Constant Current Source for Coulometric Titrations JAMES J. LINGANE Department
o f Chemistry, Harvard University, Cambridge 38, Mass.
tentiometer the voltage drop across the precision resistance R-4 and applying Ohm’s latv. Decade resistance boxes may be used
Po~F/V;r/OMP;rER
ANALYTICAL CHEMISTRY
1022 in conjunction with the Beckman autotitrator (3) an alternating current ground through a capacitor is necessary to eliminate perturbation of the Beckman instrument by stray alternating current from the Brown amplifier. The voltage dropping resistance R-2 consists of a bank of five 6000-0hm~ high wattage, resistors connected t o a terminal panel in such a way that various series and series-parallel combinations can be selected by plug-in connectors t o provide a range from 1200 t o 30,000 ohms. Control resistance R-3 is a 200-ohm, 10-turn Helipot potentiometer, connected directly t o the motor by a short length of thickwalled rubber tubing. This serves as a simple friction drive that slips if the Helipot reaches the end of its travel, so that limit switches are not required. When the controller is turned on, the Helipot is adjusted manually so that it operates near the middle of its range. The motor and Helipot operate a t 27 r.p.m. which provides amply rapid corrective action. Optimum control action results when the voltage drop across R-3 is small compared t o the total voltage output of the power supply. 4 value of R-3 of 200 ohms is suitable foi currents ranging from about 5 to 100 ma. when the pori-er supply output is between about 100 and 300 volts. K h e n the controller is turned on the power supply must always be connected to a load, because on open circuit resistor R-3 would be driven t o its lower limit. I n actual use in coulometric titrations a dummy resistor is provided whose resistance is adjusted to match the impedance (resistance plus back e.m.f.) of the cell. By means of a single pole-double throw switch the current is directed either to the cell or the dummy resistor.
With the automatic coulometric titrator previously described (3) this switching is done automatically by a relay which also controls the timing clock, By placing a switch in one of the motor supply leads the motor and R-3 can be inactivated, but the amplifier can be left on and warmed up, during periods when it is desired to open the circuit. The switch S in series with the biasing battery should be opened when the power supply circuit is opened, as othern-ise the small current drain from it detracts from the constancy of its voltage. Because of the small time lag associated with the mechanical linkage in the servo loop, this type of controller does not have the instantaneous response of all electronic devices. However, the fact that the current tends to change only relatively slowly renders instantaneous response unnecessary. The speed of response of the present instrument is more than ample in actual practice. L I T E R A T U R E CITED
Carson, W.N., Jr., ANAL.CHEY.,22, 1565 (1950); 25,226 (1953). DeFord, D. D., Johns, C. J., and Pitts, J. Y., Ibid.,23, 938, 941 (1951). Lingane, J. J., Ibid.,26, 622 (1954). Lingane, J. J., “Electroanalytical Chemistry,” pp. 377 et seq., Sew York, Interscience Publishers, 1953. Neier, D. J., Meyers, R. J., and Sn-ift, E. H., J . Am. Chem. SOC, 71, 2340 (1949). Reilley, C. K . , Cooke, W. D., and Furman, N. H., kvra~.CHEM., 23, 1030 (1951). Kise, E. K . , Gilles, P. W., and Reynolds, C. A , , Jr., Ibid.,25, 1344 (1953). RECEIIED for review J a n u a r y 27, 1954. Accepted March 25
1954
Precision Determination of l o w Concentrations of Carbon in Metals LEONARD P. PEPKOWITZ and WILLIAM D. MOAK Knolls Atomic Power Laboratory, General Electric Co., Schenectady,
The determination of low concentrations of carbon in is increasing in importance with the metals (
