Construction of a Photometer as an Instructional Tool for Electronics

Apr 15, 2014 - An introductory electronics laboratory unit for the undergraduate chemical instrumentation course is presented. In this unit, students ...
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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, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Aqueous Solution Chemistry, Laboratory Computing/Interfacing, Laboratory Equipment/Apparatus, Spectroscopy, Water/Water Chemistry

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detector.6 The main focus of the Toronto experiment is the programming of the software interface using LabView. Thal and Samide describe the construction of a simple spectrophotometer using an LED light source and a comparison of photoresistor, photodiode, and photodarlington detectors.7 Sengupta et al. at University of MassachusettsLowell describe an infrared LED source and photodiode detector spectrometer that also includes a home-built lock-in amplifier constructed in the physical chemistry laboratory.8 In addition to spectroscopic instruments, potentiostats for electrochemical measurements can be made from basic electronic components and work well for student electronics construction projects.9,10 Basic electronics is still an important part of advanced courses in analytical or physical chemistry, and a few universities have even developed complete courses in electronics for chemistry students.11−13 Although these courses were developed in the late 1980s and today’s electronic technologies are very different, the basic premises of these courses are still relevant and important. Students who seek advanced degrees in physical and analytical chemistry need to have skills in electronics and computer interfacing. Students who work with electronic circuits develop good hands-on problem-solving skills. A basic understanding of electronic signals as voltages and currents helps students understand the physics behind the “magic” of modern technologies. An introductory laboratory unit on electronics for the undergraduate chemical instrumentation course is described. In

he impact of electronics in modern society has been transformational. Current students have grown up in a time when electronic tools, toys, and gadgets are seemingly everywhere, 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 using the instrumental software associated with instruments. Although comfortable with the software, students are often mentally detached from the hardware functions of the instrument and can have a difficult time understanding how the instrument actually works. As modern instruments and their interfaces become more sophisticated, it is even more difficult for students to grasp the basic measurement principles. Getting students to think fundamentally about how instruments work, so they can critically evaluate the quality of the data the instrument conveniently provides, is an ongoing challenge in the teaching of chemical instrumentation courses. One strategy to get students thinking about instruments from a fundamental level is to have students build their own functioning instruments. This Journal has numerous examples of student-built instrumentation either at the modular level1−3 using monochromators, detectors, light sources, and so forth, or at the level of electronic components. At Penn State University, the instrumentation course includes a semester-long research project building a functional instrument from electronic components. Example instruments built by Penn State students are a light emitting diode (LED) based fluorimeter4 and a Karl Fischer titrator.5 University of Toronto students also make a fluorimeter using an LED light sources and photodiode © 2014 American Chemical Society and Division of Chemical Education, Inc.

Published: April 15, 2014 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.

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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 filters, a relaxation oscillator, a relay, and a half-wave rectifier. These circuits were made from resistors, capacitors, a discrete transistor, operational amplifiers, a light emitting diode, signal diodes, a cadmium sulfide photoconductor, and a silicon 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 students also learned to use a digital multimeter and oscilloscope during the circuit construction. This unit took five laboratory sessions, each laboratory was 3 h in length, and the students worked in pairs. The laboratory unit was scheduled closely with the coverage of electronics in the lecture portion of the course so that the students simultaneously learned both the theoretical and practical aspects of introductory electronics. Light emitting diodes (LED) are readily available in many colors covering the visible spectrum.16 Silicon photodiodes have spectral responses that also cover the visible spectrum. With the proper choice of LED, the photometer can measure the absorbance of any solution of color and can be used for a number of traditional colorimetric methods of analysis.17 The photometer was used to measure the concentration of hexavalent chromium in an unknown water sample prepared by the instructor. The chromium test was used as an example that has local interest to the students because elevated levels of hexavalent chromium have been reported in Madison, WI tap water.18 In this method,19 chromium(VI) was complexed with 1,5-diphenylcarbazide resulting in a strongly colored magenta solution. The complex has λmax = 543 nm, which is close to the λ = 525 nm maximum output of the green LED (Figure 1).

Figure 1. Absorbance spectrum of Cr−diphenylcarbazide complex and emission spectrum of green LED. The absorbance data were taken with 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 an Ames Photonics LARRY linear array CCD coupled to an Acton SpectroPro 2150i monochromator.

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

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HAZARDS Chromium(VI) is a known carcinogen, and although the Cr(VI) solutions are quite dilute, they should be handled carefully and disposed of properly. See the Supporting Information for suggested alternative methods using less hazardous materials. The diphenylcarbazide coloring reagent is prepared in 1 N H2SO4, which should be handled carefully as it can cause minor burns to the skin. Safety glasses and gloves should be used by students when handling the reagents Safety glasses should be worn at during the electronics construction since an improperly wired LED can draw enough current to heat up and rupture. There is a small chance of low level shocks when wiring, so students should be reminded to turn off the power when wiring and rewiring their circuits.

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DESCRIPTION OF THE FIVE LABORATORY SESSIONS The details of the circuit construction are found in the Supporting Information, but an overview is provided here. On the first day of the laboratory unit, students built a very simple photometer based on a cadmium sulfide (CdS) photoresistor detection element incorporated into a voltage divider circuit. They used their photometer to measure the absorbance of a series of chromium(VI) standard solutions ranging from 0.05 to 4.0 mg/L and created a calibration curve. The voltage divider is a fundamentally important circuit that is often used to introduce basic direct current (DC) electronics. The simplicity of the design of this photometer helped students get comfortable with circuit construction techniques and measurements. Over the next two laboratory periods, the students constructed a more sophisticated version of the photometer.

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MATERIALS To do this experiment in its entirety, students needed an electronics workstation that included ±12 VDC power supply, oscilloscope, digital multimeter, and bread-boarding tools. An Arduino Uno development board was used for analog-to-digital conversion and the open source Arduino for programming. All of the part numbers for the individual components are found in the 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 instructor made a series of standard solutions ranging from 0.05 to 4.0 mg/L for the experiment. The instructor made the 2 mM diphenylcarbazide coloring reagent in 10% methanol and 1 N H2SO4 by (1) adding 2.8 mL of conc. H2SO4 to approximately 748

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

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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 path cell.

Figure 2. The completed photometer with microcontroller interface. A 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 ⎛ Vsample − Vdark ⎞ Absorbance = −log⎜ ⎟· ⎝ Vreference − Vdark ⎠

(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, they used their instrument to measure the concentration of hexavalent chromium in an unknown water sample.

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

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RESULTS 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 from a commercial Jasco 570 UV/Vis/NIR spectrometer at λ = 525 nm. All of the curves showed the expected deviation from the Beer−Lambert law at higher concentrations. The curves were reasonably linear at Cr(VI) concentrations below 1.5 mg/ L; however, there was a noticeable difference in sensitivities, the slopes of the linear region, between the three instruments. The

The detection limit and limit of quantitation (LOQ) for Cr(VI) concentrations were determined by measuring the voltage and standard deviation of a water blank. The minimum detectable absorbance is given by ⎛V − 3σblank ⎞ Absorbance = −log⎜ blank ⎟ Vblank ⎠ ⎝ 749

(2)

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

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where Vblank is the blank voltage measurement and σblank is its standard deviation. The detection limit for the Cr(VI) was calculated from the minimum absorbance and the slope of the calibration curve. For the limit of quantitation, 10 σblank was used. Because sample positioning is the limiting source of uncertainty, the students measured the standard deviation of the blank by making a series of voltage measurements of the blank while removing the sample cell between each measurement. The figures of merit20 for the photodiode-based photometer are shown in Table 1.

Notes

The authors declare no competing financial interest.

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Table 1. Figures of Merit for the Student-Built PhotodiodeBased Photometer Figure of Merit

Value for Cr(VI) Measurement

Sensitivity Limit of linearity (LOL) Minimum detectable absorbance Detection limita Limit of quantitation(LOQ)a Dynamic range (LOL-LOQ)

0.55 Abs/(mg/L) 1.5 mg/L 0.003 0.006 mg/L 0.02 mg/L 0.02 to 1.5 mg/L

a

Detection limit and LOQ were calculated based on sample positioning being the limiting factor.

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

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CONCLUSION In this laboratory unit, students build a fully functional photometer, complete with a microcontroller interface, using elementary circuit components. The unit helps students learn electronic circuit construction techniques, measurements, and concepts at an introductory level in the context of chemical instrumentation. The students use their photometers to measure the concentration of hexavalent chromium in aqueous samples and determine the instrumental figures of merit for the measurement. To get their instruments working properly, the students are forced to think about the instrument from a fundamental level and troubleshoot their own problems. This is a valuable skill in research, but one that is difficult to develop when working with commercial instruments. The students can get frustrated during the construction of their instruments, which is typical for students first experience in electronics. With a little instructor guidance, the students do get through their circuit difficulties and get a strong feeling of accomplishment when completed. This unit always receives high rankings on the end of semester evaluation forms.

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ASSOCIATED CONTENT

S Supporting Information *

Circuit diagrams, descriptions, and oscilloscope images for all of the subcircuits of the photometer; a list of part numbers for the individual components; example Arduino code; three alternatives 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.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 750

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