If you are looking for a fast and easy solution : a reasonably cheap alternative that allows immediate interfacing between Arduino and pH probe is the pH/ORP Adapter - 1130_0B at Phidgets costs 30$ and has ideal inverted signal, 525mV offset and 4.75 gain. I have to admit: I tried the self-build option and failed.
This phidget has good gain which allows for more precise A/D conversions by the Arduino. Other interfaces (Vernier) only have 2.2 gain, resulting in larger margins of error.