Lab 2: Soda Pop Spectroscopy - Optical Spectroscopy | PHYS 552, Lab Reports of Optics

Download Lab 2: Soda Pop Spectroscopy - Optical Spectroscopy | PHYS 552 and more Lab Reports Optics in PDF only on Docsity! Physics 552 Optical Spectroscopy (Fall ‘08) - 1 - Lab 2: Soda pop spectroscopy The topic of today’s experiments is transmission spectroscopy. We will study the absorption of light in soda-pop beverages, correlating the listed ingredients with the features in the optical absorption spectra and testing the limits of the human eye to recognize color. The key elements to be covered include: 1. Instrumentation () UV-vis absorption spectrophotometer 2. Absorption spectra () Beer’s Law and the extinction coefficient () Oscillator strength () Correction for cuvette/solvent background 3. Organic chromophores () Conjugated bonds () Resonance delocalization Table of Contents: I. Agilent UV-vis spectrophotometer II. Lab instructions & questions III. Sample descriptions IV. Soda-pop ingredients V. Chemical structures • The quartz cuvettes we are using are expensive (~$150 @). Be careful with them. • Please do NOT turn off spectrometers when you leave. Physics 552 Optical Spectroscopy (Fall ‘08) - 2 - I. Instrumentation: Agilent UV-Vis Spectrophotometer Remarks: The Agilent UV-vis spectrophotometer differs from the more common set-ups used for optical transmission measurements. The major difference is that the sample is illuminated with “white” light (i.e. no monochromator is used). Wavelength separation is achieved by using a diffraction grating and diode array placed after the sample. This offers the distinct advantage of being able to collect all wavelengths simultaneously instead of cycling through the range one wavelength at a time. Thus, the Agilent equipment is much more rapid than most other spectrophotometers (think seconds instead of minutes). How it works: (1) Two lamps are needed to cover the optical range. The tungsten lamp emits light in the visible and near-IR from ~370-1100 nm and the deuterium lamp covers the near-UV from ~190-370 nm. (2) Light from both lamps is collected into a single beam and passes through the collimating lens to shape a parallel beam directed toward the sample. (3) After passing through the sample, the spectrograph lens focuses the beam onto the holographic grating. (4) The diffraction grating separates the transmitted light by refracting it at different angles according to the wavelength. (5) The diffracted light then reaches the detector: a diode array composed of 1,024 photodiode elements. Each diode selectively collects the signal from one wavelength. Physics 552 Optical Spectroscopy (Fall ‘08) - 5 - Measurements (1) Water. First autozero the instrument to blank air (no cuvette), then measure the absorption spectrum of water. Note the strong peak and two smaller features (bumps or shoulders) in the near-IR. These are caused by absorption through vibrational modes of the water molecules. Save the spectrum, then autozero the instrument to water. This will set the base-line for all remaining measurements. (2) Mountain Dew, Live Wire, Code Red, and Voltage (1x and 20x dilutions). Measure the absorption spectra for the four versions of Mountain Dew at 1x and 20x dilutions. Note the peaks in the near-UV and visible. The color of the sodas is due to subtractive effects (i.e. some of the wavelengths in the incident white light are absorbed and the eye sees the colors that remain). Save all eight spectra. (3) Sprite (20x dilution). Measure the absorption spectrum for this sample. Sprite is a good control measurement for the colors, as well as some of the organic compounds that are found in the Mountain Dew sodas, but not in Sprite. Note the changes in the near-UV peaks. Comparing ingredients, you should be able to guess a likely candidate for the chemical origin of at least one of the UV peaks. Save the spectrum. (4) Sugar, caffeine, erythorbic acid, and benzoic acid. Measure the spectra for these organic compounds. You should now be able to identify the sources of the near-UV peaks in the soda- pop spectra. Look at the chemical structures of the organic compounds and attempt to correlate them with the spectra. What is a common feature for all of the strong absorbers? Save all four spectra. (5) Jones Pure Cane Soda Blue Bubblegum and Coca-Cola (1x and 20x dilutions). Measure the absorption spectra for these samples. Save all spectra. (6) Food coloring. Measure the spectra for the four food coloring solutions. This may help you identify the peaks for the dyes in the beverages. (7) ‘Just barely visible’. It is time to test the limits of your color vision. The food coloring solutions have been prepared in a series of dilutions. For each series, identify the dilution that is at the limit of your ability to recognize color. Record this value for later use. Physics 552 Optical Spectroscopy (Fall ‘08) - 6 - Lab Report Questions (1) Calculate the extinction coefficient of water at its absorption peak in the near-IR. The concentration is ~55 mol/L and the path length is 1 cm. You must “correct” the measured OD for cuvette-water reflections first. Do this by eye, extrapolating from the OD values in the visible (roughly constant). water absorption / near-IR peak wavelength (nm) extinction coefficient (L/mol-cm) (2) Calculate the peak extinction coefficients for the organic compounds. Use the concentrations given on the sample vials and, if necessary, account for any dilutions made. Report the values for all significant peaks in the near-UV. If there is no discernable peak, enter N/A in the table and explain why in terms of a difference in chemical structure compared to other compounds. If you find a negative absorbance for any of the solutions, explain why, recalling what the measurement is relative to. You do NOT have to turn in absorption spectra plots for the organic compounds but it might be helpful (for this and later questions) for you to look at a plot of the absorption spectra for the four compounds in the near-UV (200-380nm). organics absorption / near-UV compound peak wavelength (nm) extinction coefficient (L/mol-cm) sugar benzoic acid erythorbic acid caffeine (3) Prepare a plot of the absorption spectra in the near-UV (200-380 nm) for Mountain Dew, Coca-Cola Classic and Sprite at 20x dilution. On the plot, label each peak with the compound(s) responsible for the absorption. Using the calculated extinction coefficients, estimate the amount of caffeine and sodium in these sodas. Are your results consistent with the product labeling (if provided)? Is the calculated amount of sodium a minimum or maximum value (refer to ingredients)? caffeine & sodium content soda caffeine (mg/L) sodium (mg/L) Mountain Dew Coke Classic Sprite Physics 552 Optical Spectroscopy (Fall ‘08) - 7 - (4) Prepare a plot of the absorption spectra in the visible (380-700 nm) for Mountain Dew, Live Wire, Code Red, Voltage and Jones Pure Cane Sugar Blue Bubblegum at 1x concentration. Identify the locations of the dyes yellow 5 or 6, red 40, and blue 1. That is, add a label for each dye with an arrow pointing to the approximate wavelength of its absorption peak. (5) The oscillator strength is a good measure of the overall utility of a dye to provide color to an object or solution – something like the ‘most bang for the buck’. A rule of thumb suggests that for the healthy human eye to detect color in a solution (just barely visible), the product of the oscillator strength (f) and concentration (C) for the dye must be greater than 5x10-7 mol/L. Check this rule of thumb for the four food coloring solutions. To do this calculation, use the equation given for the oscillator strength in the reading material and substitute the absorption coefficient and concentration to yield: ...... 1032.4 22112 2 +⋅+⋅= ⋅⋅ =⋅ ∫∑ − CfCf dA n Cf λ λ Calculate this quantity for the beverages at 1x concentration using the wavelength range from 380 to 700 nm. Properly scale the ∑f·C quantity to account for the dilution that is ‘just barely visible’. Report the value for each individual color and calculate the average value for the four food coloring solutions. Comment on the results. Are the numbers comparable for the different colors? If not, what is a possible reason why? oscillator-concentration product / visible Food coloring ∑f·C (mol/L) yellow red green blue Average Value (6) In performing today’s experiments, it was important to first equilibrate the beverages to ambient conditions. This includes bringing them to room temperature and removing the carbonation from the sodas. Why is this important? That is, how can the temperature and carbonation have an effect on the measured spectra? Hints: It has nothing to do with the chromophores, color, or absorption. The answers would be the same for a measurement made on cold, carbonated water. Why do we set cold drinks on coasters? Why don’t we shake sodas before opening them? (7) Extra Credit Repeat (3) and (4) for each of your Diet Mountain Dew and Red Bull. Compare the Mountain Dew to the Diet Mountain Dew. Are there any differences between Red Bull and the sodas. Physics 552 Optical Spectroscopy (Fall ‘08) - 10 - Code Red CARBONATED WATER, HIGH FRUCTOSE CORN SYRUP, ORANGE JUICE CONCENTRATE, CITRIC ACID, SODIUM HEXAMETAPHOSPHATE (TO PROTECT FLAVOR), SODIUM BENZOATE (PRESERVES FRESHNESS), NATURAL FLAVOR, CAFFEINE, SODIUM CITRATE, GUM ARABIC, CALCIUM DISODIUM EDTA (TO PROTECT FLAVOR), RED 40, BROMINATED VEGETABLE OIL, YELLOW 5, BLUE 1 Serving size 8 fl oz (240 mL), sugars 31 g, sodium 70 mg, caffeine 36 mg, potassium 5 mg, phosphorous 35 mg. Sprite Carbonated water, high fructose corn syrup, citric acid, natural flavors, sodium citrate, sodium benzoate. Serving size 8 fl oz (240 mL), sugars 26 g, caffeine NONE, sodium 45 mg. Jones Pure Cane Soda Blue Bubblegum Carbonated water, inverted cane sugar, natural and artificial flavors, citric acid, sodium benzoate and potassium sorbate (as preservatives), blue1. Serving size 12 fl oz (355 mL), Sugars 48g, Sodium 30mg. Coca Cola Classic Carbonated Water, High Fructose Corn Syrup, Caramel Color, Phosphoric Acid, Natural Flavors, Caffeine. Serving Size 8 fl oz (240 mL), sugars 27g, caffeine, sodium 35mg, caffeine 23 mg. Diet Mountain Dew Carbonated water, concentrated orange juice, citric acid, natural flavors, citrus pectin, potassium benzoate (preserves freshness), aspartame, potassium citrate, caffeine, sodium citrate, acesulfame potassium, sucralose, gum arabic, sodium benzoate (preserves freshness), calcium disodium EDTA (to protect flavor), brominated vegetable oil, yellow 5. Serving size 8 fl oz (240 mL), sugars 0g, sodium 35mg. Red Bull Carbonated water, sucrose, glucose, sodium citrate, taurine, glucuronolactone, caffeine, inositol, niacinamide, calcium panthothenate, pyridoxine HCl, Vitamine B12, Natural and Artificial Flavors, Colors. Serving Size 8.3 fl oz (250 mL), sugars 27g, sodium, 200mg. Physics 552 Optical Spectroscopy (Fall ‘08) - 11 - C A F F E IN E H H H H H H HH H H H H H H HH S U G A R H B E N Z O IC A C ID E R Y T H O R B IC A C ID Y E L L O W 6 (S U N S E T Y E L L O W ) B L U E 1 (N E P T U N E B L U E ) R E D 40 (A L L U R A R E D ) Y E L L O W 5 (T A R T A Z IN E ) C A F F E IN E C A F F E IN E H H H H H H HH H H H H H H HH S U G A R H H H H H H HH H H H H H H HH S U G A R H B E N Z O IC A C ID H B E N Z O IC A C ID E R Y T H O R B IC A C ID E R Y T H O R B IC A C ID Y E L L O W 6 (S U N S E T Y E L L O W ) Y E L L O W 6 (S U N S E T Y E L L O W ) B L U E 1 (N E P T U N E B L U E ) B L U E 1 (N E P T U N E B L U E ) R E D 40 (A L L U R A R E D ) R E D 40 (A L L U R A R E D ) Y E L L O W 5 (T A R T A Z IN E ) Y E L L O W 5 (T A R T A Z IN E )