Luggin-Haber probe

FIGURE 4.3.5. A Luggin-Haber probe combined with the calomel reference electrode. 

Luggin probe or Luggin-Haber capillary A device used in measuring the potential of an electrode with a significant current density imposed on its surface. (The probe minimizes the IR drop that would otherwise be included in the measurement without significantly disturbing the current distribution... 

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

Reference electrodes are generally used together with Haber-Luggin capillaries (for details, see Ref. [2]).The design and position of these capillaries pose current and potential distribution problems. In order to minimize ohmic drop they have to be placed as close as possible to the electrode surface. But if the distance is too small they act as a current shield and non-imiform current distribution arises. In practice the tip of the Luggin probe should be at a distance of about 2 d from the working electrode where d is the external diameter of the capillaiy. 

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