Precise Wide-Range Direct-Reading Electronic Integrator Edward N. Wise, Department of Chemistry, University of Arizona, Tucson, Ariz. PRECISE
wide-range direct-reading
A integrator may be assembled from
a d.c. potential to an a s . frequency converter and an electronic counter. This integrator is useful in controlledpotential coulometry and in gas chromatography. In the block diagram shown in Figure, 1, the d.c. signal to be integrated may be the voltage drop across a resistor in series with a current to be integrated. This voltage is fed to the converter, where an a.c. signal is generated the frequency of which is directly linearly proportional to the magnitude of the d.c. signal. At a typical range-setting of the converter, its output will vary from 0 to 10,000 cycles per second when its input varies from 0 to 1.000 volt d.c. The cycles of the a.c. frequency output of the converter are then counted by the electronic counter. The total number of a.c. cycles counted is directly linearly proportional to the quantity of electricity which has passed through the series resistor in the original circuit.
A selection of the values of the series resistor and of the converter range may be made to provide direct-reading of the counter in coulomb units. For example, when a series resistor of 100.00 ohms is used with a converter range of 0 to 10,000 cycles per second for a 0 to 1.000-volt input, a current of up to 10 ma. may be integrated with the counter reading directly in microcoulombs. Because in controlled-potential coulometric titrations a current of 96.5 ma. will produce one microequivalent of electrochemical change per second, the use of a 10.363-ohm series resistor in the electrolysis circuit with the same converter range as above will result in each count of the electronic counter representing 100 picoequivalents of electrochemical reaction. We have used a Hewlett-Packard Dymec Model DY-2210 converter and a Nuclear-Chicago Model 192-B scaler to integrate over periods of from a few seconds to 30 minutes with a precision of from *0.02% to 3=0.05%. Other brands of converters are available with
D.C. SIGNAL TO BE IPSTEGRK1"ED
ELECTRONIC COUNTER
Figure 1. integrator
Block
diagram
of
more or less precision, depending on price, and any electronic or electronicmechanical counter having a counting speed of a t least 10,000 counts per second may be used. This instrument was developed in work supported by the U. S. Atomic Energy Commission under Contract AT-(ll-1)-553.
High Speed Coulometer Based on a Voltage-to-Frequency Converter Allen J. Bard and Emanuel Solon, 'Department of chemistry, The University of Texas, Austin, Tex.
used in controlled potential coulometry should be accurate, linear, capable of following rapid changes in current, and easily constructed. Ideally it should indicate directly coulombs or equivalents and should require only a small voltage input. The coulometers most frequently used (4) generally fail in one or more of these requirements. Although chemical coulometers, such as gravimetric, titraACOULOMETER
COUNTER
tion, and gas coulometers, are inexpenBive and easily used, they are not useful for the coulometric determination of very small amounts of material and they require many manipulations and calculations before results are available. Electromechanical coulometers, such as those based on the ball and disk integrator, are incapable of following very rapid changes in current, while direct current integrating motor types are nonlinear and require input voltages of as much as 24 volts. Integrators based upon a high quality capacitor and an operational amplifier (2) meet most requirements. Kramer and Fischer (3) have described a coulometer based on a simple relaxation oscillator-counter combination. The accuracy of this instrument was only within 1% and its linearity was not checked over a very wide current range. We have constructed a highly accurate and sensitive coulometer based on an integrating amplifier-type voltage-to-frequency converter which may be assembled readily from commercially available components. The coulometer presents the number of coulombs in digital form on a counter
* VOLTAGE TO FREQUENCY CONVERTER
CURRENT
INPUT RESISTOR
1. Figure coulometer
Block
diagram
of
and may be used in rapid electroohemic d determinations where electrolyses are completed in about 60 seconds. EXPERIMENTAL
The assembly of the coulometer is shown in Figure 1. Its operation is based on the ability of a converter to produce output pulses of a frequency proportional to the voltage applied to its input. When this output frequency is applied to a counter, the accumulated number of counts is proportional to the voltage-time integral. For use as a
Table 1. EfFect of Variation of Input Current at Constant Input Resistance on Calibration Factor of Coulometer
(Input resistor was 1O.OOO =t0.005 ohm) Current, Calibration Factor, Ma. Countsflolbsec. 70.872 67.306 6.6794 0.7139 0.08501
10,015 10,011 10,009 10,000 10,104
VOL. 34, NO. 9, AUGUST 1962
11 81
