Laboratory Experiment

pubs.acs.org/jchemeduc

Construction of a Photometer as an Instructional Tool for Electronics and Instrumentation Robert L. McClain*

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

*S Supporting Information

ABSTRACT: An introductory electronics laboratory unit for the undergraduate chemical instrumentation course is presented. In this unit, students use basic electronic components to build a functioning photometer. Students interface the photometer 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 of hexavalent chromium in a series of standard solutions and determine the figures of merit: sensitivity, detection limit, and dynamic range of the instrument. They also used their instrument to measure the concentration of hexavalent chromium in an unknown water sample.

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

he impact of electronics in modern society has been detector.6 The main focus of the Toronto experiment is the T transformational. Current students have grown up in a programming of the software interface using LabView. Thal and time when electronic tools, toys, and gadgets are seemingly Samide describe the construction of a simple spectropho- everywhere, with many of these students using video games, tometer using an LED source and a comparison of computers, and cell phones before they start elementary school. photoresistor, , and photodarlington detectors.7 By the time students take a course in chemical instrumentation, Sengupta et al. at University of MassachusettsLowell describe they are comfortable sitting in front of a computer and using an LED source and photodiode detector spectrometer the instrumental software associated with instruments. that also includes a home-built lock-in amplifier constructed in Although comfortable with the software, students are often the physical chemistry laboratory.8 In addition to spectroscopic mentally detached from the hardware functions of the instruments, potentiostats for electrochemical measurements ffi instrument and can have a di cult time understanding how can be made from basic electronic components and work well the instrument actually works. As modern instruments and their for student electronics construction projects.9,10 ffi interfaces become more sophisticated, it is even more di cult Basic electronics is still an important part of advanced for students to grasp the basic measurement principles. Getting courses in analytical or physical chemistry, and a few students to think fundamentally about how instruments work, universities have even developed complete courses in − so they can critically evaluate the quality of the data the electronics for chemistry students.11 13 Although these courses instrument conveniently provides, is an ongoing challenge in were developed in the late 1980s and today’s electronic the teaching of chemical instrumentation courses. technologies are very different, the basic premises of these One strategy to get students thinking about instruments from courses are still relevant and important. Students who seek a fundamental level is to have students build their own advanced degrees in physical and analytical chemistry need to functioning instruments. This Journal has numerous examples 1−3 have skills in electronics and computer interfacing. Students of student-built instrumentation either at the modular level using , detectors, light sources, and so forth, or who work with electronic circuits develop good hands-on at the level of electronic components. At Penn State University, problem-solving skills. A basic understanding of electronic signals as voltages and currents helps students understand the the instrumentation course includes a semester-long research “ ” project building a functional instrument from electronic physics behind the magic of modern technologies. components. Example instruments built by Penn State students An introductory laboratory unit on electronics for the are a light emitting diode (LED) based fluorimeter4 and a Karl undergraduate chemical instrumentation course is described. In Fischer titrator.5 University of Toronto students also make a fluorimeter using an LED light sources and photodiode Published: April 15, 2014

© 2014 American Chemical Society and Division of Chemical Education, Inc. 747 dx.doi.org/10.1021/ed400784x | J. Chem. Educ. 2014, 91, 747−750 Journal of Chemical Education Laboratory Experiment this unit, students learn basic electronics while building a functioning photometer. They interface the photometer to a microcontroller and write an Arduino14 program to display the measured absorbance data on a liquid crystal display (LCD). They use their instrument to measure the concentration of hexavalent chromium a series of standard solutions and to determine the figures of merit: sensitivity, detection limit, and dynamic range of the instrument. They also used their instrument to measure the concentration of hexavalent chromium in an unknown water sample.

■ UNIT OVERVIEW This unit is unique because the circuits for the photometer were designed to incorporate electronic concepts at a level and scope consistent with a typical textbook for the upper-level instrumentation course.15 The circuits included a voltage divider, a current to voltage converter, high- and low-pass Figure 1. Absorbance of Cr−diphenylcarbazide complex and filters, a relaxation oscillator, a relay, and a half-wave rectifier. emission spectrum of green LED. The absorbance data were taken These circuits were made from resistors, capacitors, a discrete with a Jasco 570 UV/Vis/NIR spectrometer for a 1 mg/L Cr(VI) transistor, operational amplifiers, a light emitting diode, signal solution in a 1 cm path cell. The emission data were collected with an diodes, a cadmium sulfide photoconductor, and a silicon Ames Photonics LARRY linear array CCD coupled to an Acton SpectroPro 2150i . photodiode. The students used a microcontroller with an analog-to-digital converter and were introduced to software programming with Arduino. As in any electronics laboratory 50 mL of water in a beaker, (2) dissolving 0.05 g of 1,5- students also learned to use a digital multimeter and diphenylcarbazide (Sigma-Aldrich) in 10 mL of methanol in a oscilloscope during the circuit construction. This unit took fi second beaker, (3) combining the two solutions, and (4) ve laboratory sessions, each laboratory was 3 h in length, and diluting to 100 mL with water. The coloring solution should be the students worked in pairs. The laboratory unit was scheduled made fresh, but can be stored for a couple of days in a closely with the coverage of electronics in the lecture portion of refrigerator. the course so that the students simultaneously learned both the theoretical and practical aspects of introductory electronics. ■ HAZARDS Light emitting diodes (LED) are readily available in many Chromium(VI) is a known carcinogen, and although the colors covering the visible spectrum.16 Silicon Cr(VI) solutions are quite dilute, they should be handled have spectral responses that also cover the visible spectrum. carefully and disposed of properly. See the Supporting With the proper choice of LED, the photometer can measure Information for suggested alternative methods using less the absorbance of any solution of color and can be used for a hazardous materials. The diphenylcarbazide coloring reagent number of traditional colorimetric methods of analysis.17 The is prepared in 1 N H2SO4, which should be handled carefully as photometer was used to measure the concentration of it can cause minor burns to the skin. Safety glasses and gloves hexavalent chromium in an unknown water sample prepared should be used by students when handling the reagents by the instructor. The chromium test was used as an example Safety glasses should be worn at during the electronics that has local interest to the students because elevated levels of construction since an improperly wired LED can draw enough hexavalent chromium have been reported in Madison, WI tap 18 19 current to heat up and rupture. There is a small chance of low water. In this method, chromium(VI) was complexed with level shocks when wiring, so students should be reminded to 1,5-diphenylcarbazide resulting in a strongly colored magenta ff λ turn o the power when wiring and rewiring their circuits. solution. The complex has max = 543 nm, which is close to the λ = 525 nm maximum output of the green LED (Figure 1). ■ DESCRIPTION OF THE FIVE LABORATORY SESSIONS ■ MATERIALS The details of the circuit construction are found in the To do this experiment in its entirety, students needed an Supporting Information, but an overview is provided here. On electronics workstation that included ±12 VDC power supply, the first day of the laboratory unit, students built a very simple oscilloscope, digital multimeter, and bread-boarding tools. An photometer based on a cadmium sulfide (CdS) photoresistor Arduino Uno development board was used for analog-to-digital detection element incorporated into a voltage divider circuit. conversion and the open source Arduino for programming. All They used their photometer to measure the absorbance of a of the part numbers for the individual components are found in series of chromium(VI) standard solutions ranging from 0.05 to the Supporting Information. 4.0 mg/L and created a calibration curve. The voltage divider is The laboratory instructor made a 1000 mg/L stock solution a fundamentally important circuit that is often used to of hexavalent chromium by dissolving K2CrO4 (Sigma-Aldrich) introduce basic direct current (DC) electronics. The simplicity in deionized water. From the stock solution, the instructor of the design of this photometer helped students get made a series of standard solutions ranging from 0.05 to 4.0 comfortable with circuit construction techniques and measure- mg/L for the experiment. The instructor made the 2 mM ments. diphenylcarbazide coloring reagent in 10% methanol and 1 N Over the next two laboratory periods, the students H2SO4 by (1) adding 2.8 mL of conc. H2SO4 to approximately constructed a more sophisticated version of the photometer.

748 dx.doi.org/10.1021/ed400784x | J. Chem. Educ. 2014, 91, 747−750 Journal of Chemical Education Laboratory Experiment

A silicon photodiode was used as the detector in the new version. The design contained six different subcircuits providing students with experience working with resistors, capacitors, diodes, a transistor, and operational amplifiers. The six subcircuits were a current-to-voltage converter, a high-pass filter, an active rectifier, an active low-pass filter, an oscillator, and a transistor current amplifier. The students used an oscilloscope to measure the voltages at important points in the circuit to confirm the circuit was working, to help see and understand how the subcircuit functioned, and to troubleshoot problems in their circuitry. On the fourth laboratory day, the students completed a microcontroller tutorial that included coding examples for serial communication, digital input and output, mathematical calculations, and analog-to-digital conversion. The completed photometer is shown in Figure 2. Their final photometer

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 with blue diamonds and the student-built photodiode-based photometer data are represented with green triangles. The data, represented by purple x’s, were taken by the instructor with a Jasco 570 UV/Vis/NIR spectrometer at λ = 525 nm. All measurements are made in a 1 cm Figure 2. The completed photometer with microcontroller interface. A path cell. detailed description of all of the circuitry and programming is provided in the Supporting Information. design based on the photodiode was about twice as sensitive as the CdS based instrument (Figure 4). program allowed the user to store the dark voltage, Vdark, and reference voltage, Vreference, using push buttons, and to automatically calculate absorbance from the sample voltage measurement, Vsample, according to ⎛ ⎞ VVsample− dark Absorbance=− log⎜ ⎟· ⎝ ⎠ VVreference− dark (1) When the students had the microcontroller interfaced with the photometer, they were ready to use their instrument for the chromium tests. On the fifth and final day of the unit, the students used their photodiode photometers to measure the absorbance of 11 chromium(VI) standard solutions ranging from 0.05 to 4.0 mg/ L. They created a new calibration curve and compared the photodiode-based instrument calibration to the CdS instrument calibration. They determined the figures of merit: sensitivity, detection limit, and dynamic range of their instrument. Finally, Figure 4. The linear region of the calibration curves from the student- they used their instrument to measure the concentration of built photometers. The sensitivity of the photodiode-based photo- hexavalent chromium in an unknown water sample. meter (green triangles) is more than twice the sensitivity of the CdS- based photometer (blue diamonds) as shown by the steeper slope of ■ RESULTS the calibration curve for the photodiode based instrument. Calibration curves from a series of chromium(VI) standard solutions in each of the photometers are shown in Figure 3. Also included in the figure are the instructor-obtained data The detection limit and limit of quantitation (LOQ) for from a commercial Jasco 570 UV/Vis/NIR spectrometer at λ = Cr(VI) concentrations were determined by measuring the 525 nm. All of the curves showed the expected deviation from voltage and standard deviation of a water blank. The minimum the Beer−Lambert law at higher concentrations. The curves detectable absorbance is given by ⎛ ⎞ were reasonably linear at Cr(VI) concentrations below 1.5 mg/ V − 3σ ff Absorbance=− log⎜ blank blank ⎟ L; however, there was a noticeable di erence in sensitivities, the ⎝ ⎠ slopes of the linear region, between the three instruments. The Vblank (2)

749 dx.doi.org/10.1021/ed400784x | J. Chem. Educ. 2014, 91, 747−750 Journal of Chemical Education Laboratory Experiment σ where Vblank is the blank voltage measurement and blank is its Notes standard deviation. The detection limit for the Cr(VI) was The authors declare no competing financial interest. calculated from the minimum absorbance and the slope of the σ calibration curve. For the limit of quantitation, 10 blank was ■ REFERENCES used. Because sample positioning is the limiting source of (1) Strobel, H. A. Choosing the Right Instrument: The Modular uncertainty, the students measured the standard deviation of Approach Part I. J. Chem. Educ. 1984, 61 (2), A53−A56. the blank by making a series of voltage measurements of the (2) Patterson, B. M.; Danielson, N. D.; Lorigan, G. A.; Sommer, A. J. blank while removing the sample cell between each measure- Analytical Spectroscopy Using Modular Systems. J. Chem. Educ. 2003, ment. 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Educ. 2012, 89 (1), 168−172. ■ CONCLUSION (10) Gostowski, R. Teaching Analytical Instrument Design with LabView. J. Chem. Educ. 1996, 73 (12), 1103−1107. In this laboratory unit, students build a fully functional (11) Hargis, L. G.; Evilla, R. F. A Course in Electronics, Interfacing photometer, complete with a microcontroller interface, using and on-Line Techniques for Scientists. J. Chem. Educ. 1982, 59 (5), elementary circuit components. The unit helps students learn 414−416. electronic circuit construction techniques, measurements, and (12) Keedy, C. R.; Abele, J. C. Electronic Instrumentation at the concepts at an introductory level in the context of chemical Liberal Arts College. J. Chem. Educ. 1985, 62 (2), 144−146. (13) Scheeline, A.; Mork, B. J. Electronics for Scientists: A instrumentation. The students use their photometers to − measure the concentration of hexavalent chromium in aqueous Computer-Intensive Approach. J. Chem. Educ. 1988, 65 (12), 1079 fi 1082. samples and determine the instrumental gures of merit for the (14) Arduino_HomePage. http://arduino.cc/ (accessed Mar 2014). measurement. To get their instruments working properly, the (15) Skoog, D.; Holler, F.; Crouch, S. Principles of Instrumental students are forced to think about the instrument from a Analysis, 6th ed.; Thomson, Brooks, Cole: Belmont, CA, 2007; fundamental level and troubleshoot their own problems. This is Chapters 2,3 and 4. a valuable skill in research, but one that is difficult to develop (16) Mouser Electronics. http://www.mouser.com/ (accessed Mar when working with commercial instruments. 2012). The students can get frustrated during the construction of (17) Colorimetric Chemical Analytical Methods,8th ed. Thomas, L. C.; their instruments, which is typical for students first experience Chamberlin, G. J.,Eds.; John Wiley and Sons: New York, 1974. in electronics. With a little instructor guidance, the students do (18) Wisconsin State Journal. http://host.madison.com/news/local/ get through their circuit difficulties and get a strong feeling of environment/study-madison-tap-water-has-relatively-high-levels-of- chromium/article_b9d14e08-0bd1-11e0-9069-001cc4c002e0.html (ac- accomplishment when completed. This unit always receives cessed Mar 2014). high rankings on the end of semester evaluation forms. (19) EPA method 218.7. http://water.epa.gov/scitech/ drinkingwater/labcert/upload/EPA_Method_218-7.pdf (accessed ■ ASSOCIATED CONTENT Mar 2014). *S Supporting Information (20) Skoog, D.; Holler, F.; Crouch, S. Principles of Instrumental Circuit diagrams, descriptions, and oscilloscope images for all of Analysis, 6th ed.; Thomson, Brooks, Cole: Belmont, CA, 2007; Chapter 1. the subcircuits of the photometer; a list of part numbers for the individual components; example Arduino code; three alter- natives experiments for the photometer that do not use hexavalent chromium; discussion using a LabView interface instead of the microcontroller; and the student handout. This material is available via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

750 dx.doi.org/10.1021/ed400784x | J. Chem. Educ. 2014, 91, 747−750