Beer's Law: Colorimetry of Copper(II) Solutions

1 Illinois Central College CHEMISTRY 130 Name:_____Beer's Law: Colorimetry of copper (II) SolutionsObjectivesIn 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 aseries of known compounds have been used extensively in the treatment of algae in municipal water supplyimpoundments. Consequently, recent indications that copper levels in the sediments of theseimpoundments are impeding plant growth have led scientists to more closely monitor copper levels innatural white light is passed through a solution containing a colored compound, certain wavelengths of lightare selectively absorbed (taken in). The resultant color observed is due to the transmitted light (lightwhich passes through).

2 copper (II) nitrate appears blue to the eye. This is because red light is absorbedand blue light is transmitted (Table 1). The amount of red light absorbed is directly proportional tothe concentration of the copper (II) ions in the solution as defined by Beer's Law. In this experimentwe will measure the absorbance of several copper (II) 1. Correlation between wavelength, color, and complementary color in the visible region. Wavelength, nm Color (light absorbed) Complementary color(light transmitted)380-435violetyellow-green435 -480blueyellow480-490green-blueorange490 -500blue-greenred500-560greenpurple560-5 80yellow-greenviolet580-595yellowblue595 -610orangegreen-blue610-750red* blue -gree n**For copper (II) nitrate the absorbtion maximum is 630 11 Page 1 Beer's Law In 1852, Beer discovered that the transmittance of light decreases exponentially in proportion to theconcentration of the species absorbing the light.

3 The fundamental law regarding the amount of incominglight absorbed by a sample is known as Beer's Law. For example, a full bottle of cola placed beside abottle containing 1/10 cola and 9/10 water will be drastically different in appearance. This is becausethere are more molecules causing coloration in the bottle of straight cola than in the diluted bottle. Inother words, more of the visible light is being absorbed by the straight cola than by the diluted 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 , Beer's Law can be stated most simply as: A=kxC where A is the Absorbanceof light by the sample.

4 The constant, k, depends on the path length through the sample (diameter of thecontainer), 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 thesubstance appears; the solution has the color of the light that is not absorbed. As you will see, ameasurement of the absorbance, A, of a sample will allow you to find the concentration of thelight-absorbing sample. In other words, you can quantitatively identify chemicals in solution by theamount of light they absorb. So, according to Beer's Law, a plot of the absorbances vs theconcentrations of several samples should produce a straight line with a slope, k. ColorimeterA colorimeter measures the amount oflight passing through a sample; thisintensity of light is known as will use a Colorimeter (a side viewis shown in Figure 1) to measure theconcentration of each solution.

5 In thisexperiment, red light from the LED lightsource will pass through the solutionand strike a photocell. A higherconcentration of the colored solutionabsorbs 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 ofwavelengths with a peak, or most intense, wavelength near the center. The peak wavelengths for thecolorimeter LEDs are 430 nm, 470 nm, 565 nm, 635 nm for the violet, blue , green and red coloredLEDs, respectively. Due to the nature of LEDs, it is incorrect to assume that the light emitted by twoExercise 11 Page 2 Figure will generate a third color. Therefore, any practical use of the colorimeter will involve only oneLED at a given the photocell detector simply changes resistance in proportion to the intensity of the light thatstrikes it, we can use the current that passes through the cell to determine the %Transmittance of thesample where %T = or %T = Unfortunately, %T is not linearly proportionalto Concentration.

6 As stated before, it is anexponential relationship. However,Absorbance of light by the sample is linearwith concentration. If the current reading (inmicroamperes) for the photocell without anabsorbing specimen in the path is Io and thecurrent reading with an absorbing sample inthe path is I, (Figure 2.) then the absorbanceof the sample is defined as: orConnecting the ColorimeterConnect the Vernier Colorimeter to theGoLink USB interface and connect theGoLink to the USB input on your the Menu Bar select File/Open and clickon the folder Chemistry with the file Beer's You shouldnow see the window displayed here. Right mouse click anywhere on the graph andchoose Graph Options from the pop-up menu. Select the axesoptions tab and change the x-axis scaling to 0 for the left to forthe the arrow buttons on the colorimeter to select the 635 nm a single cuvette to use for both your blank and your samplesfor this 11 Page 3A=log(100%T)Light-emittingDiode (Source)Clear CuvetteCdS Cell(Detector)Ioutgoinglightincomingligh t 100samplecurrent(microamps)blankcurrent( microamps) 100A=log(IoI)IoSample Figure of your "Standards"1.

7 Label five clean test tubes A through E. Fill test tube A approximately 2/3 full with M Cu(NO3)2. Using a mL pipette, transfer mL of distilled water into test tubes Bthrough Pipette mL of solution A into test tube B and mix well. Take care not to lose any of thesolution during mixing. In a similar fashion, pipette mL of solution B into test tube C; mL of solution C into test tube D; then mL of solution D into test tube Calculate the Molarities of each of your standards and record them on the report sheet. Notethat each successive dilution cut the molarity in Label three 50 mL beakers "Unknown 1 through 3" and obtain 10 mL samples of the threeunknown Cu+2 Fill one of the cuvettes with distilled water to serve as a "blank". The blank contains all theconstituents used in the analysis except the substance to be measured. We can assume then thatthe difference in the color between the blank and the sample is due only to the substance to bemeasured.

8 Distilled water is the reference blank for this Insert the cuvette containing the distilled water into the opening of the colorimeter. Note that thecuvette is "ribbed" on two sides. IMPORTANT: Be certain that the light path is passingthrough the CLEAR sides of the cuvette facing the arrow at the top of the cuvette the lid of the colorimeter (to keep out stray light) and press the "CAL" button on thecolorimeter to calibrate it. Release the CAL button when the red LED begins to flash. When theLED stops flashing, the calibration is complete and your unit is ready to collect and with the blank still in the colorimeter, click the button. You will beprompted to enter a molarity for the sample. Enter for the molarity of the blank. Remove the cuvette from the colorimeter and empty it. Fill the cuvette with the Copper(II) nitratesolution from tube E (your most dilute standard.), insert it in the colorimeter, and close the a few seconds for the Absorbance reading tostabilize.

9 Click the button and enter the molarityfor the copper solution in tube E. Continue this sameprocess until all of the known standards have beenmeasured, working your way toward the highest Once you've finished reading your standard Solutions ,from the Analyze Menu, choose Linear Fit. (Or clickon the Linear Fit icon found on the Toolbar.) Your graphshould now look the one displayed 11 Page 410. Now fill the cuvette with the first Unknown solution. As soon as the Absorbance readingstabilizes, choose Analyze from the Menu Bar and select Interpolation Calculator. Thisshould create a dialogue box on your graph indicating the Molarity of your first unknown. Clickand drag this dialogue box to a vacant area of the graph. fill the cuvette with your next unknown and repeat the Analyze/Interpolation Calculatorprocedure. Move the dialogue box to another area of the graph. the cuvette with the third unknown andrepeat the Analyze/InterpolationCalculator procedure.

10 Once all threeunknowns have been analyzed, record themolarities of your unknowns on your reportsheet. Your graph should now look like theone shown Print a copy of this graph to be attachedto your Report Exit 11 Page 5 Exercise 11 Page 6 Illinois Central CollegeCHEMISTRY 130 Name:_____REPORT SHEET Beer's LawStandardsSampleMolarity%Transmittance AbsorbanceBlank M1000 ABCDEU nknownsSampleAbsorbanceMolarity123 Exercise 11 Page 7 Exercise 11 Page 8 Illinois Central College CHEMISTRY 130 Name:_____PRELAB: Beer's Law SHOW YOUR WORK 1. A substance that absorbs light at 495 nm appears to have what color?