Simplified Low-Cost Colorimetry for Education and Public

Article Cite This: J. Chem. Educ. 2019, 96, 1136−1142

pubs.acs.org/jchemeduc

Simplified Low-Cost Colorimetry for Education and Public Engagement J. O’Donoghue* School of Chemistry, Trinity College Dublin, Dublin 2, Ireland

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S Supporting Information *

ABSTRACT: This work describes the design, building, and testing of miniature, low-cost LED−LDR colorimeters for absorption-spectrophotometry experiments. It also describes using these colorimeters for simplified context-based-learning (CBL) activities in school teaching laboratories and for publicengagement events. The colorimeters are simple and robust, performing well against a conventional benchtop UV−vis spectrophotometer. Data collection and analysis is quick and intuitive, allowing for efficient turnaround between groups and making it particularly useful at public-engagement events. The experiments and activities described here are based on real-life examples to emphasize the usefulness of chemical analysis and quantification in everyday life. KEYWORDS: High School/Introductory Chemistry, Public Understanding/Outreach, Analytical Chemistry, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Applications of Chemistry, Laboratory Equipment/Apparatus, Quantitative Analysis, Spectroscopy, General Public

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INTRODUCTION

Reports of using light-emitting diodes (LEDs) for analyticalchemistry activities started as soon as LEDs became widely available in the 1970s. In particular, this Journal has reported an extensive range of custom-built spectrophotometers coupled with a variety of applications. In recent years, the availability of low-cost microcomputers has also led to numerous reports of multifunctional homemade spectrophotometers. However, for the most, part they require some knowledge of programming and electronic-circuit design.10−13 The use of smartphones for simplified spectrophotometry activities has also become increasingly popular in recent years, with impressive results compared with traditional instruments.9,14−17 However, multiple smartphones are necessary to run hands-on activities for larger groups, increasing the required cost significantly. Using traditional analytical instruments for public-engagement, science-communication, and outreach activities can be very difficult because of their cumbersome nature and relatively high cost. Homemade colorimeters using LEDs have been reported for high schools (secondary schools) and undergraduate teaching laboratories but less so for younger (primary or elementary) students and public-engagement events. Most spectrophotometers, including homemade colorimeters, are normally designed for laboratory use with controlled experiments overseen by trained scientists. Public events and younger audiences carry with them a greater number of

Quantifying the concentrations of analytes with spectrophotometers via the Beer−Lambert law is an important component of nearly all chemistry curricula around the world, for both schools and undergraduate teaching.1,2 However, modern laboratory instruments (particularly spectrophotometers) are often viewed as sealed (black) boxes with little or no opportunity for students to investigate the inner workings. It has been suggested that teaching experiments should be designed as a mixture of traditional techniques and modern technology in order to reduce the influence of “black boxes” on scientific literacy.3 Many learners have difficulty with abstract concepts without context, so it is important to present science (particularly chemistry) in a context that is relevant to them.4 Contextdriven courses for chemistry are reported to be more motivating and interesting as well as more likely to encourage students to study chemistry for longer.5,6 Many experiments reported for teaching spectrophotometry are based on solving real-world problems by quantifying analytes such as proteins or investigating fresh-water samples.7−10 Therefore, it would seem that spectrophotometers are well suited for context-basedlearning (CBL) activities, particularly for younger audiences and members of the public who generally possess relatively low levels of practical chemistry expertise. However, the sealed “black box” nature of modern spectrophotometers mystifies the process taking place within the instrument, which can hinder the intended learning from an otherwise well-designed experiment. © 2019 American Chemical Society and Division of Chemical Education, Inc.

Received: April 1, 2019 Revised: May 8, 2019 Published: May 23, 2019 1136

DOI: 10.1021/acs.jchemed.9b00301 J. Chem. Educ. 2019, 96, 1136−1142

Journal of Chemical Education

Article

Figure 1. Schematic diagram, drawn to scale, showing how the viewing angle of the LED (rated as 15°) compares with the distance and size of the LDR surface. The distance between the tip of the LED and the surface of the LDR (A) is 30 mm on average across the seven colorimeters built to date, and the diameter of the LDR sensitive surface is 10 mm (B). Using trigonometry, the calculated viewing angle within the colorimeter is therefore 19°, which ensures most of the light from the LED reaches the LDR surface. The plastic ABS casing is modified from a commercially available version measuring 50 × 35 × 20 mm.

LED is reported by the manufacturer as 15° as a result of its built-in lens. The total angle within the colorimeter was calculated using the distance between the tip of the LED and the surface of the LDR as well as the size of the LDR’s sensitive surface. The viewing angle (receiving angle) of the LDR with respect to the LED was calculated as 19° for this design, which ensures most of light from the LED is collimated through the sample cuvette toward the LDR, as desired (Figure 1).31

potential variables over a lab environment, making analyticalchemistry activities in particular very difficult. Therefore, multiple robust, low-cost instruments with greatly simplified activities are required to engage with large crowds of younger audiences and the general public.

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COLORIMETER DESIGN When designing the colorimeter, various detectors were considered on the basis of their previously reported success for colorimetry applications, including silicon photodiodes,8,10,11,18,19 light-emitting diodes (LEDs),7,20,21 phototransistors,22,23 and light-dependent resistors (LDR).12,24−26 After considering numerous reviews and experimental comparisons of potential detectors,27,28 a light-dependent resistor (LDR) was chosen to reduce the potential cognitive load for the intended audience of young novice science students and the general public.29,30 Unlike the other detectors, which are more frequently used for homemade colorimetry, the resistance value (measured in Ω) obtained from an LDR is inversely proportional to light transmittance.26 Therefore, the numerical values obtained from the colorimeter correspond directly with the concentration of the analyte being measured (i.e., higher resistance values correspond to higher concentrations, and lower resistance values correspond to lower concentrations). For the light source, an LED with the appropriate emission wavelength was chosen to match the analytes used for the activities as well as the detection range of the LDR. The maximum absorbance for the purple-colored analytes used in both activities was determined to be 526−531 nm using a benchtop Shimadzu UV-1601 Spectrophotometer. The dominant emission wavelength of the 5 mm LED used here is reported by the manufacturer to be 535 nm, whereas the peak spectral response for the LDR is reported as 550 nm (further details are provided in the Supporting Information). The colorimeter casing size and shape were chosen and modified to ensure the majority of light from the LED reached the LDR surface through the cuvette. The viewing angle of the

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BUILDING THE COLORIMETERS

The custom-built colorimeters described here are less expensive than many of those reported previously in the literature. They are simple in design, quick to construct, and very robust, and they all provide consistent, accurate results. The materials used to build and operate the entire device total less than €20 each. In addition to the parts list and costs provided in Table 1, some tools, solder, and glue are used during the building process and are spread out over the cost of the all colorimeters. Table 1. Cost of Parts for Each Colorimeter Part

Supplier

Pricea, €

Green (535 nm) LED CdS LDR ABS enclosure (casing) AA-battery holder Battery Strap and Lead Multimeter AA batteries EC2 electrical connectors Plastic cuvette Total

RS Electronics RS Electronics RS Electronics RS Electronics RS Electronics Maplin IKEA High Performance RC Connectors Cole-Parmer

0.21 2.04 1.95 1.00 1.55 8.99 0.40 1.00 0.35 17.49

a

All prices are calculated from bulk purchases; retail prices for individual parts are greater. Part numbers and other details are provided in the Supporting Information.

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Figure 2. Photographs of the materials used to construct the colorimeters. (a) Individual parts used for each colorimeter before assembly and (b) assembled colorimeter with the cover and cuvette removed, showing all the parts in place before the addition of cables, batteries, and the multimeter. The hole for the cuvette is created using an electric drill and a craft knife. The holes for the legs of the LED and LDR are created using a hot needle, with is protruded through the plastic. A strip of cardboard is glued to the inside of the casing and to the cover to ensure no light escapes around the edges of the cuvette during operation and to prevent the cuvette from moving.

Figure 3. Photographs of the standards used and a colorimeter in operation. (a) Standard solutions of salicylic acid reacted with iron(III) chloride in test tubes and transferred to cuvettes to determine a calibration curve. Iron(III) chloride is normally yellow in appearance and forms a purple complex upon reaction with salicylic acid. Here, this complex appears brown-purple because of the presence of unreacted iron(III) chloride left in the solution, which is used in excess. From left to right: 0 (blank), 100, 200, 300, 400, and 500 mg/L. (b) Fully operational colorimeter measuring resistance in real time.

Each colorimeter consists of a commercially available multipurpose black plastic (ABS) enclosure, an LED (green, 535 nm dominant emission wavelength) as the light source, a light-dependent resistor (LDR) as the detector, some cabling, a multimeter capable of measuring resistance, and a battery pack consisting of two AA batteries in series. The black plastic (ABS) casing comes complete with a cover and two small screws, it is modified here with 5 holes to fit the LED, LDR, and cuvette. Further details of the parts used are detailed in the Supporting Information. Photographs of the parts and the completed colorimeter are shown in Figure 2. The standard plastic casing is first modified by creating a square hole in the center of one of the long narrow sides using an electric drill and a craft knife. The opening needs to be large enough to fit a standard 10 × 10 mm cuvette. Two small holes are then created using a heated sewing needle on either side of the casing at a 90° angle to the opening for the cuvette. An LED and an LDR are both glued in place directly opposite each other, with the legs protruding through the case via the small holes. The battery cables are soldered to the legs of the LED, connecting it to the battery pack, which features a removable stud connector to control the power supply. A small

female−male EC2 electrical connector is soldiered to the protruding legs of the LDR, with the opposite side soldiered to the cables attached to the multimeter. All exposed cabling is wrapped with electrical tape where needed. Jenway macro plastic 4.5 mL UV−vis cuvettes with four visible windows obtained from Cole-Parmer are used as removable sample holders. The incorporation of the small connectors into the design ensures convenient and quick assembly and disassembly when required, which is particularly useful for transportation to and from public-engagement events.

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EXPERIMENTS AND ACTIVITIES

Performance Testing

Each colorimeter is tested using the same stock solution and standards to gauge consistency across the seven custom-built instruments. A benchmark test is also carried out using a Shimadzu UV-1601 UV−vis spectrometer, again with the same standards as those used for the colorimeters. A stock solution of 1000 mg/L salicylate is prepared from its dried sodium salt (CAS 54-21-7), which is then serially diluted to give five standards to use as a calibration curve. Each standard (1 mL) is then reacted with an excess of iron(III) 1138



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Table 2. Calculated Absorbance Values for the Standards Obtained from the Seven Custom-Built Colorimeters Compared against the Absorbance Values Obtained from a Benchtop UV−Vis Spectrometer at λmax = 535 nm Salicylate Standard (Concentration)a

1

2

3

4

5

6

7

Shimadzu UV-1601 UV−vis Spectrophotometer

A (100 mg/L) B (200 mg/L) C (300 mg/L) D (400 mg/L) E (500 mg/L) R2 (Linear fit)

0.14 0.27 0.41 0.56 0.69 0.9993

0.14 0.28 0.43 0.57 0.70 0.9990

0.14 0.28 0.42 0.58 0.70 0.9979

0.14 0.28 0.42 0.58 0.71 0.9995

0.14 0.29 0.43 0.60 0.73 0.9991

0.15 0.29 0.46 0.60 0.70 0.9973

0.14 0.28 0.43 0.58 0.72 0.9996

0.28 0.56 0.84 1.15 1.42 0.9997

a

All measured as 1 mL of salicylate standard solution in 3 mL of iron(III) chloride.

Figure 4. Calibration curves with absorbance values obtained at 535 nm, comparing the linear fits of the salicylate standards from a benchtop UV− vis spectrophotometer with the average absorbance values calculated from the custom-built colorimeters using the same standards.

from 0.9973 to 0.9996, comparing very favorably to the value obtained from the benchtop spectrophotometer (0.9997). However, the Shimadzu UV-1601 UV−vis spectrophotometer gave a much steeper curve, with each absorbance value representing roughly double the value obtained from the colorimeters. This is consistent with previous reports obtained for light-dependent resistors (LDR) when comparing calculated absorbance values to those obtained from benchtop spectrophotometers and photodiodes in terms of sensitivity.12

chloride (CAS 7705-08-0) in a test tube to form a purplecolored iron salicylate complex.32 A blank is prepared with iron(III) chloride and 1 mL of water in place of the salicylic standard solutions. All solutions are transferred to individual cuvettes for analysis at λmax = 535 nm by the Shimadzu UV1601 UV−vis spectrometer and the homemade colorimeters (Figure 3). For the colorimeters, measurements of resistance are substituted for transmittance. As stated previously, because resistance (R) is inversely proportional to light transmittance (T), absorbance (A) is calculated using the following modified version of the Beer−Lambert law, where the negative sign has been changed to a positive:26 ji Tsample zyz ji R sample zyz A = −logjjj z = logjjj z j Tblank zz j R blank zz k { k {

Teaching-Lab Activity

Salicylic acid is the active component in many commercial liquid facewashes. It is also the major metabolite of aspirin and is commonly found in medications used to treat a variety of ailments, such as acne, warts, and other skin-related afflictions. Quantifying salicylic acid in clinical laboratories normally uses visible spectrophotometry, where an excess of iron(III) chloride readily reacts with the salicylate species to form a purple-colored complex. The purple color can sometimes appear brown because of the excess of iron(III) chloride remaining in the solution. The color intensity of this complex is dependent on the concentration of the salicylate species, allowing for the Beer−Lambert law to be applied to create a standard curve and calculate the concentration of an unknown sample.32 Liquid-facewash products were chosen for analysis because of their consistency and appearance. Both products used here are 200 mL in size, colorless, transparent, and appear to have similar viscosities. The premium branded product is priced at €5.49, whereas the drugstore-owned-brand product is priced at

(1)

Table 2 compares the calculated absorbance values for the seven colorimeters against those obtained from the benchtop UV−vis spectrometer. The resistance values in ohms (Ω) obtained for each sample as well as the blank are provided in the Supporting Information. All seven custom-built colorimeters provide remarkably consistent results with only a few notable variations in the values obtained. Figure 4 displays the linear fit for the calculated absorbance values at λmax = 535 nm for various concentrations using the custom-built colorimeters as well as that of the absorbance values obtained from the Shimadzu UV-1601 UV−vis spectrophotometer for the same set of standards. The linearfit R2 values obtained for the individual colorimeters ranged 1139



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€3.99. A calibration curve is produced using the standards prepared from a stock solution of sodium salicylate, as detailed in the previous section with the absorbance values and graph displayed in Table 2 and Figure 4, respectively. The two facewash samples are first diluted to 1 mL of facewash in 100 mL of water. The absorbance values are then measured in the same way as those of the salicylate standards described previously (i.e., as 1 mL of sample in 3 mL of iron(III) chloride). Table 3 provides the absorbance values obtained for both facewash products from all seven homemade colorimeters as

Public-Engagement Activity

When designing an activity for public audiences, consideration needs to be given to the potential unpredictability that can occur when engaging people with little or no scientific training. Risk assessments and other safety precautions also need to be considered in order to run these activities in a relatively uncontrolled environment without personal protective equipment (PPE).4 It was therefore decided that traditional colorimetric experiments based around the Beer−Lambert law and involving stock solutions of restricted or hazardous chemical species could not be used. Because the age profiles of public audiences can vary enormously by event and location, all liquids used for these activities needed to be fit for human consumption in order to greatly reduce any risks associated with the activity. The participants’ prior knowledge and any potential barriers to learning also needed to be considered at the design stage.4 The activity was therefore designed with cognitive-load theory in mind, and many of the processes normally required to determine the concentration of an analyte were simplified. Asking the general public to make up standards and produce a graph as per the Beer−Lambert law was deemed to be unfeasible because of the cognitive load required and the time constraints of a public setting. Instead, a simple comparative method of determining the concentrations of three diluted juice samples from the resistance values obtained from the colorimeters was employed. Each participant (usually in groups) is first invited to arrange the samples by concentration using whatever means they wish (smell, taste, visual cues, etc.). Plastic Pasteur pipettes are provided to transfer the juice from the cups to a cuvette, which is then placed in a colorimeter. The resistance value from the multimeter is displayed instantly and recorded by the participant on their worksheet (see the Supporting Information). This process is then repeated for the other two samples, with the cuvette cleaned using a wash bottle between each sample. The three resistance values obtained are then compared by the participant to determine the correct order of the juice concentrations from highest to lowest. The entire activity takes approximately 5 min.

Table 3. Absorbance Values Obtained for the LiquidFacewash Samples at λmax = 535 nm and Their Calculated Concentrations Colorimeter 1 2 3 4 5 6 7 Average concentration (mg/L) Shimadzu UV-1601 UV−vis spectrophotometer Concentration (mg/L)

Premium Branda

Drugstore Branda

0.11 0.12 0.11 0.12 0.12 0.12 0.12 85 0.24

0.21 0.20 0.21 0.21 0.22 0.21 0.21 149 0.42

86

149

a

Facewash samples were prepared using a dilution factor of 1 mL in 100 mL of water. Absorbance was measured as 1 mL of the sample solution in 3 mL of iron(III) chloride. Concentrations (mg/L) were calculated using the standards reported in Table 2.

well as the benchtop spectrophotometer. As expected, the benchtop spectrophotometer provides absorbance values that are roughly double those calculated from the homemade colorimeters. However, the calculated concentrations of the salicylate species from the colorimeters gave remarkably similar results to those obtained from the benchtop Shimadzu UV−vis spectrophotometer when using their respective calibration curves.

Figure 5. Simplified diagram used during activities to explain the principle operation of an optical heartrate monitor of the kind usually found in modern smartwatches. The green LED flashes light onto the surface of the skin, and the photodiode detector measures the intensity of the reflections. Blood in arteries absorbs the light better than the surrounding tissue, so as the vessels contract and swell in response to the pulsating blood pressure, the intensity of the reflected light rises and falls. 1140



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Real-World Example

7 year olds) in particular initially ran into difficulty using the plastic Pasteur pipettes. They struggled to determine the correct time to squeeze and release the bulb of the pipette in order to suck up and release the liquids. However, by the time they got to the third sample of the experiment, most had mastered this skill and identified this section as one of the most enjoyable parts of the activity. The use of optical heartrate monitors in smartwatches as an example for the real-world use of the concept was very well received by all participants. In particular, the vast majority of participants at public-engagement events were wearing a smartwatch device with a heartrate monitor or were aware of such devices.

After both activities, the participants are given a real-world use for the process they used in the colorimeters. Once the activity is finished, the mechanism behind optical heartrate monitors, which are used in most smartwatches, is explained in simplified terms, particularly in relation to the green LEDs that are used because of the red color of blood. The process used by smartwatches is known as photoplethysmography and operates on the same principle as the Beer−Lambert law. Briefly, activity facilitators explain that optical heartrate monitors detect changes in the volume of blood just below the surface of the skin. The change in volume caused by the pressure pulse is detected by illuminating the skin with light from a green LED and then measuring the amount of light either transmitted or reflected to a photodiode (Figure 5).33 The principle difference between a smartwatch optical heartrate monitor and a colorimeter is that the light source and detector are placed next to each other on the rear of a smartwatch rather than opposite each other, as is the case in most spectrophotometers. All facilitators to date, through verbal feedback, have noted that the use of this anecdote is very well received by teachers, students, and the general public. Feedback from teachers also emphasized that it helps to connect the concept of colorimetry to a familiar real-world application. It is particularly effective when participants are actually wearing wrist-based heartrate monitors and can investigate the color of the light for themselves by taking off their watches.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.9b00301. Colorimeter materials and testing (PDF) Activity worksheets (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

J. O’Donoghue: 0000-0002-6130-9293

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Notes

The author declares no competing financial interest.

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CONCLUSION Multiple low-cost, simplified, robust colorimeters were made from LEDs, LDRs, some cables, batteries, small casings, and multimeters. These colorimeters have been successfully employed at multiple events, with simplified activities based around context-based learning with real-world examples. The colorimeters consistently give remarkably accurate quantitative results in comparison to those from a conventional benchtop UV−vis spectrometer, despite the significant difference in cost. The low-cost nature of the colorimeters described here meant that robustness and longevity were not considered at the design stage, because all the parts could be replaced very easily. Despite this, all seven colorimeters initially built have exhibited impressive robustness to date, with no major servicing required. They have all been used for multiple events with hundreds of students, teachers, and members of the general public. Using multiple colorimeters for a teaching practical provides students with the opportunity to work in smaller groups to run an experiment based on the Beer−Lambert law. The accuracy and low-cost nature of the colorimeters allows students to run the entire experiment at their desk rather, than waiting their turn to run samples on a single “black box” device for the entire classroom. At the beginning of the activity, students are encouraged to remove the cover of each colorimeter to observe the inner workings. This is easily achieved because it is held in place with only two screws. Interestingly, during public-engagement activities, when asked to guess the concentration of the juice, most people held the samples up to the nearest light source to visually analyze the concentration. This instinctive nature implied they already had some basic idea about the relationship between light and the concentration of analytes. Young participants (5−

ACKNOWLEDGMENTS Financial support was gratefully received from Science Foundation Ireland (SFI) to purchase the required materials and run the activities described here. Sincere thanks to the many Ph.D. students from the School of Chemistry at Trinity College Dublin who used these colorimeters to engage with students and the general public at schools, colleges, and public events. Thanks also to the Irish State Laboratory for their generous donation of the Shimadzu UV-1601 Spectrophotometer, which was used as a benchmark for testing the performance of the colorimeters. Finally, special thanks to Ger O’Donovan at Regina Mundi College; my Trinity College Dublin (TCD) colleagues; Niamh McGoldrick, Noelle Scully, Carl Poree, and the other members of the TCD ChemEd group for all of their help, support, and feedback.

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REFERENCES

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Simplified Low-Cost Colorimetry for Education and Public

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