Development of a Handmade Conductivity Measurement Apparatus

Laboratory Experiment pubs.acs.org/jchemeduc

Development of a Handmade Conductivity Measurement Apparatus and Application to Vegetables and Fruits Seng Set* and Masakazu Kita Department of Chemistry, Faculty of Education, Okayama University, Okayama 700-8530, Japan S Supporting Information *

ABSTRACT: This paper describes the development of a simple handmade conductivity measurement apparatus based on a Kohlrausch bridge with inexpensive materials. We have examined the reliability of this apparatus with standard solutions and then measured juices of vegetables and fruits as well as a sports drink. Comparisons to total alkali content as determined by titration were made. The handmade apparatus was introduced to high school chemistry classes through a lesson on the conductivity of electrolyte solutions. The results showed that the developed apparatus could be used effectively for teaching high school chemistry.

KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Conductivity, Food Science, Laboratory Equipment/Apparatus, Titration/Volumetric Analysis

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conductivity data was used to determine the amounts of total electrolyte in the solutions. Independent measures of the total amount of alkali were made by acid titration of the combustion residues of the vegetables and fruits and compared with the conductivity values obtained by this apparatus. The use and the effectiveness of the apparatus in the classroom are demonstrated through the experiments completed by high school students in Japan and Cambodia.11

he commercial electrical conductivity (EC) instrument (e.g., Shimadzu portable Khohlraush bridge BF-62A) is user-friendly and ready-to-use, but it is expensive as it costs more than $1000. A less expensive alternative EC meter uses electrodes to directly measure sample conductivities with the conductivity value being shown on a screen without calculation, for example, Horiba LAQUA EC meter B-771, which costs $240. Other methods include a handmade instrument equipped with a light bulb or a light-emitting diode.1−4 These methods measure the conductivity of different electrolyte solutions by noting the dimness or brightness of the diode. Students can use such devices easily, but the results are qualitative and cannot compare electrolyte concentrations or the extent of dissociation in solution. Some electrical conductivity apparatuses are only suitable for demonstrations because they use a high voltage 120 V ac source.5,6 Others can only detect the presence of electrolyte in a solution through noting the electrical current.7−9 Zawacky produced an inexpensive, semiquantitative handmade conductivity tester10 that could detect the conductivity of aqueous electrolyte solutions at different concentrations. However, assembly of the apparatus was complicated and it could not determine the actual conductivities of solutions. In this study, we developed a less expensive handmade conductivity meter that is easy to make and can be used in a high school chemistry classroom effectively and quantitatively. The development of an easy to make, handmade conductivity meter based on the Kohlraush bridge principle is described. The conductivity meter was used to make measurements of (1) standard alkali halide solutions, (2) solutions isolated from ground vegetables and fruits, and (3) a sports drink, and the © XXXX American Chemical Society and Division of Chemical Education, Inc.

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CONDUCTIVITY APPARATUS

Instrumental Setup

The handmade conductivity meter is based on the Kohlraush bridge principle. A 300 Ω variable resistance resistor was attached to a stiff circular paper and a steel wire fixed perpendicular to the axis to indicate the magnitude of the resistance on the paper (Figure 1A). The resistance value was determined by a multimeter. An electrode cell was setup by placing two carbon electrodes in a film canister (Figure 1B). The variable resistor and the electrode cell were assembled on a hard rectangular paper with one 200 Ω and one 1000 Ω fixed resistor and a piezoelectric buzzer to create a circuit (Figure 2). Because an electrolyte solution is easily electrolyzed by dc power, the apparatus was powered by 10 V ac power source (not shown) using an ac transformer to prevent polarization of the ions in the solution.

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dx.doi.org/10.1021/ed400611q | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 3. Diagram of handmade conductivity measurement apparatus based on the Kohlrausch bridge principle.

EAB + E BD = EAC + ECD = 10 V,

therefore,

I1R1 + I1R 2 = I2RX + I2R3 = 10 V

Figure 1. Photos of the (A) variable resistor and (B) electrode cell from two viewpoints.

(2)

From eqs 1 and 2,

I1R 2 = I2R3

(3)

which can be written as I1R1 IR = 2 X I1R 2 I2R3

(4)

and therefore RX =

R3 R1 R2

(5)

Substituting the values of R2 and R3 by 200 Ω and 1000 Ω, respectively, the sample solution resistant can be determined whenever the variable resistance is known: RX = 5R1 Figure 2. Photo of the assembly of the handmade conductivity measurement apparatus.

Measurement of Sample Solution Resistance

The apparatus was connected to an ac transformer set to 10 V. The sample solution (15 mL) was added to the film canister, the electrodes were inserted, and both ends of the electrodes were connected to the circuit by crocodile clips. With the circuits completed, in the Kohlrausch bridge current I1 and I2 pass through parallel paths splitting at A and returning at D across a set 10 V potential. If the potential difference between AB and AC is small enough, that is, they are sufficiently balanced, there is insufficient potential to drive the buzzer. In the experiment, the variable resistor was first turned to zero, causing the buzzer to emit sound. As the resistance was increased, the buzzer eventually silenced (Rlow). As the resistance was further increased, the potential was again unbalanced and the buzzer emitted sound (Rhigh). The average of Rlow and Rhigh was used as the value for R1. Finally, RX could be calculated by using eq 6. After each measurement, the electrodes and the sample solution container (canister) were washed with distilled water and wiped with tissue paper.

Sample Solution Resistance

Sample solution resistance, RX, is the key to determine the conductivity of an electrolyte solution because it is directly related to its ability to pass an electric current. The conductivity value can be calculated if RX is determined. A bridge setup to calculate the sample solution resistance, RX, is shown in Figure 3. (By analogy, if dc power is used instead of the ac power in Figure 3, the setup is called a Wheatstone bridge and is usually used to measure the resistance of a resistor.) If there is insufficient difference in electric potential between AB and AC, the buzzer does not emit sound. Assuming a perfectly balanced bridge, according to Ohm’s law, the following relationship exists: EAB = EAC ,

therefore,

I1R1 = I2RX

(6)

(1)

where EAB is the electric potential between A and B, EAC is the electric potential between A and C, I1 is the current between A and B, R1 is the variable resistance, I2 is the current between A and C, and RX is the resistance of the sample solution. The value of R1 varies depending on the sample solution. Under these conditions, and considering the circuit forms a parallel connection, the relationship can be also expressed as

Determination of the Cell Constant

The cell constant must be obtained before determining conductivity of sample solutions. The cell constant can be simply understood by the equation of electrical conductivity, κ, of an electrolyte solution.12 B



κ=

L 1 × A RX

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HAZARDS Students should be careful when crushing or ashing the vegetables or fruit. Hydrochloric acid is corrosive and causes burns to body tissue. If students suspect that they have accidentally made contact with hydrochloric acid during the titration, they must immediately rinse with plenty of water. CuCl2(aq) and AlCl3(aq) solutions may cause irritation to skin, eyes, and respiratory tract and may be harmful if swallowed or inhaled.

(7)

where L is the distance between the two electrodes and A is the surface area of electrodes dipped into the sample solution. The value of L/A is defined as the cell constant.13,14 Instead of measuring L and A, the electrical conductivity of 50 mM KCl(aq) solution, which is 0.00644 S cm−1 (standard solution), was used as a reference to assess the cell constant. From eq 7, the cell constant can be determined by cell constant = κRX

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(8)

RESULTS AND DISCUSSION

Conductivity of Electrolyte Solutions

The conductivity of each sample solution was obtained through calculations using eq 7 and the calculated cell constant value. The results determined by the handmade conductivity apparatus were plotted and compared with the results measured by a portable conductivity meter (Shimadzu portable Kohlrausch bridge BF-62A), as seen in Figure 4. The results

Using the handmade conductivity apparatus, Rlow and Rhigh were found to be 28 Ω and 52 Ω, respectively. Therefore, the variable resistance, R1, was 40 Ω as the midpoint between Rlow and Rhigh. Then, the resistance RX of 50 mM KCl(aq) solution could be calculated as 2.0 × 102 Ω by eq 6. Finally the cell constant value was found to be 1.3 cm−1 by using eq 8.

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EXPERIMENTAL DETAILS

Measurement of Electrolyte Solution Conductivity

With the obtained cell constant, the apparatus was used to measure the conductivities of sample solutions. In this study, 6.0, 8.0, and 10 mM of CuCl2(aq) and AlCl3(aq) solutions were prepared along with 10, 12, and 14 mM NaCl(aq) solution. The resistances of these sample solutions (RX) were measured starting from the solution with the lowest concentration, and then their conductivity values were calculated and plotted. Measurement of Conductivity of Vegetables, Fruit Juice Solutions, and a Sports Drink

The handmade conductivity meter was applied to measure conductivity of some solutions found in daily life such as vegetable and fruit-based solutions, and a Pocari Sweat sports drink. Fresh carrot, radish, cabbage, apple, and orange were used to produce sample solutions. First, each vegetable or fruit (20 g) was ground into a pulp, and water (20 mL) was added. The mixture was then filtered to collect the vegetable or fruit juice solution. The Pocari Sweat sports drink was prepared following the instructions on the packet: 1 pack, 74 g, was dissolved in 1.0 L of water. The resistance of each sample solution was measured using the same procedures as for the electrolyte solutions, and their conductivity values were calculated.

Figure 4. Electrical conductivity of NaCl, CuCl2, and AlCl3 aqueous solutions.

showed that the handmade apparatus provided similar values to the commercial portable conductivity meter. Also, the effect of ionic charges and ionic number on the conductivity values can be clearly understood by the results. The resistance measurements used in Figure 4 are available in the Supporting Information. Conductivities of Vegetables, Fruit Juice Solutions, and a Sports Drink

Acid−Base Titrations of Ash from Combustion of Vegetable and Fruit

To confirm the accuracy of the handmade conductivity meter, a 10 mM KCl(aq) solution was used instead of 50 mM KCl(aq) and a new electrode cell was used. For these series of measurements of vegetable and fruit juice solutions, it was necessary to determine a new cell constant value. In this case, Rlow and Rhigh were 65 Ω and 115 Ω, respectively, for the handmade conductivity measurement apparatus. Thus, R1 as the midpoint between the two resistances was determined as 90 Ω and RX was calculated to be 4.5 × 102 Ω. The conductivity of 10 mM KCl(aq) is known as 0.0014114 S cm−1 (a standard solution), and using eq 8, the new cell constant was found to be 0.64 cm−1. The electrode cell was again verified with NaCl(aq) solution as shown in Table 1. Despite this change, the resulting electrical conductivities were found to be almost the same as the ones

The ashes of the same vegetables and fruits were used in acid− base titrations. To prepare the ashes of the vegetables and fruits, 20 g of fresh vegetable or fruit was heated in a crucible via a gas flame until it became ash. It was assumed that alkali metal ions (Na+, K+, etc.) changed to alkali metal oxides and alkaline earth metal ions (Mg2+, Ca2+, etc.) changed to alkaline earth metal oxides in this process. Next, the ash was suspended in 50 mL of water and stirred for about 2 min, and then the mixture was filtered to remove the insoluble particulates in the solution. The alkaline property of the solutions was identified as they turned phenolphthalein indicator pink. The resulting ash solution (20 mL) was titrated with 10 mM HCl(aq) to determine the total alkali content of vegetables and fruits. C




Table 1. Electrical Conductivity Data from NaCl(aq) Solutions Conductivity/(S cm−1) Rlow/Ω

NaCl(aq)/mM 10 12 14

Rhigh/Ω

70 65 62

116 95 80

R1/Ω

RX/Ω

93 80 71

4.6 × 10 4.0 × 102 3.6 × 102 2

Handmade

Shimadzu

0.0014 0.0016 0.0018

0.00132 0.00153 0.00172

Table 2. Electrical Conductivity Data from Vegetable and Fruit Juice Solutions Conductivity/(S cm−1) Rlow/Ω

Solutions Carrot Radish Cabbage Orange Apple Pocari Sweat sports drink

20 25 30 50 86 50

Rhigh/Ω 48 80 80 136 170 76

R1/Ω

RX/Ω

34 53 55 93 128 63

× × × × × ×

1.7 2.7 2.8 4.6 6.4 3.1

2

10 102 102 102 102 102

Handmade

Shimadzu

0.0038 0.0024 0.0023 0.0014 0.0010 0.0020

0.00368 0.00243 0.00227 0.00136 0.000971 0.00210

The results of titration provided the total alkali content of electrolyte assumed in the vegetable and fruit juice solutions and was converted into the total alkali content in 1 g of the fresh vegetable or fruit (Table 3). This result showed consistency with the conductivity data as shown in Figure 5.

obtained with the initial electrode cell (Figure 4). Therefore, even though its cell constant was standardized with other concentration of KCl(aq), the accuracy of the handmade apparatus was confirmed. The data was also compared with values obtained by the portable conductivity meter and proved to be only slightly different (Table 1). The handmade conductivity meter was applied to measure conductivities of the carrot, radish, cabbage, orange, and apple solutions and a Pocari Sweat sports drink. The results show that the handmade apparatus detected the presence of electrolyte in our everyday solutions with acceptable accuracy, and the values compared well with values obtained by the portable conductivity meter (Table 2). Among the examined solutions, it appeared that carrot juice contains the highest electrolyte concentration, whereas apple juice contained the least. Acid−Base Titrations for Vegetable and Fruit Ashes

Electrolyte concentrations in the vegetable and fruit ash-derived solutions were calculated by assuming all alkali species in the ash-derived solutions were solely monobasic (MOH). At the phenolphthalein equivalence point of the titration, the volume of 10 mM HCl(aq) added was noted (Table 3). Hence,

Figure 5. Alkali content in vegetable and fruit versus the conductivities derived from the ashes.

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Table 3. Total Alkali Concentrations Obtained by Titration Sample Ash Derived Solutions Carrot Radish Cabbage Orange Apple

Volume of HCl(aq)a/ mL 22.4 10.5 8.0 6.2 5.7

Concentration of Total Alkali Cb/mM 11.2 5.25 4.0 3.1 2.9

Lesson Introduction

Total Alkali Contentb/(mol g−1) 2.80 1.31 1.00 0.78 0.73

× × × × ×

While learning about the conductivity of electrolyte solutions in the classroom, the handmade apparatus was used by 11th grade students in Japan and in Cambodia to evaluate its potential and applicability in a real classroom. Four lab periods were used, and the student groups needed approximately 25 min per period to complete the experiment. In the first period, the instructors presented background information on electrolytes, conductivity, and the Kohlrausch bridge principle. Students were given time to derive eq 6 and to determine the cell constant, as well as measurement procedures. In the following three periods the students examined the conductivity of electrolyte solutions, the conductivity of vegetable and fruit juice solutions, and acid−base titration for vegetable and fruit ashes. All the sample solutions and materials were prepared for students before the lab period. The students obtained similar results from the handmade apparatus to those shown in Figures 4 and 5. The results from the portable conductivity meter (Shimadzu portable Kohlrausch bridge BF-62A) were measured by the instructor.

−5

10 10−5 10−5 10−5 10−5

a

Volume of HCl(aq) used to reach the equivalence point. bTotal alkali content is per 1 g of vegetable or fruit.

concentration of the alkali species in the ash-derived solutions could be calculated by the following quantitative acid−base equation Cb =

CaVa Vb

APPLICATION TO CLASSROOM

(9)

where Ca and Va are acid concentration and volume, respectively, and Cb and Vb are base concentration and volume, respectively. D




Pedagogical Impact of the Lesson

study electrolytes. The use of everyday drinks such as vegetable and fruit juices in the classroom considerably enhances students’ interest in learning science. Therefore, addition to other student-made instruments published in this Journal,15−21 this handmade apparatus can be used to determine the electrical conductivity of electrolyte solutions as well as everyday solutions like vegetable and fruit extracts with high school chemistry students.

Before the experiment, students were asked to predict and explain what would happen to the conductivity when the concentration of electrolyte in solution decreased or increased, based on their background knowledge on how electricity passed through a material. Because Pocari Sweat sports drink is a common drink in Japan that people often use to supplement electrolytes in their body after exercising, the lesson presenter (S.S.) asked students whether people could use vegetable or fruit juices instead of Pocari Sweat sports drink after they exercised. Such introductory questions were of key importance in the lesson because the questions motivated students toward scientific investigation. The most impressive activity for students was the conductivity determination of vegetable and fruit solutions. The students could hardly wait to conduct their experiment to find out whether their predictions about whether Pocari Sweat sports drink was better than the vegetable and fruit juices were correct or not. The students understood that electrical conductivities for a certain electrolyte solution differed at different concentrations. Through the experiment, they observed that conductivity increased with increasing concentration, as shown in Figure 4. They also observed that a higher charge of cation showed a higher conductivity at similar concentrations and that the electrical conductivity of different electrolyte solutions increased with respect to the number of ions dissociated from the species. As shown in Table 2, the students surprisingly realized that some vegetable and fruit juices such as cabbage, radish, and carrot could provide more electrolytes than Pocari Sweat sports drink. This result impressed the students so much that they said they would drink juice instead of Pocari Sweat sports drink to supplement electrolytes to their body when they sweat. The total content of electrolyte was also revealed to the students through acid−base titration experiments. The students understood the clear relationship between the total content of electrolyte and alkali in the corresponding ash derived solutions (Figure 5). In addition, the students learned that although vegetables and fruits contained organic and inorganic substances, they mostly contained inorganic substances when they were burned to ash. They observed that the ash solution turned pink when a few drops of phenolphthalein were added, which clearly proved its alkalinity.

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ASSOCIATED CONTENT

S Supporting Information *

Student handout; student worksheets; prelesson test; postlesson test; student questionnaire; additional experimental details. This material is available via the Internet at http://pubs. acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Ministry of Education, Science, Sports and Culture of Japan for financial support in this research through Japan Society for the Promotion of Science. We also wish to express our thanks to David Ford, of The Royal University of Phnom Penh, for helpful correction of our English and discussion of the present manuscript, and Pen Samphea for his support to this research.

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REFERENCES

(1) Russo, T. A Cheap, Semiquantitative Hand-Held Conductivity Tester. J. Chem. Educ. 1986, 63, 981. (2) Gadek, F. Easily made electronic device for conductivity experiments. J. Chem. Educ. 1987, 64 (7), 628−629. (3) Vitz, E. W. Conductivity of solutions apparatus. J. Chem. Educ. 1987, 64 (6), 550. (4) Gadek, F. A commercially available device for conductivity experiments. J. Chem. Educ. 1987, 64, 281−282. (5) Rettich, T.; Battino, R. An inexpensive and easily constructed device for quantitative conductivity experiments. J. Chem. Educ. 1989, 66 (2), 168−169. (6) Mercer, G. D. A low-cost, portable, and safe apparatus for lecture hall conductivity demonstration. J. Chem. Educ. 1991, 68 (7), 619− 620. (7) Hans, A.; Kaelber, L. Apparatus for the demonstration of conductivity of electrolytes. J. Chem. Educ. 1955, 32, 640. (8) Colton, R.; Sketchley, G. J.; Ritchie, I. M. J. Chem. Educ. 1976, 53, 130−132. (9) Havrilla, J. W. An inexpensive device for quantitative conductivity experiments. J. Chem. Educ. 1991, 689 (1), 80. (10) Zawacky, S. K. S. A Cheap, Semiquantitative Hand-Held Conductivity Tester. J. Chem. Educ. 1995, 72 (8), 728. (11) The author is Cambodian and is conducting research in Japan. Therefore, the handmade apparatus was used in both Japanese and Cambodian high schools. (12) Atkins, P.; de Paula, J. Atkins’ Physical Chemistry, 8th ed.; Oxford University Press: New York, 2006; pp 761−762. (13) Barrow, G. M. Physical Chemistry, 3rd ed.; McGraw-Hill: Tokyo, 1973; pp 617−620. (14) Barrow, G. M. Physical Chemistry, 6th ed.; McGraw-Hill: Tokyo, 1996; pp 361−362.

CONCLUSION

Electrical conductivity measurement methods are useful for helping students understand electrolytes. The handmade conductivity meter presented here is inexpensive, made from readily available materials, and easy for teachers and high school students to assemble and use. Pre- and post-test results showed that the students’ understanding of electrolytes improved significantly. Results obtained through measurements using the handmade apparatus were consistent with the results obtained with the Shimadzu portable Kohlrausch bridge BF-62A. This study showed that, even if different electrode cells resulted in relatively different cell constant values, the calculated final conductivity values were similar. Other notable benefits were that this handmade apparatus used only a low voltage ac power source as well as a combination of 200 Ω and 1000 Ω resistors in the circuit, which contributed to the accuracy of the results and safety in operation. In addition, this experiment made students aware that everyday drinks can be used as samples to E




(15) Seng, S.; Kita, M.; Sugihara, R. New Analytical Method for the Determination of Detergent Concentration in Water in Fabric Dyeing. J. Chem. Educ. 2007, 84 (11), 1803−1805. (16) Agbeko, J. K.; Kita, M. A Qualitative Experiment to Analyze Microbial Activity in Topsoil Using Paper and a handmade reflection Photometer. J. Chem. Educ. 2007, 84 (10), 1689−1690. (17) Teerasong, S.; Maclain, R. L. A Student-Made Microfluidic Device for Electrophoretic Separation of Food Dyes. J. Chem. Educ. 2011, 88 (4), 465−467. (18) Peterson, K. I. Measuring the Density of a Sugar Solution: A General Chemistry Experiment Using a Student-Prepared Unknown. J. Chem. Educ. 2008, 85 (8), 1089. (19) Hoffman, A. R.; Britton, S. L.; Cadwell, K. D.; Walz, K. A. An Integrated Approach To Introducing Biofuels, Flash Point, and Vapor Pressure Concepts into an Introductory College Chemistry Lab. J. Chem. Educ. 2011, 88 (2), 197−200. (20) Janusa, M. A.; Andremann, L. J.; Kliebert, N. M.; Nannie, M. H. Determination of Chloride Concentration Using Capillary Zone Electrophoresis: An Instrumental Analysis Chemistry Laboratory Experiment. J. Chem. Educ. 1998, 75 (11), 1463. (21) Hiemenz, P. C.; Pfeiffer, E. A general chemistry experiment for the blind. J. Chem. Educ. 1972, 49 (4), 263.

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Development of a Handmade Conductivity Measurement Apparatus

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