Edwin E. Steinberg' and J. Nordmann
10s Angeles Valley College
Van Nuys, California
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Electrical Conductance Apparatus
The ac circuit commonly employed to introduce the concept of electrical conductance in liquids uses a light bulb indicator. Even when the device incorporates a meter to limit subjectively, it is both dangerous and inaccurate, if full line power is delivered to elect,rodes where resistance heating affects the sample.
3-wire power cable grounds the transformer frame and test circuit. The test circuit uses both meter and indicator lamp to show relative conductivity of electrolyte in the test cell. A shunt resistance of 47 ohms is used to limit lamp current to the desired maximum value. A series resistance of 10 ohms is used to limit total circuit current to its desired maximum, a value dependent on the internal resistance of the meter. Exclusive of the test cell, the combination of transformer secondary winding, meter resistance, series resistance, and parallel pairing of shunt resistance and lamp give a total of 75 ohms at full scale deflection of the meter. The ammeter plus the current limiting resistors and power supply constitute an ohmmeter.
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Figure 1. Circuit diagram. R1. 1 0 0 resi9tor. W; R1, 4 7 0 reristor, '/s W; M, C-100 milli~mrneteroc Triplen, or equivalent; P, moting 2contoct connector3 (phone jack, for examplel; L, Mozdo #A8 lamp, 2 v ot 6 0 ma; 5, witch; C, mnductonce cell containing platinum wire electroder; 1, lepdown transformer, 115 v primary, 6.3 v secondary ot 1 mmp. Color code: b, block; g.green; w, white.
The apparatus of Figure 1 obviates these disadvantages. Electrodes (not shown) are a pair of 24 gauge platinum wires fusion welded to copper leads and sealed in glass tubing. They are held at a fixed gap by t,ape or by a small %hole rubber stopper in a cell of convenient size. The exposed platinum must he totally submerged in liquid a t all times during measurements to assure constant active electrode area. Half the instrument's total cost of $25 is in the meter. One hundred fifteen v line power is applied to a stepdown (filament) transformer. Under the small load conditions, the operating potential is a safe 7.5 v. A 'Present address: Helicopter Air Lift of California, Inc., Los Angeles International Airport, P.O. Box 90188, Los Angela, Cd., 90009.
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8 12 16 mi. of 0.6 M Na OH a d d e d
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Figure 2. Titrotion of 0.1 M acetic acid with 0.6 M sodium hydroxide, or monitored by the apparatus.
If the electodes are short circuited, the current is limited to 100 ma maximum and full lamp brilliance. The lamp is a #48 Mazda (bayonet base). While this simple circuit was planned only for qualitative demonstrations, it substitutes semiquantitatively for a conductance bridge in monitoring, for example, Volume 43, Number 6,June 1966
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acid-base titrations. I n this application a variation in transformer voltage output becomes evident. The result is seen as curves instead of straight lines when current, taken as equivalent to conductance and corrected for dilution, is plotted against volume of titrant.
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Journal o f Chemical Education
Nevertheless, in favorable cases (Fig. 2), end point determinations from curve extrapolation intersections are comparable in reproducibility to other methods requiring similar operations. Readings are most reliable in the upper three-fourths of the meter's range.
