Any microcontroller should work. The real cost of a pH monitoring setup is in the probe. Most pH probes are made of very thin glass. Despite their craftsmenship they are nothing more than electrodes. Inside the probe is a salt solution and a submerged electrode. The outside like I said was very thin glass and when it is submerged in a solution a voltage potential is created between the solutions inside and outside the probe.
Theoretically you should be able to buy any replacement probe, hook it up to the arduino and measure the voltage across the electrode and then using the Nernst equation
be able to calculate the pH of the solution. If you have a thermometer in the same solution you can input that into the calculation rather than always measuring at the same temperature.
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