Hi,
Some suggestions from my side:
1) In normal industrial standard to connect sensor module to plc it is recommended to use 1V-5V DC or 4-20mA, and the distance between sensor and PLC is long then its always best to use 4-20mA signal
2) I dont about the power supply at your end but in industry I think 24V DC power is available on Power line
So if you use the opamp which you have shown in schematic you will have to generate two power supplies +5V and -5V but if you use the above signals (1-5VDC, 4-20mA) then this can be done by a single positive power supply itself
3) As per the schematic you can check the below link
it consists of a 4-20mA transmiiter ckt on page 28 and they have given quite a good explaination (only read once, I have not gone into details yet)
if you have problem understanding it than you can ask me for more details
https://www.intersil.com/data/Xicor/an177.pdf
in above circuit you will only need is a circuit to convert you PH output into 0-2.5V voltage
which can be done by a simple addres/amplifier circuit (sorry I dont have time to draw it now, you try finding it on net if you are not able to find then I will try to make one and send you
Found one more link
**broken link removed**
Best of luck