Exercise 11 Page 1 Illinois Central College CHEMISTRY 130 Name: Beer's Law: Colorimetry of Copper(II) Solutions Objectives In this experiment, we will use Beer's Law to determine the unknown concentrations of Copper(II) solutions by comparing the amount of light absorbed by the unknowns to the absorbtion of light by a series of known concentrations. Copper compounds have been used extensively in the treatment of algae in municipal water supply impoundments. Consequently, recent indications that copper levels in the sediments of these impoundments are impeding plant growth have led scientists to more closely monitor Copper levels in natural waterways. Background If white light is passed through a solution containing a colored compound, certain wavelengths of light are selectively absorbed (taken in). The resultant color observed is due to the transmitted light (light which passes through). Copper (II) nitrate appears blue to the eye. This is because red light is absorbed and blue light is transmitted (Table 1). The amount of red light absorbed is directly proportional to the concentration of the copper (II) ions in the solution as defined by Beer's Law. In this experiment we will measure the absorbance of several copper (II) solutions. Table 1. Correlation between wavelength, color, and complementary color in the visible region. Wavelength, nm Color (light absorbed) Complementary color(light transmitted) 380-435 violet yellow-green 435-480 blue yellow 480-490 green-blue orange 490-500 blue-green red 500-560 green purple 560-580 yellow-green violet 580-595 yellow blue 595-610 orange green-blue 610-750 red* blue-green* *For Copper (II) nitrate the absorbtion maximum is 630 nm.
Exercise 11 Page 2 Beer's Law In 1852, Beer discovered that the transmittance of light decreases exponentially in proportion to the concentration of the species absorbing the light. The fundamental law regarding the amount of incoming light absorbed by a sample is known as Beer's Law. For example, a full bottle of cola placed beside a bottle containing 1/10 cola and 9/10 water will be drastically different in appearance. This is because there are more molecules causing coloration in the bottle of straight cola than in the diluted bottle. In other words, more of the visible light is being absorbed by the straight cola than by the diluted cola. From this example, it seems reasonable that the amount of light absorbed by a sample, denoted by A, should be proportional to the amount (or concentration, C) of light absorbing molecules in the sample. Therefore, Beer's Law can be stated most simply as: A = k x C where A is the Absorbance of light by the sample. The constant, k, depends on the path length through the sample (diameter of the container), the wavelength of the light used, and the type of absorbing sample. As shown in Table 1., the color of light that a substance absorbs is the "opposite" of the color the substance appears; the solution has the color of the light that is not absorbed. As you will see, a measurement of the absorbance, A, of a sample will allow you to find the concentration of the light-absorbing sample. In other words, you can quantitatively identify chemicals in solution by the amount of light they absorb. So, according to Beer's Law, a plot of the absorbances vs the concentrations of several samples should produce a straight line with a slope, k. Colorimeter A colorimeter measures the amount of light passing through a sample; this intensity of light is known as the transmittance. You will use a Colorimeter (a side view is shown in Figure 1) to measure the concentration of each solution. In this experiment, red light from the LED light source will pass through the solution and strike a photocell. A higher Figure 1. concentration of the colored solution absorbs more light (and transmits less) than a solution of lower concentration. The light sources in the colorimeter are light emitting diodes (LEDs). The LEDs emit a range of wavelengths with a peak, or most intense, wavelength near the center. The peak wavelengths for the colorimeter LEDs are 430 nm, 470 nm, 565 nm, 635 nm for the violet, blue, green and red colored LEDs, respectively. Due to the nature of LEDs, it is incorrect to assume that the light emitted by two
Exercise 11 Page 3 LEDs will generate a third color. Therefore, any practical use of the colorimeter will involve only one LED at a given time. Since the photocell detector simply changes resistance in proportion to the intensity of the light that strikes it, we can use the current that passes through the cell to determine the %Transmittance of the sample where outgoing light 100 incoming light %T = or %T = sample current (microamps) blank current (microamps) 100 Unfortunately, %T is not linearly proportional to Concentration. As stated before, it is an exponential relationship. However, Absorbance of light by the sample is linear with concentration. If the current reading (in microamperes) for the photocell without an absorbing specimen in the path is I o and the current reading with an absorbing sample in the path is I, (Figure 2.) then the absorbance of the sample is defined as: Light-emitting Diode (Source) Clear Cuvette Sample I o I CdS Cell (Detector) A = log( I o I ) or A = log( 100 %T ) Figure 2. Connecting the Colorimeter Connect the Vernier Colorimeter to the GoLink USB interface and connect the GoLink to the USB input on your computer. From the Menu Bar select File/Open and click on the folder Chemistry with Computers. Open the file Beer's Law.cmbl. You should now see the window displayed here. Right mouse click anywhere on the graph and choose Graph Options from the pop-up menu. Select the axes options tab and change the x-axis scaling to 0 for the left to 0.15 for the right. Use the arrow buttons on the colorimeter to select the 635 nm LED. Select a single cuvette to use for both your blank and your samples for this experiment.
Exercise 11 Page 4 Procedure Preparation of your "Standards" 1. Label five clean test tubes A through E. Fill test tube A approximately 2/3 full with 0.125 M Cu(NO 3 ) 2. Using a 5.00 ml pipette, transfer 5.00 ml of distilled water into test tubes B through E. 2. Pipette 5.00 ml of solution A into test tube B and mix well. Take care not to lose any of the solution during mixing. In a similar fashion, pipette 5.00 ml of solution B into test tube C; then 5.00 ml of solution C into test tube D; then 5.00 ml of solution D into test tube E. 3. Calculate the Molarities of each of your standards and record them on the report sheet. Note that each successive dilution cut the molarity in half. 4. Label three 50 ml beakers "Unknown 1 through 3" and obtain 10 ml samples of the three unknown Cu +2 solutions. 5. Fill one of the cuvettes with distilled water to serve as a "blank". The blank contains all the constituents used in the analysis except the substance to be measured. We can assume then that the difference in the color between the blank and the sample is due only to the substance to be measured. Distilled water is the reference blank for this experiment. 6. Insert the cuvette containing the distilled water into the opening of the colorimeter. Note that the cuvette is "ribbed" on two sides. IMPORTANT: Be certain that the light path is passing through the CLEAR sides of the cuvette facing the arrow at the top of the cuvette slot. Close the lid of the colorimeter (to keep out stray light) and press the "CAL" button on the colorimeter to calibrate it. Release the CAL button when the red LED begins to flash. When the LED stops flashing, the calibration is complete and your unit is ready to collect data. 7. Click and with the blank still in the colorimeter, click the button. You will be prompted to enter a molarity for the sample. Enter 0.0 for the molarity of the blank. Click OK. 8. Remove the cuvette from the colorimeter and empty it. Fill the cuvette with the copper(ii)nitrate solution from tube E (your most dilute standard.), insert it in the colorimeter, and close the lid. Allow a few seconds for the Absorbance reading to stabilize. Click the button and enter the molarity for the copper solution in tube E. Continue this same process until all of the known standards have been measured, working your way toward the highest Molarity. 9. Once you've finished reading your standard solutions, from the Analyze Menu, choose Linear Fit. (Or click on the Linear Fit icon found on the Toolbar.) Your graph should now look the one displayed here.
Exercise 11 Page 5 10. Now fill the cuvette with the first Unknown solution. As soon as the Absorbance reading stabilizes, choose Analyze from the Menu Bar and select Interpolation Calculator. This should create a dialogue box on your graph indicating the Molarity of your first unknown. Click and drag this dialogue box to a vacant area of the graph. 11. Now fill the cuvette with your next unknown and repeat the Analyze/Interpolation Calculator procedure. Move the dialogue box to another area of the graph. 12. Fill the cuvette with the third unknown and repeat the Analyze/Interpolation Calculator procedure. Once all three unknowns have been analyzed, record the molarities of your unknowns on your report sheet. Your graph should now look like the one shown here. 13. Print a copy of this graph to be attached to your Report Sheet. 14. Exit LoggerPro.
Exercise 11 Page 6
Exercise 11 Page 7 Illinois Central College CHEMISTRY 130 Name: REPORT SHEET Beer's Law Standards Sample Molarity %Transmittance Absorbance Blank 0.0 M 100 0 A B C D E Unknowns Sample Absorbance Molarity 1 2 3
Exercise 11 Page 8
Exercise 11 Page 9 Illinois Central College CHEMISTRY 130 Name: PRELAB: Exp.11 Beer's Law SHOW YOUR WORK 1. A substance that absorbs light at 495 nm appears to have what color? (refer to Table 1.) 2. Referring to the colorimeter in this experiment, if a sample transmits sufficient light to cause a current of 488 microamperes in the photocell compared to a blank solution that allows a current of 622 microamperes, what is the %Transmittance of the solution? 3. What would the Absorbance value be for the solution in problem #2? 4. Why does the procedure for measuring the concentration of a solution photometrically require the use of a "blank"?
Exercise 11 Page 10