Chemistry Everyday for Everyone Tested Demonstrations
Solution Conductivity Apparatus submitted by:
Daniel T. Haworth,* Mark R. Bartelt,* and Michael J. Kenney Department of Chemistry, Marquette University, Milwaukee, WI 53201-1881; *064HAWORTHD@vpop3. csd.mu.edu (DTH);
[email protected] (MRB)
checked by:
Reed Howald Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717
When embarking upon the design of a conductivity meter that would be safe and easy to use and, at the same time, provide more than a simple binary response, we reviewed the pages published in this Journal and various general chemistry textbooks and found references to a number of devices. Older devices, which are still illustrated in current general chemistry textbooks, are all of the same basic design. Two exposed wires, which, when dipped into a conductive solution or touched to a bare metal surface, complete a circuit that includes an indicator (usually a light bulb) and a power source (frequently a wall socket) (1, 2). More recent designs presented in this Journal consist of a 9-volt battery as the power source and an LED or piezoelectric buzzer as the indicator (3–11). Zawacky’s conductivity meter (3) represents a good improvement over meters reported earlier. This meter is a hand-held bar-graph LED tester powered by a 9-V battery, which gives more than a “yes/no” answer to whether a solution does or does not conduct. Electrode polarization after the electrodes are immersed for longer than one second is reported to occur with this tester as with other dc methods of measuring conductivity (11).
Figure 1. Hand-held conductivity apparatus. Test solution is vinegar.
Design of Apparatus The conductivity meter described in this communication is an improvement over these designs, especially over the archaic design of electrodes in series with a 110-volt line. Our design includes a readout display, which like the Zawacky model, provides semiquantitative information; its major improvement is the use of a timer to provide a square wave ac voltage to the probe. Furthermore, the meter has been constructed in both a hand-held model (Fig. 1), and a lecture-hall version (Fig. 2), which is six feet in length. Both devices use the same basic electronic configuration and consequently give equivalent responses. The primary purpose of this apparatus is to keep the attention of the audience through a visual display rather than a numerical reading of conductance. The meters use a 10-element LED or incandescent lamp array to indicate conductivity strength. The probe is a standard stereo headphone plug (4). We have also used a 1/8-in. phone plug, which gave excellent results for analyzing small volumes of solution as in a microscale laboratory. Power is supplied by a 9-volt battery for the hand-held version and a standard 110-volt wall outlet for the larger version. The lights on the larger model are 30-W incandescent lamps similar to those found in an aquarium, which has a greater audience impact than a LED display.
Figure 2. Lecture-hall conductivity apparatus. Test solution is 0.1 M HCl.
Table 1. Meter Response to Various Aqueous Solutions Solution Deionized water Tap water
Elements Lit (No.) 0 3
0.1 M HCl
10
0.1 M NaCl
10
0.1 M CaCl2
10
Vinegar
6
Glacial acetic acid
0
Glacial acetic acid + deionized water
varies
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Chemistry Everyday for Everyone
Figure 3. Diagram and components for the conductivity meters.
Table 2. Parts List for Conductivity Apparatus a
b
Item No. LM78M12CT-ND ICM7555IPA-ND LM3914N-ND MOC3010QT-ND
Description 12-V regulator CMOS timer LED bar-dot display driver Opto-triac driver (10 each)
R1 R2 R3 R4 R5 R6 R7
1.0KQBK-ND 10KQBK-ND 1.0KQBK-ND 3009P-104-ND 390QBK-ND 1.0KQBK-ND 180H-ND
1k 5% 1/4-W 10k 5% 1/4-W 1k 5% 1/4-W 100k 10T trimmer 390 Ω 5% 1/4-W 1k 5% 1/4-W 180 Ω 5% 1/2-W (10 each)
C1 C2 C3 C4
P6269-ND P4923-ND P3104-ND P3474-ND
220-µF 50-V electrolytic 0.10-µF 50-V ceramic 0.1-µF polypropylene 0.47-µF polypropylene
T1 T2
T138-ND Q4008L4-ND
120:12.6-V ac transformer 400-V 8-A triac (10 each)
F1 S1 D1 L1
F115-ND CKN1018-ND KBP01G-ND –
1-A 250-V normal fuse SPDT switch 100-V 1.5-A bridge rectifier 120-V 25-W lamps (10 each)
Label U1 U2 U3 U4
Note: Adjust R4 for full scale with the most conductive solution. Substitute 9-V batery and LEDs for 120-V ac parts for hand-held unit. aLabel in the diagram, Fig. 3. bDigi-Key Corp., 701 Brooks Av. South, Thief River Falls, MN 56701-
Testing of Apparatus We have tested these meters using various aqueous solutions. The meters are set to a full-scale response, using an external adjustment, with a solution of concentrated HCl. The relative conductivity of a solution is estimated for the handheld model by counting the number of bar graph elements that are lit. A NaCl solution is very conductive, as expected, and all 10 elements are lit. When a 0.1 M acetic acid solution is tested, 6 of the elements are lit. When vinegar is tested, 6 elements are lit (Fig. 1). None of the elements are lit when deionized water is tested. Tap water in our laboratory, as evidenced by 3 lit elements, contains a significant amount of ionic material. A summary of some typical results is presented in Table 1. Figure 2 demonstrates the response of the largescale version of the meter for a 0.1 M HCl solution. A glacial acetic acid solution shows no conductivity, but becomes conductive upon addition of a small amount of deionized water to the acid. Safety Both meters have been student and faculty tested for safety. The probe of the larger meter is as safely handled as the smaller model’s probe (the electronics were designed to be equivalent for the two models, the difference being the method of display). The current in both meters is so low at the probe end that the probe may be held between the fingers at no risk to the investigator. In fact, the meter will indicate the relative conductivity of the individual’s hand when this is attempted.
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Journal of Chemical Education • Vol. 76 No. 5 May 1999 • JChemEd.chem.wisc.edu
Chemistry Everyday for Everyone
Description of Apparatus Figure 3 presents a schematic diagram and list of components required to construct these conductivity meters. A source (Digi-Key Corporation) for the components with part numbers is displayed in Table 2. The cost is about $15.00 for the hand-held model and about $70.00 for the lecturehall model. The conductivity demonstration unit has three sections: power supply, oscillator, and detector with its output options. The hand-held unit uses a single 9-volt battery for the circuitry and LED bar graph display. The lecture-hall version has a transformer-isolated 12-volt supply for the circuitry and 120 V ac for the lamps, which are switched by optoisolators, providing isolation on the output side of the device. The oscillator uses a CMOS version of the venerable 555 timer because it can be configured to provide a true square wave and its output swings rail to rail. Passing the square wave through a capacitor (C4) provides an ac signal with no dc component, to avoid any polarization effects (3, 11). Care should be exercised in the construction of this meter, since it uses a 120-V circuit. It has been suggested that a 12-V transformer using low-voltage lamps can be substituted for our described unit; however, proper construction techniques are still required. One of the authors (MRB) is available for assistance in the assembling of such an apparatus. The current flowing through the solution (5 mA max) develops a voltage across the input resistor (R5) of the detector circuit. The LM 3914 Dot/Bar Display Driver measures this voltage and drives the LED bar graph or optoisolators.
The hand-held version uses a 10-segment bar graph linear display (33996) with the driver IC built in available from Jameco Electronics, 1355 Shoreway Road, Belmont, CA 94002-4100. The probes, 1/8 in. and 1/4 in. phone plugs, are available from Radio Shack (parts no. 42-2444 and 42-2373, respectively). Student Response We have had a very good response from students when these meters are used. They enjoy measuring the conductivity of solutions in both a lecture-hall demonstration and in a laboratory setting. The semiquantitative nature of both displays enables a better description and—we hope—a better understanding of the concept of conductivity. Literature Cited 1. Hill, J. W.; Petrucci, R. H. General Chemistry; Prentice Hall: Upper Saddle Press, NJ, 1996; p 446. 2. Kotz, J. C.; Treichel, P. Chemistry and Chemical Reactivity, 3rd ed.; Saunders: New York, 1996; p 165. 3. Zawacky, S. K. S. J. Chem. Educ. 1995, 72, 728. 4. Mercer, G. D. J. Chem. Educ. 1991, 68, 619. 5. Havrilla, J. W. J. Chem. Educ. 1991, 68, 80. 6. Battino, R. J. Chem. Educ. 1991, 68, 79. 7. Rettlich T. R.; Battino, R. J. Chem. Educ. 1989, 66, 168. 8. Gadek, F. J. J. Chem. Educ. 1987, 64, 628. 9. Vitz, E. J. Chem. Educ. 1987, 64, 550. 10. Slevens, D. J. Chem. Educ. 1986, 63, 981. 11. Ewing, G. W. J. Chem. Educ. 1974, 51, A469.
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