In the Laboratory
A Quantitative Conductance Apparatus Danny Burns and Don Lewis Bee County College, 3800 Charco Road, Beeville, TX 78102 Katz and Willis described the evolution and improvement of a safe student conductivity apparatus as originally proposed by Russo (1, 2). They proposed a conductivity scale associated with the brightness of a green-and red-light emitting diode (LED). Their design is simple to construct. However, in order to exhibit the low category of conductivity (1 on the Katz and Willis scale), the electrode depth of immersion and spacing is such as to obscure the distinction between the medium and high conductivity categories (2 and 3 on the Katz and Willis scale). Our apparatus is only slightly more difficult to construct than the Katz and Willis circuit; on the other hand, the design we propose has reproducible quantitative features (including standard units) not available from the other device. Design and Construction Figure 1 illustrates the circuit configuration that was used, and Table 1 lists the parts required to construct the circuit. When the conductivity probe or cell is connected between points A and B, the circuit operates as a free-running multivibrator. The LED flash repetition frequency is determined by the resistance of the sample, Rc; the value chosen for R1; and the capacitance, C. The circuit response monitored was the time in seconds, τ, between the end of one
flash of the LED and the beginning of the next flash. Figure 2 illustrates the four electrode designs tested. Designs A, B, and C are referred to as probes, while design D, a container form, is referred to as a cell. Probes were mounted on strips of acrylic windowpane material. The portion of the electrodes that was not intended to come in contact with the sample solution was sealed with epoxy within drinking straws or thick-wall glass capillary tubing. The probe-style electrodes were 20-gauge wire (design A), carbon rod 0.5 cm in diameter (design B), and 0.25-watt carbon resistors (R < 10 Ω) cut into two parts (designs C and D). Although carbon resistors have been largely been replaced by metal-film resistors, it was possible to obtain a few dozen resistors from various sources. The carbon resistors had an outside radius of 0.5 cm. When cut perpendicular to the cylindrical axis, a circular carbon surface (0.32cm diameter) was exposed. The carbon resistors were particularly convenient to use because the external phenolic surface is resistant to most solutions and the resistor leads can be soldered. The wire electrodes used in probes were either copper or 12-carat gold (available from manufacturing jewelers for about $1.00/ in.). In the probe configuration, carbon rod or wire electrodes had an exposed length of 1 cm and the cylindrical axis of the electrodes separated by about 1.5 cm. Electrodes were mounted on scrap strips of acrylic windowpane material. The adhesives used were epoxy or hotmelt glue. In those instances where the cell or probe was used with organic solvents, only epoxy sealant with glass capillary tubing was used. Experimental Results The flash interval, τ, is linearly related to the equivalent resistance between points A and B of the circuit (Rc in Fig. 1). We used combinations of 1% tolerance metal film resistors between points A and B and measured the resulting flash intervals. A spreadsheet linear regression application generated eq 1. Rc = 2553τ – 1511
Figure 1. Circuit schematic. Electrodes are attached between points A and B.
(1)
For 1.5 kΩ < Rc < 2 MΩ and for 2.7 s < τ < 700 s, eq 1 described the measurement with calculated values of Rc dif-
Table 1. Parts List for Circuit Construction Part
Catalog Number
Integrated circuit, TLC555 or equivalent
Radio Shack no. 276-1718
Dual general-purpose IC PC board
Radio Shack no. 276
Resistors R 1= 1.5 kΩ R 2= 220 Ω
Radio Shack no. 271-309 contains 50 resistors, tolerance 1%
Light-emitting diode
Radio Shack no. 276-1622 contains 20 assorted LED
Capacitor, C = 470 mF
Radio Shack no. 272-1030
Battery snap connectors
Radio Shack no. 270-3250
A
B
C
Figure 2. Electrode construction styles.
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Journal of Chemical Education • Vol. 74 No. 5 May 1997
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In the Laboratory
Figure 3. Conductivity as a function of cell conductance.
Table 2. Conductance Proper ties for Selected Alcohols Samplea
Flash Interval τ (s)
Conductance Σ (µmho)
methanol
3.8
8.21
ethanol
10.2
3.17
1-propanol
20.5
1.59
2-propanol
33
0.995
1-butanol (n -butyl alcohol)
22.7
1.44
2-butanol (sec -butyl alcohol)
37.3
0.881
2-methyl-2-propanol (ter t -butyl alcohol)
204.3
0.161
a The alcohols listed were technical-grade alcohols that may have had more than the trace of water (< 0.3%) that was sometimes specified on container labels.
fering from values for R taken from the resistor specification sheet by less than 5%. It should be noted that neither the Katz and Willis design nor the apparatus described in this paper measures conductivity (mho/cm). Nevertheless, the circuit we have proposed measures the cell resistance, Rc (ohms). Conductance is the reciprocal of resistance; hence, the cell conductance (mho) can be correlated with conductivity measurements made with a commercial conductivity meter (Beta Technology Incorporated, Model 01645). Figure 3 displays the relationship for a carbon resistor cell electrode configuration. The solutions tested were serial dilutions of filtered water taken from Nueces Bay, near Corpus Christi, Texas. Using a cell configuration with carbon rod electrodes, it was possible to compare the conductance of seven alcohols. A preliminary investigation indicated that the flash interval would be quite long. We constructed another circuit similar to that illustrated in Figure 1, but with C = 47 µF. We also placed an infrared (915 nm) LED in series with the red LED of Figure 1. The infrared LED used was the emitter part of the emitter–detector pair available from Radio Shack as part number 276-142. The reduction in C reduced the flash interval time by a factor of 10. The device was calibrated using precision resistors and a new equation was constructed to replace eq 1. The infrared LED was included so that we could use the photogates and timing system (Apple II computer, Pasco photogates, Vernier Software program) available in the physics laboratory. Flash interval times listed in Table 2 are for a series of 10 flash intervals with a standard deviation less than 2% of the average value. Literature Cited 1. Katz, D. A.; Willis, C. J. Chem. Educ. 1994, 71, 330. 2. Russo, T. J. Chem. Educ. 1986, 63, 981.
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