A Simple Experiment Demonstrating the Relationship between Response Curves and Absorption Spectra Chia-yu Li East Carolina University, Greenville, NC 27834 Although the forefront of the development of today's commercial optical instrumentation has shifted to such things as the rompukr-contn,lled Fouriw transform infrared speitrophotumeters; rapid-scanning, computing ilVlvisihle spectrophotometers;combinedsimultaneous &d sequential, inductively coupled plasma emission spectrometers; and other sophisticated devices ( I ) , the conventional UVIvisihle spectrophotometers prohahly still remain as the most widely used optical instruments in a chemistry lah. At our institution, the use of a basic spectrophotometer is first taught in the general chemistry lab. More extensive discussion of the analytical aspects of UVIvisihle spectroscopy is given in the sophomore quantitative analysis course. This is followed by a senior-level instrumental analysis course in which the instrumentation asDects of this technioue are discussed. From the basic laws of molecular absorption to instrumentation and applications, there is no shortaee of material for instructors t o use in their lectures. The s u b k t is covered virtually in every current instrumental analysis text (2-6) and some special topics can also he found in the literature (7-13). In teaching this subject to our instrumental analvsis students. we feel that one of the most important spectroscopy terms for s t u d m a to learn is the definition ol'a spectrophotometer. Accordinr to Anolvrisol Chemistry (in the ~ k u a r yissues), a spectrophotom&er is defined as the "spectrometer with associated equipment, so that i t furnishes the ratio, or a function of the ratio, of the radiant power of two heams as a function of spectral wavelength. These two heams may he separated in time, space, or hoth."The two heams are commonly known as the reference beam and the sample beam. However, we found that the concept of obtaining the ratio of two heams appears too ahstract to many average students. Despite excellent coverage given by many instrumental texts on how this is accomplished
Figure 1. Schematic diagram of asingle-beam specoophotometer assembled from modules.
in a commercial instrument, there is no straightforward exnerimental ~rocedureeiven that students can use to record and measure these two heams separately. In this article, a simple experiment is described for recording two individual spectrophotometer response curves. The two response curves are directly related t o the power of transmitted heams that have passed through the solvent and the solution. From the calculated ratios of these two curves as a function of wavelength, one can construct an absorption spectrum of the solution in question. This experiment has been tried for the past two years in our instrumental analysis lab. We feel that it did give students a better understanding of the behavior of a spectrophotometer and the nature of absorption spectrophotometry. The Experiment Students are first asked to assemble a singe-beam spectrophotometer from discretemodules as shown in Figure 1.
Volume 61 Number 9
September 1984
827
The modules are part of a set of McKee-Pedersen modular systems which were ohtained from Pacific Precision Instruments (Concord, California 94518). They include a tungsten lamp light source, a6-V power supply for the tungsten lamp, a monochromator, a cell compartment, a photomultiplier (PM) tuhe, a P M tube power supply, a general purpose operational amplifier, a 10 K, 10-turn potentiometer, and a strip-chart recorder (minimum sensitivity-10 mV full scale). The operational amplifier is wired as the current follower to track the cnrrent flowing from the PM tuhe. The typical value for the stabilizing capacitor, Cf,is 0.1 pF. The feedback resistor, Rf, is adjustable and is normally set a t 10 KQ. The current follower output is attenuated by the potentiometer so that a signal of suitable size can he recorded within the recorder range. The system's monochromator is a CzernyTurner type with a focal length of 45.7 cm, adjustable slits from 5 pm to 5 mm, and six selectable scan rates. We used a diffraction matine of 1180 lineslmm ruline blazed a t 0.3 um in the mon&hroiitor. With this ruling, tile available waveleneth is hetween 200 and 1000 nm. Kfter the assembly is completed, the first task is to record a dark-current remonse curve in the visible ranee with an opaque object in t hc sample cumpartment.'l'he recorder's zero adiustable knoh is used to t~rinethe startine noint of the curve tozero on the recorder. w h e n t h e record&is set a t 1V full scale, the dark current response curve is indistinguishable from the base line. With the instrument settings unchanged, students then proceed to record the response curve of the pure solvent, i.e., with water in the cuvet. A typical curve is shown in Figure 2% In a like manner, the response curve of 5.0 X M KMnO4 (Fig. 2h) is obtained. The monochromator scan rate used was 100 nmlmin and the slit width was fixed a t 450
Figure 2. Actual recorder traces of the spectrophotometer response curves with a 1.2-m cwet containing water (curve a)and 5.0 X MKMnO, in 0.1 MHISOI (CUNB b).
w.
The spectrophotometer response curve of Figure 2a is really a combination of the spectral response curve of the P M tuhe, the spectral distrihution curve of the radiant energy source, and the optical characteristics of the cell. All of which are, of course, a function of wavelength. The response of 1P28A has a relatively flat region within the visible region, but the black-body radiator, the tungsten lamp, has a continuous changing energy output with a maximum near 1000 nm. Therefore, strickly speaking, the curve in Figure 2a does not reoresent onlv the Dower of the incident beam. It. nevertheless, can be c&ed Po, the radiant power that has tiansrnitted throueh the cell containine the solvent. Likewise. Fieure 2b represents P, the radiant power that has passed thro&h the solution. From the appearance of the curves, it can he clearly seen that when a colored solution is present, the radiant power that reaches the detector is reduced. At a given wavelength, the voltage recorded by the recorder is uno = hi& (1) where k is the potentiometer attenuation factor and io is the I'M tuhe current registered when a solvent is in the cuvet. The latter is a function of 1'0 and the dark current id. io = cPo+ id
(2)
where c is the proportionality constant. Combining eqns. (1) and (2), we have U R= ~ kcP& + kidRf (3) Since the second term is arbitrarily set to zero by the re, then directly proportional to corder zero-adjust knoh, u ~ is
Po In reality, we found that the second term is less than 0.3% of the first term. This value was ohtained by using a precision 828
Journal of Chemical Education
Figure 3. Abswption specbum of KMnO. constructed trom tiw spectropb tometer response curves using eq. (6).
digital voltmeter to measure the current follower output a t a wavelength near the peak transmission. Thus, for practical purposes, i t can be neglected. Similarly, when a solution is placed in the cell, we have Since k , c, and Rfare constants, the ratio of UR to UR, is directly proportional to P versus Po, i.e., the transmittance. Thus, an absorption spectrum can he constructed by using the relationship A = -log PIP0 = - log u n l u ~ ,
(6)
The result is shown in Figure 3. From the spectrum, the molar absorptivity of KMn04 was calculated to he 2.4 X 103 llmol-cm a t 520 nm, which is in good agreement with the ac: cepted textbook values.
Alternatively, a Spectronic 20 can he used to perform this experiment if a modular system described here is not available. The instrument meter should first be zeroed with the dark-current control knob when no cuvet is in place. With water in the cuvet, the wavelength of maximum sensitivity can he determined by rotating the wavelength knoh until the meter shows a maximum deflection. This wavelength is near 530 nm for a combination of a tuugsten-lamp and a bluesensitive phototube. The meter can he adjusted to near 100% T with the light-control knob. The response curve of the solvent can then be recorded manually from 350 to 650 nm a t 5 nm increments with the light-control knoh in the fixed position. The process is repeated with the KMnOa solution. Treat the first curve as Paand the second curve as P, an absorption spectrum of KMnOl can thus be plotted. (It should be noted that the spectrum is normally not obtained in this manner. This method is merely intended to show how the transmitted beams behave with a fixed slit width.) Since the spectral band width of the Spectronic 20 is 20 nm, no fine structures as shown in Figure 3 can be obtained. The monochromator of the modular system used here has a spectral band width of 1.3 nm (based on a slit width of 45Opm), it naturally producesa better resolved spectrum. Besides, the nonscanning characteristics of the Spectronic 20 makes the data collection tedius. In principle, the response curves can be recorded from any single-beam instrument. For example, the anode of the P M tube in a Beckman DU can be connected to a current follower. The output is then fed into a digital voltmeter to complete the circuit. In conclusion, we believe that the use of the modular system in this type of experiment offers some distinct advantages. First, the assembly work itself is quite educational to the students. They can learn what goes into a spectrophotometer and how each individual component functions. Second, the modular approach is flexible. The effect of light source stability on the power of the transmitted beams can be demonstrated by deliberately varying the light source power supply
voltage. Also, by replacing the tungsten lamp with a mercury vavor lamv. the effect of slit width on the resolution of merc& emis& lines can be dramatically demonstrated. Finally, the modular system is versatile. By repositioning the optical modules and the detector, a fluorescence spectrophotometer or a nevhelometer can be built (14). Interested readers mav wish toconstmct their own modkes: The optical modules wi& scanning capabilities may not be easy t o build, but the electronic modules can he constructed with relative ease; for instance. the Dower s u v ~ l i e scan be assembled convenientlv from the kits. The integrated circuit operational amplifiers can be mounted on the verforated or minted circuit boards with very little cost. 0nEe the modules are built, they can be used over and over again for different purposes. Acknowledgment
This work was supported in part by a grant from the National Science Foundation-Instmdional Scientific Equipment Program (SER-7913320). Literature Cited (1) Chsm. and Eng. Nama.41 (March 22,1982).
(2) Willard, H. H., Merritt, L. L., Jr.. Dean, J. A,. and Settle, F. A , Jl., "lostrumental Methods of Analysis,"6th 4.D . .Van Noatrand Co., New York, 1981. (3) Ewing, G W.,"lnstrumentsl Methods of C h e m d Analysis," 4th ed.. MeOraw.HiU, New York, 1975. Skoog, P. A,, and Weat, D. M., "Prineipleaof Inattumental Analysis." 2nd ed.,W. 8. Ssunders. Philadelphia, 1980. Menn, C. K., Viekers,T.J..sndGulick,W . M.,"lnstrumentelAnalysis,"H~rand Row, New Vork, 1974. B m q H . H.,Christian.G.D..andO'Reilly,J. E.,"htrumentdAmlysis,"AUynand Bacon. Boston. 1978. Swinehart. D. F.. J. C-. EDUC. 89.333 (1962). Cmk, R. 8.. and Jankow, R.,J. CHBM. EDUC., 49,405 (1972). Bruzzone, L., and Rmlli, M. E.,J. CHEM Eouc.,50, 701 (1973). In&, J. D., Jr., J.CHEM. EDUC., 51,IW (1974). Billmeyer. F.W., Jr., J. CHEM. EDUC.,51,530 (1974). Eriekson, J. 0.. and Surles, T., Amer Lob., 41 (June 1976). Cheng, K. L., "Spectrochemica1 Methods of Analyaia."(Edilar: Winefardner. J. D.), Chapter VI. Wilcy~lntcrseienee.New York. 1971. "MP-SYSTEM 10004peration and Applications,"4th ed., McKee-Pedersen Insfrumentsmacifie Precision Instruments. Conmrd. CA 91518. 1971.
Volume 61
Number 9
September 1984
829
