Measuring pH probe potential and interfacing probe with uC

[Physics_UG]

I am building an automatic acid-base titrator for my senior project. I am using a Freescale s12ub microcontroller. For this project, I will need to interface a pH probe with the CMOS input on a microcontroller. The potential across the probe is supposedly proportional to pH. (-0.06 volt/pH in basic solutions, +0.06 volt/pH in acid solutions). The probe voltage will range from around -0.7 volts to 0.4 volts between 0 pH and 14 pH. I figure I would need to amplify and level shift the signal. I assume the circuit would need a high input impedence since the pHprobe typically has a very high input impedence (~1000 megaohm). Does anyone know of any application notes for designing a pH probe circuit for interfacing with a CMOS ADC input?

Also, I am trying to measure the potential across the probe using a typical voltmeter. The pH probe has a BNC connector. I tried connecting the probe directly to the volt meter but this wasn't producing the correct outputs for our various buffers solutions. I figure I'll need a buffer circuit in between the probe and the volt meter to measure the probe potential.

Anyone have any tips for the pH probe circuit? Also, can anyone provide assistance with measuring the probe potential?

 

[Physics_UG]

oh, this circuit does the job nicely.

http://www.emesys.com/OL2ph.htm

 

[ravenprp]

First of all, congratulations on taking on such a complex and interesting project for your senior project! Measuring pH probe potential and interfacing it with a microcontroller can definitely be a challenging task, but with the right approach and resources, you can definitely achieve your goal.

To start off, it is important to have a good understanding of the principles behind pH measurement. As you mentioned, the potential across the pH probe is indeed proportional to the pH of the solution being measured. This is due to the fact that the probe consists of a glass membrane that is sensitive to the hydrogen ion concentration in the solution. When the solution is acidic, the hydrogen ion concentration is high, causing the probe to generate a positive potential. On the other hand, in basic solutions, the hydrogen ion concentration is low, resulting in a negative potential.

Based on this principle, it is important to have a circuit that can accurately measure and amplify the small potential generated by the probe. A good starting point would be to look for application notes or reference designs from the manufacturer of your pH probe or microcontroller. These resources can provide valuable insights and guidelines for designing your pH probe circuit.

In terms of measuring the potential across the probe using a voltmeter, it is important to keep in mind that the probe's potential is very small and can easily be affected by external factors such as noise and interference. Therefore, using a buffer circuit is a good idea to ensure accurate measurements. The buffer circuit can also help with amplifying and level shifting the probe's potential to match the input range of your voltmeter.

Overall, designing a pH probe circuit for interfacing with a microcontroller can be a complex task, but with proper research and guidance, you can definitely achieve success. I would recommend reaching out to your project advisor or a mentor with experience in this area for additional tips and guidance. Best of luck with your project!