What Is Color? The origin of an object's color depends upon the nature of the object. The simplest color to investigate is the color of light itself.

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1 What Is Color? "Light and color." These two concepts may seem so simple, we hardly even think about them. Perhaps it is because of the pervasive nature of these two phenomena in our world that we tend to take them for granted, seldom pausing to consider the principles that make light and color the way they are. The concepts of light and color transcend the (artificial) boundaries of art, chemistry, biology and physics, and influence our day-to-day lives in countless ways. To chemists, light and color provide basic and powerful tools for exploring chemical systems, as you will see throughout this semester. The origin of an object's color depends upon the nature of the object. The simplest color to investigate is the color of light itself. THE COLOR OF LIGHT To observe the color of light, you will use a spectrometer (Spec20) as the light source. The instructor will briefly describe the workings of the spectrometer to you, but for now it will suffice to say that a spectrometer breaks up white light into its constituents. The constituents of white light, the colors, are distinguished by their wavelength, which is related to the frequency with which the light wave vibrates. The dial on top of the spectrometer chooses the wavelength of light that is used. Turning the wavelength dial changes the reading on a scale, which shows the wavelength of the chosen light in nanometers (nm, 10-9 meter). The knob on the right front of the spectrometer changes the intensity of the light, which is completely independent from the wavelength of the light. We will deal with the left knob later. To observe the color of light, turn on the spectrometer by turning the left front knob clockwise until it clicks. Now turn up the intensity of the light by turning the intensity knob (left knob) clockwise until you feel resistance. Set the wavelength dial to around 550 nm. Slowly lower the test tube, containing boiling chips, into the sample compartment until the boiling chips pass through a light beam and are colored. Leaving the test tube at this height, slowly turn the wavelength dial, noting the effect of wavelength on color of the light. 1

2 In your notebook, make a table as follows: Color Observed red red-orange orange orange-yellow yellow yellow-green green green-blue blue blue-violet violet Wavelength (nm) Use the wavelength dial to find the wavelength (or wavelength range) corresponding to each color and record the results in your table. Now you have a reference table to refer to when wavelengths are discussed that can connect wavelength to color. THE COLOR OF MATERIALS The boiling chips are green when green light comes from them to your eye. They could not look green if green light were absorbed by them, because then no green light would reach your eye. By inference, any other material is green when green light comes from it, and is therefore not absorbed. To further explore the color of objects, you will again use the spectrometer to quantitatively measure the amount of light at each wavelength that is passed or absorbed by a liquid material. To measure the amount of light that passes through a material, chemists use the terms transmittance and absorbance. Transmittance simply reports the fraction of light that passes through a material. For example, a transmittance of 0.25 means that 25% of the light passes through a material and makes it out the other side. Often, transmittance will be reported as percent transmittance (%T). Absorbance (A), which can be very useful at times, is given by the equation, this calculation is automatically performed by the LoggerPro A = -log 10 (T) 2

3 Therefore, a material with %T = 25% will be have an absorbance of -log 10 (0.25) = A high transmittance and low absorbance results from most light passing through the sample. Low transmittance and high absorbance results from most light being absorbed. General: It is important to note that for every material, the absorbance or transmittance strongly depends on the wavelength of light passing through the material. The SpectroVis Plus will scan through all the wavelengths in the visible spectrum ( nm). The intensity of the light that passes through the sample is compared to the intensity of light that passes through a blank (distilled water for this experiment), which absorbs none of the light. The intensity of light passing through the sample is expressed as a percentage of the blank s intensity, called percent transmittance (%T). Calibration: Open LoggerPro, and plug in the USB cable for the SpectroVis Plus once LoggerPro is fully open, and you should see the visible spectrum on the screen. Place a blank in the sample compartment (cuvette ¾ full of distilled water). Usually, the blank will be a cuvette containing the solvent to be used in the experiment, such as water. Be sure to wipe the blank with a Kim-Wipe and line up the cuvette so that the clear side is in the light path of the SpectroVis Plus. In LoggerPro choose Experiment Calibrate Spectrometer:1. Wait for the SpectroVis Plus to warm up if necessary and follow the directions on the calibrate dialog box and click OK when calibration is complete. Collecting Spectra for Colored Solutions: Fill a vial ¾ full of the colored solution to be tested, place in SprectroVis Plus, making sure to align the white mark on the tube with the violet mark, and hit Collect to obtain the spectra. Once the spectra has been collected click Stop. Repeat with the other two colored solutions, clicking on store latest run after hitting collect button. Once all three colored solutions have been monitored print the graph and label which spectrum belongs to which color. Using a table similar to the one below, compare the color you see to the colors strongly absorbed (high absorbance) and transmitted (low absorbance). Determine the colors absorbed and transmitted by comparing the wavelengths in the spectrum to the reference table you constructed earlier. 3

4 Observed Wavelengths Colors most Wavelengths Colors least color of most absorbed absorbed least absorbed absorbed solution (nm) A>0.3 A>0.3 (nm) A<0.3 A<0.3 Lab concept and introductory text from "What Color is Your T-Shirt? Reflections on an Absorbing Question," Anne M. Wagner, Virginia Larner, Esra Yavuz and Judith Ann Halstead, Department of Chemistry and Physics, Skidmore College, Saratoga Springs, NY 12866; Mary Campbell, Department of Chemistry, Mt. Holyoke College, South Hadley, MA 01075; Susan B. Piepho, Department of Chemistry, Sweet Briar College, Sweet Briar, VA

5 The Chemist s Use of Color Objects can absorb light in a process that raises their energy level. The energy of the light becomes part of the object s energy. As one might expect, a greater concentrations of light-absorbing substance will absorb more light than a lower concentration. Chemists take advantage of this fact in quantitative spectroscopy, which is the focus of this lab period. Collection of the data You will need to calibrate the SpectroVis Plus, same procedure as above. Prepare a blank by filling a cuvette ¾ full with distilled water. To correctly use a colorimeter, remember: All sample tubes should be wiped clean and dry on the outside with a tissue. All solutions should be free of bubbles. Always position the tube with its reference mark (if it has one) facing toward the reference mark in front of the sample holder on the colorimeter. Choose the wavelength of a color that is strongly absorbed by the dye solution. Discuss your choice of wavelength with your instructor before proceeding. Pipette 3.0 ml of red (1.8 x 10-5 M) solution into an empty screw-top test tube. Return the tube to the spectrometer. Hit the collect to collect the full spectra of the red solution. LoggerPro will automatically monitor all future measurements at the peak wavelength of the spectra. Prepare the spectrometer to measure the absorbance of the solution, by choosing Experiment Data Collection Events with Entry. The column name should be concentration and the units are M. Hit Collect, and then Keep to store the absorbance data. You will be asked to input the concentration which must be entered in the proper format (1.8 x 10-5 M, you have to enter 1.8E-5 into LoggerPro). Pipette 0.5 ml of distilled water into the tube. Cap the tube and shake to mix thoroughly. Record the absorbance of this solution. Calculate the concentration of the red dye in the solution (as described below), and store that value as the keyboard entry. Repeat the addition of water and record the absorbances of at least eight total solutions. Analysis of the data 5

6 the equation Calculate the concentration of red dye, [red], in the solution, based on its dilution by water using (concentration of red in the diluted sample) * (volume of the diluted sample) equals (original concentration of red)* (volume of the original dye solution) or, M 1 *V 1 = M 2 *V 2 Your job now is to determine the relationship between [red] and absorbance. Create a plot of absorbance vs. [red]. The concentration of the red and the absorbance of the solution should be linearly related. Use the Linear fit button on the toolbar of LoggerPro to determine the slope, intercept and correlation of the data. Devise a mathematical equation (Beers Law) that relates [red] to absorbance, so that if you are presented with the absorbance of any dye solution, you could identify the concentration of dye responsible for the absorption. Print out this plot. Analysis of an unknown solution As directed by the instructor, record the absorbance of an unknown test solution of red dye, provided in lab. From the absorbance, determine the concentration of red in the solution using the equation of the line you used to fit your data. 6