how to offseting voltage for ph electrode?
A glass pH sensor has an output of (nominally) zero volts at a pH of 7, and a negative coefficient. So without an offset a single-supply circuit without an offset as shown would be unable to measure any pH greater then 7, since it cannot provide a negative output

An input offset of 512 mV gives you about a 500 mV margin for neutral solutions, and since the output sensitivity of a glass sensor is about -60 mV/pH, the circuit shown can read the pH of solutions up to about a pH of 16. 

The reference amp is not strictly necessary, but buffering references is always a good idea, and should be the norm for high-precision circuits. What it does not do is provide useful temperature drift compensation. The LMP7721 input offset voltage only drifts about 1.5 uV/deg C, and the temperature characteristics of a pH cell change much more than this with temperature.