A Simple Solid-state pH Meter J. Kenneth Jacobsen, Milton Roy Co., St. Petersburg, Fla.
HE RECENTLY developed insulated Tgate field effect transistor has made possible the use of a transistor directly in a pH meter. This type of transistor has a continuous insulating layer of silica between the input lead and the rest of the transistor, resulting in an input resistance of approximately loL4 ohnis (1, 2 ) . This is far in excess of the input impedance required for the use of glass pH electrodes. The circuit developed, as shown on Figure 1, is a Kheatstone bridge with the transistor acting as one arm of the bridge. A thermistor and resistor combination are placed in series with the transistor to balance out most of the transistor’s temperature sensitivity. This temperature sensitivity results in an amplifier drift of 0.05 pH per O C.; the thermistor reduces this by a factor of about 10. The 5000-ohm pot varies the balance
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of the bridge so that the zero can be set anywhere. The particular meter used is a zero-center 50-wa. meter, so that the electrical zero reads around pH 7 on the meter. If desired, a standard meter with left hand zero can be used; in this case, the bridge is unbalanced with the pot until the correct meter reading is obtained. The circuit has more output than required and, if desired, the 50-kohm. pot can be adjusted so that full scale is 2 pH units. Because the zero pot has a wide range of adjustment, any two pH segments can be expanded. The linearity of the circuit is better than 0.02 pH unit over the range of pH 2-14. Below pH 2 , there is increased nonlinearity; a t pH 0, a correction of -0.2 pH unit must be added to the meter scale. As an added feature, a 100-pf. polystyrene capacitor is connected across
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Circuit diagram of solid-state pH meter
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the input, reducing a.c. pick-up and banging of the meter when the electrodes are removed from the solution. It also reduces the noise level and cuts down any high voltage input spikes that might damage the transistor. During the assembly of this unit, precaution should be taken t o prevent static electricity or any other source of high voltage from coming in contact with the transistor gate lead, as it might destroy the transistor. Care should also be taken t o use only Teflon or a similar high resistance niaterial as insulation in the input connections. The pH meter was assembled in such a may that 11-hen the glass electrode fitting is renioved from the socket, the input is shorted. This affords a quick check of zero drift because once the pH meter is standardized with a particular electrode and the shorted input reading noted, the zero can be checked simply by pulling out the electrode fitting. With the circuit maintained a t a constant temperature and the input shunted by a 10L2-ohni resistor, the input noise is approximately 0.001 pH unit with 0.003 pH unit drift over a 2-hour period. The noise and drift remain a t approximately the same level when a shunting resistor of a lower resistance is used. If greater linearity or voltage span is desired, the circuit can be used as a null detector with a bucking voltage applied from a precision potentiometer. The same thing can be done automatically by connecting the circuit as part of the feedback loop of an operational amplifier. The amplifier will then automatically vary its output as required to exactly balance out the input. The device is easily made and is ideal for use in schools or laboratories where an inexpensive pH meter of moderate accuracy is desired. LITERATURE CITED
(1) nlotorola Semiconductor Data Man-
ual, 1st Edition Xotorola Inc., 1965. (2).Seviq: L. J. Jr., “Field Effect Transistors, McGraw Hill, New York, 1965. Presented at the 1966 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy.
VOL 38, NO. 13, DECEMBER 1966
1975
