Communication pubs.acs.org/jchemeduc
A Simple Oxygen Detector Using Zinc−Air Battery Yoong Kin Hooi, Masayoshi Nakano, and Nobuyoshi Koga* Department of Science Education, Graduate School of Education, Hiroshima University, 1-1-1 Kagamiyama, Higashi-Hiroshima 739-8524, Japan S Supporting Information *
ABSTRACT: The construction of a simple oxygen detector using a zinc−air battery as an oxygen sensor is described. It is a user-friendly device that can be employed in various laboratory activities in both junior and senior high schools. A short circuit can be introduced to reduce the O2 concentration in the air-diffuser layer of the battery that causes a decrease in the voltage. This phenomenon provides the basis to make an electrical device that can produce a voltage increase whenever oxygen gas is present. If the surroundings do not contain any available oxygen gas, the voltage in the zinc−air battery would remain static. In addition, time-to-operate of an electric component attached to the battery can also be used for semiquantitative determination of O2 concentration in a gas sample.
KEYWORDS: Elementary/Middle School Science, High School/Introductory Chemistry, Demonstrations, Hands-On Learning/Manipulatives, Gases, Laboratory Equipment/Apparatus
T
he detection and identification of O2 are common laboratory activities, which are introduced to students as early as elementary school. A flame test, in the form of relighting of glowing splinter, acts as a common and basic qualitative test for students to confirm the presence of O2. At a high-school level, a solution containing indigo carmine can be used to indicate the presence of O2. A semiquantitative measurement of O2 concentration can be performed using a gas detector tube.1 The measurement of O2 concentration using an electric sensor with a data logger is currently being introduced in school laboratories. This paper describes a new and simple method to qualitatively detect the presence of O2 and semiquantitatively measure the concentration. Commercially available zinc−air batteries have been used in various experiments in high schools.2−4 The zinc−air battery works in such a way that oxygen gas becomes the active cathode, while zinc is the anode contained within the battery. The respective electrode reactions of the zinc−air battery and the corresponding standard potentials of the equations as written are5
According to eq 3, the battery consumes atmospheric O2 and gains mass in the form of ZnO. Therefore, Faraday’s law can be confirmed by measuring the change in the volume of O2 consumed or the gained mass of the zinc−air battery with increasing quantity of electricity in a circuit using the zinc−air battery as the power source.2−4 At the operating voltage of the zinc−air battery (1.4 V), the electric current in a circuit, with a small resistance, linearly changes with respect to the atmospheric O2 concentration.5 This makes it possible to use the zinc−air battery as a sensor for measuring O 2 concentration. This idea is further developed by applying an equivalent circuit in a device.6 In this communication, we report a method to use a zinc−air battery in a very simple circuit for detecting O2 and semiquantitatively determining the concentration. With a very small residual O2 concentration in the air-diffuser layer of the zinc−air battery, the electromotive force of eq 3 dramatically decreases.7 The re-establishment of the electromotive force by exposure to O2 from a gas sample provides evidence for the presence of O2, and the recovery time is an approximate measure of the O2 concentration in the sample. The basic principle is described in the Supporting Information.
( −)Zn(s) + 2OH−(aq) → ZnO(s) + H 2O(l) + 2e− (E° = 1.25 V)
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(1) −
DETECTOR SETUP A simple handmade detector was constructed using a zinc−air battery (1.4 V Panasonic PR44 or 1.4 V Panasonic PR2330), rubber stoppers (size 2, 3, and 8), a glass tube, wires, a switch, and an electronic melody box P70−3934 (Narika, Japan).
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( +)O2 (g) + 2H 2O(l) + 4e → 4OH (aq) (E° = 0.4 V)
(2)
The overall reaction of the battery is 2Zn(s) + O2 (g) → 2ZnO(s)
(E° = 1.65 V)
© 2013 American Chemical Society and Division of Chemical Education, Inc.
(3)
Published: December 24, 2013 297
dx.doi.org/10.1021/ed400169z | J. Chem. Educ. 2014, 91, 297−299
Journal of Chemical Education
Communication
There are several air holes in these models of the zinc−air batteries. Leaving one hole open for gas sample intake, other holes were sealed with a transparent tape. Then the zinc−air battery was fitted to a shaped rubber stopper. This was done to reduce the rate of voltage increase by slowing the oxygen gas adsorption in the air-diffuser layer of the zinc−air battery. It is important, particularly after short circuit, to provide ample time for the voltage hike to be detected by the electrical component; in this case, an electronic melody box. The detector was constructed with a switch acting as an alternative circuit path for the short circuit, as can be seen in Figure 1.
Figure 2. Relationship between duration time of the short-circuit treatment and the voltage recorded immediately after completing the short-circuit treatment (initial voltage) for the PR44 zinc−air battery. SW is the switch and V is the voltmeter.
Figure 1. (Bottom) A completed detector and (top) the corresponding circuit diagram.
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APPLICATION IN SCHOOL LABORATORIES
Short Circuit and Detector Performance
When a new zinc−air battery is first exposed to the atmospheric air, the voltage gradually rises and subsequently remains constant at around 1.4 V. The idea was to reduce the voltage to a value lower than the minimum requirement of the melody box, which, in this case, was 0.760 V. By implementing the idea of using a short circuit, the negative and the positive terminals were connected to cause a voltage drop. By connecting the battery to a voltmeter, the voltage immediately after the shortcircuit treatment (initial voltage) was investigated. When a PR44 battery was tested, the voltage dropped to ca. 0.2 V after a short-circuit time of 30 s (Figure 2). When a PR2330 battery was tested, the voltage dropped to ca. 0.5 V with a short-circuit time of 30 s and resulted in an air-charging time of 30 min or more, and a 15 s short−circuit time resulted in an air chargingtime of less than 30 min. A maximum voltage difference between the minimum requirement of the melody box and the voltage immediately after completing the short-circuit treatment of the battery gives optimum results for the qualitative and semiquantitative tests. The relationship between the O2 concentration in the test gas and time-to-sound using the PR44 battery detector is shown in Figure 3. The time-to-sound values at the respective O2 concentrations are reproducible. The relation between O2 concentration and time-to-sound can be empirically expressed by an inverse proportion. This relation was used as a calibration curve to determine the O 2 concentration in a test gas.
Figure 3. A calibration curve for the semiquantitative determination of O2 concentration using the PR44 battery detector setup.
box, to investigate the voltage change (Figure 4). When two melody boxes were wired to the oxygen detector at each
Figure 4. Voltage change at the anode and cathode in detectors during the electrolysis of water using the PR44 battery detector setup.
electrode instead of the voltmeters, the melody box at anode produced sound after 2 min, but no sound at cathode for more than 5 min. The audio results can be explained by the voltage changes at each electrode shown in Figure 4 and the line showing the minimum voltage requirement of the melody box.
Electrolysis of Water
Two detectors, similar to those shown in Figure 1, were employed to verify the gas sample evolved from the electrolysis of water while being wired to a voltmeter instead of the melody 298
dx.doi.org/10.1021/ed400169z | J. Chem. Educ. 2014, 91, 297−299
Journal of Chemical Education
Communication
Table 1. Performance of the Oxygen Detector Using Zinc−Air Battery in Various Experiments Time to Sound/min Gases
PR44a
PR2330b
O2 evolved from Anode (+) H2 evolved from Cathode (−) O2 by H2O2(aq) decomposition H2 by reaction of HCl(aq) + Zn(s) CO2 by reaction of HCl(aq) + Na2CO3(s) O2 consumed from air by chemical body warmer (kairo)c Oxygen inhaler (95% O2) Inspiration (18.8% O2)d Expiration (16.2% O2)d Before (18.8% O2)d After (15.1% O2)d
1.9 No sound (5 min) 0.8 ± 0.1 No sound No sound No sound 0.9 ± 0.1 8.3 ± 0.3 11.4 ± 0.8 8.3 ± 0.3 11.6 ± 0.4
2.5 No sound (10 min) 1.0 No sound No sound No sound 1.2 ± 0.2 9.4 ± 0.2 11.5 ± 0.2 9.9 ± 0.1 12.9 ± 0.1
Experiment Water Electrolysis Comparison of Gases
Human Breath
Combustion of Candle a
The initial voltage right after the short-circuit treatment is 0.2 V. bThe initial voltage right after the short-circuit treatment is 0.5 V. cSee the Supporting Information. dMeasured using an oxygen detector tube (Gastech, Japan).
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Application in Other Experiments
The performance of the oxygen detector using the zinc−air battery in various experiments in student laboratory experiments is summarized in Table 1. As can be seen from these data (comparison of different gases), the detector selectively works for oxygen. In addition, if the initial voltage of the zinc−air battery, after the short-circuit treatment, is set as constant in each experimental run, the concentration of oxygen can be semiquantitatively determined by comparing the time required by electronic melody box to make sound. This can be observed in the results of the experiments for the human breath and the combustion of a candle. The details of all the experiments listed in Table 1 are described in the instructor information found in the Supporting Information.
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CONCLUSION The simple device with a zinc−air battery as O2 sensor can be applied to various student experiments at schools. The detection and determination of O2 concentration is more quantitative than the conventional flame test, easier to use than an indigo carmine solution, and more cost-effective than a gas detector tube.
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REFERENCES
(1) Collings, A. J. Performance Standard for Detector Tube Units Used to Monitor Gases and Vapours in Working Areas. Pure Appl. Chem. 2001, 54 (9), 1763−1767. (2) Kamata, M.; Kawahara, T. Teaching Materials using Zinc−Air Batteries. I. Educational Experiment for Faraday’s Law. Kagaku to Kyoiku 2000, 48 (3), 192−194. (3) Kamata, M. Teaching Materials using Zinc−Air Batteries. II. Educational Experiment for Faraday’s Law (2nd report). Kagaku to Kyoiku 2000, 48 (5), 330−331. (4) Kamata, M.; Paku, M. Exploring Faraday’s Law of Electrolysis Using Zinc−Air Batteries with Current Regulative Diodes. J. Chem. Educ. 2007, 84 (4), 674−676. (5) Tanaka, Y.; Koga, N. A Convenient Measurement of Oxygen Concentration using Zinc−Air Battery. Chem. Educ. J. 2009, 13(1), No.13-10. http://chem.sci.utsunomiya-u.ac.jp/v13n1/10_2d4_1.pdf (accessed Dec 2013). (6) Takahashi, M.; Yamauchi M. Equivalent Circuits of Zinc−Air Battery and Analysis of Zinc−Air Battery Oxygen Sensor using the Equivalent Circuits. Abstract PRiME2012, 2012, #3553. http://ma. ecsdl.org/content/MA2012-02/51/3553.full.pdf (accessed Dec 2013). (7) Smith, G. C.; Hossain, Md. M.; MacCarthy, P. Why Batteries Deliver a Fairly Constant Voltage until Dead. J. Chem. Educ. 2012, 89 (11), 1416−1420.
ASSOCIATED CONTENT
S Supporting Information *
Instructor information. 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 The present work was supported partially by a grant-in-aid for scientific research (A)(25242015), (B)(22300272) and challenging exploratory research (23650511) from Japan Society for the Promotion of Science. One of the authors (Y.K.H.) acknowledges the Ministry of Education, Culture, Sports, Science, and Technology in Japan for the financial support in the Teacher Training Program. 299
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