It is a difficult question because of the position of your counter electrode.  Normally one would try to place the counter electrode in a symmetrically oriented position in opposition to the working electrode, i.e., facing the working electrode and not along one side.  Clearly, there will be an unsymmetrical electrical field gradient across the working electrode in your current configuration.  At best, your proposed positioning of the Lugging capillary will provide you with a reproducible reference, but not indicative of the potential at different locations across the WE.

Ideally, the opening of your capillary should be facing the WE in the same plane so there is no ohmic gradient across the opening. You don't need to be in the center but you should be at least the distance from the edge as from the face of the WE.

It is a difficult question because of the position of your counter electrode.  Normally one would try to place the counter electrode in a symmetrically oriented position in opposition to the working electrode, i.e., facing the working electrode and not along one side.  Clearly, there will be an unsymmetrical electrical field gradient across the working electrode in your current configuration.  At best, your proposed positioning of the Lugging capillary will provide you with a reproducible reference, but not indicative of the potential at different locations across the WE.

Facing of electrode towards WE or position of  to the WE is much matter in case of if event is sensitive to electrophoretic and dielectrophoretic forces. 

Practical suggestion:

-a 5 mm longer capillary

- cut its end obliquely

- ignore the issue if the working solution is aqueous and  highly conductive.

I think the colleagues who wrote before were very precise. There is just more one contribution: Try to have a thin capillary, once a big "body" in front of the electrode can affect the current lines and disturb your measurement in some condittions.

I'm not sure what was meant by the statement, "ignore the issue if the working solution is aqueous and highly conductive".  Even in a highly conductive aqueous solution there will be an electrical field gradient across the large electrode surface.  In addition, depending on the working electrode material, there can also be an iR drop through the electrode itself.  This is not a trivial problem.

To have a parallel position, use a close end and made a lateral hole.

@Namal. This cell will be a 2 or 3 electrodes one?

If it is a 3 electrode cell , the potentiostat can compensate for the iR drop.

If it is a 2 electrodes cell for an analytical application, the calibration will compensate the iR drop.

Example of a typical calculation for iR drop: 30 ohm x 10 x10^-6 A = 0.3 mV

Don,t forget, you have to allow for a small permanent leakage from the reference electrode.

@Richard.  I agree with you, it is not a trivial problem, but only if there is a significant iR drop. 

I appreciate everyone's help to resolve my problem. I am really grateful. Yes, this is a 3 electrode cell. The counter-electrode is perfectly parallel to the working electrode. From what I understand from all of your suggestions: I will improve the design so that the capillary tip is facing toward the WE at a minimum distance of 3 mm. I think in that way the placement of the capillary will not disturb the electric field between WE and CE much. I will also perform the impedance measurement to determine the IR drop and correct it before the electrochemical measurement. Please be kind enough to correct me if I am wrong. :) Thank you all

Hi

The purpose of the Luggin capillary in an electrochemical cell is to reduce the Ohmic resistance of the solution.   The closer the tip of the Luggin Capillary , a long with its position facing the working electrode, the less the IR would be for that particular cell.   The above is necessary steps if the solution has low ionic conductivity.