Sfanley E. Wood and c. B. Anderson University of California Santa Barbara
A
Potentiometer for the General Chemistry Laboratory
recent article on construction of potentiometers1 prompts us to report the design of a durable low-cost potentiometer for the general chemistry course. These potentiometers were used during the last year by 500 students with no service problem. It was found that freshman students could understand the operation of the instrument since only Ohm's law is involved. The circuitry is shown in Figure 1. The standard cell is a 1.34 v mercury cell which has an accuracy of about +0.5%. A ten-turn helipot, R., linear to +0.25%, is used as the slide wire. Thus, although readings are accurate to only 0.5-1.0% because of the limits of the standard cell and 1% resistors in the voltage divider network, the error between readings on the helipot is about +0.25'%. The voltage divider network, consisting of the 680Cl and 2KCl resistors, provides a 01.0 v range so that the helipot can read directly in millivolts. For the 1.34 v range, the measured voltage is obtained simply by multiplying 1.34 v by the helipot dial reading divided by 1000. A six-pole, five-position rotary switch, S,, combines many switchmg functions into one knob; the five positions are off, calibrate, and measure for the 1.00 v range, and calibrate and meas ure for the 1.34 v range. A spring return switch, St, with positions for high sensitivity, off, and low sensitivity closes the circuit actuating the meter. The cost of the parts listed is $42. The assembled meter is shown in Figure 2. The students measured the voltage of a Daniel1 cell in which the concentrations of copper and zinc sulfates
' BATTINO, R., J. CEEM.EDUC., 42,211 (1965).
were both 0.1 M, obtaining avalue within 1% of 1.10 v. Upon ten-fold dilution of either half-cell, the students obtained voltages which were 0.03 + 0.005 v larger or smaller than the original reading, in agreement with the Nernst equation. I n part two of the assignment, the solubility product of copper hydroxide was determined using the cell: CulC~(OH)~10.1 M KOH /KC]/0.1 M CuSOIICu
Students obtained voltages in the range 0 . 3 4 . 5 v. Results were in the range of published values. I n part three, the formation constant of the copper tetrammine complex ion was determined in a similar way from the emf of the cell : Cu ICu(NH8)4+P, NHaJ/ C U + ~ ~ C ~
Values obtained were in the range, K ,
Figure 2.
=
10'1-1014.
Assembled potentiometer.
100-0-1 0 0 po Simpson 21/* in. round 6 pole, 5 position, rotary CRL 2020 double pole, 3 position, spring
Meter Switch, S, Switch, Sr Helipot, RX Helidial Potentiometer, R Rerimr Resistor Rerirtor Rerirtor Bottery Bottery Holder
~AL
-
.
I 0 0 il lineor carbon 1 0 Kn, w a n 10% 1 5 KG '12 w a n 1Oyo 680il wan 1% 2 ~ n , w a n 1% 1.5 v Eveready 7 3 5 Eveready E-3 Keystone 103
Y1
Figure 1.
Volume 42, Number 12, December 1965
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659
