An Inexpensive Device for QuantitativeConductivity Experiments John W. Havrilla University of Pittsburgh at Johnstown, Johnstown, PA 15904 The laboratory manual we use has a quantitative conductivity experiment1, however, we do not have access to a conductivity apparatus. T o overcome this problem, we reviewed some recent articles from this Journal2-5 and came up with a design incorporating ideas from these articles. The design we present has the advantages of being inexpensive and safe and giving reasonable results. Figure 1 shows a schematic'of the apparatus, which uses a 9-V battery as a source of current. A 555 timer is used to produce approximately 1000 Hz of pulsed current t o avoid any polarizing of the electrodes. Figure 2 shows the experimental setup. The current is measured using a digital multimeter (DMM) (Metex 3610) connected in series to one of the electrodes. The electrodes are carbon rods (0.5 cm diameter) mounted in a two-hole no. 7 stopper. The carbon rods were covered with paraffin, and the paraffin was scraped off the tips to provide a constant surface area. Table 1 lists the parts purchased from All Electronics Corp., P. 0. Box 567, Van Nuys, CA 91408. The total price t o
huild five sets of the a .~.o a r a t u was s $27.15. This does not . -~ include the price of the multimeter. Table 2 shows the results for a few solutions that we use in the experiment. The DMM was set on the 2-ma scale. The values for the correlation coefficients were calculated using a calculator. We found i t necessary to wait 20 s for the current to stabilize when measuring the current. We feel the apparatus can be used to distinguish between strong electrolytes, weak electrolytes, and nonelectrolytes. We were also able t o distinguish between the conductivity of C a ( N 0 h (3ionslmol) and KN03 (2 ionslmol). However, the best results occurred a t lower concentrations. ~
~
~
-
~~~
The author wouldlike to thank Joseph Carney for the idea for this amaratus and Stanley Pikarski for his help in de. signing the project. Wentworth. R. A. D. Experiments in General Chemistry; Houghton Milfiin: Boston. 1987; p 163. Rettich, T. R.; Battino, R. J . ~..Chem.Educ. 1989, 64, 168. Vitz. E. J. Chem. Educ. 1987, 64. 550. ' ~ a d e k .F. J. J. Chem. Educ. 1987.64.628. 5 ~usso;T.J. C h e ~Educ. . 1986,63. 981-982.
Figure 1. Schematic of me conductivity apparatus Table 1.
Parts Llst for the Co~uctlvltyApparatus
Figure 2. Experimental setup lor the mndunivity experiment 2
5 5 5 5 16
5 lo lo lo 10 5
an
Item
Catalog No.
Cost
4 X 6-111. Perf. Board 9-V Battery Snaps Black Banana Plug Jacks R e d Banana Plug Jacks CMOS 555 Timers
ICPB-46
$5.50 $1.00 $2.00 $2.00 $6.25
0.01-MFD Capacitors 10-MFO Electrolytic Cap. loo-0. Y.-W Reslstars 10.0, Y,-W Reslstars 2.7-KO. &W Resistws 66.K0, '/,-W Resistors I.C. Sockets
BST-4
WL-B WL-R ICM 7555
$4.00
$0.65 $0.50 $0.50 $0.50
HRICS-8
.lo~~rnal of Chemical Education
$0.50 $3.75
Table 2.
Currents (mA) Measured Uslng the Apparatus
Concentration (M) 0.0250 0.0100
Solution Tap Water Demineralized Water HCI
CalNOJ. KNOJ HC2Hs02 NHa
0.0500
0.010 0.000
-
-
0.282 0.260 0.248 0.039 0.026
-
0.207 0.224 0.169
0.027 0.017
-
r -
1.00
0.159 0.176
0.975
0.126 0.016 0.010
0.967 0.99 1.0
