How to calculate redox potential of enzyme

Respected Sir/mam

I want to know the redox potential of enzyme. Purified enzyme is dissolved in citrate phosphate buffer pH 4.0. I have query :

1) Is their any standardized soltion for millivolt calibration??

2) How can i exactly know the redox potential of enzyme?? Should i measure the pH meter with C.P. buffer pH 4.0 for nullifying its value form enzyme containing c.p. buffere pH 4.0.

With Regards

Amit Kumar