Purpose:
In this experiment, you will determine the method by which Brita Filters filter out solute from water. You will attempt to answer: “does the filter remove a particular fraction of a given solute, or bring the solute concentration to a particular level?” (Weaver, 2018).
Background and General Information:
The goal of this experiment is to determine the method of filtration via atomic absorbance. You will do this by repeatedly running a sample of dissolved copper sulfate pentahydrate through a Brita filter, then tracking the solute’s concentration as it gets filtered out. If the sample becomes less and less concentrated each time it is run through the filter, then the filter likely removes a fraction of the solute with each subsequent pass-through. If the sample remains at a constant solute concentration throughout each round of filtration, then the filter likely works by bringing the solute to a specific, basal level upon the first filtration. If it is the former, what percent of solute is removed each time? If it is the latter, what is this particular level of solute concentration? Copper concentration will be defined in units of milligrams of copper per liter of solution.
To do this experiment, you must create a series of standards to calibrate the spectrometer. The aim is to have the lowest concentration standard serve as a lower bound for your measurement, and have the highest concentration standard serve as an upper bound. That way, the spectrometer will find the concentration of each filtered sample via interpolation rather than extrapolation, which is known to be less accurate (Wahab, 2016).
The Brita Filter serves to remove solute from water to make it cleaner and safer for drinking. The Environmental Protection Agency has set the legal limit for copper in drinking water at 1.3 ppm (“Lead and copper rule,” n.d.). That being said, the Brita Filter is likely only functional at copper concentrations near 1.3 ppm. Therefore, the starting concentration of copper should not exceed ten times the legal limit (13 ppm) so that it is not too far out of the filter’s functional range. It should not be too low either to ensure you can observe any trends in the concentration change between filtrations.
Safety:
The only chemicals that will need to be handled for this lab are copper sulfate pentahydrate and water. Gloves and goggles should be worn to ensure proper lab safety, however no special precautions must be taken to handle these chemicals. All waste can be disposed of down the sink. Make sure to handle the atomic absorption spectrometer with care, as it does contain an open flame.
Materials:
• Copper sulfate pentahydrate (CuSO4 • 5H2O) – at least 0.0100 g
o This will be used to prepare both the standard solutions and the unknown solution that will be filtered by the Brita.
• Atomic absorption spectrometer with a copper hollow-cathode lamp
o The hollow-cathode lamp must match the metal being studied, which in this case is copper. This is because the wavelength of the light emitted by the lamp must match the wavelength of light that can be absorbed by the sample in order for the spectrometer to detect absorption.
• Deionized water
• Brita filter
Procedure:
This lab’s procedure will be broken up into tasks that need to completed before running the spectrometer. That being said, the order of these tasks is not important, as long as the spectrometer is prepared, standard solutions are created and samples are filtered before running the spectrometer.
Preparing the Spectrometer:
1. At the start of the lab period, either the lab TA’s or the instructor will show you how to use the Atomic Absorption Spectrometer. They will turn on the spectrometer to allow the flame to heat up so it reaches a constant temperature – one high enough to atomize your sample.
Preparing your standard solutions:
1. Weigh out about 0.01 g of pure copper sulfate pentahydrate (CuSO4 • 5H2O; FW = 249.68 g/mol). Be sure to record the exact weight of copper sulfate used. Transfer the sample in to a 100 mL volumetric flask and dilute to the mark with dIH2O. Find the copper concentration in the solution, which should be approximately 25 ppm.
2. Transfer at least 20 mL of the solution into your buret. Dispense the following amounts into six separate 10 mL volumetric flasks: 0.40 mL, 1.0 mL, 2.0 mL, 3.0 mL, 4.0 mL and 5.0 mL. Record the actual amounts dispensed into each flask, as you will need them to calculate the final copper concentration in each of the flasks. Dilute each flask to the 10 mL mark with dIH2O.
3. Calculate the final concentration in each flask (in ppm) using the starting concentration obtained in Step 1 and the exact volumes dispensed into each flask. You will need to enter these values into the atomic absorption spectrometer before running your filtered samples.
Preparing your Brita Filter:
1. To activate the Brita filter, run at least one full bottle of dIH2O through it. Make sure to properly clean and dry it before putting in any of your sample.
Preparing and filtering your sample:
1. Transfer 50 mL of the solution prepared in Step 1 of the previous task (“Preparing your standard solutions”) into your buret (after emptying it from the prior use). Dispense all 50 mL into a 100 mL volumetric flask. Dilute the solution to the mark with dIH2O. Using a 10 mL pipette, transfer 10 mL of this diluted solution into a 10 mL volumetric flask and set it aside.
2. Transfer the remaining sample into the Brita Filter water bottle. Run the sample through the filter and collect it in a beaker. Transfer 10 mL of the filtered solution into a 10 mL volumetric flask and set it aside. Rinse the Brita Filter with dIH2O between samples.
3. Repeat Step 2 two more times for a total of 3 filtrations, plus the unfiltered sample you began with.
Collecting data with the Atomic Absorption Spectrometer:
1. Fill a 250 mL beaker with dIH2O. This will be used to blank the spectrometer before running the standard solutions, and between each measurement to clean the instrument.
2. With the help of the lab TA’s or the lab instructor, check all of the settings to ensure that it is in program mode and that the units are set to “MG/L.” Enter the concentrations you calculated in Step 3 of “Preparing your standard solutions” into the S1 to S6 slots to two digits after the decimal place.
3. Place the capillary tube into the beaker of dIH2O. Press the “Auto-Zero” key and wait for the spectrometer to read “0.00” in the upper-right corner of the display.
4. Then, place the capillary tube into your first standard solution. Press the S1 key and the machine will give a reading on the concentration of copper in that standard solution. Then, place the capillary tube into the beaker of dIH2O for at least 5 seconds to clean the instrument.
5. Repeat Step 4 with standard solutions 2-6, pressing the corresponding ‘S’ key to measure each standard’s concentration. Note that the readings on standards 3-6 may not exactly correspond with the concentrations you calculated in Step 3 of “Preparing your standard solutions.” This is alright, just make sure to record the readings as they appear.
6. After placing the capillary tube into the beaker of dIH2O, the spectrometer will be calibrated. Place the tube into the first diluted solution (the one set aside before being run through the Brita filter) and press “Read.” The spectrometer display will show the concentration of copper in ppm. Record the reported concentration and place the capillary tube into the beaker of dIH2O for at least five seconds to clean it.
7. Repeat Step 6 to take three measurements of each sample, for a total of twelve measurements (three for the unfiltered sample, and three for each of the three filtered samples). Record all displayed concentrations and be sure to clean the capillary tube using the dIH2O between each measurement.
Original procedure: I began this experiment with a drastically different set up. I originally addressed the question of Brita filtration mechanism by preparing two standard solutions with known concentrations of copper, running them through the Brita filter, and then using atomic absorption spectrometry to measure the final concentrations. If the two solutions had final concentrations that are at the same ratio as the starting two solutions, then the filter works by reducing the solute concentration by a certain percent. If the filtered solutions have statistically similar final concentrations, then the filter works by bringing the solute down to a certain level. The issue with this method is that to begin at two starting concentrations that are drastically different, the concentrations must be relatively high (e.g. 50 ppm versus 100 ppm). This is problematic for two reasons: because the filter is meant to filter out drinking water, which should not exceed 1.3 ppm, and because the spectrometer does not report concentrations greater than 100 ppm. Therefore, I adjusted the procedure in order to keep the starting concentration low, but not too low as to not be able to observe changes in copper concentration after filtration.
The data collected in this original experiment is presented in the table labeled “EXP 1” in the appendix. Using the atomic absorption spectrometer, I found the copper concentration in solutions A, B, C, and D. Solution A contained a fluid that was originally about 20 ppm copper and had been run through the filter. Solution B contained a fluid that was originally about 40 ppm copper and had been run through the filter. Solution C was a solution that was originally about 200 ppm copper and then filtered. Solution D was a fluid that was originally 400 ppm copper and then filtered. The results from this experiment were unclear, as there were no readily observable trends in the resultant concentrations.
Data Analysis:
1. Find the average and standard deviation of each of the four measured samples.
2. Using a graphing software, construct a graph of copper concentration vs. number of filtrations. Is it a flat line? Is it linearly decreasing? Use the graph to determine by what mechanism the Brita Filter removes copper from solution.
a. If you determine that the filter works by removing a specific percent of solute from solution, what percent does it remove?
b. If you determine that the filter reduces solute concentration down to a specific level, to what level does it reduce copper?
3. Present your data, graph and justify your response using support from your results.
Results:
As shown in the attached graph (which represents the data from the table titled “EXP 2,”) each repeated time the sample was run through the Brita, the filter removed a consistent amount of solute. The slope of the graph is -2.696 and represents the average amount of copper the Brita removes solute per each round of filtration. In other words, these results indicate that the Brita functions by removing a constant amount of copper from water each time it is run through the filter, regardless of starting copper concentration. This is not the same as a constant proportion, as the amount of solute removed is not dependent on the initial concentration before filtering. If it were a percent of copper removed each filtration, the filter would remove more copper from a higher concentration solution and less copper from a lower concentration solution. This is not the case, since each successive filtration yielded a line that decreases by a constant amount of copper per round of filtration.
References:
Atomic Absorption Spectroscopy and Atomic Emission Spectroscopy. University of Illinois at Chicago. http://www.chem.uic.edu/chem421/aa.PDF (accessed 12/4/18).
Harris, D.C. Atomic Spectroscopy. Quantitative Chemical Analysis, 6th Ed. W. H. Freeman and Company. New York, 2003, pp. 494-516.
Lead and copper rule. (n.d.). Retrieved December 3, 2018, from United States Environmental Protection Agency website: https://www.epa.gov/dwreginfo/lead-and-copper-rule
Wahab, M. A. (2016). Interpolation and Extrapolation. Topics in System Engineering.
Weaver, J. (2018). Determination of copper by atomic absorbance. In Chem 260L: Quantitative analytical chemistry lab. Atlanta, GA.
Appendix A