The Luggin-Haber Capillary or Luggin Probe

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A while ago I got a question about constructing a Luggin capillary (Luggin probe or Luggin-Haber capillary) for a reference electrode. My answer formed the basis of the notes below.

A Luggin capillary is a way of effectively bringing the potential sensing tip of the reference electrode close to the surface of the working electrode, thereby reducing the uncompensated iR drop. The Luggin probe may be open at the bottom end and filled with the test electrolyte. It is often closed at the end with a Vycor® frit and filled with a salt solution to form a salt-bridge. A Luggin is fairly well described on the Gamry website.

Since the goal of using a Luggin is to bring a narrow outside diameter capillary close to your working electrode, most manufacturers will design a Luggin to be compatible with the geometry of their cells.  Your best route is either to order one from the manufacturer of your cells, or to make one yourself.

The design criteria are:

The outside diameter of the tip should be small. ASTM G5 recommends that a Luggin tip be no closer than two tip diameters or so from the surface of the working electrode (Ref 2).  Otherwise the tip will shield part of the working electrode and disturb the current flow path.  So, the smaller the tip, the closer you can get to the electrode surface. This helps to minimize the uncompensated resistance of the cell setup.

Unfortunately, if the outside diameter is small, the inside diameter is smaller! This leads to a high resistance in the Luggin/salt bridge. See the next section.

Keep the length of the narrow capillary short.  Otherwise the resistance of the Luggin can be high, even if it is filled with a very conducting solution!  At one time I measured the resistance of a Luggin filled with 0.1M salt and it was about 100 Kohm.  High resistances in the reference electrode path can cause problems by causing a potentiostat to oscillate.  Many commercial potentiostats have problems with reference electrode impedances above about 10 Kohm.

The problem of high resistance salt bridges or Luggin probes can be overcome in several ways. One (Ref 3) is to simply short out the high resistance with a thin piece of platinum wire placed in the narrow section of the capillary. No external connection to the platinum is required. Another (Ref 4, Ref 5) is to place a piece of platinum close to the tip of the Luggin. The platinum is coupled to the reference electrode input of the potentiostat using a 0.1-0.01 µF capacitor. This has the extra advantage of couteracting the effects of a high resistance reference electrode as well as the effects of a high resistance capillary.

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References:

(1) http://www.gamry.com/App_Notes/ReferenceElectrodes/luggin_capillary.htm
(2) "Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements", ASTM Standard G5-94, (2004) ASTM International.
(3) "A reference half-cell capillary that improves the high frequency performance of the potentiostat/whole-cell combination", S Fletcher and M Horne, J. Electroanal. Chem., 297 (1991), 297-299. doi:10.1016/0022-0728(91)85375-Y 
(4) "Practical Problems in Voltammetry 3: Reference Electrodes for Voltammetry," Current Separations, 14:2 (1995), 64.
(5) http://www.gamry.com/App_Notes/ReferenceElectrodes/ReferenceElectrodes.htm
 

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