Unfortunately, the potential of a chemical half-cell cannot be measured directly but requires to be used with a reference cell whose potential is known and remains sensibly constant.
There are certain criteria required for a satisfactory reference electrode; the chemical system must be reversible and the output must be accurately described by the Nernst equation. It’s potential should remain constant over significant period of time and should permit some current to be taken from the cell without altering its chemical characteristics; finally the electrode should exhibit no thermal hysterisis. Unfortunately, there are relatively few electrodes that meet the rather stringent requirements for a reference electrode.