Circuit Project Electronic

Here’s a schematic diagram for the simplest possible pH meter. 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.
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. [...]
Source: http://testmeasurementcircuit.blogspot.com/2008/07/
simplest-possible-ph-meter.html