Building the Simplest Possible pH Meter
You can build a simple pH meter with just an inexpensive operational amplifier ("op amp") IC (a TL082 Dual JFET-input IC, about $2.00), 2 batteries, a digital volt meter, and a pH probe.
Schematic Diagram
Here's a schematic diagram of the circuit.
 Two 9-volt batteries power a high-input-impedance operational amplifier, such as a TL082. The pH probe is connected to the non-inverting input. The output voltage (V out), which is directly proportional to pH, is read with a voltmeter.
How It Works
The TL082 (or any other operational amplifier with a high impedance input) works as a unity gain buffer between the high impedance of the pH probe and the digital multimeter.
View of the Circuit
Here's a view of the entire circuit.
 The TL082 is mounted directly to a BNC panel connector.
Pictorial View of the Circuit
Here's a view of the entire circuit.
 This diagram shows the physical layout of the circuit. The voltmeter is connected to ground and pins 6 and 7 of the TL082 op amp, which are shorted together. In this circuit, pins 1, 2, and 3 of the IC are not used.
 Close-Up View
This circuit is so small and so simple that no circuit board is required. The prototype is soldered directly to the BNC panel connector.
Calibration and Use
In theory, a pH probe produces about 59 millivolts (mV) per pH unit, and at pH 7 (neutral pH) the probe produces 0 volts. Acid pHs produce negative voltages. Basic pHs produce positive pHs. For example, the system shown here reads +7.6 mV with the inputs shorted (due to the input offset error of the op amp). With the probe in pH 7.01 calibration buffer, the voltage is +4.6 mV. With pH 4.00 calibration buffer, the output voltage is 168.8 mV. This gives a range of 168.8 - 4.6 = 164.2 mV for 3.01 pH units, or 54.55 mV per pH unit.
If, for example, I measure 100 mV, the pH is 100 mV / 54.55 mV = 1.83 pH units deviation from 7.01, so I subtract 1.83 from 7.01 for a pH of 5.18. Creating a graph or a programming a calculator to do the arithmetic makes the process of determining pH simple, if not quite as simple as reading it off a pH meter's display.
Graphing the results
Here's a graph of pH and voltage from the calibration of the setup described here.
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