: Absorption Spectroscopy of Cobalt(II) OBJECTIVES In successfully completing this lab you will: prepare a stock solution using a volumetric flask; use a UV/Visible spectrometer to measure an absorption spectrum; measure the molar absorptivity for the aqueous cobalt(ii) ion; and determine the concentration of cobalt(ii) in an unknown. INTRODUCTION Much of what we know about the world comes through the analysis of the color of light absorbed or emitted by atoms and molecules (i.e. spectroscopy). The observation that energetically excited atoms emit light of specific wavelengths that are unique for each element led Bohr, Schrödinger, and others to formulate our present day quantum theory of electronic orbitals. Chemists use spectra for the analysis of composition, structure and concentration. Studies of spectra also led to the development of our current theory of the electronic structure of atoms and molecules. Organic chemists routinely use infrared and nuclear magnetic resonance spectroscopy to provide important information for the determination of the structures of compounds. Beer-Lambert Law. Analytical chemists utilize the relationship that the amount of light of a specific wavelength absorbed is proportional to the concentration of the absorbing species. This is known as the Beer-Lambert Law (or, frequently, Beer s Law): A = ε l c (1) where A is the absorbance, ε is a proportionality constant determined by the nature of the absorbing species (the molar absorptivity or molar extinction coefficient), l is the path length of light through the sample, and c is the concentration (molarity) of the absorbing species. The absorbance is related to the amount of light transmitted through the sample by, A = log 10 T (2) where T (transmittance) is the fraction of light at the specified wavelength that is transmitted through the sample. For example, if 10% of the light is transmitted through the sample (the other 90% being absorbed by the sample), then T = 0.10 and A = 1.0. Larger A means more light is absorbed by the sample, thus less light is transmitted. When the absorbance is in the visible part of the spectrum, larger A means that the sample looks darker or more strongly colored to your eyes. In preparation for this experiment, you should review pp. A-16 through A-19 in Zumdahl & DeCoste s Chemical Principles, which deals with the principles of spectroscopy and Beer s Law. The visible region of the electromagnetic radiation spectrum is only a small part of the spectrum utilized by chemists. The visible and ultraviolet regions of the spectrum encompass the energies necessary to promote electrons from ground state orbitals to excited orbitals. The energy of a photon of light is proportional to its frequency, E = hv. Author: Dr. David Kelley 1 1 Last Revised on 6/18/13 by JCV
Given that the speed of light is the product of its frequency and wavelength, c = λ v, the energy can be expressed as E = hc/λ. It is most important to recognize that the longer the wavelength, the lower the energy. Gamma rays, x-rays and ultraviolet radiation have shorter wavelengths and higher energy than visible light. Infrared, radio and TV have longer wavelengths and lower energy than visible light. In this experiment, you will use visible light to determine the value of the molar absorptivity, ε, for aqueous cobalt(ii) solutions. You will then use Beer s law to determine the molarity of the cobalt(ii) ion in a solution of unknown concentration. Equipment and Supplies: Ocean Optics UV/Vis Spectrometer; 2 glass cuvettes; 50 ml volumetric flask; transfer pipet with bulb; 50 ml buret; test tubes and beakers. Chemicals: cobalt(ii) nitrate hexahydrate. SAFETY PRECAUTIONS Always wear safety goggles: many reagents are corrosive, especially to your eyes. If any reagent gets in your eyes, flush with water for at least 15 minutes. Wear gloves and rinse-off any residual solutions: aqueous solutions of many transition metals can be corrosive and/or toxic. If any chemicals get on your skin, immediately rinse with water for at least 10 minutes. Neutralize any spilled reagents by sprinkling with sodium bicarbonate (NaHCO3) before disposing in a properly labeled waste container. WASTE COLLECTION Dispose of all aqueous cobalt(ii) solutions in the labeled waste container. At the end of the laboratory, wash your hands well to remove any potential residue from the aqueous cobalt(ii) solutions. EXPERIMENTAL PROCEDURE NOTE: Work in pairs (or groups of 3), but each student must write down all information in his/her own notebook. Read the provided procedure for the Ocean Optics spectrometer. Plug in the spectrometer and make sure that the computer program recognizes it. Allow the spectrometer to warm up for at least 30 minutes before taking a reference spectrum of water and a dark spectrum (no signal). A set of glass cuvettes is provided for each group and must be treated with care to avoid scratching the glass. Part 1: Preparation of the Cobalt(II) Solutions. Calculate the amount of cobalt(ii) nitrate hexahydrate [Co(NO3)2 6 H2O] needed to prepare 50 ml of a 0.15 M cobalt(ii) nitrate stock solution. Weigh out the cobalt(ii) nitrate hexahydrate and record the precise mass in your notebook. Dissolve the cobalt(ii) nitrate hexahydrate in 25 ml of deionized water in a clean small beaker. Transfer the solution to a clean 50 ml volumetric flask (you may want to rinse the volumetric flask and stopper before using them). Be sure to rinse the beaker and add these rinsings to the flask. Dilute to the mark and thoroughly mix as shown in Appendix 1. 1 2
Clean a 50 ml buret according to the instructions in Appendix 2. Rinse the buret with the cobalt(ii) nitrate solution and then fill the buret. Clean, dry, and number six test tubes from 0-5. Deliver the cobalt(ii) nitrate solution with the buret into each numbered test tube according to the table below. Table 1 1. Aqueous Cobalt(II) Sample Composition. Test Tube Volume of ~0.15 M Cobalt(II) Nitrate Volume of Deionized Water 0 0 ml 5.00 ml 1 1.00 ml 4.00 ml 2 2.00 ml 3.00 ml 3 3.00 ml 2.00 ml 4 4.00 ml 1.00 ml 5 5.00 ml 0.00 ml Record the exact amount dispensed into each test tube in your lab notebook. Pour the remaining cobalt(ii) nitrate solution in the buret into the Cobalt(II) Nitrate Solutions bottles. Rinse the buret several times with deionized water. Fill the buret with deionized water. Deliver the deionized water with the buret into each numbered test tube according to the table above. Thoroughly mix the contents of each test tube. Obtain a cobalt(ii) nitrate solution of unknown concentration from your instructor and record the unknown number in your lab notebook. At this point, have your TA check your work and initial your lab notebook before continuing. Part 2: Beer s Law. Measure the absorption spectrum of each of the knownconcentration (test tube) solutions and your unknown. Use a pipet or carefully pour each solution into the cuvette to ¾ full. Take a new water reference just before each spectrum. (Note that when you dump out a solution, some of the solution will remain in the cuvette. To avoid contaminating the next solution, you must rinse the cuvette with a little bit of the new solution a couple of times.) Clean Up. Remove all cuvettes and unplug the power cable from the spectrometer. Clean all spills (solid and liquid) containing cobalt(ii) nitrate immediately. Pour your cobalt(ii) nitrate solutions from your test tubes, volumetric flask, and cuvettes into the Cobalt(II) Nitrate Solutions bottles in the hoods. Place any used weighing paper, plastic pipets, and paper towels that contain cobalt(ii) nitrate in the Chemically Contaminated Items container. Clean your buret the directions in Appendix 2. Rinse your cuvettes and volumetric flask with distilled water multiple times and allow to air dry. Place the volumetric flask upside down in drying rack with the glass stopper next to it. Rinse all of the other glassware with tap water and then with distilled water before drying. Have your TA check your work and initial your notebook before leaving. 1 3
CALCULATIONS Use your recorded weight of cobalt(ii) nitrate to calculate the exact concentration of your stock cobalt(ii) nitrate solution in the volumetric flask. Use the concentration of the stock solution to calculate the cobalt(ii) nitrate concentration of each sample and summarize this data in an appropriate table, such as: Sample # 1 2 3 4 5 Unknown Cobalt(II) Concentration, mol/liter (M) Absorbance Using a program such as Microsoft Excel, plot the absorbance (y-axis) at the absorption maximum (near 510 nm) versus the known concentration of cobalt (II) ion (x-axis) for samples 1-5. Insert a best-fit line through the data, with a fixed intercept of zero. The plot you obtain should be linear; verify this by including the R 2 -value. Using the equation of the best-fit line, solve for the molar concentration (x) of cobalt(ii) in the unknown from its absorbance (y). Also, determine the value of ε for Co 2+ (aq) you obtained from the slope of the line. RESULTS Include the following results in the summary portion of your report. 1. Report the molar concentration of Co 2+ (aq) in your unknown, along with the corresponding unknown ID#. 2. Report the molar absorptivity (ε) for Co 2+ (aq), including appropriate units. 3. How well was Beer s Law obeyed? Using your R 2 -value, explain your answer. PRE-LAB CALCULATIONS 1. How many grams of CuSO4 5 H2O are needed to prepare 100.0 ml of a 0.15 M CuSO4 (aq) solution? 2. In order to make a solution that is 0.030 M in CuSO4, what volume of the above solution should be diluted to 5.00 ml with water? 3. A 0.10 M solution of compound B is measured in a 1.00 cm path length cuvette at a wavelength of 500 nm. The percentage of light transmitted through the cell is 38%. What is the absorbance of this sample (A) at 500 nm? Assuming Beer s Law holds, what is the value of ε for compound B at 500 nm? 4. A sample of compound B having an unknown concentration is measured to have an absorbance of 0.90 in a 1.00 cm path length cell at 500 nm. What is the concentration of B in this sample, assuming Beer s Law holds? 1 4
APPENDIX 1: Use of volumetric flask for dilution. 1 5
APPENDIX 2: Cleaning and Using Burets. Before Use: Pour out liquid in buret through the top. Refill halfway with distilled water. Drain ~5 ml out of the tip and then pour the rest out of the top. Repeat the rinse with distilled water and open the stopcock when pouring out the water to allow the water to drain out of the tip. Wipe off the top of the buret and fill with your solution (from a clean beaker) to about the 48 ml mark. Try to rinse all the way around the sides. Drain a little out of the tip into a different, waste beaker. Into the waste beaker, pour the rest out of the top of buret slowly while rotating it to coat the walls. Repeat this process. Fill the buret to the specified amount in your lab with your solution, making sure to drain a small amount out of the tip to remove any air bubbles in the tip. During Use: Start your titration by reading the initial volume in the buret (should not be 0.00 ml). Add the solution while swirling the receiving flask to mix the solutions. Read the buret to the nearest 0.01 ml after every addition. After Use: Drain remaining solution and place in proper Used Solution bottle. Rinse with distilled water as you did before using the buret, making sure to also rinse the outside of the tip. When done, fill buret with distilled water to just above the 0.00 ml mark and leave mounted in the buret holder. 1 6