It looks fairly straightforward to me assuming one side of the ph probe can be grounded, which I guess is the case since the probes normally use bnc connectors. You need 2 op amps (1 with low input current) and 4 close tolerance resistors. Also 2 capacitors, 2 diodes and a resistor to generate a negative supply for the op amp from a pwm pin. I'd post a schematic, but i'm sending this from a mobile. You'll need to calibrate it at one point, e.g. short the probe and calibrate to ph7.