Communication pubs.acs.org/jchemeduc
Determining the Amount of Copper(II) Ions in a Solution Using a Smartphone Marc Montangero* Lausanne University of Teacher Education, HEP Vaud, Lausanne 1014, Switzerland Gymnase de Morges, Morges 1110, Switzerland S Supporting Information *
ABSTRACT: When dissolving copper in nitric acid, copper(II) ions produce a bluecolored solution. It is possible to determine the concentration of copper(II) ions, focusing on the hue of the color, using a smartphone camera. A free app can be used to measure the hue of the solution, and with the help of standard copper(II) solutions, one can graph a calibration curve to determine the concentration of copper in solution and calculate the mass of copper that has been dissolved or, for example, the percentage of copper in an alloy.
KEYWORDS: High School/Introductory Chemistry, Analytical Chemistry, Laboratory Instruction, Calculator-Based Learning, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Problem Solving/Decision Making, Aqueous Solution Chemistry, Laboratory Equipment/Apparatus, Quantitative Analysis
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OVERVIEW
This laboratory experiment can be done during a 90 min high school lab lesson, with 12 students working in pairs. Students do not need to be in a scientific class in order to undertake this lab. It can also easily be done individually or in groups of three or more, as needed. Before the lesson, students should install a free app on their smartphone (e.g., Color Grab on Android and ColorAssist on iOS; similar apps can also be used). The aim of the experiment is to determine the amount of copper dissolved in nitric acid. The hue of the color of the blue solution obtained is measured with a smartphone, and the result is compared with copper solutions of known concentrations. The students can determine the mass of copper dissolved by using a calibration curve. This lab provides students the opportunity to work with the scientific method, to collect data, and arrange them in a graph. Some stoichiometry calculation can also be done, although this is not necessary, depending of the class. Students particularly enjoy this lab because they can use their own smartphone to measure the color of the solutions. This lab shows them that they can study science with a tool they always have with them. It is also a good way to show a different lab that uses current technology without having to purchase new equipment. © XXXX American Chemical Society and Division of Chemical Education, Inc.
EXPERIMENTAL SECTION
Tasks for the Teacher
At the beginning of the lesson, present the purpose of the laboratory exercise. Measure the mass of a piece of copper (between 1.5 and 3.5 g; do not tell the students how much copper you weighed) and dissolve it in nitric acid under a fume hood (Figure 1). Use approximately 5 mL of HNO3 65% per gram of copper; for example, if you want to dissolve 2 g of copper, use 5 × 2 = 10 mL HNO3 65% in a 50 mL beaker. When the dissolution is complete and no further release of brown gas is observed, dilute the resulting solution with approximately 30 mL of distilled water; the solution should be blue. At this point it is no longer necessary to work under a fume hood. Transfer and dilute this solution in a 100 mL volumetric flask and close it. The students will use this volumetric flask as the unknown solution. Tasks for the Students
The students prepare some copper solutions of known concentration by dissolving copper(II) nitrate Cu(NO3)2 in distilled water in a 100 mL volumetric flask. They should prepare at least two solutions: the first one containing about 20% more copper than the amount of dissolved copper; and the second one containing about 20% less copper than the amount of dissolved copper (tell the students the approximate masses they have to weigh). It is not significant whether the mass is
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DOI: 10.1021/acs.jchemed.5b00167 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
Figure 1. Copper that is dissolved in nitric acid produces nitrogen dioxide.
precisely 20% more or less; however, the students have to measure the exact mass used. It is also possible to ask each group to prepare one or two solutions and then ask the different groups to share their solutions with one another. In this case, instructors could prepare a wide range of solutions (each one containing from 1 to 4 g of pure copper). Of course, the students will have to know the exact mass of the salt they usefor example, Cu(NO3)2·3H2Oand then calculate the equivalent mass of pure copper they have used as well. When all the solutions are ready, students can measure the color (H-value) of the solution using their smartphone cameras. They should put the volumetric flask on a white sheet of paper (or paper towel), with a circle drawn on it in order to define the exact position of the volumetric flask. Then, they should put another white sheet of paper (or paper towel) in the background. The smartphone should be positioned so that it is perfectly vertical and points to the center of the volumetric flask (Figure 2). Students should make sure that there is sufficient ambient light, or use the built-in camera flash, if necessary. Finally, students should measure the color with the HSV (or HSL) color model, and report the H-value (Figure 3); they repeat the same task with each volumetric flask, including the “unknown”. Students plot the H-values of the known solutions as a function of the mass of pure copper on the graph. They use the H-value of the unknown solution to determine the mass of copper that is contained in the unknown solution from the graph.
Figure 2. Students measuring the H-value of the solution with their smartphone.
Variants
Copper(II) sulfate could be used instead of copper(II) nitrate. The results would of course be exactly the same, as both anions are colorless. An alloy of copper could also be used, for example, a Swiss 5-cent coin, which contains 92% Cu, 6% Al, and 2% Ni.1 Offering this variant enables students to determine the percentage of copper in the coin. Do not use an alloy with too much nickel or another metal whose ions will color the solution (for example, the Swiss 10-cent coin contains 75% copper and 25% nickel;1 the solution gets too greenish for this experiment to work).
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HAZARDS Nitric acid is oxidizing and corrosive, it produces a toxic gas (NO2) when dissolving copper; therefore, dissolve the copper under a fume hood and use protective clothing, gloves, and glasses. Copper(II) nitrate is oxidizing, harmful, and dangerous for the environment; again, use protective clothing, gloves, and glasses. All solutions must be collected at the end and labeled for treatment in an environmentally appropriate manner. B
DOI: 10.1021/acs.jchemed.5b00167 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 4. Graph of the H-value as a function of the mass of copper.
and understand it, which is not the case with a spectrophotometer.
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ASSOCIATED CONTENT
S Supporting Information *
Student handout for the lab; notes for the teacher; Excel file allowing teachers to have a quick look at student results and check calculations. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS I would like to thank my colleagues of Science on Stage involved in iStage2, especially Daniel Bengtsson, who tested the app with his students and who helped me a lot. Many thanks to SAP who supported iStage2, which was the starting point of this lab development.2−5 Finally, I would like to thank Francis Mingard for the idea of using a coin in this activity, and Nicolas Schurter for editing the text with meticulous care.
Figure 3. Screenshot of the smartphone. Simply point the center of the volumetric flask with the spot (circle on Android, square on iOS) and read the H-value (the first of HSV, 191° on this example). If the app does not give the HSV values, click on the settings button (gears on Android, circled i on iOS) and choose HSV.
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RESULTS The graph of the H-value as a function of the mass of copper is not linear. But one can accept it is almost linear between two scale points, if the two points are close enough. Figure 4 shows a typical result of a group that tested three unknown solutions (the red points). With a 5-cent Swiss coin, 8 groups of students found an average of 1.66 g of pure copper for a 1.80 g coin; that is to say, 92.2% of copper instead of 92.0%,1 which is a very good result.
REFERENCES
(1) Wikipedia entry, Coins of the Swiss Franc. http://en.wikipedia. org/wiki/Coins_of_the_Swiss_franc (accessed Jun 2015). (2) Montangero, M.; Bengtsson, D.; Szabo, M.; Miroslaw, L.; Jonas, L. How Deep Is Your Blue? Coloured Chemistry with Smartphones. In iStage2: Smartphones in Science Teaching ; Science on Stage Deutschland e.V.: Berlin, 2014; Vol. 1, issue 1, pp 20−24. http://www. science-on-stage.de/download_unterrichtsmaterial/iStage_2_ Smartphones_in_Science_Teaching.pdf (accessed Jun 2015). (3) Byoung-Yong, C. Smartphone-Based Chemistry Instrumentation: Digitization of Colorimetric Measurements. Bull. Korean Chem. Soc. 2012, 33 (2), 549−552. http://koreascience.or.kr/article/ ArticleFullRecord.jsp?cn=JCGMCS_2012_v33n2_549 (accessed Jun 2015). (4) Kehoe, E.; Penn, R. L. Introducing Colorimetric Analysis with Camera Phones and Digital Cameras: An Activity for High School or General Chemistry. J. Chem. Educ. 2013, 90 (9), 1191−1195. (5) Moraes, E. P.; da Silva, N. S. A.; de Morais, C.; de, L. M.; das Neves, L. S.; de Lima, K. M. G. Low-Cost Method for Quantifying Sodium in Coconut Water and Seawater for the Undergraduate Analytical Chemistry Laboratory: Flame Test, a Mobile Phone
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CONCLUSION Nowadays, many students have a smartphone and are willing to install a free app. They are then quite excited to use the app during a chemistry lab. Although the results are not as accurate as those expected with a spectrophotometer, the smartphone camera results are good enough for a high school lesson, and obtained at a much lower cost. Another benefit of this approach is that students can see with their eyes what they are measuring C
DOI: 10.1021/acs.jchemed.5b00167 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Camera, and Image Processing. J. Chem. Educ. 2014, 91 (11), 1958− 1960.
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DOI: 10.1021/acs.jchemed.5b00167 J. Chem. Educ. XXXX, XXX, XXX−XXX
