Accurate, Photoresistor-Based, Student-Built Photometer and Its

Article Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Accurate, Photoresistor-Based, Student-Built Photometer and Its Application to the Forensic Analysis of Dyes Anna L. Adams-McNichol,† Rayf C. Shiell,*,‡ and David A. Ellis† †

Department of Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9J 0G2, Canada Department of Physics & Astronomy, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9J 0G2, Canada

‡

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S Supporting Information *

ABSTRACT: Student-built photometers are recognized as being useful tools in teaching laboratories that help students understand the concepts behind experimental measurements, and several devices have been reported that employ photoresistors as the detector of choice. We demonstrate here an improvement over these instruments and the analyses thereof, providing more rapid deployment in the laboratory and also considerable increases in both linearity and accuracy. This was confirmed by conducting quantitative absorption spectroscopy of known concentrations of potassium permanganate solution. A laboratory exercise utilizing this approach was employed in two postsecondary institutions to simulate the forensic analysis of unknown fabric dyes to determine the identity and concentration of an unknown dye solution.

KEYWORDS: Second-Year Undergraduate, Analytical Chemistry, Laboratory Instruction, Applications of Chemistry, Spectroscopy, Dyes/Pigments, Forensic Chemistry, Qualitative Analysis, Quantitative Analysis, Hands-On Learning/Manipulatives

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INTRODUCTION

The precise structure of a molecule determines the wavelengths of electromagnetic radiation that can be absorbed or emitted.1−3 Generally, incident UV−vis light results in molecules being excited to a higher-energy state, and when they then relax toward their ground-state, light may also be emitted.1 The wavelengths of light absorbed or emitted correspond to the differences in energy between the relevant energy states, and thus the relative amount of light absorbed at each wavelength is unique to each individual chemical species.1 When light passes through a sample that has not been previously excited, the transmitted intensity for any particular narrow band of wavelengths, I, is less than the corresponding incident intensity, I0. The absorbance at each wavelength, A, which is unitless, is defined by the following, where the logarithm base-10 is usually used:1

Spectroscopy is a valuable technique in analytical chemistry and forensic science.1 Ultraviolet−visible (UV−vis) spectroscopy is used widely in chemical research and in biochemical, environmental, and pharmaceutical laboratories.1 Quality and impurities are commonly assessed using UV−vis spectroscopy in the food and beverage industry.1 Quantitative determination of DNA and RNA can also be accomplished using such data together with the known optical properties of these molecules at the wavelengths of interest.1 Spectroscopic measurements of samples are usually conducted using one of two modalities: emission, where the light emitted by the sample is characterized according to its intensity as a function of wavelength, or absorption, where the light transmitted through a sample at one or more wavelengths is measured and then reported at these wavelengths. 2 A spectrometer is a device that focuses primarily on recording spectral information, such as the wavelengths of strong absorption or emission; a photometer typically provides absolute intensity-related quantities, such as the power per unit wavelength per unit solid angle of emitted light, or the absolute f raction of light absorbed at a wavelength. Absorption photometers thus employ a single, narrowband wavelength source, whereas absorption spectrophotometers irradiate a sample with a range of incident wavelengths. Here, we report on an improved circuit and approach to data analysis for an absorption photometer utilizing a single narrowband wavelength source, which can, if desired, be adopted within an absorption spectrophotometer. © XXXX American Chemical Society and Division of Chemical Education, Inc.

A = −log10

I I0

(1)

Beer’s law states the equivalence between this absorbance and the product of the path length of the light through the sample, l; the species concentration, c; and its attenuation coefficient at that wavelength, ε: A = εcl

(2)

Received: October 21, 2018 Revised: March 25, 2019

A

DOI: 10.1021/acs.jchemed.8b00862 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Article

Figure 1. Schematic of an optical spectrometer or spectrophotometer, including a light source, wavelength selector, sample, detector, and signal processor−readout unit.

results from these studies have generally neither demonstrated the linearity expected from Beer’s law nor determined the correct molar attenuation coefficients, with inconsistencies that are apparent from the data or the discussion sections within these papers or their supporting information. Here, we expand on these reports by adopting a particularly simple design for such a photometer to enable rapid deployment in an educational setting; we apply physical principles to the circuit and its components to arrive at improved data analysis that provides robust linearity and accurate attenuation coefficients; and we report student responses from using this approach to test fabric dyes within two different postsecondary institutions. Such an exercise provides experiential learning opportunities for chemistry and forensic-science majors to learn the core concepts of a spectrophotometer, to successfully build such a device, and to understand and trust Beer’s law as a physical law with real utility. Reporting experimental results increases students’ technical-communication skills through the preparation of calibration plots and reinforces their knowledge of statistical analysis through calculations of standard deviations from repeated trial data. This laboratory exercise is extendable as it can provide detailed spectra through the use of either a set of LEDs or a white-light source with a dispersive component such as a grating; thus, it can become a widespread and trustworthy tool within teaching laboratories throughout the postsecondary sector.

and thus a given path length of a particular species reflects a linear relationship between absorbance and species concentration. This applies provided that uniform solutions are prepared, stray light is negligible, and the species concentration is not so high that intermolecular interactions result in collective behaviors such as clustering. Note that some older texts refer to the attenuation coefficient as the extinction coef f icient or absorptivity.4 Convenient and consistent units for the quantities in eq 2 are decimeters cubed per centimeter per mole (dm3/cm/ mol) for ε, moles per liter (M) for c, and centimeters for l; with moles used as here, the qualifier molar is often prepended to the relevant terms. Thus, the slope of absorbance (on the y-axis) against species molar concentration (on the x-axis) for narrowband incident light with a known path length can yield the molar attenuation coef f icient at this wavelength, and knowing these for a range of incident wavelengths can then identify an unknown compound by comparison of them with reference absorption spectra and attenuation coefficients.3 Note also that some authors instead adopt the natural logarithm, ln, within eq 1, in which case eq 2 still applies, but then the e-based, rather than the decadic, attenuation coefficient results, which is a factor of ln(10) ≈ 2.3 times greater.5 Several studies have demonstrated the pedagogical benefits of introducing students to simple, modular photometers or spectrophotometers comprising only a few components to help them understand the principles behind commercial instruments used in analytical-chemistry laboratories. These studies have used a range of technologies for both the light source and the detector. For the former, these include a white light followed by a diffraction grating,6,7 a light-emitting diode (LED),8−15,18 and a laser pointer;16 the latter, include a photodiode or LED,9,10,12−14 a photoresistor,8,11,14,18 and a digital camera or cellphone.6,7,15,17 Of these combinations, arguably the most simple instructional setup that uses its constituent electronic components in the way they are most commonly configured is that of an LED source coupled with a photoresistor detector.8,11,14,18 For here, the LED is simply powered by a voltage source with a resistor in series to avoid current overload, and the resistance of the photoresistor (itself alternatively called a light-dependent resistor, photoconductive cell, or photocell) dictates the voltage drop across it when powered by a battery in series with a second fixed resistor. Several projects reported in the educational literature8,11,14,18 have enacted this or close variants thereof, where the data analysis assumed that the parameter of interest (here, the resistance of the photoresistor) is proportional to the lightintensity incident on this electronic component. Although such linearity is generally true for photodiodes, this does not usually apply to photoresistors over the working range of interest. Thus,

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BUILDING THE PHOTOMETER Many chemical- and biochemical-research and industrial sectors use UV−vis spectroscopy as a powerful tool for both quantitative and qualitative analysis.1 Understanding the composite components of the instrument is crucial to help eliminate a student’s “black box” mindset of simply introducing a sample and obtaining a result without understanding how or why the specific results materialize. In general, opticalabsorption spectrometers consist of a light source, a wavelength selector, the sample, a detector, and a signal processor−readout unit,2 as illustrated in Figure 1, although a set of distinct narrowband sources may replace the first two items. By understanding the constituent components within such an instrument, students can truly appreciate the connections among its operation, the signal obtained, and the optical properties of a sample. Inspired by the photoresistor-based circuit in refs 8 and 18, with some adaptations we adopted a setup containing only four electronic components placed on a solderless breadboard: an LED, 1 and 10 kΩ resistors, and a photoresistor. A 1.0 cm pathlength cuvette was situated between the LED and photoresistor, and the circuit was powered with a 9 V battery through jumper B



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Figure 2. (a) Photograph of the photometer circuit board. A photograph box obtained from a craft store was used over the setup to minimize signal obtained from stray light. The red and black leads of the multimeter are seen in the right side of the image. (b) Circuit diagram of the key components composing the photometer. LED, light-emitting diode; CUV, cuvette containing the analyte of interest; PC, photocell−photoresistor with associated resistance RPC.

wires, as depicted in Figure 2. The precise resistances of the two resistors are not critical; the series LED resistor prevents overloading the LED, and good circuit stability results when the resistance of the second resistor is approximately ten times greater than this. For reasons we expand upon below, voltage measurements were taken across this resistor (in series with the photoresistor), using a multimeter set to a full-scale reading of 20 V. A detailed list of components and possible light sources can be found within the Supporting Information located on this Journal’s website. Compared with the photometer adopted in refs 8 and 18, the multimeter alone replaced the op-amp and feedback resistor, with the advantage of simplicity in design. For analytes such as those studied here with visible absorption spectra that are hundreds of nanometers wide, an LED with a bandwidth of ∼5 nm can usually be considered a sufficient narrowband source.19 Depending on the solution to be analyzed, different LED wavelengths may be selected to determine wavelength-dependent absorbance. Typically for peak absorption the LED selected should be the complement color to that of the solution being analyzed. For the student laboratory exercises discussed below, two yellow fabric dyes were studied, Rit dye and Tulip dye, with wavelengths of maximum absorbance for each determined with a commercial instrument to be ≈390 and ≈420 nm, respectively. Thus, for the teaching laboratory exercise, the LED selected was in the violet range, with a manufacturer-reported range from 395 to 403 nm.

The photometers were assembled by students following a specific procedure, which is available in the Supporting Information. Students also had access to a demonstration photometer and images of an assembled instrument were provided on the course learning platform for reference prior to the lab. Figure 2a is an image of such a sample student-built photometer, and Figure 2b shows the circuit diagram. Application of Ohm’s law to the circuit in Figure 2b indicates that the output voltage is given by Vout =

VbatR R + RPC

(3)

The dependence of Vout on the light intensity striking the photoresistor comes from the variation of RPC with this intensity; we incorporate this in the following section. After construction, the apparatus was placed inside the upturned lid of a photograph box purchased from a local craft store and then covered with the box to decrease stray light contamination and minimize experimental drift. Students were provided with laboratory tape to secure the cuvette in position on the circuit board and instructed to keep the cuvette in place while different concentrations of each solution were analyzed to further minimize spurious effects from changing the path traversed by the light. C



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QUANTITATIVE VALIDATION OF THE PHOTOMETER USING BEER’S LAW We now provide the appropriate equation with which to obtain the molar attenuation coefficient of a sample using the photometer shown in Figure 2, and in doing so, we correct some oversights that have previously been made in the literature.8,11,14,18 The approximately half-page step-by-step derivation leading to the defining eq 5 (below) is provided in the Supporting Information. Photoresistors are thin-film devices made by depositing a layer of a photoconductive semiconductor, such as CdS or CdSe, onto a ceramic substrate between two connecting terminals.20 The physical property of interest is RPC, the resistance of the photoresistor−photocell, and its dependence on I, the light intensity incident on this component. As this intensity increases, electrons are increasingly excited from the valence band of the semiconductor into the conduction band, resulting in decreased resistance due to more holes and more electrons in each of these respective bands. The datasheets for such devices (see, for example, refs 20 and 21) demonstrate that the relationship between RPC and I is nonlinear, but for most intensity ranges used, the relationship between ln(RPC) and ln(I) is very close to linear. We label the slope of ln(RPC) against ln(I) as −b, and we call parameter b the photoresistor slope, which typically takes a value between 0.4 and 0.8.21 (Note that if b ≈ 1, then it could be said that the conductance is linearly proportional to the light intensity.) Thus, the relationship between the resistance of the photoresistor and the intensity of the incident light is given by RPC = aI −b

dependence on Vsolvent. Then, if desired, Vsolvent could be obtained simply from the y-axis intercept of this plot. For comparison, we briefly compare the analysis above with that of previous photoresistor-based circuits and respective data analyses,8,11,14,18 where a resistor we label here as R4 is placed in series with the photoresistor, and the photoresistor is then connected to an inverting operational amplifier with feedback resistor R3. The output voltage for this circuit takes prima facie a similar functional form to eq 3, although with a subtly different dependence on its parameters: Vout′ =

VbatR3 R 4 + RPC

(7)

By repeating the process from above, we arrive at the following defining equation for these photoresistor-based photometers: ÄÅ ÉÑ R3 ÅÅÅ ÑÑÑ ′ − V V ′ V bat solvent ÅÅ ÑÑ R4 1 sample Å ÑÑ = εcl − log10ÅÅ R ÑÑ 3 Å ′ b Vsolvent ′ ÅÅ R Vbat − Vsample ÑÑ ÅÇ 4 ÑÖ (8) There are two points of note: the first and more minor point is that the stability of these photometers depends on the stability of R3 and R4; second and more importantly, what should be plotted against concentration to obtain both a linear agreement with Beer’s law and the correct molar attenuation coefficient is the left-hand side of this equation (or, as per eq 6, the logarithm of the first term in brackets on the left-hand side), and not the expression ÅÄÅ V ÑÉ Å sample − Vzero ÑÑÑ ÑÑ −log10ÅÅÅÅ ÅÅÇ Vsolvent − Vzero ÑÑÑÖ

(4)

where a and b can be obtained from the relevant datasheet. We shall see that for photoresistor-based photometers as described here, all results are independent of parameter a, and so only the photoresistor slope, b, is relevant. By applying eqs 3 and 4 to the solvent-only case (i.e., when Vout = Vsolvent), and then to the case with a sample in the cuvette (i.e., Vout = Vsample) and then substituting these two cases into eq 2, we arrive at the defining equation for the photometer shown in Figure 2: ÅÄÅ ÑÉ ÅÅ Vsample Vbat − Vsolvent ÑÑÑÑ 1 Å − log10ÅÅ ÑÑÑ = εcl ÅÅ Vbat − Vsample b Vsolvent ÑÑÖ (5) ÅÇ

as has been previously plotted.8,11,14,18 (Note that this latter expression also included a constant offset voltage, Vzero, intended to addresses any issue of stray light). Indeed, using a Taylor series expansion of eq 8 for both the decadic logarithm and resulting exponential terms leads to ÄÅ É ij y ÅÅ Vsample ÑÑÑ jj Vsolvent zzzz Å Ñ j Å Ñ zz −logÅÅ Ñ ≈ εblc jjj1 − R3 ÅÅÇ Vsolvent ÑÑÑÖ jj Vbat zzz R 4 k { ij yz jj 2 z zz (εblc)2 jj Vsolvent V z + 2.3jjj R − solvent 2 zzz + ... jj 3 V zz 2 R 3 jj R 4 bat z zz V j R 4 bat k {

(

Several observations can be made from eq 5. First, the calibration plot that results is independent of both parameter a and R, the value of the resistor in series with the photoresistor. The advantage here is that any long-term drift in either of these two quantities does not affect the accuracy of the data analysis. Second, the absorbance is given by the left side of eq 5, and so a plot of this quantity on the y-axis against sample concentration on the x-axis should produce a straight line through the origin with a slope of εl. Third, this equation can be further rearranged into the following more obviously linear (y = ax + b) form: ÉÑ ÄÅ ÅÄÅ ÑÉÑ ÑÑ ÅÅ Vsolvent ÅÅ Vsample ÑÑ 1 1 ÑÑ Å Å Ñ ÑÑ = εlc − log10ÅÅÅ − log10ÅÅ Ñ Å Ñ ÅÅÇ Vbat − Vsolvent ÑÑÑÖ b b ÅÅÇ Vbat − Vsample ÑÑÖ

)

(9)

where 2.3 is simply the rounded value of ln(10). Equation 9 demonstrates that plotting log10(Vsample/Vsolvent) against concentration when expecting a slope of εl will give instead, at low concentrations, an erroneously reduced molar attenuation coefficient of ij y jj Vsolvent zzzz j zz εbjj1 − R jj 3 Vbat zzz j R 4 k {

and also manifest quadratic behavior with concentration, which then serves to increase the apparent slope. To clarify the significance of the corrections associated with using eq 5, rather than simply assuming the absorbance to be −log10(Vsample/Vsolvent), it can be noted by comparing these two expressions that a first correction involves the photoresistor

(6)

which shows that for a known path length, the molar attenuation coefficient, ε, can be most simply determined from only the slope of the left side of eq 6 against molar concentration, with no D



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slope, b, not being unity, and a second is associated with the term (Vbat − Vsolvent)/(Vbat − Vsample) also not being unity. The photoresistor slope is determined from the relevant data sheet; the magnitude of the second correction is determined from examination of the circuit under particular operating conditions. As RPC increases at lower incident-light intensities, and Vout is measured across resistor R in Figure 2b, then Vsolvent will, within experimental error, always be greater than Vsample. Thus, the second correction will be less significant if it is engineered that Vsolvent ≪ Vbat, which can be achieved using low-light conditions through adopting either neutral density filters or a highly dispersive element positioned after a white-light source. In the experiments outlined below and in others from the literature, a 9 V battery is often used, and Vsolvent ≈ 8.5 V, so clearly both corrections remain important. If, however, Vsolvent ≪ Vbat, then only the first correction applies. To confirm the accuracy of eq 5 for our photometer, a series of dilutions was performed on solutions of potassium permanganate with concentrations ranging from 3.13 × 10−5 to 1 × 10−3 M. The solutions were then analyzed using a yellow LED with a central wavelength of 591 nm. Two readings of all measurements were sequentially taken, and the averages were used for analysis. A calibration plot was constructed by using absorbance values calculated from eq 5 and, for comparison, also by simply assuming the absorbance to be −log10(Vsample/Vsolvent). The data and resulting absorbances are listed in Tables 1 and 2, with the

incorrect molar attenuation coefficient) with quadratic behavior also evident from the data. Having now established the validity of the photometer and the protocol for data analysis, we describe a laboratory exercise in which students compare two commercial dyes by undertaking a mock study within the field of forensic science.

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Spectroscopic Analysis of Dyes

The forensic analysis of fabric dyes and fibers has been demonstrated using a variety of spectroscopic techniques, including Fourier-transformed infrared (FTIR), Raman spectroscopy (RS), infrared-chemical-imaging spectroscopy, X-ray fluorescence spectroscopy, and UV−vis microspectrometry.23 Of the many spectroscopic techniques, UV−vis spectroscopy has been used for a wide variety of applications. Extraction of dyes from textiles followed by separation using high-performance liquid chromatography (HPLC) and detection using UV− vis spectroscopy has been utilized to identify a variety of classes of colored agents used to dye historical textiles.24 Furthermore, UV−vis spectroscopy has been used to characterize the acidity constants of azo dyes in water through spectrophotometric titrations.25 Forensic characterization of fiber-dye analysis has also been demonstrated by combining HPLC with UV−vis and mass spectrometry in a single instrument allowing separation and characterization of dyes with similar UV−vis spectra.26 Given the accuracy and ease-of-use of our photometer, it can reasonably be expected to be used within the visible range to create calibration plots and allow for quantitative and qualitative analysis of fabric dyes by students. A patent-infringementinvestigation laboratory exercise was therefore created to increase interest among students majoring in chemistry or forensic science.

Table 1. Input Parameters for the Analysisa Input Parameters

Values

Vsolvent Vbat b (dimensionless)

8.47 V 9.155 V 0.765

a

Either measured or derived from the photoresistor datasheet.

Table 2. Recorded Voltages Obtained Using the Photometer Shown in Figure 2 and the Resultant Absorbances for Various KMnO4 Concentrations with an LED Centered at 591 nm Concentration, M Vsample, V 1.00 × 10−3 5.00 × 10−4 2.50 × 10−4 1.25 × 10−4 6.25 × 10−5 3.13 × 10−5

8.09 8.335 8.425 8.465 8.495 8.49

Correct Absorbance from Equation 5

Apparent Absorbance Using −log10(Vsample/Vsolvent)

0.277 0.111 0.0391 0.0045 −0.0228 −0.0182

0.0199 0.00698 0.00231 0.000256 −0.00128 −0.00102

USE OF THE PHOTOMETER IN A STUDENT LAB FOR THE FORENSIC ANALYSIS OF DYES

Dye Selection

The dyes used in this laboratory experiment were manufactured, powdered, patent-protected fabric dyes produced by Tulip and Rit. Because of patent protection, the identities of the chromophore components in the two dyes are unknown, and thus all concentrations below are in terms of mass concentrations (g/L) rather than molar concentrations (mol/L). To confirm that the dyes did indeed have different chromophore components, UV−vis spectra were first obtained using a commercial Cary UV−vis 50 spectrophotometer (Figure 4). It was found that the Rit dye had a maximum absorbance at 395.0 nm, and the Tulip dye had a maximum absorbance at 425.1 nm. In what follows, the assumption was made that any chromophores composing the dyes acted chemically independently of each other (i.e., no clustering or dimerization occurred), and thus the absorbance, A, would indeed be linearly dependent on the dye concentration, c. The appropriate starting concentration for dilution into stock dye solutions for student use was determined to be 1.25 g/L for both dyes. This was selected to provide students with two distinct samples that would remain visibly undistinguishable across the full dilution range considered. An unknown dye concentration that was given to students then fell within the midrange of these dilutions.

calibration plots shown in Figure 3. Figure 3a shows the result of analyzing the data using eq 5; the plot is highly linear (r2 = 0.997), and the resulting molar attenuation coefficient is 309 dm3/cm/mol. This value agrees well with the literature value of 305 dm3/cm/mol at 590 nm, which was obtained by careful examination of Figure 6 of ref 22 and which reveals an absorption coefficient at 590 nm of 0.0328 cm−1 for a 17 ppm (i.e., 17 μg/dm3) solution. Agreement was also noted with the molar attenuation coefficient at 591 nm determined using two of our commercial instruments: a new VWR UV−vis spectrophotometer (r2 = 0.997, ε = 303 dm3/cm/mol) and also an older Cary UV−vis 50 spectrophotometer (r2 = 0.999, ε = 272 dm3/ cm/mol). Conversely, adopting the simpler expression given in the right-hand column of Table 2 provides the results depicted in Figure 3b and gives erroneous absorbances (and thus an

Solution Preparation

During both Fall 2016 and Spring 2017, the laboratory exercise was deployed within a second-year analytical-chemistry course E



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Figure 3. Calibration plots for a 1 cm cuvette containing KMnO4 using a yellow LED at 591 nm, with a fitted line that is unconstrained other than being a linear fit. (a) Data analyzed and plotted using eq 5. The slope is 309 dm3/cm/mol. (b) Data analyzed and plotted using the simpler expression given in Table 2. The slope is 21.8 dm3/cm/mol.

Figure 4. Absorption spectra of (a) 1.25 g/L Rit yellow dye solution showing a maximum absorbance at 395 nm and (b) 1.25 g/L Tulip yellow dye solution showing a maximum absorbance at 425 nm.

at Trent University. In the prelaboratory exercise, students were asked to calculate the volumes needed for preparation of 1.00, 0.500, 0.250, and 0.125 g/L solutions from a 1.25 g/L stock solution. After removing the portion needed for the next solution in the series of dilutions, the prepared solutions were stored in labeled snap-cap vials or centrifuge tubes for later spectroscopic analysis. This portion of the experimental procedure allowed students to gain experience with serialdilution calculations, solution preparation, and proper pipetting techniques while using nonhazardous materials.

battery while changing samples to ensure the battery maintained sufficient voltage throughout the laboratory (a typical 9 V battery holds ∼200−400 mA h of charge, and the circuit draws ≈10 mA). They were also instructed to rinse the cuvettes with the new solution twice before taking measurements. It was found that using a disposable transfer pipet while leaving the cuvette in place minimized drift in voltage measurements due to the variation of path length that could occur by disturbing the LED or photoresistor upon removing and replacing the cuvette or by slightly rotating the cuvette.

Data Collection

Quantitative Analysis

During the Fall 2016 semester, eight sections of approximately 22 students per section completed the data collection in the first 2 h of a 3 h laboratory session. Prior to running the experiment again in the Winter 2017 term, repeated trials were added to allow for a more robust data-analysis portion of the postlab questions. In the Winter 2017 semester, three sections of 22 students completed all components of the data collection within the 3 h laboratory session. A shorter version of the laboratory with a single set of serial dilutions was given to college-level students in the Biotechnology-Advanced Diploma program at Fleming College during both the Winter and Spring 2017 semesters. These students were given the abbreviated version because the laboratory session had only a 2 h duration and was completed by approximately 100 students. All students ran the trials by moving through the solutions from the most concentrated to the least concentrated. This is opposite of the normal protocol and was implemented to effectively use the time limits provided for the scheduled laboratory sessions. Students were instructed to disconnect the

Quantitative analysis allows chemists to determine the exact concentration of a chemical species present in a given sample.1 UV−vis spectrometry has been used for the quantification of textile dyes in water. By obtaining calibration plots through UV−vis spectrometry, the percentage of dyes in effluent water from textile factories has been obtained.25 In this experiment, students prepared calibration plots showing the average absorbances of dye solutions taken from three repeated measurements and the associated concentrations of the dyes (g/L). Students were also given a dye sample of unknown concentration with their stock dye solution. After running the trials for the known concentrations, they were asked to obtain the absorbance for the unknown solution, and using eq 5, they then calculated the concentration of the unknown solution. Further quantitative analysis was added to the Trent University students’ postlab data analysis by incorporating standard-deviation calculations and error bars into their calibration plots for both the Rit and Tulip calibration plots. Some example data showing calibration plots for each dye taken at 470 nm, where the absorbances are very similar, are F



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Figure 5. Calibration plots showing the absorbance of (a) Rit dye and (b) Tulip dye obtained using a blue LED at 470 nm and absorbance values calculated with eq 5.

shown in Figure 5. The excellent fit of the linear trendline confirms the sensitivity and utility of this photometer and indicates that attenuation coefficients at a range of wavelengths (in this case, mass attenuation coef f icients) can be reliably obtained. Through this data-analysis portion of the exercise, students gained knowledge of, practice in, and trust in creating and using calibrations in quantitative chemical analysis.

At the end of each trial for the Rit and Tulip dyes, students obtained the voltage measurement for their unknown dye. They then compared the absorbance at the given concentration to determine if the dye fell on the calibration plots for either of the known dyes. Pedagogy and Student Responses

The laboratory exercises were designed to provide reinforcement of theory taught in the lecture components of courses at both academic institutions. An increased understanding of the components of a UV−vis spectrometer, applications of Beer’s law to industry scenarios, reinforcement of solution-preparation skills, and technical data analysis were the primary learning outcomes for the experiments at both academic institutions. A survey was conducted to obtain feedback on several new laboratories implemented in the courses for ongoing improvement. Overall student feedback was positive, with 68.5% of respondents agreeing with the statement that the laboratory experiment increased knowledge of spectrophotometry, 75.0% of students agreeing that the lab increased solution-preparation skills, and 62.5% of students feeling that the lab increased knowledge and application of Beer’s law. In all cases, 8.3% or fewer of students disagreed with the statements. Feedback indicated an overall increase in knowledge of spectrometry and Beer’s law as a result of the experiment. Specific comments from the students indicated that they enjoyed being able to build their own photometer and use it to analyze solutions they prepared. Suggestions for improvement were mainly related to the report format and discussion questions, with a few respondents stating that more time in the lab was needed and two others stating that a bit more clarification was needed in the procedure. A major advantage of this experiment is that no hazardous chemicals are used, and the waste generated is considered safe to dispose of in the sinks. Other advantages of using solderless bread boards and simple electrical components to build the photometer include

Qualitative Analysis

As previously established, the Rit and Tulip dyes were determined to be two different chemical species on the basis of their respective UV−vis spectra. Through the development of the two calibration plots, it is evident that the slopes of the trendlines are slightly different for the two dyes, even as shown here at 470 nm. This slope is determined by the attenuation of the absorbing dye. Hence the mass attenuation coefficient for the two dyes at almost all wavelengths are demonstrably not the same, which allows for quantitative identification. Recall that the molar extinction coefficient cannot be determined as the identity of the chromophore is unknown for these dyes. However, on the basis of the premise that the two dye solutions absorb differently on the basis of their chemical structures, students can determine if an unknown dye of a given concentration falls within one calibration plot or the other on the basis of absorbance. The Trent University students were given two stock dye solutions and an unknown dye solution at a given concentration. The scenario stated in the introduction to the experiment was the following: Suppose that the Rit and Tulip Dye manufacturers have filed a lawsuit for patent infringement, alleging that a new fabric dye manufacturer, Freshlook, has used the patented dye components contained in their fabric dyes. During this experiment you will determine if a patent infringement has occurred using a spectrometer to determine the absorbance of Rit and Tulip Dyes at different concentrations. Due to patent protection, the identity of the dye compound used in commercial dyes is unknown and, therefore, molar concentrations cannot be determined. The dyes submitted for analysis were in a powder form and stock solutions have been prepared for you in units of g/L. You will prepare serial dilutions of each dye solution, then measure the voltage to calculate the absorbance of each dilution. Using the absorbance values and the mass concentration (g/L) a linear relationship can be established. You will then determine the absorption of the Freshlook sample to determine if it falls within the calibration curve for either the Rit or the Tulip dyes.

• Ease of connecting the circuits • Fast assembly of the photometer (approximately 20 min) • Visualization of the components and mechanics of a spectrometer • Low cost per student set (totaling approximately $25− 30). Although the lab exercise had numerous advantages, some drawbacks of running the experiment must be taken into consideration. Variability in the voltage signal obtained and loose connections require students to tape down the leads to the G



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multimeter. It was also found that students needed to secure the cuvette in place with a piece of tape. Furthermore, several replacement LEDs were needed because of occasional short circuiting of the LED. Students should be cautioned to remove the LED prior to unhooking the battery. Laboratory staffing should also be considered, as the student-built photometers should be checked prior to connecting the battery to ensure proper connections are in place.

SUMMARY Spectroscopy and spectrometry are widely used in academic laboratories and industry. Through construction of an accurate photometer using a very small number of simple electrical components, students gained knowledge of the components and principles behind a UV−vis spectrophotometer. Through the analytical-chemistry and forensic-science applications, students learned valuable solution-preparation and quantitative- and qualitative-analysis skills in a hands-on learning environment. By constructing their own photometers, students eschew the “black box” idea of putting a sample in and receiving results. Through analysis of their data they can form accurate links between the concepts of concentration and absorbance and relate them to Beer’s law. This experiment can be adapted to several areas of study depending on the chemical compounds studied. The case scenario can be adapted to meet the interests of a variety of students, academic majors, and levels within chemistry. One possible use for the photometer in an upper-year spectroscopy course is to use a known-molarity two-component mixture and measure the absorbances at two different wavelengths to construct calibration plots. These could then be used to determine the concentration in an unknown mixture. The scenario could also be changed to perform a quality assurance− quality control experiment for academic programs with an emphasis in that area. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00862. Notes for electrical-component ordering and laboratory preparation (PDF, DOCX) Derivation of eq 5 (PDF, DOCX) Appendix with pictures of the spectrometer setup provided to students (PDF) Data set with analysis (XLSX) Analytical-chemistry-lab student handout (PDF, DOCX)

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REFERENCES

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Article

General-chemistry student handout: condensed procedure (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rayf C. Shiell: 0000-0002-9281-8103 Notes

The authors declare no competing financial interest. H



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Accurate, Photoresistor-Based, Student-Built Photometer and Its

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