William B. Thomas Bates College Lewiston, Maine
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Electrolytic Conductivity A demonstration experimenf
Experiments on electrolytic conductivity have long been an important part of the general chemrevealed that istry course. A search of THIS JOURNAL most of the methods that have been proposed for studying electrolytic conductivity fall into one of the following categories: (a) qualitative observations employing modifications of the familiar light bulb apparatus; (b) a milliammeter is used to measure thc dc current flowing through the cell; (c) an ac eurrent through the cell is rectified and the resulting ac eurrent is measured; (d) other methods suitable for advanced courses. A quantitative method is essential in order to bring out some of the more subtle comparisons of electrolytes, such as those based on ionic mobilities. In this paper a simple method of measuring electrolytic conductivity is described which is based on Ohm's Law. The circuit is showu in Figure 1. A variable voltage transformer is used as a source of 60-cycle ac current. which is measured with a milliammeter. The voltage drop aeross thc cell is determined by means of a vacuum-tube volt,meter. This type of meter is mandatory in order to obtain an adequate resistance through t,his part of the circuit. A switch permits either measurement of the current or visual observation of thc intensity of the light bulb. The electrodes dipping into the solution may be of any suitable design. We have cmployed electrodes made by sealing a short length of platinum wire into a soft glass tube and then bending the wire back in a spiral around the outside of the tube. Two such electrodes are held rigidly in a large cork. Electriral contact is made through mercury in the glass t,~tbrs. The electrodes must be plat,inii;nd.
The apparatus is operated on a variable-voltage, constant-current basis. To make a measurement, a 50-milliliter beaker is filled with enough test solut,ion to completely cover the platinum wires. The applied voltage is slowly increased by means of the transformer until the desired current is attained. Both meters are then read. In ordcr to obtain good precision, it is desirable to maintain a constant eurrent through the various test solutions by adjusting the voltage drop aeross the cell. A current of 50 milliamperes represents a suitable compromise between those for too low meter readings and undesirable heating of the solution. If the solutiou is a very poor conductor it will be necessary to use a current smaller thau 50 milliamperes. Ohm's Law is used to calculate the resistance, the reciprocal of which gives the conductance. A t,,vpical set of data is praserrt,ed in Table 1. The method of measuring electrolytic conduet~ivity prescntcd in this paper is intended only for demonstration purposes and for a comparison of solutions of electrolytes. The results given in the last column of Table 1 indicate that this objective has been realized. However, it is of interest to consider the validity of the Ohm's Law approach. To this end, the solutions were tested with an Industrial Instruments Conductivity Bridge, Model RC-IB using the same electrode system. The resulting data are given in the third column of Table 1. To attain the order of agreement between the two methods shown in the table, it was necessary to calibrate the ammeter and voltmeter against precision meters. The calibration is not necessary for demonstration purposes. Although the apparatus cannot be assembled for the proverbial few dollars, the only expensive items-the transformer and the voltmeter-will find other uses in
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Toble 1. .. Electrolvtic Conductance Measurements ~
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Test solution
Figure 1. Circuit for meowing electrolytis conductivity. TR Variablevoltage tronrformer, e.9.. a Powerrtat V Vacuum-tube voltmeter, e.g., RCA Senior Voltohmyrt M A Muitironee milliammeter, e.g., Simpron Model 378
-Resistance Ohm's law
(ohms)Meter
ConductanceU (ohms-')
Distilled water 5% Sugar 0.01 M HC1 0.01 M H8Ol 0.005 M H2SOI 0.01 M H3PO4 0.01 M HOAc 0 . 5 M H,BOa 0.01 M NaOH 0.01 M NHIOH 0.01 M NaCl 0.01 M NH&1 0.01 M PblOAch , ,a
Calculated from the resistances in the second column. Volume 39, Number 10, October 1 9 6 2
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most laboratories. Also, inexpensive models of the meters and transformer are available from most radio supply houses. The pedagogic value of the Ohm's Law approach as compared with the use of a rondurtivity meter is a
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matter of opinioii. There is an advantage iu a straightforward method that the student in an introductory course can readily understand. Furthermore, the theoretical limitations of the method call be disrussed, thus addinq to the value of the demonstration.
