Ultra-Low Copper Concentrations Determined by Rotating Disk Electrode Stripping Voltammetry

Transcription

1 Ultra-Low Copper Concentrations Determined by Rotating Disk Electrode Stripping Voltammetry 1. Purpose Copper ion concentrations in the parts-per-billion (ppb) range will be determined using anodic stripping voltammetry at a rotating glassy carbon electrode. The concentration of copper in the unknown will be measured using the standard addition method. 2. Background/Theory 1 Among the many analytical methods, there are a select few that are capable of determining ultralow concentrations. Anodic stripping voltammetry is one such technique. Sub-ppb detection limits are possible because the analyte is collected and effectively pre-concentrated at the working electrode surface during what is called the deposition step prior to the stripping step that provides the analytical current (signal). In this experiment, you will use anodic stripping voltammetry for the determination of copper on thin mercury layer on a rotating glassy carbon electrode. While the experimental parameters vary depending upon the solution conditions, there are three important steps in the operation of the instrument: 1) deposition time, 2) quiet time, and 3) the stripping step that provides the current (analytical signal). During the deposition step, the potential applied to the electrode is held at a relatively negative value so as to reduce the copper ion in solution for a pre-determined time. During this time, a portion of the copper in solution amalgamates with the mercury electrode: Cu 2+ (aq) + 2e - Cu (in Hg) This makes the concentration of copper at the electrode much greater (typically 2 or 3 orders of magnitude) than the original concentration of copper in the solution (consequently, the deposition step is also sometimes called the pre-concentration or collection step). After the deposition step, the stirring is stopped and the system is allowed to reach equilibrium during the quiet time. During the stripping step, the applied potential is scanned in a positive direction and the copper in the mercury electrode is oxidized back to copper ions in solution; that is, the copper is "stripped" from the electrode and the current measured. Cu (in Hg) Cu 2+ (aq) + 2e - The magnitude of the current of the stripping peak is proportional to the concentration of the analyte in the mercury electrode. Since the concentration of the analyte in the electrode is related to its concentration in solution, the stripping peak current is therefore proportional to the solution concentration. The potential at which the stripping peak is observed is related to the redox potential of the above reaction. However, it also depends upon solution conditions and so varies somewhat with different users. 3. Safety 1

2 Mercury is a toxic heavy metal. While you should not be handling mercury metal directly, it is recommended that you wash your hands frequently and especially at the end of the laboratory. Wear your safety glasses at all times. For more general safety in the laboratory, please refer the appendix at the end of the procedure. 4. Experimental Method Special Note This laboratory involves the determination of copper at very low concentrations. As such, contamination of the unknown, especially once diluted, is of special concern. Copper, like some other cations, tends to stick to glass and only comes off over time. Thus, if a particular piece of glassware (e.g., a volumetric flask) was/is used to make a concentrated copper solution, that glassware could have excess copper still attached to it despite extensive rinsing. To minimize this possibility, the glassware used for making the unknown solutions should be filled with nanopure water for a week prior to its use (and then that water discarded before making the dilution). This step is unnecessary if you have used the glassware to make other solutions not containing copper. Conversely, you should transfer any diluted unknown from the volumetric flask to a plastic bottle (a special Teflon bottle is necessary for the most dilute unknown solution) as soon as possible to minimize Cu sticking to the glass and thus being lost from solution. See your instructor if you have questions. 4.1 Preparing the 250-ppm and 5-ppm copper standards: Calculate the approximate mass of copper needed to prepare 1-L of 250-ppm copper. Cut an approximately one-inch square piece of copper foil (picture and example on the tube) and weigh on the top-loader balance to determine if it is reasonable close to the desired mass. Tear a bit off, if needed. Then clean the copper foil with methanol and a Kimwipe before weighing the cleaned copper foil on your analytical balance and recording the mass in your notebook. Place the copper foil in a 250 ml beaker, add about 10 ml of 4 M nitric acid, and cover with a watch glass. Place the beaker on a hotplate, and heat until all the copper dissolves in the acid. Once the copper has dissolved, remove the beaker from the hot plate, and allow it to cool. When the solution is cool enough to handle, transfer the dissolved copper solution to a mlvolumetric flask. Dilute to the mark when the entire solution is room temperature. Label the solution "copper stock standard". This standard is too concentrated and must be quantitatively diluted down to about 5-ppm copper. Make the necessary dilution(s), labeling the final standard solution "copper working standard". Record the exact concentration of this working standard solution in your notebook, showing all relevant calculations. 4.2 Preparing and diluting the unknown Obtain a Teflon bottle and the unknown from the instructor. Quantitatively transfer the unknown to a 250-mL volumetric flask and label it "anodic unknown A". If you intend to store this solution for any length of time, transfer it to a plastic bottle. Subsequently, transfer a 10-mL aliquot of "A" to a 100-mL volumetric flask and dilute not with water, but instead with "anodic 2

3 stripping supporting electrolyte. Add this supporting electrolyte to the flask until just below the mark and then dilute to the mark with nanopure water. The unknown copper is now in the ppb range and must be stored in the Teflon bottle. Transfer to the clean Teflon bottle and label it "anodic unknown B". Note: your sharpie will not write on Teflon. Write your label on a piece of tape and attach to the bottle. You are now ready to begin the instrumental analysis portion of this laboratory. Take the following materials to the Epsilon electrochemistry system (Figure 1): 5-ppm standard copper solution Unknown Solution B 250-mL beaker (for rinsing) notebook clean 15-mL pipette and bulb 100-µL pipette wash bottle Kimwipes (if not already at the instrument station) 4.3 Calibration of a micro-pipette Before you use the micro-pipette, you should review and check/calibrate the micro-pipette. See Appendix A at the end of the procedure for more information on this. 4.4 Use of the Epsilon Rotating Disk Electrode Figure 1. The Epsilon Controller, RDE, and the computer Note: The analysis also involves the conditioning of the glassy carbon electrode. Before starting to use the instrument, ask the laboratory instructor if the electrode has been conditioned for the use. Start the nitrogen flow to the Epsilon system with the following sequence: 1. The green regulator handle (Figure 2) must be set fully counter-clockwise (shuts off the regulator) 2. Open the valve on top of the tank fully counter-clockwise (opens the tank to the regulator) 3. Slowly turn the green regulator handle clockwise until gas bubbles gently into your unknown solution. 3

4 Figure 2. Nitrogen regulator Figure 3. The back of RDE unit The power switch is located next to the power cord Press the on/off switch on the rear right hand side of the rotating disk electrode apparatus, if needed (See Figure 3). At this time, see your instructor about degassing your sample. Dissolved oxygen can slightly interfere with the eventual signal you will get for copper. The glass sample cup under the electrode sets on a round magnetic stirrer (Figures 4-5). Hold the cup in place and move the stirrer to the right (Figure 5) below before lowering the cup so it clears these three items: 1. Glassy carbon electrode (in center) 2. Reference electrode (on the left) 3. Platinum wire counter electrode (on the right) Figures 4 and 5. 4

5 The sample/electrodes assembly of RDE unit with the cup (Figure 4, left) and without the cup, the holder to the side (Figure 5 above). Holding a beaker underneath at an angle, rinse these three items with copious amounts of water from the wash bottle. Transfer a ml aliquot of unknown "B" to the cup, replace under the electrodes, and rotate the stirrer back to the left so it supports the cup. Go to the computer and start Epsilon application by double-clicking Epsilon icon near the upperleft corner of the desktop (Figure 6). Once the Epsilon software has started, go to "File" and select c:\epsilon EC\Data\466.dpsv0 from the menu. This will initialize the computer to start a new run. Figure 6. Windows desktop Figure 7. The screenshot of the Epsilon software At this point, you must ensure that the experimental parameters are correct. Go to the Experiment menu and click on Setup/Manual Settings. Verify that under the Cell Stand/Accessories heading that RDE-2 mode is checked. If not, select this mode, apply, and exit. Figure 8. Setup/Manual Settings Window Next, open the Change Parameters dialog box (or just hit the F6 key) also in the Experiment menu. Check with your instructor to confirm the experimental conditions. The deposition 5

6 potential (nominally -800 mv), initial potential (-300 mv) and the final potential (-25 mv) can change over the semester. Also confirm the step potential (2 mv) and deposition time (100 s). Ask your instructor to verify your settings (which may differ than those below) and record them in your notebook. Afterwards, click apply, then exit. Figure 9. Experimental Parameter Window Click the "Run" button on the Epsilon software to start the run. The electrode will rotate at 3000 RPM for about 60 seconds; during which time copper will be reduced to copper metal and dissolve (amalgamate) into the mercury film on the glassy carbon electrode. The electrode rotation will stop during the quiet time before beginning the stripping step of the experiment. During that time, you should see a curve with a negative (anodic) current displayed on the screen as the copper is oxidized and "stripped" from the drop into the solution. Eventually, the trace should form an inverted bell shaped curve and some data will display on the right of the screen. The peak current height (µa) is proportional to the concentration of Cu 2+ in solution and should be recorded in your notebook (to at least three and preferably four decimal places), along with the mv reading. While recording the data in your notebook, print a copy of your results by clicking the print icon. You are now ready to run a spiked sample. If you need a review of the proper use of micropipettes, ask your instructor. Obtain the 100-µL micropipette and insert a new plastic tip. This pipette should be set at 50-µL. Fill the pipette with your copper working standard solution. Remove the small black plug in the electrode holder, insert the tip through the hole, and transfer the aliquot to the sample in the cup. Replace the black plug, press the run button to start a new scan and wait for the run to finish. Record the new peak current (µa) and voltage (mv) values in your book while also printing a copy of your data. This completes the first trial. Remove the cup, dispose of the copper solution in your waste beaker. Use your wash bottle and 250-mL beaker to rinse everything before starting your second trial. Perform 3 or 4 trials. If you must do more than 4 trials (total of 8 runs), consult your laboratory instructor for help, as mercury layer will degrade after several trials. It is likely that the current for these trials will differ somewhat, but ideally the spiked sample current will always be larger than the unspiked sample current by the same approximate 6

7 percentage. You can track the ratio between the spiked and unspiked peak current values to track the reproducibility of your results. Once you are finished, turn off the RDE apparatus (leave the computer on), rinse the sample cup and electrodes with water and add about 10 ml of water to the cup before returning it to the stirrer so the electrodes soak. Turn off the nitrogen by: 1. Slowly turning the green regulator knob completely counter clockwise (shuts off). 2. Turning the tank valve fully clockwise (shuts off). For more accurate results, you must experimentally determine the exact volume delivered by the micropipette. Fill a small beaker with nanopure water and obtain a small empty vessel, such as the 50 ml Erlenmeyer flask. Tare the flask on your analytical balance and determine the mass of water delivered by the micropipette. Repeat this procedure for a total of three times. Use the density value of g/ml to determine the volume delivered by the pipette. Use this volume in your calculations. A sample table for your notebook is shown next. Micropipette Calibration For fixed volume use d = mg/µl Trial* Set volume (µl) Delivered mass (mg) Delivered volume (µl) Mean Stdev RSD (ppt) calculate* calculate calculate *use the mean volume in your calibration calculations for you unknown 5. Calculations and Reporting Requirements In all cases, the signal (µa) should be directly proportional to Cu concentration. There are two microampere readings for each trial. Set up the following equations for standard addition for each trial: before spike: µa=k[conc unk Cu] after spike : µa =k([conc unk Cu] + [conc Cu from spike] ) If we assume the trials were performed under identical conditions, then the k values are the same and the second equation can be solved in terms of the first. Note, however, that spiking the sample also dilutes the original unknown concentration and also your working standard. Calculate the copper concentration for each trial separately and report the average ppb of the unknown copper in Solution B to one decimal place. Also report the percent relative standard deviation for your trials. 6. Reminder/Clean-up 7

8 All volumetric flasks and pipettes used MUST be rinsed with generous amounts of nanopure water. Copper from more concentrated solutions will adsorb onto glass and can cause large positive errors at the ppb level if not copiously rinsed. It may be advisable to soak the glassware in water prior to measuring the dilute unknown. All copper solutions can be disposed in the sink. Return the Teflon bottle to your instructor immediately upon completion of the lab. 7. References 1. Online Instruction Manual for BAS Epsilon for Electrochemistry. See 8

9 Appendix A: Operation of Eppendorf Adjustable Pipettes A1. Volume Setting The volume is adjusted by pressing down the lateral catch and turning the control button at the same time. It is advisable to carry out volume setting from the higher down to the lower value (i.e. first go above the desired volume and then return to the lower value). A2. Pipette tips Typically the color of the control button will correspond to the color of the eppendorf tip or tip rack. For the best precision and accuracy, pre-wet all new tips by aspirating and dispensing liquid 2-3 times before pipetting. A3. Aspirating liquid Attach suitable pipette tip to the pipette firmly. Press down the control button to the first stop (measuring stroke). Immerse the pipette tip vertically ~3 mm into the liquid. Allow the control button to slide back slowly. Pull the tip out of the liquid slowly. To remove any remaining droplets, dab with non-fibrous cellulose material, ensuring that liquid does not come out of the tip. You can also dab on the side of the beaker containing the liquid you are pipetting. A4. Dispensing liquid Hold the tip at an angle against the inside wall of the tube/flask. Press down the control button slowly to the first stop (measuring stroke) and wait until the liquid stops flowing. Press down the control button to the second stop (blow-out) until the tip is completely empty. Hold down the control button and pull the tip out of the inner wall of the tube/flask. Allow the control button to slide back slowly. Tip is ejected by pressing the control button to the final stop. Do not lay down the pipette when a filled pipette tip is attached as this may result in liquid entering the pipette. A5. Verification of pipette You can verify that the pipette is performing accurately by dispensing nanopure water from a pre-wetted tip into a tared flask or tube onto an analytical balance. Typically, for this experiment, test at the volume that you need to use for the 100 μl pipette. You should do so BEFORE you start using the pipette. Convert the mass to volume by dividing by the density at room temperature. For example, the density of water is mg/μl at 20 C. This number is the volume actually delivered by the pipette. Determine the error relative to the set value. Repeat a few times to verify that the pipette is accurately delivering water. If not, consult your instructor. 9

10 Instrumental Analysis Laboratory Safety Rules A. Instructions: Carry out all manipulations in accordance with instructions and the safety rules and procedures given herein. B. Eye Protection: All students and staff working in the laboratory must wear safety glasses at all times. If a student needs to be reminded more than three times to wear goggles, she/he will be dismissed from lab for the remainder of the day, and will not be given an opportunity to make up the work. C. Apparel: The clothes you wear in lab are an important part of your safety equipment, and should offer protection from splashes/spills. Closed toed shoes (sneakers are fine), Full-length pants or a full-length skirt, and A shirt that completely covers your torso (i.e. at minimum, a t-shirt). In other words, you must NOT wear shorts to lab. You must NOT wear flip-flops, sandals, or crocs. You must NOT wear tank tops, halter tops, spaghetti-strap tops, or low cut jeans to lab. Exposed abdomens, hips, and backs are not safe in the lab. D. Gloves: Gloves are an important part of personal protection. Gloves will be available at all times in the laboratory. Your instructor will require their use when appropriate. E. Food: Food, drinks, and gum are not allowed in lab. None at all, not even water bottles. F. Sanitation Issues: Be sure to wash your hands before leaving lab, before you eat anything outside of lab, and before you answer your cell phone. G. Music: Individual headphones are not allowed. Your may choose to play music for the entire class. H. Cell Phones and Other Electronic Devices: Cellular phones and other electronic devices that you do not need to perform your laboratory work should be put away. I. Other: All students are explicitly prohibited from: 1. conducting any unauthorized experiments. 2. removing chemicals or apparatus from the laboratory for any reason. 3. working in the lab alone, or at other than regularly scheduled lab periods. 4. smoking in the laboratory or within 20 feet of any doorway. 5. impeding movement in aisles or through doorways with bags, skateboards, etc.