Activity Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Activity Analysis of Iron in Water Using a Simple LED Spectrophotometer Benjamin J. Place* Chemical Sciences Division, National Institute of Standards & Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States
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S Supporting Information *
ABSTRACT: Access to clean water is a vitally important part of the lives of all people, yet there is often a concern about the contamination of water by heavy metals. The detection and measurement of heavy metals in water can be performed using colorimetric reagents paired with ultraviolet−visible light (UV−vis) spectrophotometry. The presented activity includes construction of a simple, inexpensive single-wavelength spectrophotometer using LEDs as the light source and detector. Using a colorimetric reagent, 2,4,6-tripyridyl-s-triazine, the concentration of iron can be measured by the LED spectrophotometer. Multiple natural water samples were collected and analyzed by this technique, and the performance of the LED spectrophotometer was compared to that of a conventional UV−vis spectrophotometer. The LED spectrophotometer had comparable results to those of the conventional UV−vis spectrophotometer while teaching students about simple circuit design, spectroscopy, and water quality measurements. KEYWORDS: Analytical Chemistry, Environmental Chemistry, High School/Introductory Chemistry, First-Year Undergraduate/General, Hands-On Learning/Manipulatives, Computer-Based Learning
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INTRODUCTION Water is a vital part of everyday lives, and the access to clean water can be a concern as the result of changes to municipal water treatment or natural disasters.1,2 According to the United States Geological Survey, the total withdrawal of water by the U.S. is approximately 1.3 trillion liters of water per day (including irrigation and mining), with the total publicsupplied water use at 159 billion liters of water per day.3 With such a large dependency of the U.S. on water, it is important that the water has low levels of toxic chemical contaminants. Under the authority of the Safe Drinking Water Act 40 CFR 141, the U.S. Environmental Protection Agency (USEPA) sets regulatory standards for drinking water; the USEPA requires maximum concentration limits on toxic metal concentrations in drinking water, including arsenic, lead, chromium, and mercury.4,5 The compound of interest for this activity is iron in natural waters. There is no regulatory limit to the concentration of iron in drinking water,4,5 but there can be nontoxic effects from water with high concentrations of iron, such as damage to plumbing fixtures (ref 6, pp 170−224). In addition, there is a wide range of iron concentrations in natural waters,6 which allows the activity to provide varying results with different water sources without the need for spiked water samples. The purpose of this activity is to teach students about water quality via spectrophotometry and environmental analytical chemistry. Through the creation of a simple and inexpensive spectrophoThis article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
tometer and a fast, observable reaction with an iron indicator, students can measure the concentration of iron in natural waters, directly linking them with a natural, relevant phenomenon. The construction of the light-emitting diode (LED) spectrophotometer introduces students to the concept of analytical instrumentation, including circuit design and spectroscopy. Spectrophotometers with a single multiwavelength light source require a diffraction grating to isolate single wavelengths.7,8 Student users of these commercial spectrophotometers may see them as “black boxes” with complicated circuitry, many mechanical parts, and software that returns interpreted data. In contrast, the LED spectrophotometer is a simple design that gives students an intimate look at the function of a spectrophotometer and may provide a better understanding of the interaction of light with a solution. LEDs can be manufactured to emit light over a narrow range of wavelengths, and LEDs are commercially available and relatively inexpensive. A unique feature of LEDs is their function as both a light source and detector at similar wavelengths, as demonstrated in other published LED spectrophotometer activities.9,10 Received: July 3, 2018 Revised: January 28, 2019
A
DOI: 10.1021/acs.jchemed.8b00515 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Activity
Figure 1. Graphical display (top) and electrical circuit diagram (bottom) for the LED spectrophotometer.
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HAZARDS The organic chemicals and nitric acid should be handled and disposed according to their respective safety data sheets. Goggles and gloves should be worn at all times while handling solutions. While the current and voltage from the battery pack with 2 AA batteries are low, care should be taken to make sure the wires are placed into the breadboard before the battery pack is turned on.
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The approximate cost for a single spectrophotometer was $9.00, not including batteries or the multimeter. Calibration Solutions
An iron(II) [Fe2+] stock solution (nominally 100 mg/L) was made in the lab using laboratory distilled water, and nitric acid was added for a nominal concentration of 1.5% (mass percent) nitric acid. Solutions of 100 mg/L Fe2+ are commercially available. Calibration solutions were prepared by volumetric dilutions in laboratory distilled water. The procedures are described in the Supporting Information; the calibration solutions concentrations were 1, 0.5, 0.04, and 0 mg/L Fe2+ in water.
EXPERIMENTAL SECTION
LED Spectrophotometer Construction
The spectrophotometer consists of a miniature breadboard, two LEDs (bright yellow; 585−590 nm maximum emission wavelength), four wire connectors with alligator clips, a AA battery pack for two AA batteries in series (with on/off switch), one 100 Ω resistor, and a multimeter capable of measuring 0−2 V, which were acquired from Spark Fun Electronics (Niwot, CO). Polystyrene cuvettes were purchased from VWR International, LLC (Radnor, PA). The construction of the LED spectrophotometer is detailed in the Supporting Information. Briefly, the cuvette is glued to the center of the breadboard, and the LEDs are placed on opposite sides of the cuvette and bent to 90° (parallel to the breadboard) so that the top of the LEDs are aimed directly at each other through the cuvette. The wires with alligator clips are placed to create a circuit between the detector LED and the multimeter, and between the battery pack, the resistor, and the source LED. A graphical display and circuit diagram are shown in Figure 1. The typical range for the voltage response is between 0 and 200 mV, but this is dependent on the position of the LEDs and the brightness of the source LED (which can vary by manufacturer). With distilled water in the cuvette, the LEDs can be carefully moved until the output voltage is as high as possible, which indicates the optimal position (highest transmittance of light).
Collecting the Water Samples
For the present study, water was collected from five different locations in Maryland: a river, a pond, water from a household well (the sample was taken prior to household water treatment), and Atlantic Ocean coastal water. The water samples were collected in polypropylene bottles and stored in a refrigerator until analysis. In addition to the natural water samples, a laboratory-spiked water sample was prepared by adding the stock solution to distilled water to create a 0.2 mg/ L solution and labeled as an unknown spiked solution. Measuring Iron in the Solution
The iron indicator reagent (2,4,6-tripyridyl-s-triazine; TPTZ) was purchased from the Hach Company (Loveland, CO) in reagent powder pillows for 25 mL water samples. In addition to the iron indicator, the reagent powder includes a reducing agent that converts all precipitated or suspended iron to Fe2+.11 In most aquatic systems, the dominant form of ionic iron is in the Fe2+ state, although in highly oxidizing and acidic conditions, Fe3+ can be present (ref 6, pp 100−103). The structure and reaction of TPTZ with Fe2+ are shown in Figure 2. The maximum applicable Fe2+ concentration for the reagent is 1.8 mg/L.11 The absorption spectrum for the iron-TPTZ coordination compound is shown in Figure 3, with the maximum absorption between 580 and 600 nm. B
DOI: 10.1021/acs.jchemed.8b00515 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Activity
log10
V0 =ε×b×c V
(2)
where V0 is the measured voltage response of the LED detector from the light through the solvent (distilled water) without the compound of interest, and V is the measured voltage response of the LED detector from light through the solution with the compound of interest. Conventional UV−Vis Spectrophotometer
Figure 2. Observed reaction between Fe2+ and the TPTZ iron indicator reagent. The resulting coordination compound is an observable violet color.
To evaluate the performance of the LED spectrophotometer, the calibration and sample solutions described above were analyzed using a Cary 100 UV−vis spectrophotometer (Agilent Technologies, Santa Clara, CA). The absorbance of the solutions was measured at 585 nm with spectral bandwidth of 1.5 nm and collection time of 0.1 s.
To prepare samples, one packet of the iron indicator reagent was added to 25 mL of each calibration solution and water sample. The solutions were shaken briefly and allowed to sit for at least 3 min before analysis. Sequentially, the calibration solutions and samples were added to the cuvette (with distilled water rinses between each measurement), and the measured voltage was recorded. Each solution was measured three times to determine the variability of the detector response. In addition, the measured voltage of distilled water (no reagent) was recorded to determine V0 (see equation below). As described by the Beer−Lambert Law, the light intensity through a solution at a specific wavelength is I log10 0 = ε × b × c (1) I where I is the intensity of the light through the test solution, I0 is the intensity of the light through the solvent without the compound of interest, ε is the absorptivity of the compound of interest at the specified wavelength, b is the path length of the cell (often 1 cm), and c is the compound concentration of the test solution. The value on the left side of the equation is equivalent to the absorbance (A), which is the typical response reported by a conventional UV−vis spectrophotometer. As the voltage produced by the LED detector is proportional to the intensity of the light detected, the measured voltage can be substituted for the intensities, therefore
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RESULTS The results from the calibration solutions, using both the LED and conventional spectrophotometers, are shown with linear regression statistics (y = mx + b; with the slope m and the intercept b) and calibration curves in Table 1 and Figure 4, Table 1. Comparison of Linear Regression Results of the Calibration Curve of the Two Spectrophotometers Spectrophotometer Statistic a
Slope Intercept Correlationd
b
LED
UV−Visc
0.36 0.013 0.99822
0.46 0.0061 0.99992
a
m within y = mx + b regression. bUsing log10(V0/V) for y. cUsing measured absorbance for y. dThe correlation value represents the Pearson product−moment correlation coefficient (based on 4 points).
respectively. As can be noted in Table 1 by the correlation coefficient for the linear regression, there is a good-fitting linear response (R2 = 0.998, based on 4 points) for the LED spectrophotometer, which is comparable to the conventional spectrophotometer. As noted in Kvittingen et al., LED
Figure 3. Absorption spectra of 1 mg/L Fe2+ solution with TPTZ (orange solid line) and distilled water (blue dotted line). C
DOI: 10.1021/acs.jchemed.8b00515 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Activity
Figure 4. Calibration curve of the mean responses for the individual calibration solutions using the LED spectrophotometer (top) and the conventional UV−vis spectrophotometer (bottom).
spectrophotometers do have a nonlinear response over a large range of concentrations;10 for the purposes of this experiment and within the range of the calibrants of this activity, we treated the response as linear for ease of calculation. Using the standard deviation (σ; n = 3) of the signal of the 0 mg/L solution and the slope of the calibration curve, the limit of detection (LOD) and limit of quantitation (LOQ) can be determined for both the LED and conventional spectrophotometer using the following equations: LOD =
3.3 × σ slope
(3)
LOQ =
10 × σ slope
(4)
Table 2. Calculated Results for Iron Concentration in the Water Samples Mean Fe2+ Concentration Values, mg/L (SD) by Spectrophotometer Sample Source
LED
UV−Vis
Spiked unknown River Pond Well Ocean
0.215 (0.012)
