaks seen on the various voltammograms. Discuss

any differences between the manual and auto results for the bovine liver sample results. If your

blank potassium nitrate solution contained a significant amount of copper, apply corrections to

your results and compare them to the original values.

4. Discuss how the amount of the Roman coin sample in the cell vial, as given on the

Report sheet, compared to the size of the bovine liver sample supports the objective of the

experiment. If you were able to weight out directly this small amount of a sample, what

concern should you have about the reported percent copper for your Roman coin sample?

5. Include a short discussion about the analysis of Roman coins, comparing ASV with

other methods of analysis for the composition of the coins

6. Be sure to include all the items required for the short report introduced at the appropriate places.

7. The long report for this experiment should be written in the style of Analytical Chemistry articles as described in the introductory part of the lab manual. For example, Results and Discussion are usually combined together as one section. Appropriate tables should be prepared to present your data and results, i.e., do not just attach your Report sheets. You can, however, attach the printed graphs as the figures for the long report paper. Be sure to reference your paper fully in the style of Analytical Chemistry articles.

ACKNOWLEDGMENT

The development of this experiment and the initial writing of these instructions were done by Maple Fung as part of a summer 1995 research project on anodic stripping voltammetry. Further developmental work and writing was done by Kiyomi Parish during

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summer 1996.

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VOLTAMMETRIC DETERMINATION OF COPPER IN COPPER BASE ALLOYS AND A BIOLOGICAL SAMPLE

NAME_________________________________Date experiment completed_________________

Percent Copper in Known Alloy_____________Date report submitted _____________________

Calibration of the Eppendorf Pipet

Average Mass of Additions

Standard Deviation Relative Percent Standard Deviation

Average Calculated Volume of Additions

milligrams µl

Standard Copper Foil Solution

Amount weighed out (mg)_________________________________

Concentration in 250 ml (g/ml) ____________________________

For Standard Additions of Standard Copper Solution to Alloy Sample Solutions

Volume of Standard 50 µl 100 µl 150 µlg of Standard

g/ml of Standard (in 15.05ml) (in 15.10 ml) (in 15.15 ml)

For Standard Additions of Standard Copper Solution to Bovine Liver Sample Solution

Volume of Standard 25 µl 50 µl 75 µlg of Standard

g/ml of Standard (in 15.03ml) (in 15.05 ml) (in 15.08 ml)

Precision of DPSV with Known Alloy Sample

Ep, Average of three trials, (mV) ________________

Standard Deviation ________________

Ip, Average of three trials, (mA) ________________

Standard Deviation ________________

Relative Percent Standard Deviation ________________

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Determination of Copper in Known Alloy Sample

Milligrams of known alloy sample weighed out ________________

Concentration from standard addition graph (g/ml) ________________

Standard Addition Correlation Coefficient ________________

Percent copper in known alloy (without blank correction) ________________

Relative Percent Error (with respect to known value) ________________

Determination of Copper in Roman Coin Sample

Milligrams of Roman coin sample weighed out ________________

Micrograms of sample in the CGME vial cell ________________

Concentration from standard addition graph (g/ml) ________________

Standard Addition Correlation Coefficient ________________

Percent copper in Roman coin (without blank correction) ________________

Determination of Copper in Biological Sample

Milligrams of bovine liver sample weighed out ________________

Concentration from standard addition graph (g/ml) ________________

Standard Addition Correlation Coefficient ________________

Percent copper in bovine liver (without blank correction) ________________

Current Differences for Standard Addition Runs

SamplesAverage Current

Difference Between Runs

Standard Deviation of Currents

Relative Percent Standard Deviation

Known Alloy

Roman Coin

Bovine Liver

Estimated Amount of Copper in Blank Potassium Nitrate Solution __________ g/ml

Calculated Maximum Amount of Copper in the Blank Solution __________ g/ml

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