A Simple Conductivity Bridge for Student Use This note describes a simple, self-contained unit suitable for student use for conductometrie titrations. It was intended to replace an electron-ray "tuning-eye" apparatus1 widely used for this purpose for many years. Although it operates on the same principle as the older instrument, namely on the basis of a comparison methad,* it incorporates several improved features and is constructed using modern integrated circuits. Since the theory of the measurement is well described in the I l t e r a t ~ r e , ' only , ~ a brief description of the new circuit features is given below. * 5
K-IOT
lCl.558
The circuit uses a pseudo-bridge,2an unbalance of which is sensed by one-half ofa dual op-amp connected as a gain-of-one differential amplifier. The unbalance is amplified by the second op-amp which drives an N-channel FET used as a phase detector.Vhe filtered output of the detector is registered on a zero-center meter, the sensitivity of which is determined by the scale resistor R2 (=47K ohms for a 500-0-500microamp meter). Manual adjustment of the op-amp feedback resistor (5K-l0T) serves to null the meter to detect the balance point. The reference phase for the detector is taken from the sameside of the power transformer as the bridge excitation. This providesan important feature of the circuit in that the reactive component of the bridge unbalance due to the cell capacity is in quadrature with the excitation and hence not detected to first order.Vhe effect is a marked improvement in the ease and precision in locating the null-point. The cell is placed in the non-inverting input side of the differential amplifier so that the readout is directly in conductance rather than resistance units.2 Also and of more importance, in this configuration the o p ~ s m pis not requircd to supply cell current; thus wide changes in the conductance of the solution during the course of the titration can he accommodated. The conductance readout is taken directly from the dial of the 5K ohm ten-turn potentiometer after multiplication by an appropriate scale factor determined by R1. This and the remaining features of the circuit are identical with the electron-ray version and the original reference1 should he consulted for details. Aside from the cost of the meter and cabinet, the unit can he duplicated for under fifteen dollars. None of the components specified are critical. The transistor, diodes, and IC op-amp shown in the figure were chosen solely because they are inexpensive and readily available on the "surplus" market. The 558 op-amp is a dual unit, two amplifiers in a single package; other types can be substituted if proper attention is paid to possible changes in pin connections. In the schematic, capacitors are specified in microfarads, resistors in ohms. With an inexpensive imported FM-tuning meter (Lafayette Radio), we found that a conductance change of 0.5ppt could beeasily detected. Five of theseunits have been constructed, and over the period of three years, have given excellent and trouble-free service. 'Reilley, C. N.. and Sawyer, D. T., " ~ x p e ~ i m e n tfor a Instrumental Methods," McGraw-Hill, Inc. New York, 1961, p. 361. ZEwing, G. W., J. CHEM. EDUC., 52, A239, A469 (1975). W'Haver,T. C., Chem. Instrum., 3,1(1971). 4Schmidt, K.,Reu. Sci. Inst., 37,671 (1966). Rutgers University New Brunswiek, New Jersey 08903
G. M. Muha
Volume 54. Number 11, November 1977 1 677
