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