Laboratory Experiment pubs.acs.org/jchemeduc
Microscale Electrolysis Using Coin-Type Lithium Batteries and Filter Paper Masahiro Kamata*,† and Seiko Yajima‡ †
The United Graduate School of Education, and ‡Graduate School of Education, Tokyo Gakugei University, Tokyo 184-8501, Japan S Supporting Information *
ABSTRACT: An educational experiment illustrates the electrolysis of water and copper chloride to middle school science students. The electrolysis cell is composed of filter paper soaked with Na2SO4 or CuCl2 aqueous solution sandwiched, along with a sheet of platinum foil, between two coin-type lithium batteries. When the electrolysis is carried out, the electrolysis cell is connected to a current indicator with a light-emitting diode (LED) and to a current-regulating diode (CRD). Because no elaborate components such as an ammeter, dc supply, or Hoffman Cell are needed, the experiment is easy and inexpensive to carry out. In addition, the time needed to obtain the result is less than 5 min and the results are visually clear. The water electrolysis experiment was tried in science classes of ninth grade students, and the responses from the students were positive.
KEYWORDS: Elementary/Middle School Science, High School/Introductory Chemistry, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Electrochemistry, Oxidation/Reduction he term “electrochemistry” is not used in middle school science in Japan. However, students discuss the electrolysis of water and copper chloride in 8th grade in addition to thermal decomposition of silver oxide when they learn that a compound is made of different elements.1 Laboratory work is helpful in encouraging students to become interested in some phenomena and to understand the associated scientific concepts. However, when it comes to electrochemistry, some teachers experience difficulty in preparing and conducting experiments because expensive and elaborate apparatus such as ammeters, dc supplies, and electrolysis cells (e.g., Hoffman cells) are needed. Downsizing the apparatus is an effective way to improve the situation. A small-scale cell made of inexpensive plastic is useful because the quantity of chemicals required for the experiment can be reduced and students can easily handle the apparatus by themselves. On this basis, several kinds of easy and low-cost experiments have been developed.2−6 In general, the key structure of the downsized apparatuses, or microscale apparatuses, is almost the same as that of the original ones although the size is different. This means that most small- or microscale apparatuses are made by isotropic downsizing. However, when it comes to electrochemical cells, the merit of isotropic downsizing is limited because this kind of downsizing also reduces the electrode size. Because reactions occur on the surface or in the vicinity of the electrodes, downsizing the electrode makes the students’ visual observation more difficult. In this experiment, only the distance between the two electrodes was uniaxially downsized. This was done by using a filter paper soaked with electrolyte solution sandwiched
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© 2013 American Chemical Society and Division of Chemical Education, Inc.
between two coin-type lithium batteries. This new electrolysis cell has the following advantages: • The required volume of electrolyte solution (∼0.1 mL) is less than 1/10 of that needed for other forms of microscale electrolysis. • The surface of the electrode (∼3 cm2) is large enough for students to easily recognize reactions occurring on the surface, even when the current density is low. In addition, a low current density is important because it reduces the possibility of side reactions (e.g., hydrogen evolution on the electrode for copper electrodeposition), which may confuse the students’ understanding. • Because the distance between the two electrodes is small and their surface area is large, the overpotential and ohmic resistance of the electrolysis cell is small, and therefore, a high voltage is not needed. (Two lithium batteries are enough for water electrolysis using one current indicator and one current regulator.) Lithium batteries (CR2032) and a light-emitting diode (LED) are used as well as current-regulating diodes (CRD) instead of the elaborate devices used in traditional experiments. The microscale experiment is simple and inexpensive and can be carried out by every student. In addition, the time needed to obtain the result is less than five minutes and the results are visually clear. The details of the experiment will be presented below, as well as typical results and the responses from the students who carried out the water electrolysis. Published: January 11, 2013 228
dx.doi.org/10.1021/ed300365d | J. Chem. Educ. 2013, 90, 228−231
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Laboratory Experiment
Figure 1. Aspects of the water electrolysis cell: (A) diagram of the cell, (B) components of the cell, (C) pH testing paper put in a punched hole of filtering paper, and (D) experimental cell held in a metallic clip.
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EXPERIMENTAL METHOD
Electrolysis of Water
The water electrolysis cell is made of several components (Figure 1A,B): (a) two coin-type lithium batteries (CR2032), (b) platinum foil with a thickness of 0.02 mm, (c) three annular sheets of filter paper soaked with 1.0 M Na2SO4 aqueous solution, (d) three circular pieces of 1−14 universal pH-test paper whose size is the same as the size of a hole in the filter paper (Figure 1C), (e) a current regulator using a CRD (E103), and (f) a current indicator using an LED. When these components are assembled in series and held together with a metallic clip (Figure 1D), electric current starts to flow as indicated by the arrows in Figure 1A, and water starts to be electrolyzed on the surfaces of the lithium battery and platinum foil. Because the body of the lithium battery is made of stainless steel and is tightly sealed, the surface of the negative side can be used as a cathode. However, the surface of the positive side may become oxidized and corroded when it works as an anode during the electrolysis. Because the reaction of oxidizing stainless steel competes with the oxygen evolving reaction that produces hydrogen ions, the pH change will be suppressed if the positive side of the battery is used as an anode. Therefore, a platinum foil was put between the positive side of the lithium battery and the filter paper soaked with Na2SO4 aqueous solution so that the positive side of the battery was not in contact with the electrolytic solution. As illustrated in Figure 2, the current passing through the CRD (E-103) is constantly about 10 mA when the applied voltage is larger than 5 V.7 Thus, the component made of one CRD, which bypasses the plastic disk as illustrated in Figure 3A, works as a current regulator of 10 mA.8 The structure of the current indicator is presented in Figure 3B. Because the LED bypasses both sides of the plastic film, the LED lights up when electricity is passing through the indicator in the correct direction. If the contacts between the components are loose or the direction of the cell is wrong, the LED will not light up, and
Figure 2. Property of CRD (E-103).
therefore, the students can easily understand what they have to do. The reactions at the cathode and anode are cathode:
anode:
2H 2O + 2e− → H 2 + 2OH−
2H 2O → O2 + 4H+ + 4e−
(1) (2)
Although detection of odorless gases such as hydrogen and oxygen evolving on the surface of the lithium battery or platinum foil is not easy, a pH change, which takes place at both electrodes as shown in eqs 1 and 2, can be easily detected using pH-test paper. After 3 min of electrolysis, the clip is removed and the three pieces of pH-test paper are removed from the assembly and the students can confirm how the color of each piece has changed. Electrolysis of Copper Chloride
The electrolysis cell for copper chloride is made of several components (Figure 4). The filter paper between the two lithium batteries is soaked in 1.0 M CuCl2 aqueous solution. When the components are assembled in series and held together with a metallic clip, the CuCl2 starts to be electrolyzed on the surfaces of the lithium battery and platinum foil. cathode: 229
Cu 2 + + 2e− → Cu
(3)
dx.doi.org/10.1021/ed300365d | J. Chem. Educ. 2013, 90, 228−231
Journal of Chemical Education
Laboratory Experiment
Figure 3. Setup for 10 mA (A) current regulator and (B) current indicator, showing (C) the components and final product.
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TYPICAL EXPERIMENTAL RESULTS
Electrolysis of Water
A typical result of a 3-min electrolysis experiment is presented in Figure 5. Assuming the current is 10 mA, both protons and
Figure 5. Typical result of water electrolysis.
Figure 4. Experimental cell for electrolysis of CuCl2.
anode:
2Cl− → Cl 2 + 2e−
(4)
The cathode reaction of the electrolysis can be easily identified by observing the copper metal deposited on the negative side of the lithium battery after a few minutes of electrolysis and the anode reaction can be identified by the smell of chlorine soon after the electrolysis starts. If visual detection is preferred, another filter paper soaked with the CuCl2 solution and marked with red ink can be inserted between the original filter paper and the platinum foil (Figure 4). Because Cl2 bleaches the red ink stain adjacent to the anode, chlorine evolution can be visually recognized.
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HAZARDS
Figure 6. Typical result of electrolysis of CuCl2: (A) copper deposited on the cathode and (B) red ink bleached by chlorine evolving on anode.
Copper(II) chloride and sodium sulfate may cause irritation to skin, eyes, and respiratory tract and may be harmful if swallowed or inhaled. Students are recommended to use tweezers when disassembling the cell because acid and hydroxide are produced during the electrolysis.
hydroxide ions produced during 3 min can be calculated as 1.9 × 10−5 mol using Faraday’s law based on the eqs 1 and 2. As 230
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classes with a total of 110 students responding. The responses to two questions are shown in Figure 7. The students’ responses to the experiment were positive and there were many who were impressed with the color change of the pH-test paper after the electrolysis. On the basis of the results of the questionnaires and the students’ behavior during the class, the microscale experiment is considered useful, especially for middle school science classes.
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CONCLUSION Although the results of our experiments are qualitative rather than quantitative, students can obtain clear results in a short time without technical difficulties. This indicates that the miscroscale method is especially useful in middle school science. In addition, as the method is inexpensive and does not require special equipment, it would be a great help in developing countries where schools have limited financial resources.
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ASSOCIATED CONTENT
S Supporting Information *
Student worksheet; notes for the instructor. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Figure 7. Results of questions: (A) Was the experiment difficult? and (B) Did you enjoy the experiment?.
Notes
The authors declare no competing financial interest.
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the water contained in one piece of filter paper is approximately 0.02 mL, the change of pH is expected to be over 7. Because protons and hydroxide ions migrate toward each other and recombine into water, the observed changes are not large. However, a change in the color of the pH-test paper was clearly visible, as shown in Figure 5.
REFERENCES
(1) Yoshikawa, H.; Tsukada, M.; Yamagiwa, T.; Mori, K.; Ohya, Y. Science 2 (School Textbook); Keirinkan: Osaka, 2012; pp 117−122. (2) Habich, A.; Hausermann, H. R. J. Chem. Educ. 1987, 64, 171. (3) Singh, M. M.; Pike, R. M.; Szafran, Z.; Davis, J. D.; Leone, S. A. J. Chem. Educ. 1995, 72, A4. (4) Suzuki, C. J. Chem. Educ. 1995, 72, 912. (5) Eggen, P.-O.; Kvittingen, L. J. Chem. Educ. 2004, 81, 1337. (6) Hugerat, M.; Abu-Much, R.; Basheer, A.; Basheer, S. Chem. Educ. J. (CEJ) 2010, 13, 2. (7) Although 3-min electrolysis is neither too short nor too long for students, if it is necessary to shorten the electrolysis time, modifications to the electrolytic cell are available in the Supporting Information. (8) Semitec Corp. http://semitec.co.jp/products/led_device/2011/ 02/28/pdf/crd_113I_e.pdf (accessed Dec 2012).
Electrolysis of Copper Chloride
A typical result of a three-minute electrolysis experiment is presented in Figure 6. A small amount of copper was deposited on the surface of the negative side of the lithium battery and the stain of the red ink was bleached with chlorine evolving on the platinum foil. Use in Middle School Science Class
To clarify whether the experiment would be accepted by middle school students, a special class was conducted to teach water electrolysis to ninth grade students using the microscale electrolysis cell. The class was 50 min long and involved the following activities: (1) Review of water electrolysis: As the students had learned about water electrolysis using a conventional method, the teacher gave them a quick review on the subject. (2) The experiment: After a short explanation on how to assemble the cell, each student made his or her own cell and electrolyzed water for three minutes. (3) Results and Discussion: Every student confirmed his or her results (a color change of the pH-test paper) and discussed how it had happened, using models of water molecules. (4) The teacher’s explanation and questionnaire The same class was repeated five times and the total number of students was 196. The questionnaires were carried out in three 231
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