Constant Current Source for Student Use in Voltammetric Titrations This note describes a simple constant current source suitable for student use in the usual laboratory exercises1 involving voltammetric titrations with a rotating platinum (RPE) or dual polarized microelectrodes. Recently Weingartner and Robertson2 have noted that a digital multimeter (DVM) operated as an ohmmeter can serve quite eonveniently.in this capacity if only a single value of the cell current is required in the particular application and high current values are permi~sible.~ However in our student exercises1the ability to vary the current over a wide range (&ZOO pA, applied electrode potential (0-2.5 V), and conveniently switch the electrode polarities led us to devise a simple circuit in which the DVM serves onlv as a measurine instrument. The circuit used (see f i b r e ) is based on a N-channel field effect transistor (FET) operated as a constant current source.' It is a simplified version of a precision circuit designed by Leach5 for a different application. In its essentials, i t involves the use of an operational amplifier connected as an inverting gain-of-one amplifier to sample the FET source voltage and apply a correction to the gate to compensate for current variations. For further details concerning the design, the paper by Leach5 may be consulted. The choice of semiconductor devices for use in this circuit has proven not to be critical. In the three units constructed. we have used tvoes 741.558. or 301 for ,$ the OD amo 1 ~ 2 1 0o r -l ~ 9 1 for 4 . (ICII. . .. MJF102 or 2N3819 forthe FET PJlL and~-~ the dwde tD1 I. All pnrr* w r e prorurcd on the "~urplu\"market. Nuune type or 9v comh~nnrionuf types rrsulrrd in a n impro\,ement :n the circuit perfurmanre 11, a degree tc, warrant recummrndntion hrrc. I n t r s u with thew unitb. tnu p w t r r d 1)). alkaline hatteries and the third hy a zcner-regolacd power s ~ p p l vall , ~hc,rrds h m term (2-5 min, ~tahilitytypically of the order of 1 p u t in 5Wl urth Imp, term smhil~tv\ I hrl and rrmahilits d half this value. Fnr these mu. the rrll was rrnlnwd by a fixed val& resistor. As shown in the figure, the 20K potentiometer in the inverting input circuit of ICl is specified as a ten-turn unit. Although not essential. this feature oermits mod resolution in the settine" of the ckll current. Of the two toe& switches shown. S- -t " permits the DVM to be used as an ammeter t o measure the cell current ("I")or as avoltmeter tomonitor the electrode potential difference YE).If R1 is a precision resistor (lK/l%), then the DVM reading is given directly in microamperes. The second toggle switch (52) is not needed if only dual polarized electrodes are to be used. Its sole function is to provide aeonvenient means ta reverse the electrode polarity in titrations using the RPE. Note that the DVM used must have a differential input capability, a common feature on most currently available units.
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Reilley, C. N., and Sawyer, D. T., "Experiments far Instrumental Methods," McGraw-Hill, Inc., New York, 1961, p.
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Weingartner, C. E., and Robertson, D.A,, J. CHEM. EDUC., 54,484 (1977). Lingane, J. J., "Electroanalytical Chemistry," Interscience, New York, 1958, p. 281. 'Cooper, W. D., "Solid-State Devices: Analysis and Applications," Prentiee-Hall Ca., New York, 1974, p. 165. Leach, M. F.,J. Sei. Inst., LO, 879 (1977).
Rutgers University New Brunswiek, N J 08903
878 1 Journal of Chemical Education
