An Integrated Circuit Power Source for Constant-Current Coulometry
G. M. Muha Rutgers University NewBrwsw~ck.New Jersey 08903
Laboratory exercises in constant-current coulometry are usuallv included in some ohase of the undereraduate chemistry curriculum to illustrate certain of the principles of electrolvtic methods of analvsis . (.I ) . E x a m ~ l e sof suitable tmnsiitorized instrumentaticm forstudent use have been given nffurded hv the use of inh\, Stuck (2,. ~. The circuit simnlicitv . tegrated circuit voltage regulators prompted our effort to incorporate such devices in circuitry for instructional use. The resulting design is the subject of this note. The circuit. based on the readilv available and inexnensive IC723 voltage regulator (31,' is shown in Figure 1. his particular IC was chosen for the present application because it proved to have the required thermal stability without the use of heat sinks or auxiliary transistor circuitry and because it incorporates as an integral part of the package, short-circuit protection against accidental overload as well as a Zener;egulated reference voltage suitable for use to set the current output of the regulator. The circuit operates as a conventional voltage-feedback amplifier connected as a constant-current source (4). The control voltage is derived from the divider network consisting of Rl and R2 in series with the electrolytic cell (cf. Fig. 1). The actual values of these resistors are determined by the internal resistance of the electrolytic cell and are chosen to bring the desired current range within the control limits of the 5K potentiometer connected across the internal Zener reference voltage provided a t pin 6 of the IC. For the cells used in our student experiments ( 5 ) ,the values R1 = R2 = 100 ohms permit a current setting in the range 6-60 mA which is quite adequate for the intended purpose. On-off control of the coulometer current is conveniently obtained by means of the switch shown in Figure 1.The apnlication is rather novel. for the oneration is such that the regulator is "tricked" into sensing an overload condition when the switch is closed causing the current output to cease. If this same switch is made double-pole (rather than single-pole as shown in Fig. I),the extra set of contacts can he used to control
-
'This particular integrated circuit is available in two different packages: TO-5and TO-116("DIP"). The pin arrangements in these two packages are different. All pin numbers shown in the figures in this note refer to the TO-116package.
D
C
-
~
~
5K
Q
100
100
7 13
IK
IK
R2 100
OFF
F y e 1 CKCUIIW a r n 1- me constlntc.nsm c ~ o m s l e rRes stlnce V ~ L K n ohms, capacmnce in p cofaraan The ntegrated clrcu t san IC723 The pmn connecllons Shawn are for the TO-1 16 ID P p a c r q e The ac n p 9~ +20 ~ V derived from a full-wave rectifier (notshown).
the operation of.an electro-mechanical timer for the required time interval measurement. In our application rather than use nlechanical timers, we have nvailahle for srudent usr electronic dirital rimers using decade counting units. We give the detail'of the extra circuitry required because with the current low prices on the surplus market for the decade counters (-$9 per decade), others may he interested in this approach. Basically the additional circuitry required is that needed to generate timing pulses and to gate these pulses into the electronic counter only when the coulometer is on. The details are given in Figure 2. A train of pulses at a 0.1-s interval generated from the 60 Hz power line by IC4 are fed to a synchronous gate (6) composed of IC2 and IC3. This gate serves the dual function of nassine the timine sienals to the counter when appropriate a& turnyng the cou!o&ter current on and off hv means of the connection to the TIS58 field-effect transistor (see below). The reason for the choice of this svnchronization scheme is that it minimizes the contribution to the titration error due to the ~ossibilitvthat the first and last timine.. ~. u l s e sin a non-synchrmized circuit ma\. not occur exactly at the instant thnt the coulomcrer current is turned on ur ofit:). Thc maximum absolute error in the present circuit from this source is
Volume 53,Number 7, July 1976
/ 465
endpoint, and through the use of appropriate circuitry (not shown in the fimres) to automatically turn off the coulometer. Calihration of the coulometer current is simply effected by connecting a precision resistor of known value in place of the cell and p&&iometrically measuring the resulting voltage drop (2). Tests conducted over a period of several months using a recordinen "zalvanometer to monitor the current for ~~- n~~ sessions of two to four hours in length, showed the current to hold constant within f0.02% within a session and reoroducible within +0.07% between sessions. In the usual student experiments, results obtained over the course of a year for a particular titration (5) indicate that a orecision of a~oroximatelv .. " 1Dart . in 250 is tvoical. .. . the .orincipal error not being due to the instrument, hut rather in matchine" the color at the endpoint with the oretitration color. More details on the construction and operation of these circuits are available from the author upon request. ~~~
Figure 2. Circuit diagram for the synchronized gate. Resistance M l W S in ohms.
The wiring of ICI is the same as shown in Figure 1 excspt for Me resistor in place of Me switch at pin 13 (see texi). me remaining integrated circuits are ICZ = 7402, IC3 = 7473. and IC4 = 7490.
capacitance in microfarads.
constant and is given bv the product of the time interval (0.1 S) and the coulo&eter current; of more importance is the fact that the error is independent of the number of times that the coulometer is turned on and off during the course of the titration (7). The on-off coulometer switch (Fig. 1)is replaced in the more elaborate version shown in Figure 2 by a field-effect transistor (TIS58) serving as an analog switch (8).We have found this electrical control feature quitcuseful in that it permits the use of a separate set of indicator electrodes (2, 9) to detect the
486 / Journal of Chemical Education
Literature Cited
p. 1723. (41 Malmstadt, H. V.. and Enke, C. C.. "Electronics for Scienfisis.I1 W. A. Benjamin. Inc, New York. 1963, p. 368 et. seq. 151 Referenee 11). p. 99. (6; Lanearter,D. C.. "RTL Cookhwk." Bobbp~MemiliCo.. lnc, Indianspolis. 1970, p. 123 Malmstadf.H. V.,and Enke, C.G.."Digitsl Elsefronim for Seientiair,"W. A. Benjamin.lnc.,New York. 1970.p.273. (7) Lin~ane. J. J.,"EleetroanalyticalChemistry,"Znd Ed.. Wiley-lnierncience. New York. 1958.p. 512. (8) 0'Haver.T. C.. Chem Instrum.. 3. 1~1971i. 19) Jakrs, F..and Clem R. C.. M P I A p p I i c o t i o n Note*, 4.9 (1971).
