How the pH Meter Works
You can build a simple digital 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 (mine's a Pinpoint brand probe that I got from Amazon.com for about $40).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.




Schematic Diagram of the pH Meter
Here's the simple schematic diagram of the pH meter circuit. Two 9-volt batteries power a high-input-impedance operational amplifier, such as a TL082. The pH probe of the meter is connected to the non-inverting input. The output voltage (V out), which is directly proportional to pH, is read with a voltmeter.






View of the pH Meter Circuit
Here's a view of the entire pH meter circuit.The IC is mounted directly to a BNC panel connector.












Pictorial View of the pH Meter
This diagram shows the physical layout of the meter. A digital voltmeter is connected to ground and pins 6 and 7 of the  op amp, which are shorted together. In this circuit, pins 1, 2, and 3 of the IC are not used.

 

 

 

Close-Up View
This pH meter circuit is so small and so simple that no circuit board is required. In this prototype, the IC is soldered directly to the BNC panel connector.

 

 

 

 

 

 

Calibration and Use of the pH Meter
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  benchtop pH meter's display.

Graphing the Results
Here's a graph of pH and voltage from the calibration of the setup described here.

 

 

 

 

 

 

 

pH Circuit for Arduino
Finally, and even simpler, if you are looking for a way to connect a pH probe directly to Arduino without building your own circuitry, you can buy a complete pH development kit for Arduino with a pH probe, calibration solutions, and pH-Stamp circuit for about $100. The pH-Stamp, which you can also buy separately, is a complete pH monitoring system that enables you to accurately monitor pH without having to add any additional circuitry or components to your design. Communication with the pH-Stamp is done with 11 simple commands. It provides scientific-grade readings to any embedded system that has an RS232 connection interface (voltage swing 0-VCC, not +/- 12 volts).