THE SPEC-20. Table 1: Absorbed Wavelength, Absorbed Color, and Perceived Color Absorbed Wavelength (nm) Absorbed Color

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1 THE SPEC-20 The electromagnetic spectrum consists of radiation ranging from high energy cosmic and gamma rays to low energy radio waves. Various regions of the electromagnetic spectrum can be used to probe molecules and atoms for information about molecular and atomic structure, bond energies, concentration etc. The Spec-20 is an instrument that uses the visible region of light to collect information about colored solutions by observing the attenuation or "diminishment" of a wavelength of light as it passes through the solution. Visible light consists of wavelengths ranging from 380 nm (blue violet) to 720 nm (red). When all wavelengths of visible light are present, the light appears "white" to our eyes. If any wavelength is removed (absorbed), the remaining combination of wavelengths of light that fall on our eyes is perceived by our brains as the "complimentary" color (Table 1). Table 1: Absorbed Wavelength, Absorbed Color, and Perceived Color Absorbed Wavelength (nm) Absorbed Color Perceived (Transmitted) Color 400 violet green - yellow 450 indigo yellow 480 blue orange 490 blue-green red 530 green purple 570 yellow-green dark blue 600 orange blue 650 red green Many transition metal complexes absorb wavelengths of light in the visible range and, as a result, their crystals and solutions are brightly colored. The energy of the absorbed photon is used to excite d-orbital electrons from lower to higher energy levels. If we allow white light to pass through a test tube containing a solution of copper (II) ion, the solution will appear blue colored because the Cu 2+ ion absorbs a photon of light in the

2 nm range. In order to describe the amount of light passing through the solution, we use two terms called "transmittance" and "absorbance". Transmittance (T), or percent transmittance (%T) is defined as the ratio of the amount of light leaving the sample to the amount of light entering the sample (or transmitted light, I, to incident light, Io) as shown in Figure 1 below. (1) T = I or %T = I x 100 I 0 I 0 I 0 I 0 : Incident Light (light entering sample) I I: Transmitted Light (light leaving sample) Figure 1. Absorbance and Transmittance of Light by a Sample Absorbance (A) is a term that refers to the amount of light reduction as it passes through a solution. Absorbance and transmittance are interchangeable terms: (2) A = log (1/T) = log T or A = log %T For very dilute solutions, the amount of light absorbed at a specific wavelength is directly proportional to the concentration of the solution. This relationship is commonly called Beer's Law (3). Refer to your textbook for a detailed discussion of Beer's Law. (3) A = ε C l A = absorbance (no units) ε = molar absorptivity coefficient (units = L/mol-cm) C = concentration of absorbing species (units = mol/l) l = path length (units = cm) 2

3 An instrument that can measure the absolute or relative intensities of light passing through a sample is called a spectrophotometer or spectrometer. The spectrometer mechanism consists of a light source, focusing lenses, a diffraction grating or prism to split light into different wavelengths, a sample holder or "cell", a photosensitive detector which measures the light passing through the sample, an amplifier, and an output device such as a meter or recorder. A schematic of a spectrometer is shown in Figure 2. Spectrometers can be constructed to utilize any region of the radiation spectrum. In General Chemistry Lab students will use an instrument that has a tungsten light source and movable diffraction grating constructed for the visible region of the spectrum. A movable cam allows students to manually scan a range of wavelengths from 300 to 900 nm. This instrument is called a "Spec 20" and is manufactured by Bausch and Lomb. Procedure for Using a Spec 20 General Information Pour the solution to be measured into a special test tube called a "cuvette" filling it to about 2/3 full. Cuvettes often are distinguished by markings such as "B&L" (for Bausch and Lomb), a daisy, or a vertical white line on the upper lip of the tube. At least two cuvettes are to be used when measuring a sample. One will contain the reference or "blank" solution (solvent only) and the other contains the sample dissolved in the same solvent. Wipe the outside of a cuvette with a tissue before placing it in the sample holder. Beads of water, fingerprints, or bubbles in the solution will interfere with transmission of light through the sample. Replace any cuvette that has a scratch on the glass. When inserting a cuvette into the sample holder, use gentle pressure (never extreme force), and take care not to spill solutions into the cell. To ensure reproducibility of the path length of light between measurements, line up the index mark on the edge of the sample holder with the markings on the lip of the cuvette (daisy, etc.). Immediately report any problems with a Spec 20 to your TA or to the stockroom. 3

4 [1] [10] [2] [3] [6] [8] [7] [9] [5] [4] [1] Tungsten Lamp [2] Field Lense & Entrance Slit [3] Objective Lens [4] Grating [5] Wavelength Cam [6] Light Control [7] Exit Slit [8] Sample [9] Filter [10] Measuring Phototube Figure 2. Internal View of a Spectrophotometer Meter Sample Compartment Wavelength Control Zeroing & On/Off Light Control Figure 3. External View of a Bausch and Lomb Spectrophotometer ( Spec 20 ) Absorbance and percent transmittance are read directly from the meter and increase (or decrease) in OPPOSITE DIRECTIONS. Both A and %T values for a sample should be recorded but generally the %T readings are more accurate. Note that the meter has a mirrored back. The observer's eye, needle, and the needle's reflection in the mirror must be in a straight line to avoid a parallax error (The perspective in Figure 4 would give an incorrect reading, the needle s reflection should not be seen). When read correctly, the reflected image of the needle should be directly beneath and essentially obscured by the needle itself. (Figure 4.) 4

5 Figure 4. Close up view of Spec 20 Meter. To Measure the Absorbance (or %T) of a Sample Turn the machine on and let it warm up for 10 minutes before use. Turn the wavelength meter to the desired setting. With NO cuvette in the sample holder, close the sample holder cover and turn the zeroing knob until the meter reads infinite absorbance (T = 0%). The Spec 20 is now "zeroed"; that is, set to ignore any stray light that might leak into the closed sample cell. Wipe the reference cuvette containing only the solvent solution (the blank) and insert it. Align it with Figure 5. Sample Insertion into Spec 20. Note: White dash mark near top of cuvette is pointing forward. 5

6 the mark in the sample holder, close the cover, and turn the light control knob until the absorbance reads zero (T = 100%). The Spec 20 is now "calibrated" (set to ignore any absorption caused by the solvent or the glass). Remove the blank cuvette. Repeat if necessary until the machine reads T = 0% without a sample and T = 100% with the blank. The machine is now calibrated. Take the cuvette containing the sample solution, wipe it, align the mark in the holder (Figure 5), close cover, and record both A and %T. Absorption Spectrum The absorption spectrum is a plot of the absorbance of a sample as a function of wavelength. This plot can be used to help identify an unknown sample since some species have characteristic absorption spectra. To construct an absorption spectrum, zero and calibrate the instrument at a selected wavelength using the blank solution. Next, measure A and %T for the sample. Remove the sample and change the wavelength to a new setting. Rezero, recalibrate, and measure A and %T at the new wavelength using the same sample. Continue to make measurements over a specified range of wavelengths. Figure 6 shows the absorbance spectrum in the visible region for a complex metal ion. Maximum absorption for this ion occurs at a wavelength of approx. 560 nm. 0.5 Absorbance vs. Wavelength Absorbance Wavelength (nm) Figure 6: A Plot of Absorbance vs. Wavelength for a Metal Ion Complex 6

7 Beer's Law Plot A Beer's Law Plot is a graph of absorption vs. concentration of a species at a single wavelength and can be used to determine the concentration of a solution of the same species. The single wavelength setting is usually determined by first doing an absorption spectra and then choosing the wavelength at which maximum absorption occurs. This wavelength setting is used for two reasons: (1) Measurements at this wavelength can provide the greatest sensitivity. (2) There is little change in absorbance in this region if the wavelength should accidentally be changed or if the voltage should fluctuate slightly. The absorbance is then measured at this wavelength for a series of solutions of known concentration. A graph of absorbance versus concentration is constructed and a best fit straight line is drawn through the data points. Then the absorbance of a solution of unknown concentration is measured and its concentration is determined by comparison to the Beer's Law Plot. Figure 7 shows a typical Beer's Law plot for a series of cobalt solutions of known concentration taken at 510 nm. Example: An unknown cobalt solution is measured at the same wavelength and absorbance is By comparison to the plot below, the concentration of the unknown is mol/l. Absorbance vs. Concentration Absorbance Concentration (M) Figure 7: A Beer's Law Plot for a Cobalt Solution at 510 nm. 7

8 (Note that the best-fit line in the plot passes through the origin; that is, A equals zero when concentration equals zero, as expected. However, Beer's Law is linear only for very dilute solutions and may deviate from linearity at higher concentrations. The range of dilution must be determined by experiment. For some species the range may be M, for others it may be closer to 10-5 to 10-6 M. This is determined by experiment.) Interferences Because absorption is dependent on concentration, identity of the absorbing species, and path length, experiments need to be conducted carefully so that there is no error caused by changes in the path length of light during the experiment or interferences caused by other absorbing species in the solution. The path length of light can be held constant if the same test tube or identical test tubes are used for each measurement. Special test tubes provided for the Spec 20 have standard, reproducible diameters and volumes. The solvent, impurities in the solvent, and/or the cuvette glass can absorb light in the selected region. These interferences can be corrected or eliminated by setting the Spec 20 to 100% transmittance using a reference test tube filled only with the solvent. This reference solution is referred to as a "blank". In effect, setting the Spec 20 to 100% transmittance through the blank solution instructs the instrument to ignore any absorbance from materials in the glass or solvent and to detect only the absorbance from the particular species to be measured in the sample. (A similar analogy is the taring of a balance.) Review Questions: Define: absorbance, transmittance, blank, path length, Beer's Law, cuvette. If a solution is blue-colored, what wavelength of light (in nm) is being absorbed? What is absorbance when percent transmittance is equal to 58%? When absorbance is infinite, what is %T? 8

9 If fingerprints are left on the cuvette, how does it affect A and %T? A student rinses a cuvette with DI water and then pours his solution into the wet cuvette. How does this affect A and %T? How does a student correct for an absorbing impurity present in the solvent? How does the student correct for any stray light that might enter the sample holder? What is the purpose of the mark on the edge of the sample holder? What is being measured in an "absorption spectra"? What is being measured in a Beer's Law Plot? What two factors are held constant when constructing the plot? Why is the wavelength of light set at maximum absorbance when making a Beer's Law Plot? Does the wavelength change while doing the experiment? Which solution is expected to deviate from the straight line in a Beer's Law plot: the more concentrated or more dilute solution? Is the straight line of a Beer's Law plot expected to pass through the origin of the graph or at some point above the origin? 9