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 WisconsinMadison, 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, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Aqueous Solution Chemistry, Laboratory Computing/Interfacing, Laboratory Equipment/Apparatus, Spectroscopy, 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 light source and a comparison of computers, and cell phones before they start elementary school. photoresistor, photodiode, and photodarlington detectors.7 By the time students take a course in chemical instrumentation, Sengupta et al. at University of MassachusettsLowell describe they are comfortable sitting in front of a computer and using an infrared 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 monochromators, 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 spectrum 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 monochromator. 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 photodiodes 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