EMISSION AND ABSORPTION OF LIGHT

Transcription

1 EMISSION AND ABSORPTION OF LIGHT I. RATIONALE Light is a form of energy. The amount of energy corresponds to the color of the light - blue light is more energetic then red light. Atoms and molecules can absorb or release energy as light. The particular color or colors of light involved are characteristic of the substance. How much light is absorbed or emitted is a measure of how much of the substance is present. In this unit we want to see what colors are characteristic of certain elements, and how we could use the absorption of light to measure how much of a substance is present in solution. II. PREREQUISITES 1. As required In GENERAL INFORMATION - LAB, be ready to turn in a COPY of your: Equipment list 2. Because you will doing calculations today, bring a calculator with you. 3. Be ready to answer the following questions: A. We perceive different wavelengths of light as: B. A spectrophotometer is an instrument that: C. When using the spectrophotometer, a blank is: D. What must you do every time that you change the wavelength on your spectrophotometer? E. The term "wavelength of maximum absorption" means: F. If a sample whose concentration is 10 mg/ml has an absorption of 0.80 at the wavelength of maximum absorption, what is the concentration of a sample whose absorption is 0.40? G. If you fill a cuvette over half full, what should you do? III. EXERCISE 1. INTRODUCTION Light is a form of radiant energy and has wavelike properties. The distance between the peaks of the wave is called the wavelength. Visible light has wavelengths between 380 and 750 nanometers (nm). We perceive the different wavelengths of light as different colors. For example, light that has a wavelength of about 650 nm looks red to us. 57

2 What we perceive as white light is really composed of all visible wavelengths, or all colors combined. If white light is passed through a substance that absorbs red light, the light that passes through looks bluish because the red was absorbed. A spectrophotometer can measure the intensity (brightness) of each of these wavelengths from a light source, so passing this bluish light through a spectrophotometer would show us how much less intense the red light was. Different compounds absorb different wavelengths of light -- and this is the reason that they look different colors to us. The absorption spectrum (the graph of wavelength vs. absorption) shows which wavelengths are absorbed most and which are absorbed least. This pattern is characteristic for that compound, and can be used to identify it. In today s exercise you will measure the absorbance of different wavelengths of light by a solution of cobalt nitrate. You will find what wavelength of light is most strongly absorbed by cobalt nitrate, and, for extra points, can draw a graph of its absorption spectrum. Although we won t do it in this exercise, you could find which one of a bunch of unknown solutions was cobalt nitrate by comparing their absorption spectrums to this. Beer's Law says that the amount of light of any specific wavelength that is absorbed is proportional to the concentration (the amount dissolved in a milliliter of solution) of the absorbing substance. Therefor, light absorption can be used to determine the concentration of that substance in solution. The wavelength most sensitive for measuring the concentration of a particular substance will be the wavelength that is most strongly absorbed. The wavelength which is most strongly absorbed is known as the wavelength of maximum absorption, or ëmax (ë is the Greek letter pronounced "lambda"). This wavelength can be determined by measuring the absorbance of a substance at various wavelengths. You will determine the concentration of two solutions of cobalt nitrate today. Atoms which have absorbed energy can release this energy as specific wavelengths of light. The wavelengths emitted can be used to identify the element emitting the light. Although a spectrophotometer for this purpose (called a flame photometer) would be more reliable, you will identify an unknown metal in this exercise by visually observing the light it emits. A spectrophotometer is a sensitive instrument that 1) produces light of any specific wavelength (color) and then 2) measures the intensity of light after it passes through a sample. 58

3 59 We have two similar spectrophotometers in the lab: the Spectronic 20 and the Unico. Spectronic 20 Unico Spectrophotometer 2. WAVELENGTH OF MAXIMUM ABSORBANCE If it is not already on, turn on the Spectronic 20 with the Infinity Adjust knob and the Unico with the switch on the back right. Allow it to warm up for about 10 minutes. If you are using a Unico spectrophotometer, also make sure the Filter Lever (front left side) is pulled out so the green dot can be seen in the round opening and make sure Mode switch is set to Abs (absorbance.) Rinse two cuvettes (pronounced Q-vets - special tubes for use with the spectrophotometer) with purified water, drain, and wipe the outside dry. NOTE: There are glass bottles of each of three pink samples to be used today. Two are labeled as Unknowns and are to be used in Part 3. The third has the name cobalt nitrate and formula, Co(NO 3) 2, of the compound to be used now and its concentration (how much is dissolved per ml). This contains your "known" sample for Part 3.

4 Use a disposable dropper to place some of the 40 mg/ml cobalt nitrate solution (not an Unknown) into a cuvette until the cuvette is at least half full. (If you fill to a little over half full, don t bother to take the excess out - it doesn t matter.) Use a disposable dropper to place purified water into the second cuvette until it is also at least half full. Be sure that both cuvettes are clean and dry, free of lint, bubbles, or fingerprints over their lower halves. Stand them up in a clean, dry test tube rack. Determine the wavelength of maximum absorbance according to this procedure: A. Repeat the following Steps B to D every 10 nanometers from 450 nm to 560 nm (450 nm, 460 nm, 470 nm, nm) B. Set the wavelength to the value desired, starting at 450 nm. C. Skip to step D if you are using a Unico spectrophotomer. With the sample holder empty and the cover closed, set the meter to read (INFINITE) ABSORBANCE (lower scale) by turning the Infinity Adjust Knob (front left) on the Spectronic 20: Set the needle to the LINE at the MIRROR which runs to the symbol, not to the symbol itself. D. Place the blank (the cuvette containing water) into the sample holder and close the cover. Set the meter to read 0 ABSORBANCE (on the lower scale if you have a Spectronic 20) with the Zero Adjust Knob (front right). E. WITHOUT TOUCHING ANY OF THE CONTROLS, remove the blank and place the cuvette containing your sample into the sample holder. Close the cover and record the ABSORBANCE (On the Spectronic 20 this is the lower scale; read from RIGHT to LEFT) in the appropriate spaces below. If you do not read an absorbance at 450 nm of 0.30 to 0.40, ask for help. From this data you can see what the wavelength of maximum absorption (ë max) is for the sample. 450 nm 480 nm 510 nm 540 nm 460 nm 490 nm 520 nm 550 nm 470 nm 500 nm 530 nm 560 nm According to these results, the WAVELENGTH of maximum absorbance is nm. 60 Pour all waste solutions today into the "Inorganic" container in the hood.

5 61 3. THE CONCENTRATION OF THE COBALT NITRATE UNKNOWNS Make sure that you use the Cobalt Nitrate Unknowns (pink solutions) in this part of the exercise. According to the label on the bottle, the concentration of your Known is mg/ml The absorbance of your Known at ëmax was Rinse and drain the cuvette from the previous sample, then fill at least half way with the Unknown A. Set the wavelength control to the wavelength of maximum absorbance that you determined in Part 2, not to 450 nm. Skip this paragraph if you are using a Unico spectrophotomer. With the sample holder empty and the cover closed, set the meter to read (INFINITE) ABSORBANCE (lower scale) by turning the Infinity Adjust Knob (front left) on the Spectronic 20: Set the needle to the LINE at the MIRROR which runs to the symbol, not to the symbol itself. Place the blank (cuvette containing water) into the sample holder and close the cover. Set the meter to read 0 ABSORBANCE (lower scale of the Spectronic 20) with the Zero Adjust Knob (front right). WITHOUT TOUCHING ANY OF THE CONTROLS, remove the blank and place your cuvette with Unknown A into the sample holder. Close the cover and record the ABSORBANCE. Unknown A: Discard the solution, rinse, and drain the cuvette, then fill at least half way with the Unknown B. Place it into the sample holder and measure the absorbance. DO NOT MAKE ANY ADJUSTMENTS! Record the absorbance: Unknown B: Using the absorbance at ë max (determined in Part 2) for the compound of known concentration, you should be able to calculate the amount of unknown in each sample using the following equation (once for each unknown): Concentration of unknown = (absorbance of unknown) x (concentration of known) (absorbance of known)

6 62 Concentrations of the Unknowns (round off to a whole number): Unknown A: Unknown B: Waste Disposal: Pour all solutions from your cuvettes into the container labeled "Inorganic". NOTE: Rinse your cuvettes with PURIFIED WATER, not tap water, then drain and return them to the cuvette box. 4. EMISSION SPECTRA Light a bunsen burner and adjust the flame so that it is tall and hot. Bring each solution containing a metallic ion to your desk. Remove a little of the solution with the loop of the wire in the solution. Hold the loop just above the inner blue cone of the bunsen burner flame and watch for the color imparted to the flame. Watch carefully for the first sputter of color, which sometimes lasts only for a moment. Ignore the "false color" created when the wire alone cools the flame, which then becomes dark orange, after the real color disappears. Record the color seen: barium lithium sodium copper potassium Characteristic colors of light given off by hot ions are called emission spectra. Emission spectra are so characteristic of the elements producing the light that they can be used to identify elements in stars. Emission spectra are also responsible for the colors seen in the flames of flares and fireworks. Do a flame test for one of the Metallic Ion Unknowns (not the cobalt nitrate unknowns!). The Unknown is # The Unknown flame color is Based on your results, which metal ion is in your Unknown? Last updated: 17 March 2009