Potentiometric methods involve the measurement

a) a potential difference between the indicator and reference electrode under non equilibrium conditions.
b) a potential difference between the indicator and reference electrode under equilibrium consitions.
c) the current at the indicator electrode under non equilibrium conditions
d) the current at the indicator electrode under equilibrium conditions

Option b) a potential difference between the indicator and reference electrode under equilibrium conditions.

Explanation:

Potentiometric method is often used  to find the concentration of a solute in solution.  

In potentiometric measurements, the potential between 2 electrodes is measured employing a high electric resistance meter.  

 Use of a high electric resistance meter in vital, because it ensures that current flow is negligible. Since there's no flowing current, there will be no  chemical reactions, therefore the system is in equilibrium.  

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