An RC Filter for Automatic Differential Potentiometry M. Jaffar and S. Nooruliah Hussani Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan Ijaz-ur-Rehman Department of Physics, Quaid-i-Azam University, Islamabad. Pakistan There has been a growing trend in recent years to develop simple chemical instrumentation for the teaching of experimental analytical chemistry. Many simplified versions of sonhisticated instruments have a .~.~ e a r featuring ed . hiah .. accuracy and precision in addition t o the usual advantages of easy ooeration and low cost of electronic circui~ryinvolved. The f;eld of electrochemistry, in particular, has witnessed many such positive additions, especially in the area of potentiometry. This paper describes a new differential potentiometric technique, based on a simple RC phase inverter circuit that can he used in a variety of automated applications for specific experimental requirements in an analytical laboratory. The electrical components of the RC circuitry are inexpensive and are readily available in any average laboratory. The technique described warrants exact quantitative estimations for titrations of stronglweak acid and base comhinations and has the potential of application to other types of potentiometric estimations. The estimated average accuracy of the results ohtained lies within i1.5 %with a precision of about *I%.
cording of pH, was used with a conventional combination electrode. An Accudate 122 (Honeywell) DC differential amplifier was used at a fixed gain of 1X 50. A multispeed X-Y recorder, Sewogor 310, was used for recording the differential curves. The component specification details of the RC are given in Figure l(h). The complete apparatus for carrying out differential potentiometric titrations is shown in Figure 2. The reservoir is an inverted conical flask (250 ml) fused hetween the two parts of the stem of a graduated buret; the upper stem helps in recording the flow rate of the titrant. Normally, a flow rate of about 1%20 mllmin is found to be adequate. The differential automatic plot recorded by using this set-up appears in Figure l(c). The peakmaximum corresponds to 0 mV (pH = 7), and the volume of titrant used at the equivalence point can he calculated as a function of time to determine the desired quantitative concentrations of the unknown species. Other RC comhinations with short capacitance time have also been found suitable for obtaining differential curves.
Clrcult Design and Perlormance The circuit plan is shown in Figure 1. A Kent pH meter, Model 7020, which allows either manual or automatic re-
pH Meter
lOOkn
E
Differenlid Amplifier
Circuit
(cl Titront Volume-
Figure 1. Electronic circuit for automatic differential potentiometric titrations (a) Basic Circuit. (b) High-pass phase inverter (RC). (c)A typical differential autapiot.
88
Journal of Chemical Education
Figure 2. Autotitration set-up,
