Spectroscopic Determination of the Concentration of Manganese Objectives The objective of this laboratory is to use absorption spectroscopy to determine the amount of Mn 2+ in an aqueous solution. Background Manganese Manganese is a steel-gray, lustrous metal that resembles iron, except that it is harder, more brittle, and more resistant to high temperatures. It tarnishes slowly in air and is not particularly reactive to nonmetals except at elevated temperatures. Manganese makes up about 850 ppm (0.085%) of the Earth s crust, making it the 12th most abundant element there; among the heavy metals, only iron is more common. It is used in a variety of industrial applications, including steel production, corrosion-resistant aluminum alloys, alkaline batteries, and as a pigment for the coloring of ceramics and glass. Manganese occurs in many oxidation states, including +2, +3, +4, +6 and +7. The +2 cation is the most stable oxidation state for manganese and has a pale pink color in aqueous solution. The +7 cation is most often found in compounds of the intensely purple-violet permanganate anion, MnO 4. Manganese is a required trace mineral for all known living organisms. A normal human diet typically includes 2 9 mg of manganese per day. The +2 cation is the state used in living organisms for essential functions; other states are toxic for the human body. While ingesting too little manganese is not known to cause adverse effects, too much can lead to serious health problems, such as impaired motor skills and cognitive disorders. Absorption Spectroscopy Spectroscopy is one of the most powerful analytical techniques in modern science. In this experiment we will determine the quantity of manganese in an aqueous solution using visible Absorption Spectroscopy. When visible light is directed at a colored species in solution, the species will absorb some of that light at a given wavelength. The higher the concentration of the colored species, the more intense the solution color, and the more light it will absorb. In other words, the absorption of light (A) is directly proportional to the concentration of colored species in the solution. This relationship is known as Beer s Law, and may be expressed as: A = ELC where E is the molar extinction coefficient (a constant that depends characteristics of the species analyzed and the wavelength used for analysis), L is the path length (the distance traveled by light through the sample), and C is the concentration of the solution in mg/ml. In most cases L is a constant (1.0 cm in our experiment) and can therefore be factored into E. Spectroscopic Determination of Manganese Concentration Page 1 of 7
This means that Beer s Law can essentially be simplified as: A = EC Note that this is the equation of a line where the y-intercept is through the origin (b = 0): y = mx With this in mind, in this lab you will prepare a series of solutions with known manganese concentrations and measure their light absorbance. According to Beer s Law, a plot of Absorbance as a function of solution concentration should display a linear relationship; a best-fit trendline can be applied and the line equation determined. This line equation is a called a calibration equation, and the plot a calibration curve. The calibration equation may then be used to determine the manganese concentration in an unknown solution by measuring its absorbance under the same conditions. UV-Visible Spectrophotometer A UV-Visible spectrophotometer is the device used to measure how much light is absorbed by a colored species in solution at a specific wavelength. A schematic diagram of a typical spectrophotometer is shown below. incident light Io transmitted light It Sample detector lamp Rotatable prism or grating monochrometer Computer Light Absorbed, A = log(i o /I t ) A Wavelength Experimental Considerations One complication in this lab is that the aqueous solutions supplied for analysis contain manganese as Mn 2+ cations, which are a very pale pink color practically colorless. Visible absorption spectroscopy works best for solutions of deeply colored species. Fortunately, the Mn 2+ cations are easily oxidized in acidic solution to form permanganate anions, MnO 4, an intensely purple-violet species. The very intense color means that the spectrophotometric analysis can be very sensitive because the light absorption will be relatively large, even with small amounts of manganese in the sample. Potassium periodate, KIO 4, will be used to oxidize Mn 2+ to the purple-violet MnO 4 ion, according to the following balanced equation: 2 Mn 2+ (aq) + 5 KIO 4 (aq) + 3 H 2 O (l) 2 MnO 4 (aq) + 5 KIO 3 (aq) + 6 H + (aq) pale pink/colorless intense purple-violet Spectroscopic Determination of Manganese Concentration Page 2 of 7
A second complication that arises is determining a suitable wavelength to use to measure the light absorbance of the colored species in solution. It is important to choose a wavelength where the solution strongly absorbs light. Consider, for example, a red compound in an aqueous solution. If white light (containing all colors) illuminates this solution, the compound will absorb wavelengths in the yellow/green/blue region of the color spectrum, while transmitting (or reflecting) wavelengths in the red region of the spectrum. In other words, a solution that appears red in color to the eye looks red because it absorbs most colors except for red. It is the transmitted light that gives rise to the color that we see. Thus when studying a red solution it would be far better to use green light which is strongly absorbed by the solution; rather than orange or red light because these colors would be much less strongly absorbed. The stronger the absorption at a particular wavelength the more sensitive the instrument will be at that wavelength and the more accurate your results. Notice that green is the complementary color to red. A Color Wheel is a useful guide for choosing complementary colors, and one is provided below. Given that the solutions you will analyze in this lab are purple-violet what color of light would be most suitable for your absorbance measurements? Color Wheel Since each color of visible light corresponds to a characteristic wavelength, shown in the Visible Color Spectrum below, what wavelength of light would be most suitable for your absorbance measurements? Visible Color Spectrum Wavelength (nm) Spectroscopic Determination of Manganese Concentration Page 3 of 7
Due to time constraints in this lab, students will simply use the Color Wheel and Visible Color Spectrum to select a suitable wavelength to use. However, the standard best practice is to measure the absorbance of the solution in question across a range of wavelengths, and then select the wavelength for which the solution has the highest absorbance. For example, for the hypothetical red solution described, the experimenter might choose to measure the absorbance of the sample over the range of wavelengths between 480 nm and 560 nm, and then narrow-in on the wavelength that gives the greatest numeric value of the absorbance. While some spectrometers (scanning spectrometers) can do this automatically; others require changing the wavelength manually and each time the wavelength is adjusted these instruments need to be re-zeroed or they will not work properly. Procedure Chemicals Mn 2+ stock solution (~0.3 mg/ml), 9 M H 3 PO 4 (aq), KIO 4 (s), Mn 2+ unknown solution Equipment In Locker/Lab: 50-mL beaker, 400-mL beaker, 100-mL graduated cylinder, glass stirring rod, watch glass, stand, ring clamp, wire gauze, Bunsen burner, wash bottle with deionized water. From Stockroom: two 100.0-mL volumetric flasks, four 25.00-mL volumetric flasks, two cuvettes, two 5-mL volumetric pipets, one 1-mL volumetric pipet, one 2-mL volumetric pipet, one 10-mL volumetric pipet, KimWipes, UV-VIS Spectrophotometer. Note: Each pair of students will use their own locker/lab equipment. The stockroom items will be shared by teams of 4 students. Safety and Waste Disposal The PPE for this lab includes safety goggles, lab coat and nitrile gloves. Exercise appropriate caution when using the phosphoric acid (in Step 3) as it can cause serious chemical burns to your skin. Wear your nitrile gloves while handling this chemical! If any acid comes into contact with your skin or eyes, immediately rinse the affected areas with copious amounts of water for 15 minutes, and inform your instructor. Exercise appropriate caution when using the potassium periodate (in Steps 4 and 5). It is a powerful oxidant and must be kept away from open flames and combustible materials, including paper and wood. It is highly irritating to skin, eyes and mucous membranes (if inhaled). Wear your nitrile gloves while handling this chemical! If any comes into contact with your skin or eyes, immediately rinse the affected areas with copious amounts of water for 15 minutes, and inform your instructor. Many of the chemicals used in this lab are hazardous to the environment. All waste must be disposed of in the hazardous-waste container in the fume hood. Rinse all glassware directly into the waste container using a small squirt bottle to be certain all hazardous waste ends up in the waste container. Spectroscopic Determination of Manganese Concentration Page 4 of 7
Experiment Instructions General Notes Students will work in teams of 4 for this lab, but they will split into two groups of 2 Group A and Group B for some parts of the lab. Group A will prepare the Standard Solution (Part A) and Group B will prepare the Unknown Solution (Part B) at the same time. The two groups will perform the rest of lab together and share the data they collect. Several steps of this lab involve pipetting solutions with volumetric pipets. Be sure to rinse the pipets before use first with deionized water and then with a small amount of the solution you will be pipetting. There will be several spectrophotometers set out in the lab for student use. Your instructor will provide directions on how to use them correctly at the beginning of the lab session. Students must use the same spectrophotometer to collect all of their absorbance data. Part A: Preparing the Standard Solution (Group A) 1. In a clean, dry 50-mL beaker obtain roughly 10 ml of the Mn 2+ stock solution from the reagent bottle in the hood. Then, using a 5-mL volumetric pipet, accurately pipette 5.00 ml of this stock solution into a clean 400-mL beaker. Record this volume and the concentration of the stock solution (on the label of the bottle, ~0.3 mg/ml) on your report form. 2. Using a clean 100-mL graduated cylinder to measure volumes, add about 30 ml of deionized water to the Mn 2+ solution in the 400-mL beaker. 3. Wear gloves during this step. Measure out approximately 10 ml of 9 M phosphoric acid, H 3 PO 4 (aq), using the same 100-mL graduated cylinder, and add this acid to the solution in the 400-mL beaker. Be extremely careful when handling the phosphoric acid as it will cause serious burns if it comes in contact with your skin. Thoroughly rinse out the graduated cylinder with deionized water after performing this step, and pour the rinse water into the waste container. 4. Wear gloves during this step. Using a watch glass, weigh out approximately 0.03 grams of solid potassium periodate, KIO 4 (s). Be extremely careful when handling the potassium iodate as it is highly irritating to the skin and eyes. Keep it away from open flames and combustible materials, including paper. 5. Add the KIO 4 (s) to your solution and stir to dissolve it using a glass stirring rod. Rinse any KIO 4 (s) that sticks to the watch glass directly into the beaker using deionized water. Leave the stirring rod in the beaker to help control bubble formation when boiling. 6. Perform this step in the hood. Boil the solution gently in the hood (you will need to assemble a stand, ring clamp, wire gauze and Bunsen burner to do this). Be careful to avoid splattering and boiling over. The solution should turn purple within a few minutes. If this doesn t happen, you will need to add an extra 0.01 grams of the KIO 4 (s) to the solution. Continue boiling gently for another two minutes after the color changes to purple in order to be certain that the color change is complete. Then turn off the flame to avoid over-oxidation of the solution, which results in a rust-brown color. If this happens you will need to start all over again! Spectroscopic Determination of Manganese Concentration Page 5 of 7
7. Cool the solution, then pour it into a clean 100.0-mL volumetric flask using a funnel. Then rinse the beaker and the funnel with deionized water and pour this rinse water to the volumetric flask. This is to ensure that all of the manganese solution is transferred. 8. Dilute the solution in the volumetric flask with deionized water to exactly the index mark and swirl to mix thoroughly. 9. Label the flask Standard Solution to avoid any mistakes or confusion. Record the color and the total volume of this solution on your report form. 10. Calculate the concentration of MnO 4 in the Standard Solution. Since the Standard Solution was obtained by diluting the stock solution, simply by apply the dilution equation to determine its concentration: C 1 V 1 = C 2 V 2. Part B: Preparing the Unknown Solution (Group B) 11. Obtain an original sample of Mn 2+ unknown solution in a large test tube from your instructor. Record the ID code of this solution on your report form. Then, using a 5-mL volumetric pipet, accurately pipette 5.00 ml of this unknown solution into a clean 400-mL beaker. Record this volume on your report form. 12. Now follow Steps 2 8 to prepare your unknown sample. 13. When finished with Step 8 label the flask Diluted Unknown Solution to avoid any mistakes or confusion. Record the color and total volume of this solution on your report form. Part C: Preparing Dilutions of the Standard Solution (Groups A and B together) 14. Label four clean 25.00-mL volumetric flasks: 1, 2, 3, and 4. 15. Each student in the team of 4 is required to prepare one of the dilutions. Decide among yourselves which dilution each team member is assigned to prepare. Flask 1: Accurately pipette 1.00 ml of the standard solution into Flask 1 (using a 1-mL well. Label the flask Solution 1. Record the volume of standard solution pipetted and the total volume of Solution 1 on your report form. Flask 2: Accurately pipette 2.00 ml of the standard solution into Flask 2 (using a 2-mL well. Label the flask Solution 2. Record the volume of standard solution pipetted and the total volume of Solution 2 on your report form. Flask 3: Accurately pipette 5.00 ml of the standard solution into Flask 3 (using a 5-mL well. Label the flask Solution 3. Record the volume of standard solution pipetted and the total volume of Solution 3 on your report form. Flask 4: Accurately pipette 10.00 ml of the standard solution into Flask 4 (using a 10-mL well. Label the flask Solution 4. Record the volume of standard solution pipetted and the total volume of Solution 4 on your report form. Spectroscopic Determination of Manganese Concentration Page 6 of 7
16. Calculate the concentrations of MnO 4 in Solutions 1 4. Since these solutions were obtained by diluting the standard solution, simply by apply the dilution equation to determine their concentrations: C 1 V 1 = C 2 V 2. Part D: Measuring Solution Absorbances (Groups A and B together) 17. Your team will only be supplied with two cuvettes. Rinse them with deionized water and dry the outside of each cuvette using a KimWipe. 18. Use one cuvette for the blank and fill it ¾ full with deionized water. You will use the other cuvette to measure the absorbance of MnO 4 in all your prepared solutions. 19. Before measuring the absorbance of your solutions you will need to choose a wavelength and zero your spectrometer. Your instructor will show you how to zero the spectrometer using the blank. However, it is up to your team to determine the best wavelength to use for this experiment a poor choice of wavelengths will result in poor data. See the introduction to this experiment for information on how to choose a good wavelength (if necessary). 20. Once you have selected a wavelength and zeroed the instrument, measure the absorbance of MnO 4 in all your prepared solutions using your second cuvette (fill ¾ full each time) in the following order: Solution 1, Solution 2, Solution 3, Solution 4, Standard Solution, and Diluted Unknown Solution. Note that you will have to rinse this cuvette thoroughly between measurements, each time using a small amount of deionized water and then a small amount of the sample to be added. Record all your absorbance measurements on your report form. Part E: Data Analysis 21. Using the data collected for these 5 solutions (with known MnO 4 concentrations), prepare a graph of Absorbance versus Concentration with Microsoft Excel. Apply a best-fit line to the plotted data, and obtain the equation of the line and the R 2 value. Print out a full-sized copy of your graph (fully labeled!) and attach it to your report form. 22. Calculate the MnO 4 concentration in the Diluted Unknown Solution using the equation of the trendline and the measured solution absorbance. 23. Because this is a diluted solution, you will now need to calculate backwards from this concentration to determine the actual concentration of Mn 2+ in your original unknown sample. Spectroscopic Determination of Manganese Concentration Page 7 of 7