A Simple Device for Conductivity Experiments Mohammad H. Ghatee Shiraz University, Shiraz, Iran 71454 Conductance measurements are among the most reliable techniques in physical and electroanalytical chemistry, but accurate and meaningful measurements of electrolytic conductance require attention to the design of electrodes, cells, and the measuring circuitry. The details of all experimental techniques of conduc- tance measurement are almost impossible to be considered in undergraduate courses, but the introduction of some basic ~ractical as~ects of electrode Drocesses and measur- ing clrcunry would strcngthcn the insight toward the sub- lect. We have madc devircs that allow students in ohvsiral chemistry laboratories to carry out an experiment& Set up to measure conductance of solutions. A stabilized power supply, a sinusoidal oscillator, a Wheatstone bridge, and a null detector are the main components of the set up (I). A conductivity device has been designed by Havrilla (2) that uses a CMOS 555 Timer to produce approximately 1000 Hz of pulsed current. The pulse is applied to two elec- trodes in a solution, and the current through the solution is assumed to be a measure of conductance. We recognized several noticeable points in this method. First, the 555 Timer pulls its output through an active pull-up resistor (e.g., transistor in series with a diode) that could, in the cutoff region of the diode, keep the output impedance cou- stant. When a load on the output pulls the transistor iuto cutoff, the output impedance adjusts itself so much that the timer output can retain a constant voltage. This cou- stancy in voltage is good enough when one is concerned only with high and low logic in driving TTL circuits. We checked on the circuit in reference 2, and found, in a sim- ilar experiment with distilled water, that the output volt- age of the 555 Timer dropped by 8.7%. If the external di- vider (2) drives the active pull-up resistor iuto the cutoff region, one still has to be concerned with the change in current due to the voltage drop. Second, because the duty cycle1 (3) of the Timer (as it could be designed) is less than 0.5, the pulses subject the charge carriers to an uneven field in each cycle. Third, the timer output is coupled to an external differentiator circuit to make a bipolar current pulse. The output waveform of the differentiator does not vary steadily in each cycle. On the other hand, the charg- ing and discharging time constants of the capacitor (eqs 1 and 2, respectively) are different: where, R, is the resistance of the 10 pF capacitor and R, is the resistance of the solution. The resistances of the volt- age divider are 100 and 10. The value ofRc is estimated by extrapolation to be about 50 R (4). For a typical solution withR, about 400 R, z&a:dr is about 1.5. This makes the waveform even more unsymmetrical in a cycle. Thus, some i n a m a c i e s occur with the pulsed current method. A simple device for conductivity experiments that 'The duty cycle of the 555 Timer is equal to the ratio of the time period of the successive high and low voltages of the output wave- form of the timer. Practically, duly cycle can be calculated from the relationship Rd(Ra+ 2Rb), where 4 and R, are the eaernal adjusta- ble resistors (6). Figure 1. Schematic circuit diagram ofthe conductivity device, operates at a highly constant voltage is that of an oscillator with a sinusoidal bipolar waveform. Figure 1 shows the schematic circuit diagram of the si- nusoidal oscillator that uses +9 and -9 V DC to run a 741 Operational Amplifier. The positive feedback loops main- tain the frequency of the oscillator at approximately 1000 Hz (5). At the present time, the electrodes are standard conductivity electrodes (coated with platinum black and a cell constant of 0.75 cm-'1. A digital multimeter (DMM), the Hioki 3200, is used in series with one of the electrodes to measure the current, and a second meter, the Uni Volt DT-830, to measure the voltage across the electrodes (see Fig. 2 1. We designed two different circuits. In one, we grounded the output leadofthe 741 through one diode. In the second, we grounded the output through two diodes in series. Therefore, the voltage at point VOmt would be about the nominal cutoffvoltage of a diode for the first arrangement (e.g., 0.6 V) and about twice as much for the second ar- rangement. This method allows us to see the effect of the (from the oscillator) Hioki JGq Uni Volt Conudctivity Electrodes Figure 2. Experimental set up for measurement of the current and voltage. 944 Journal of Chemical Education