Construction of a Photometer as an Instructional Tool for Electronicsand InstrumentationRobert L. McClain*

Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States

*S Supporting Information

ABSTRACT: An introductory electronics laboratory unit for the undergraduatechemical instrumentation course is presented. In this unit, students use basicelectronic components to build a functioning photometer. Students interface thephotometer to a microcontroller and write an Arduino program to collect the signal,calculate the absorbance, and display the result on a liquid crystal display (LCD).Students use their home-built instruments to measure the concentration ofhexavalent chromium in a series of standard solutions and determine the figures ofmerit: sensitivity, detection limit, and dynamic range of the instrument. They alsoused their instrument to measure the concentration of hexavalent chromium in anunknown water sample.

KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives,Aqueous Solution Chemistry, Laboratory Computing/Interfacing, Laboratory Equipment/Apparatus, Spectroscopy,Water/Water Chemistry

The impact of electronics in modern society has beentransformational. Current students have grown up in a

time when electronic tools, toys, and gadgets are seeminglyeverywhere, with many of these students using video games,computers, and cell phones before they start elementary school.By the time students take a course in chemical instrumentation,they are comfortable sitting in front of a computer and usingthe instrumental software associated with instruments.Although comfortable with the software, students are oftenmentally detached from the hardware functions of theinstrument and can have a difficult time understanding howthe instrument actually works. As modern instruments and theirinterfaces become more sophisticated, it is even more difficultfor students to grasp the basic measurement principles. Gettingstudents to think fundamentally about how instruments work,so they can critically evaluate the quality of the data theinstrument conveniently provides, is an ongoing challenge inthe teaching of chemical instrumentation courses.One strategy to get students thinking about instruments from

a fundamental level is to have students build their ownfunctioning instruments. This Journal has numerous examplesof student-built instrumentation either at the modular level1−3

using monochromators, detectors, light sources, and so forth, orat the level of electronic components. At Penn State University,the instrumentation course includes a semester-long researchproject building a functional instrument from electroniccomponents. Example instruments built by Penn State studentsare a light emitting diode (LED) based fluorimeter4 and a KarlFischer titrator.5 University of Toronto students also make afluorimeter using an LED light sources and photodiode

detector.6 The main focus of the Toronto experiment is theprogramming of the software interface using LabView. Thal andSamide describe the construction of a simple spectropho-tometer using an LED light source and a comparison ofphotoresistor, photodiode, and photodarlington detectors.7

Sengupta et al. at University of MassachusettsLowell describean infrared LED source and photodiode detector spectrometerthat also includes a home-built lock-in amplifier constructed inthe physical chemistry laboratory.8 In addition to spectroscopicinstruments, potentiostats for electrochemical measurementscan be made from basic electronic components and work wellfor student electronics construction projects.9,10

Basic electronics is still an important part of advancedcourses in analytical or physical chemistry, and a fewuniversities have even developed complete courses inelectronics for chemistry students.11−13 Although these courseswere developed in the late 1980s and today’s electronictechnologies are very different, the basic premises of thesecourses are still relevant and important. Students who seekadvanced degrees in physical and analytical chemistry need tohave skills in electronics and computer interfacing. Studentswho work with electronic circuits develop good hands-onproblem-solving skills. A basic understanding of electronicsignals as voltages and currents helps students understand thephysics behind the “magic” of modern technologies.An introductory laboratory unit on electronics for the

undergraduate chemical instrumentation course is described. In

Published: April 15, 2014

Laboratory Experiment

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© 2014 American Chemical Society andDivision of Chemical Education, Inc. 747 dx.doi.org/10.1021/ed400784x | J. Chem. Educ. 2014, 91, 747−750

this unit, students learn basic electronics while building afunctioning photometer. They interface the photometer to amicrocontroller and write an Arduino14 program to display themeasured absorbance data on a liquid crystal display (LCD).They use their instrument to measure the concentration ofhexavalent chromium a series of standard solutions and todetermine the figures of merit: sensitivity, detection limit, anddynamic range of the instrument. They also used theirinstrument to measure the concentration of hexavalentchromium in an unknown water sample.

■ UNIT OVERVIEW

This unit is unique because the circuits for the photometerwere designed to incorporate electronic concepts at a level andscope consistent with a typical textbook for the upper-levelinstrumentation course.15 The circuits included a voltagedivider, a current to voltage converter, high- and low-passfilters, a relaxation oscillator, a relay, and a half-wave rectifier.These circuits were made from resistors, capacitors, a discretetransistor, operational amplifiers, a light emitting diode, signaldiodes, a cadmium sulfide photoconductor, and a siliconphotodiode. The students used a microcontroller with ananalog-to-digital converter and were introduced to softwareprogramming with Arduino. As in any electronics laboratorystudents also learned to use a digital multimeter andoscilloscope during the circuit construction. This unit tookfive laboratory sessions, each laboratory was 3 h in length, andthe students worked in pairs. The laboratory unit was scheduledclosely with the coverage of electronics in the lecture portion ofthe course so that the students simultaneously learned both thetheoretical and practical aspects of introductory electronics.Light emitting diodes (LED) are readily available in many

colors covering the visible spectrum.16 Silicon photodiodeshave spectral responses that also cover the visible spectrum.With the proper choice of LED, the photometer can measurethe absorbance of any solution of color and can be used for anumber of traditional colorimetric methods of analysis.17 Thephotometer was used to measure the concentration ofhexavalent chromium in an unknown water sample preparedby the instructor. The chromium test was used as an examplethat has local interest to the students because elevated levels ofhexavalent chromium have been reported in Madison, WI tapwater.18 In this method,19 chromium(VI) was complexed with1,5-diphenylcarbazide resulting in a strongly colored magentasolution. The complex has λmax = 543 nm, which is close to theλ = 525 nm maximum output of the green LED (Figure 1).

■ MATERIALS

To do this experiment in its entirety, students needed anelectronics workstation that included ±12 VDC power supply,oscilloscope, digital multimeter, and bread-boarding tools. AnArduino Uno development board was used for analog-to-digitalconversion and the open source Arduino for programming. Allof the part numbers for the individual components are found inthe Supporting Information.The laboratory instructor made a 1000 mg/L stock solution

of hexavalent chromium by dissolving K2CrO4 (Sigma-Aldrich)in deionized water. From the stock solution, the instructormade a series of standard solutions ranging from 0.05 to 4.0mg/L for the experiment. The instructor made the 2 mMdiphenylcarbazide coloring reagent in 10% methanol and 1 NH2SO4 by (1) adding 2.8 mL of conc. H2SO4 to approximately

50 mL of water in a beaker, (2) dissolving 0.05 g of 1,5-diphenylcarbazide (Sigma-Aldrich) in 10 mL of methanol in asecond beaker, (3) combining the two solutions, and (4)diluting to 100 mL with water. The coloring solution should bemade fresh, but can be stored for a couple of days in arefrigerator.

■ HAZARDSChromium(VI) is a known carcinogen, and although theCr(VI) solutions are quite dilute, they should be handledcarefully and disposed of properly. See the SupportingInformation for suggested alternative methods using lesshazardous materials. The diphenylcarbazide coloring reagentis prepared in 1 N H2SO4, which should be handled carefully asit can cause minor burns to the skin. Safety glasses and glovesshould be used by students when handling the reagentsSafety glasses should be worn at during the electronics

construction since an improperly wired LED can draw enoughcurrent to heat up and rupture. There is a small chance of lowlevel shocks when wiring, so students should be reminded toturn off the power when wiring and rewiring their circuits.

■ DESCRIPTION OF THE FIVE LABORATORYSESSIONS

The details of the circuit construction are found in theSupporting Information, but an overview is provided here. Onthe first day of the laboratory unit, students built a very simplephotometer based on a cadmium sulfide (CdS) photoresistordetection element incorporated into a voltage divider circuit.They used their photometer to measure the absorbance of aseries of chromium(VI) standard solutions ranging from 0.05 to4.0 mg/L and created a calibration curve. The voltage divider isa fundamentally important circuit that is often used tointroduce basic direct current (DC) electronics. The simplicityof the design of this photometer helped students getcomfortable with circuit construction techniques and measure-ments.Over the next two laboratory periods, the students

constructed a more sophisticated version of the photometer.

Figure 1. Absorbance spectrum of Cr−diphenylcarbazide complex andemission spectrum of green LED. The absorbance data were takenwith a Jasco 570 UV/Vis/NIR spectrometer for a 1 mg/L Cr(VI)solution in a 1 cm path cell. The emission data were collected with anAmes Photonics LARRY linear array CCD coupled to an ActonSpectroPro 2150i monochromator.

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A silicon photodiode was used as the detector in the newversion. The design contained six different subcircuits providingstudents with experience working with resistors, capacitors,diodes, a transistor, and operational amplifiers. The sixsubcircuits were a current-to-voltage converter, a high-passfilter, an active rectifier, an active low-pass filter, an oscillator,and a transistor current amplifier. The students used anoscilloscope to measure the voltages at important points in thecircuit to confirm the circuit was working, to help see andunderstand how the subcircuit functioned, and to troubleshootproblems in their circuitry.On the fourth laboratory day, the students completed a

microcontroller tutorial that included coding examples for serialcommunication, digital input and output, mathematicalcalculations, and analog-to-digital conversion. The completedphotometer is shown in Figure 2. Their final photometer

program allowed the user to store the dark voltage, Vdark, andreference voltage, Vreference, using push buttons, and toautomatically calculate absorbance from the sample voltagemeasurement, Vsample, according to

= −−−

·⎛⎝⎜

⎞⎠⎟

V V

V VAbsorbance log sample dark

reference dark (1)

When the students had the microcontroller interfaced with thephotometer, they were ready to use their instrument for thechromium tests.On the fifth and final day of the unit, the students used their

photodiode photometers to measure the absorbance of 11chromium(VI) standard solutions ranging from 0.05 to 4.0 mg/L. They created a new calibration curve and compared thephotodiode-based instrument calibration to the CdS instrumentcalibration. They determined the figures of merit: sensitivity,detection limit, and dynamic range of their instrument. Finally,they used their instrument to measure the concentration ofhexavalent chromium in an unknown water sample.

■ RESULTSCalibration curves from a series of chromium(VI) standardsolutions in each of the photometers are shown in Figure 3.Also included in the figure are the instructor-obtained datafrom a commercial Jasco 570 UV/Vis/NIR spectrometer at λ =525 nm. All of the curves showed the expected deviation fromthe Beer−Lambert law at higher concentrations. The curveswere reasonably linear at Cr(VI) concentrations below 1.5 mg/L; however, there was a noticeable difference in sensitivities, theslopes of the linear region, between the three instruments. The

design based on the photodiode was about twice as sensitive asthe CdS based instrument (Figure 4).

The detection limit and limit of quantitation (LOQ) forCr(VI) concentrations were determined by measuring thevoltage and standard deviation of a water blank. The minimumdetectable absorbance is given by

σ= −

−⎛⎝⎜

⎞⎠⎟

VV

Absorbance log3blank blank

blank (2)

Figure 2. The completed photometer with microcontroller interface. Adetailed description of all of the circuitry and programming is providedin the Supporting Information.

Figure 3. Calibration curves for a series of standard chromium(VI)solutions complexed with diphenylcarbazide in various instruments.The student-built CdS-based photometer data are represented withblue diamonds and the student-built photodiode-based photometerdata are represented with green triangles. The data, represented bypurple x’s, were taken by the instructor with a Jasco 570 UV/Vis/NIRspectrometer at λ = 525 nm. All measurements are made in a 1 cmpath cell.

Figure 4. The linear region of the calibration curves from the student-built photometers. The sensitivity of the photodiode-based photo-meter (green triangles) is more than twice the sensitivity of the CdS-based photometer (blue diamonds) as shown by the steeper slope ofthe calibration curve for the photodiode based instrument.

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where Vblank is the blank voltage measurement and σblank is itsstandard deviation. The detection limit for the Cr(VI) wascalculated from the minimum absorbance and the slope of thecalibration curve. For the limit of quantitation, 10 σblank wasused. Because sample positioning is the limiting source ofuncertainty, the students measured the standard deviation ofthe blank by making a series of voltage measurements of theblank while removing the sample cell between each measure-ment. The figures of merit20 for the photodiode-basedphotometer are shown in Table 1.

The students generally obtained good results for theunknown water sample, which the instructor prepared at aconcentration of about 1 mg/L of hexavalent chromium.

■ CONCLUSIONIn this laboratory unit, students build a fully functionalphotometer, complete with a microcontroller interface, usingelementary circuit components. The unit helps students learnelectronic circuit construction techniques, measurements, andconcepts at an introductory level in the context of chemicalinstrumentation. The students use their photometers tomeasure the concentration of hexavalent chromium in aqueoussamples and determine the instrumental figures of merit for themeasurement. To get their instruments working properly, thestudents are forced to think about the instrument from afundamental level and troubleshoot their own problems. This isa valuable skill in research, but one that is difficult to developwhen working with commercial instruments.The students can get frustrated during the construction of

their instruments, which is typical for students first experiencein electronics. With a little instructor guidance, the students doget through their circuit difficulties and get a strong feeling ofaccomplishment when completed. This unit always receiveshigh rankings on the end of semester evaluation forms.

■ ASSOCIATED CONTENT*S Supporting Information

Circuit diagrams, descriptions, and oscilloscope images for all ofthe subcircuits of the photometer; a list of part numbers for theindividual components; example Arduino code; three alter-natives experiments for the photometer that do not usehexavalent chromium; discussion using a LabView interfaceinstead of the microcontroller; and the student handout. Thismaterial is available via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author

*E-mail: mcclain@chem.wisc.edu.

Notes

The authors declare no competing financial interest.

■ REFERENCES(1) Strobel, H. A. Choosing the Right Instrument: The ModularApproach Part I. J. Chem. Educ. 1984, 61 (2), A53−A56.(2) Patterson, B. M.; Danielson, N. D.; Lorigan, G. A.; Sommer, A. J.Analytical Spectroscopy Using Modular Systems. J. Chem. Educ. 2003,80 (12), 1460−1463.(3) Bernazzani, P.; Paquin, F. Modular Spectrometers in theUndergraduate Chemistry Laboratory. J. Chem. Educ. 2001, 78 (6),796−798.(4) Wigton, B. T.; Chohan, B. S.; Kreuter, K.; Sykes, D. TheCharacterization of an Easy-to-Operate Inexpensive Student-BuiltFluorimeter. J. Chem. Educ. 2011, 88 (8), 1188−1193.(5) Dominguez, V. C.; McDonald, C. R.; Schunk, D.; Kreuter, R.;Sykes, D. The Characterization of a Custom-Built Coulometric KarlFischer Titration Apparatus. J. Chem. Educ. 2010, 87 (9), 987−991.(6) Algar, W. R.; Massey, M.; Krull, U. J. Assembly of a ModularFluorimeter and Associated Software: Using LabView in an AdvancedUndergraduate Analytical Chemistry Laboratory. J. Chem. Educ. 2009,86 (1), 68−71.(7) Thal, M. A.; Samide, M. J. Applied Electronics: Construction of aSimple Spectrophotometer. J. Chem. Educ. 2001, 78 (11), 1510−1512.(8) Sengupta, S. K.; Farnham, J. M.; Whitten, J. E. A Simple Low-Cost Lock-In Amplifier for the Laboratory. J. Chem. Educ. 2005, 82(9), 1399−1401.(9) Sur, U. K.; Dhason, A.; Lakshminarayanan, V. A Simple and Low-Cost Ultramicroelectrode Fabrication and Characterization Methodfor Undergraduate Students. J. Chem. Educ. 2012, 89 (1), 168−172.(10) Gostowski, R. Teaching Analytical Instrument Design withLabView. J. Chem. Educ. 1996, 73 (12), 1103−1107.(11) Hargis, L. G.; Evilla, R. F. A Course in Electronics, Interfacingand on-Line Techniques for Scientists. J. Chem. Educ. 1982, 59 (5),414−416.(12) Keedy, C. R.; Abele, J. C. Electronic Instrumentation at theLiberal Arts College. J. Chem. Educ. 1985, 62 (2), 144−146.(13) Scheeline, A.; Mork, B. J. Electronics for Scientists: AComputer-Intensive Approach. J. Chem. Educ. 1988, 65 (12), 1079−1082.(14) Arduino_HomePage. http://arduino.cc/ (accessed Mar 2014).(15) Skoog, D.; Holler, F.; Crouch, S. Principles of InstrumentalAnalysis, 6th ed.; Thomson, Brooks, Cole: Belmont, CA, 2007;Chapters 2,3 and 4.(16) Mouser Electronics. http://www.mouser.com/ (accessed Mar2012).(17) Colorimetric Chemical Analytical Methods, 8th ed. Thomas, L. C.;Chamberlin, G. J.,Eds.; John Wiley and Sons: New York, 1974.(18) Wisconsin State Journal. http://host.madison.com/news/local/environment/study-madison-tap-water-has-relatively-high-levels-of-chromium/article_b9d14e08-0bd1-11e0-9069-001cc4c002e0.html (ac-cessed Mar 2014).(19) EPA method 218.7. http://water.epa.gov/scitech/drinkingwater/labcert/upload/EPA_Method_218-7.pdf (accessedMar 2014).(20) Skoog, D.; Holler, F.; Crouch, S. Principles of InstrumentalAnalysis, 6th ed.; Thomson, Brooks, Cole: Belmont, CA, 2007;Chapter 1.

Table 1. Figures of Merit for the Student-Built Photodiode-Based Photometer

Figure of Merit Value for Cr(VI) Measurement

Sensitivity 0.55 Abs/(mg/L)Limit of linearity (LOL) 1.5 mg/LMinimum detectable absorbance 0.003Detection limita 0.006 mg/LLimit of quantitation(LOQ)a 0.02 mg/LDynamic range (LOL-LOQ) 0.02 to 1.5 mg/L

aDetection limit and LOQ were calculated based on samplepositioning being the limiting factor.

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