Inexpensive Scale Expansion for Ion Selective Electrodes We have found that the scale of an ordinary pH meter can be expanded by connecting a l-mV recorder or a 1amp galvanometer, or by installation of an additional amplifier. The latter is the more convenient and inexpensive alternative, costing approximately $20. A circuit is shown (Fig. 1) with resistance values suitable for use with the Tacussel 2G8N meter. When operated in the expanded mode the meter M is bypassed by the rotary switch SI (4 pole, 3 position, 2 wafer) and the potential across the rest of the meter circuit (R)is fed to an operational amplifier (Philbrick 1026) which then drives the meter. SI also allows reversal of polarity a t the meter for the measurement of negative potentials. The 2K potentiometer is adjusted to give ten-fold expansion or a readability of 0.2 mV. The amplifier requires a power supply of between * 9 and f18V, obtainable from a dry-cell battery, a mains-operated supply (e.g. Philbriek 2210) or from the pH meter itself. Although offset potential can he obtained using the "pH adjust" control we connected a mercurv cell a t the reference electrode inout as shown in Figure 2. The circuit was calibrated to give f 700 mVoffset in 100-mV increments OUT
Figure 1. Amplifier circuit for scale expansion.
Figure 2. Circuit tor offset potential.
This unit and a similar modification of the Metrohm E350B pH meter performed satisfactorily with silver, iodide and calcium electrodes (Orion) and with simulated source resistances of up to 1000 megohms. Accuracy is limited only by the stability of the existing circuit. The modified Tacussel meter showed a drift of 0.2% per 24 hr and 0.1% per "C.
P. G . Parsons University of Queenslsnd St. Lucia, Queensland, 4067 Australia
Volume 50. Number 3. March 1973
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