A Device To Demonstrate the Principles of Photometry
and Three Experiments for Its Use R. Del Delumyea P.O. Box 621, Walhalla, SC 29691
The recent attention given to the sciences in secondary education has resulted in an increased awareness of the lack of affordable equipment that can he used to demonstrate basic principles of science. This is particularly true in the area of photometry. The typical instrument used, and for which many experiments are specifically written, is the Spectronic 20 (Bausch and Lomb, Inc., Rochester, NY). The price of this instrument is on the order of $875, which limits the number of them which can he purchased by most schools. Most of the exuense of such devices is involved with capabilities not required for simple demonstrations of the principles of photometry. Further, the sophistication of the device and the isolation of students from the actual working components of the photometer tend to obscure the simplicity of the technique. Attempts have been made to produce simpler devices for use in teaching laboratories, the objective often being to reduce costs. A iecent article1 describes a dual-beam photometer for use in the teaching laboratory that can he assembled ". . . with average workshop facilities." I t requires construction of a Wheatstone bridgelnulline circuit and a Dower sunolv. Even such a relativelv s i m ~ l e device may be beyond thkskill or courage of an e ~ e k r o ~ i c s novice. Further..the . Darts are not readilv available.. reauirina . access to an electronics store and familiarity with use of
,SAMPLE
HOLDER
TUBE BATTERY
Figure 1. Schematic diagram of photometer
BAT
Table 1. Parts Llsi for Photometer t(ard!"are
Description
Cost. $
Wood block 3 X 5 X 1 in. lwith hole) . kin.dia. .. Sample Compartmem, opaque PVC pipe
DE F3 Figure 2. Circuit diagram 01 electronic components.
0.49
%-in.o.d. X 4 in. long, threaded bath ends Compartment lid %-in.0.d. PVC end cap Mounting bracket, meter Misc. screws (2 #4 X X in.. 2 #6 X Y2 in.) 2 breadboard, 1 battery strap. 1 meter bracket Subtotal
0.59 0.59 0.40 0.10 2.17
electronics catalogs. The photometer presented here is of even lower cost (under $50) and greater simplicity. The electronic circuit has onlv seven parts-a battery with a 5-V voltage regulator (VR); light-emitting diode (LED) and a controllina resistor for the LED: the solid-state ~hotodetector (PT); meter; and a breadboard on whichthe components are assembled. The sample compartment is a 4-in. section of 3/4-in.-o.d. opaque plastic (PVC) pipe and a 3/4-in.0.d. PVC end-cap mounted on a wood base. The source and detector are aliened across the center of the n i ~ ea ~ ~ r o x i mately 1in. from the base. The sample is contained in 13 X 100-mm test tube. An opaque tube is used to check that the meter reads zero when no light reaches the detector (dark current) and the maximum meter reading is set with a blank solution in the sample compartment. The photometer is shown in Figure 1. Its simplicity is apparent in its circuit, shown in Figure 2. Most of the parts are readily available either a t the local hardware or retail electronics store (e.g., Radio Shack in the United States, Tandy in Great Britain). Table 1lists the major components required for assembly of the photometer and gives Radio
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Description
#/pkg
Breadboard Battery holder Battery snaps Variable resistor Meter (50 p A FS) Transistor sockets
1 2 5 1 1 6 2 2 1 1
LED, red
green voltage regulator Phototran~istor
Part Number
Cost. $
276175 270326 270-375 271-216 270-1715 276-548 276-041 276022 275-1770 MRC-3006
6.95 0.59 0.99 0.59 7.95 1.69 0.79 0.79 1.59 3.59 $25.52 $27.69
subtdai Approximate tota
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This work was presented at the 1985 Southeastern Association of
Chemists Convention in Athens. GA. sRadioShackpansgivsnasexampleo. ~ r n e ~ ~ ~ m m a ~ b s a ~ b ~ t i ~ e d s s a ~ p r o pAnalytical riat~. 0 NO^ available ~ r o m ~ a dshack. ia ~ u sbeorderedfromsn t eisctranico rpecianymuoe. Isaacs, N. J. J Chem. Educ. 1983, 60, 607.
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Journal of Chemical Education
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Table 2. Parts List Included in Photometer Klt Electronics
Hardware
Breadboard Voltage regulator Banery strap Phototransistor Variable resistor Meter. 50 @ A FS LED'S, red and green
sample compartment (predrilledl Wood block (predriiied) Meter bracket (+ washer) Compartment lid Battery holder Screws. 4
Shack parts numbers as examples; however, the detector must be purchased from an electronics wholesale house, usuallv in minimum mantities of 100. Further, the hardware parts ;equire some d;illing (e.g., for mounting the LED and P T in the sample compartment and the compartment to the base). To make it easier for the instructor to obtain the components, the author has assembled a kit containing the items listed in Table 2, which can he purchased for $48.50, including shipping. Additional information on ordering the kits may he obtained hy contacting the author d i r e ~ t l yIn .~ addition to those components listed in the tables, black electrical tape, solder, a soldering iron, and a t least two 13- X 100-mm test tubes, one of which is blackened on the inside (flat black enamel snravpaint works well), are required. The . .. iatter is used to check the dark current. The internal diameter of these tubes is approximately 1.0cm. This size is convenient in that molar absorptivity, discussed in Experiment 3, is calculated in units of absorbance units per molar concentration per centimeter of path length, and no correction for the tube diameter is required. The circuit is assembled on an "experimenter's board" or "breadboard," consisting of two bus strips of 5 X 7 contact points each. The holes in any column of either the upper or lower bus strip are connected electrically from below. Figure 3 shows the location of the connections on the breadboard, and the row (letter) and column (number) of the breadboard nosition is shown on the circuit diaaram (Fip. 2). Using a breadboard minimizes the amount of&lderingrequired and allows the svstem to be assembled and disassembled repeatedly u s i n g reasonahlt. cure. Iktailrd instructions for assemIlly ui the circuit are prtn.ided with the kit. as arr test procedure.. to check each portion of the circuit. The photumetrr is H simple direct-rurrrnt circuit consisting of a powrr supply thattery and \wltage regulator) and two loops-lbe LED and a vnrinhle reslstor ( p o i n r IWD3-F;I IY2-B2 in Fig. 21, and thc phtrtudetectur and ia meter (puints B3-C'3-C'L 1)?-E"1. 'The meter can ht: used to ohservt the eft'ecr of chnngittg the rcsisranve in t h ~ LEI) . loon IIV monitoring LED mensin' (visually and with the phdtot;ansistor) o;to demonstrate the effect of light intensity on the response of the PT. Once assembled, almost any photometric procedure that uses red or green light can he adapted for use with the photometer; however, two experiments on the principles of photometry, written specifically for it, are provided with the kit (also available from the author on request2). The two photometry experiments are arranged with an introduction to the principles to he investigated in the experiment, a detailed step-by-step procedure, a list of equipment required for the experiment, a section on analysis of the data, and a report section with questions to he answered by the student. The exueriments are suitable for hiah - school and introductory college chemistry courses and do not involve hazardous or difficult-to-find chemicals. Clear water and a small amount of potassium permanganate (KMnO4) are required. The latter is available at the local drugstore, being a recommended treatment for "jungle rot." Experiment 2 introduces and discusses the relationship between light absorbance and concentration, known as Beer's Law. A calibra-
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tion curve is prepared and used to determine the concentration of a n unknown. E x p e r i m e n t 3 discusses a n d demonstrates the effect of the absorbance efficiency (molar absorptivity) on the slope of the calibration curve (sensitivitvi. .. The instructor should have or obtain some familiarity with electrunic vrinciules. Ans introdurtorv text i; suititlde.' Similarly, the photometry introductions serve only to introduce the concepts. and the student should he referred to a textbook for mbrerigorous discussions of the principles involved (or for different presentations). For example, the concept of molar absorptivity may also he presented as the probability that an electronic transition will occur a t a specific wavelength for agiven species, if the students are familiar with this concept, rather than as an efficiency term as described below. Texts such as Skoog and West4 are suggested for additional reading. The three experiments were tested on the 1985 Clemson University Science Summer Camp (grades 9-12) and Experiments 2 and 3 were performed by the 12th-grade science class of Pendleton High School, Pendleton, SC. Both groups responded favorably to the photometry experiments. The first experiment on assembly of the device can be used as an introductory project in electronics or as part of the physics laboratory. Alternatively, the instructor can assemble the photometer and use the other experiments to teach the photometric principles described below. Experlment 1. Demonstrating Bask Electronlc Principles Using a Simple Pholometer Electron flow in a wire can be compared to water flow in a hose. The rate of water flow depends on the water pressure the same way electron flow (current) depends on the "voltSend inquiries with self-addressed, stamped envelope. Orders for kits must be prepaid. Diefenderfer, A. J. Principles of Electronic lnsfrumeniaiion, 2nd ed.; Saunders: Philadelphia. 1979. Skoog. D. A.: West. D. M.. Eds. Analytical Chemistry. 4th ed.; Saunders: Philadelphia. 1985. Volume 64
Number 7 July 1987
631
age" of the power supply. Increasing the voltage increases the current flow. The unit of electrical current ~ ~ is the amnere. ~ ~, or "amp," and a meter used t o measure current flow is called an "ammeter." Since the ohotometer works on low Dower. its current is very small (10-6 amps) and a microakmete; is reauired. Ammeters are installed between the Dower suonlv .. . and the device being measured. The flow of water in a hose can be reduced by kinking the hose. The electrical eauivalent of a kink in ahose is a7'resistor." The size of a resistor is determined by its "resistance" to electron flow (the larger its resistance, the more it inhibits electron flow). The amount of current flowing through the meter is determined by the resistance of the phototransistor and the voltage of thhpower supply. The relationship between voltage, current, and resistance is a fundamental law of electronics called Ohm's Law. If a fixed voltage ( V ) is placed across a device, the amount of current flow ( I )through the circuit depends on its resistance ( R ) .Mathematically, Ohm's Law is written: ~
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where V = voltage, in volts, I = current flow, in amperes, and R = resistance, in ohms. Ohm's Law can he demonstrated using the photometer. The device used to measure the atnwnt i d light passing through the snrnple is a phutorransistor , P ' ~uI how r(:..isIa~weIS ~ ~ r o o o r t i m 10 dI he amuunt of light striking its surface. ~ e f e r i i n g t oFigure 2, the outer loop of the circuit (B3-C3-C2-D2-B2) is the measurement loop, consisting of the phototransistor and a meter to measure the current flowing through the loop. In the dark (when the black tuhe is inserted), the P T has a high resistance (over 107 n), and very little current (called the dark current) flows through it so the meter reads zero. When lieht strikes the photo&ansistor, its resistance drops to a value proportional to the intensity of light striking the surface. This permits more current flow and the microammeter reading increases to a value proportional to the light intensity. While it is light that causes a change in the detector, i t is current flowing in a wire that is actually measured. T h e phototransistor is acting as a "transducer,"~changing light intensity to a measurable current. The measurement is a simple application of Ohm's Law, rearranged to the form I = VIR, where V is a fixed value. Ohm's Law is used in another way in the photometer. Exposing the P T to too much light will lower its resistance so far that too much current flows throueh the looo " . (which . could damage the meter), so it is important that the intensity of the LED he controlled. Since the amount of light put out by the LED increases with increasing current and current depends on the resistance, the maximum readina of the meter can be set by adding the correct size resisto;to the circuit. The resistance of a circuit can he selected by using either a resistor with a fixed value (this, however, would require a large assortment of resistors to have one of the exact size needed) or a variable resistor (VR), which is often called a "pot." The second loop in the circuit (B3-D3-F3D2-B2) shown in Figure 2 is used to control the light intensity. Increasing the resistance of the pot decreases the current flow through the LED. reducine its intensity. The decreased light inteniity strikingthe photodetector increases the resistance of the P T , reducing the current flow throueh the looo containing the meter theieby reducing the meterreading. In the photometer, this procedure is used to set the meter reading to the maximum value for the hlank solution. Experiment 2. Demonstrating the Use of Beer's Law To Determine the Concentration of an Analyte in Solution A plot of the emission intensity of a source versus wavelength is called a "spectrum." The spectra of the LED's are shown in Figure 4. The bandpass (width of the emission band of the source) is approximately 50 nm for the red and
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Journal of Chemical Education
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Figure 4. Emission spectra of LED's.
30 urn for the green LED. For comparison, the handpass of the ~ ~ e c t r o n i c is 2 020 nm. ~ h e r e f o i eany , experiment using the "Spec 20" isalso suitable for the photometer. The following~isa brief review of the concepG to be demonstrated by this experiment and the procedure. Typical results are discussed separately below. T h e amount (A) of a given color of light absorbed by a solution deoends on three factors: the concentration (c) of light-ahsoriing material present, the distance the light travels throueh the solution ( b ) .and the efficiencv ( a )with which it absorbs the light. The amount of light ahsirbed (A) can be measured by comparing the intensity of light passing through the sample (its "transmittance," T ) to that of a nonahsorhing, "hlank" solution. The mathematical relationship is callled Beer's Law. The equation for Beer's Law is: A = -log(TITb) = abc where A = absorbance, T = transmittance of the sample, Tb =transmittance of a hlank solution, b = diameter of the tuhe holding the sample, c = concentration of the analyte, and n = molar absorptivity. T h e molar absorutivitv" (.a.. ) . and therefore the absorbance, is dependent o n t h e wavelength of the light source. A plot of molar absorptivity (or absorbance) versus wavelength is also called a spectrum. Although any photometric procedure which involves measurement a t the wavelengths emitted by either LED can he used, the permanganate ion is particularly well suited for demonstration oirhe ciircrsotwilrelrngth &I thr sensit:vity oithe determipermanganare solunntiun. T h r sprrfrum of n 1.2 X lion is shown in Fieure 5. Coninarism ot C'ieurei J and 5 shows that the green LED will 'be absorbed Kore strongly than the red LED (better overlao of the LED emission with the permanganate absorbance). In this experiment, the green LED is used. As observed in Exoeriment 1. the meter rvndiny is propurfional to the light inrknsiry striking the P T and is rherefure a direct mmsure of thr rranjmitrnnre of rhe solution. A stock solution of potassium permanganate is prepared (or can be supplied by the instructor). Using graduated cyl-
.
CALIBRATION CURVES
PERMANGANATE, X 10 -4 M Figure 6. Photometer response 10 permanganate salutions using green and red LED'S. For aomparison,the response of the Spectronic 20 at 569 nm is also shown for the same solutions. Table 3. Data Entrv Sheet for ExDerlments 2 andlor 3 WAVELENGTH,
nm
Sarnpie
Molarity
Blank 1.0 2.5 5.0 7.5 10.0
0.00
Figure 5. Absorbance specbum of 1.2 X lo4 M permanganale solution.
inders. five standard solutions are orenared hv dilution of the stdck solution with clear water. bs& the sHme tube for each measurement, the transmittance (meter reading) is determined for the blank, each standard, and the unknown. The raw data are recorded in the form of Table 3 and Beer's Law used to calculate the ahsorbance of each solution. Aplot of absorbance versus concentration is prepared for the stand a d s and the concentration of permariganate in the unknown determined from this "calibration curve." The students are asked toreport the concentration of their unknown and suggest improvements in the procedure that would immove the results. Possible errors include the dilution error resulting from using graduated cylinders, alignment errors durine insertion of the tube into the ohotometer. and errors in the data. The instructor h a y wish to introduce olottinn techniaues a t this ooint. Students most often fit a straight (least squares) linethrough the data; however, this may not he correct as shown in Figure 6. Experiment 3. Demonstrating the Effect of Molar Absorptivity on the Sensitivity of a Photometric Procedure T o demonstrate the effect of wavelength on molarabsorptivity and sensitivity of an analysis, ~ x p e r i m e n 2t is repeated using the red LED, and the results of the two experiments are compared. In Experiment 2, Beer's Law was used to prepare a calibration curve of absorbance versus concentra-
T
Green TIT
1.00
A
0.00
T
Red TIT
A
1.00
0.00
tion usinz the ereen LED. The done of the line drawn through the d a t i is a direct measure'of the efficiency with which the light is absorbed. This efficiency factor is called the "molar absorptivity" and varies with the wavelength (color) of the light being absorbed. A plot of ahsorbance versus wavelength for a given solution in a cell of fixed thickness (the test tube replaces a "cell" in this experiment) is called a "spectrum," which will be the same for any conM centration of permanganate. The spectrum of a 1.2 X permanganate solution is shown in Figure 5. Since the same solution in the same cell is measured a t all wavelengths, changes in the absorbance must be due to differences in the molar absorptivity at each wavelength. The most sensitive wavelenath to analvze for nermanaanate is where the molar i~ highest. ~ i Q u r4es h w i the emissim sprrtra nl,sorpr~;~itY of t h ~LED'S . used in the phutometer. Cumparison of F i ~ u r e s k and 5rhowi that the emiiiion from thr green LEI) overlaps with the abwrbnnce nf permanganate better than the red. ,O\erlny the two figures, and holrl them 141to thv light to5ee this l~etter,.Since piwnang;inatr has a higher molar n l ~ s o r p t i v i l v in this renion. the rreen LET)shm~ldhe more sensitive " u to changes in concentration of permanganate Volume 64 Number 7 July 1987
633
The calibration curve produced using the photometric procedure can he used to calculate the molar ahsorptivity as follows. Beer's Law can be rearranged to the form: Alc = be. In these exneriments. b is constant (1.0 cm). Therefore. the slope of the calibratidn curve is the iatio A?C and is a direct measure of the molar ahsorptivity. In this experiment, the molar absorptivity is determined using LED's of two colors. The data from Exneriment 2 are used for the ereen LED data, and a second set of data is obtained h i repeating Experiment 2 using the red LED. The students are asked to report the concentration of the unknown solution calculated from each calibration curve and to compare the results. The molar absorptivities determined using the different LED's are also reported. The students are asked to rank a series of wavelengths in order of sensitivity, using the permanganate absorbance spectrum shown in Figure 5 for reference. Typlcal Results Obtalned from Experiments 2 and 3 Figure 6 shows calibration curves obtained using the two colors of light to determine the absorbance of M potas-
634
Journal of Chemical Education
sium nermaneanate solutions (Exoeriments 2 and 3). The response of a-~pectronic20 thk same standards is also shown. The monochromator of the Snectronic 20 was set to the emission maximum of the green-LED, 569 nm. As expected, the slopes for the two sets of data taken using green light are greater than that using red light (peak maximum 688 nm). Response to the concentration changes using the red LED is much lower, as expected from the lower molar ahsorptivity of permanganate in that wavelength region (Experiment 3). The photometer shows somewhat greater nonlinear behavior than the Spectronic 20 at the higher concentrations. Strictly speaking, Beer's Law requires a truly "monochromatic" (only one wavelength of light) source, rather than the hand emissions used. This accounts for at least some of the nonlinear behavior observed. Overall, the response of the two instruments is comparable, and the photometer presented here is an adequate substitute for more expensive devices, a t least to demonstrate simple principles of photometry.
