In a 3 electrode system, which electrochemical reaction could be taken as standard, so as to know whether the CH instrument used giving proper values or not.
Ferricyanide/ferrocyanide is used as standard electrolytic solution and glassy/graphitic carbon electrode can be used as a working elcetrode. For GCD, you may check the instrument by using any rechargeable battery of 1.2 V and below than 600mAh (eveready or Duracell).