Educational Objectives The student will determine the concentration of an unknown.

Transcription

1 Spectrophotometric Analysis 2011 by L. Dickerson and H. Patterson Lab Type Greener Lab. Quantitative wet lab. Students work in pairs. Educational Objectives The student will determine the concentration of an unknown. Safety Goggles and proper shoes must be worn. The instructor should consult relevant MSDS sheets prior to lab. Dispose of all chemicals down the drain. Lab Notebook Content Title, date, purpose, procedure; data tables. Equipment Red Dye #40 stock solution (~5.0 x 10-5 M) or other chromophore check stock concentration ml volumetric flask with stoppers Spectrophotometers (use λ max ~503.5 nm for red dye #40) Cuvettes with rack and caps SpectroVis Plus Spectrophotometer Waste If FD&C Red Dye # 40 is used as the chromophore, it is discarded down the drain. Solutions containing cobalt, copper, nickel, permanganate, etc. must be discarded into the chemical waste containers Page 1 of 21

2 Pre-Lab Exercise 1. Define the following terms: Chromophore, Absorbance. 2. Calculate the concentration of each of the following dilutions if the stock solution is 5.0 x 10-5 M: Molarity of target solution = Volume of stock solution used x 5.0 x 10-5 M ml Table Error! No text of specified style in document.-1 Dilution of Solutions Volume of Stock solution used Volume to be made 0.00 ml ml 4.00 ml ml 6.00 ml ml 8.00 ml ml Molarity of target solution Introduction The purpose of this experiment is the determination of the unknown concentration of a solution. The technique used to determine the concentration is UV/Visible spectrophotometry. To better understand the principles of this experiment some definitions are needed. Figure 1 shows a schematic diagram of a UV/Visible spectrophotometer. This device allows the wavelength of the incident light to be selected and measures the intensity of transmitted light passing through a sample. A narrow beam of the white light from the lamp is focused on a wavelength selection device. In the many spectrophotometers, the wavelength selection device is a grating. Other instruments may use a prism or filters to select the wavelength. From the wavelength selection device the beam of desired wavelength is passed through the sample and then strikes a sensitive photocell that produces an electric current that operates the liquid crystal display (LCD) output. Page 2 of 21

3 Figure 1: Schematic of a UV/Visible spectrophotometer. The basic law that is applicable in this experiment can be stated as follows:the absorption of light as it passes through a solution is proportional to the concentration of the absorbing species, the length of the light path, and a fundamental property of the material called the molar absorptivity. This law which written as a linear equation is: A = l c Where: A is the absorbance. The Greek letter represents the molar absorptivity. It is a measure of how well a solution will absorb light of a given wavelength λ. This is the slope of the line. The letter l is the light path length (1 cm). It is a measure of how far the light must travel through the sample. The letter c is the concentration of the solution in moles per liter (M). Page 3 of 21

4 This equation has the form of y=mx + b and a graph of this equation will pass through the origin and have a slope equal to the molar absorptivity. It is named in honor of the men who formulated the statement. The correct name is The Beer- Lambert Law, but it is commonly shortened to just Beer s Law. The solution studied in this exercise is a substance that absorbs light in the visible range, therefore the solution is colored. Three parameters must be known to carry out an experiment. 1. The length of the light path 2. The proper wavelength for the measurement 3. The molar absorptivity of the absorbing species. The light path length is determined by the size of the sample cell that holds the solution. The cells available for this experiment are one centimeter in cross section therefore the light path is one centimeter. The proper wavelength of maximum absorbance, λ max, is the best wavelength to make measurements of absorbance for the determination of concentration. It is found experimentally. The molar absorptivity of the compound will be experimentally determined by making a Calibration Curve; a graph of the absorbance of several solutions of known concentrations. The slope of the best straight line through this set of data is the molar absorptivity. The equation of the line will allow the concentration of the unknown solution to be calculated based on its experimentally determined absorbance. Page 4 of 21

5 The tasks to be completed in this lab period are: 1. Determine the wavelengths of light that correspond to colors in the visible spectrum. 2. Make serial dilutions of the lab stock solution of and calculate the concentration of each dilution. 3. Experimentally determine λ max for the solution. 4. Determine the absorbance of each of the serial dilutions. 5. Determine the absorbance of the unknown solution 6. Prepare a Calibration Curve using and determine the slope and equation of the best straight line through the data. Procedures Part 1. Observations of the Wavelength of Light. (Using Spec 20 spectrophotometers - groups of 4 students) The first activity of this experiment is an observation of the color to wavelength relationship of light. Turn on the spectrophotometer so that the lamp will heat up and its temperature will stabilize. On the left side of the instrument, the wavelength button moves the grating to select the wavelength of light in use. Set the wavelength at 400 nm. Take a strip of white paper and place it in the sample cell. The piece of paper should be aligned so that a beam passing right to left in the cell compartment will strike it. It is necessary to look down into the cell at the paper for a small colored spot. It usually helps to darken the room and in some cases shield your eyes from the room light. Record the wavelength you observe for each of the colors: red, orange, yellow, green, blue, and violet. Different students usually get slightly different values. Make a table and record the colors and their corresponding wavelengths in your notebook Page 5 of 21

6 Part 2. Dilutions of Stock Solution Preparation of the Blank Solution Fill a cuvette with DI water, label it and set it aside. This is the reference cell used to adjust the spectrophotometer. Preparation of Standard Dilutions Record the concentration and type of the stock solution and the code number on the bottle. Rinse a buret with a small washing of stock solution then fill the buret with about 18 ml of this solution. Label 3 clean, dry, ml volumetric flasks #4, #6 and #8. Prepare 3 serial dilutions. Transferring 4.0mL, 6.0 ml and 8.0 ml of the lab supplied stock solution, to each of three labeled volumetric flasks. Bring the total volume of each of the flasks to the ml mark with DI water. Stopper each flask and mix thoroughly by inverting and shaking. Transfer 4-5 ml of each solution to clean, dry cuvettes with clean plastic pipets. These are the sample cells used to prepare the calibration curve. Label the caps of each cuvette and keep the outside of the cuvettes clean, dry and free of finger smudges. Set these cuvettes aside for now. Unknown Solution Transfer approximately 5 ml of the lab supplied unknown solution to a square cuvette. Cap it, label the cap E1 and set it aside. Record the code number of the unknown in your notebook. Page 6 of 21

7 Part 3. Software and Hardware Setup 1. From the computer desktop, click the LoggerPro icon. 2. Connect the Vernier SpectroVis Plus to the computer USB port. 3. From the menu bar choose File Open. 4. Choose<smartrooms> folder [if this drive is not present choose Local Disk (D:) and open the <thawspace> folder] 5. Open the <CHEM 1211L> folder. 6. Open the Spectroscopy template file. 7. The visible spectrum should now display in the computer screen. Calibrate the SpectroVis Plus See the appendix for this procedure. Follow those steps to calibrate the SpectroVis Plus. Page 7 of 21

8 Part 4. Collection of Data Determination of λ max for the Solution with the SpectroVis Plus Follow the steps in the appendix for operation of the SpectroVis Plus. Insert one of your sample cuvettes into the sample holder and select Collect on the computer screen. The entire spectrum will be acquired for the sample and may then be printed. Acquire Beer s Law Data See the appendix for this procedure. Follow those steps to acquire the Beer s Law data. It is a much faster way to acquire data and the computer will generate your plot, slope and straight line automatically. Note for Red Dye #40, the spectrophotometer is set to wavelength ~ nm. 1 Clean Up Discard the plastic pipets in the trash can. Discard red dye solutions down the drain. If other solutions were used, these go into the chemical waste containers. Wash all glassware and rinse with tap water. Unplug the spectrophotometers and return them to the prep bench. Return the volumetric flasks to the prep bench or shelf. 1 Vol.81 No.10 October J. Chemical Education Page 8 of 21

9 Name: Date: Time: Day: M T W R F S Student Report List the wavelengths corresponding to visible light. Red Orange Yellow Green Blue Violet Experimental results: 1. The stock solution number is: 2. The concentration of the stock solution is: M 3. Wave length of maximum absorbance (λ max ) : nm 4. The unknown solution number is: 5. The concentration of the unknown solution is: M Dilution CONCENTRATION (M) ABSORBANCE (0) 0.00 M (blank) 0 (4.00/50) x molarity of stock solution (6.00/50) x molarity of stock solution (8.00/50) x molarity of stock solution Continued on next page Page 9 of 21

10 Fill in the blanks below with your data. Attach a properly titled and labeled graph of the calibration curve. Possible points Points earned Notebook 15 Set up of test specimens; dilutions and following directions 10 Lab Technique (handling glassware, having goggles, shoes, safe handling of chemicals, finishing on time) 15 GRAPH attached (both axis labeled with units, title, date name/s of student/s. 10 What is the equation of the straight line? 5 What is the molar absorptivity? (this is the slope of the line) 5 What is the absorbance of the unknown solution? 10 What is the concentration (molarity) of unknown solution? M 15 Cleaning up your area; check out. 5 Pre Lab Quiz 10 total 100 Page 10 of 21

11 Appendix 4: Using the Vernier Spectrometers Connect the SpectroVis Plus via the USB port. The screen will display the visual spectrum. Select Experiment" from the tool bar. Scroll down to Calibrate. Page 11 of 21

12 Select Spectrometer 1 A warm up dialogue box will appear for 90 seconds. Page 12 of 21

13 Place the blank (DI water) in the sample holder. And click the Finish Calibration button. When the calibration is complete, click OK. Page 13 of 21

14 Place the sample (~5.0 x10-5 M Red Dye 40 or other standard) in the spectrometer. Click the green Collect button. Page 14 of 21

15 The spectrum is collected and the plot, absorbance as a function of wavelength is displayed. Click the red Stop button. The example shown is for KMnO 4 Each student should print his/her own copy of the absorbance spectrum. Page 15 of 21

16 Select and click the Configure Spectrometer tool (rainbow). Page 16 of 21

17 In the dialogue box select Absorbance vs. Concentration. For red dye work at λ max = absorbance. Select this absorbance and click OK. Page 17 of 21

18 Beer s law data collection is ready to begin. Start with the blank (DI water) in the spectrometer, click the green Collect button. Page 18 of 21

19 Now click the blue Keep button Type in the concentration in the dialogue box and click OK. (no units) Page 19 of 21

20 Place the diluted samples in the spectrometer, click the green COLLECT button and enter the volume. No blanking between samples is required. The data table fills up and points are added to the plot. Page 20 of 21

21 When all sample data is collected, click the regression tab (arrow); the best line and slope is added to the graph. Students should print their graph after adding their name to the footer. Data takes only a few seconds to acquire. End of Experiment. Page 21 of 21