Electromechanical Integrator for Coulometric Analysis JAMES J. LINGANE AND STANLEY L. JONES Harvard Uniuersity, Cambridge 38, Mass.
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The disk is rotated st 45 r.p.m. by a Type KWC 2505 HoltzerCubot synchronous motor. The integrating unit includes u. swine-loaded screw adiustment which enables the compressioii
OULOMETRIC analysis based on controlled potentia1,elcctrolysis requires the scourate measurement of the quantity of electricity passed through the electrolysis cell (8). Any of the elissical types of chemical coulometers can be used for this purpose. However, they require considerable manipulation and it is very desirable to ha% a. more convenient, direct-reading device to perform this function of integrating the current-time curve, In controlled potential coulometric analysis the current usually decreases more or less exponentially with time from a relatively large value (which may be anywhere between a few tenths and several amperes) to a final value which is practically zero. The elapsed time may vary from about 10 minutes to an hour or more. These characteristics require that the integrsting instrument function accurately over the entire range of current values from zero up. The design of ah instrument which will fulfill this requirement, will have the necessary precision and accuracy of the order of 1% or better, and will not be so complicated and temperamental that its use is more involved than the chemical coulometer i t is intended to replace, is not sc simple as it might seem a t first thought. The instrument described in this paper is one solution to this problem.
nected by two s i u r g& t o the cylinder, the gear ratio being 1 to 1. Because this counter counts each tenth of a revolution, the maximal counting rat.e is 900 counts per minute for full scalt. recorder deflection. To minimize error and dead zone caused by lost motion in the gears and m e w which positions the balls, a strip of spring bronze is mounted to maintain constant tension in one direction against the sliding carriage in which the balls are mounted. To measure and integrate current a precision resistor (1 to 10 ohms depending on the current range) is placed in series with the electrolysis cell and the recording pokntiometer is connected aerqss this resistor to record the iR drop. The integrator is calibrabed with a known constant current ahieh is evaluated by measuring the iR drop across the precisian resistor with a potentiometer. The counter reading over a measured time interval with the known current is observed, These data are combined to eompute the calibration factor expressed as coulombs per count. A voltage-divider arrangement ( 1 ) can be used conveniently to change the range of the integrator over very wide limits to meas-
the integrator to be set t o correspond t o m y desired position of the recorder pen. The zero of the recorder and integrator oan thus be set to an intermediate scale position and the instrument used to inteerate curves in cmes where the measured auantitv changes signhuring the course of an experiment. ~~~~~
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CONST. CHART
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SPEED DRIVE.
MOTOR
Figure 1. Ball and Disk Transmission
A s indicated schematically in Figure 1, the heart of the device is B precision ball and disk transmission. The disk is rotated a t precisely constant speed and this motion is transmitted to a cylinder by two steel balls. The speed of the cylinder, and hence the counting rate of the revolution counter driven by it, are diiectly proportional to the position of the balls dong the radius of the disk. Because the balls me positioned by the same mechanism that drives the pen of the recording potentiometer, the counting rate a t any instant is directly proportional to the reading of the recording potentiometer, and the total counts which accumulate are directly proportional to the area under the recorded curve. B y amrouriate selection of the mechanical characteristics and the
