Luggin capillary

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... 

Fig. 1.22 Spontaneous corrosion of zinc in acid illustrated by the reversible cell ZnlZn IH30", H2 Pt. The individual potentials of the electrodes are determined by a reference electrode (ReQ and a Luggin capillary to minimise the IR drop in the solution...

By means of a resistance in the circuit the spontaneous corrosion reaction can be made to proceed at a predetermined rate, and the rate can be measured by means of an ammeter A. At the same time the potentials of the individual electrodes can be measured by means of a suitable reference electrode, a Luggin capillary and high-impedance voltmeters and Kj. At equilibrium there is no net transfer of charge (/ = A = 0). the e.m.f. of the cell is a maximum and equals the difference between the reversible potentials of the two electrodes... 

Fig. 19.7 Reference electrodes and capillaries, (a) Reference electrode, salt bridge and Luggin capillary (A) calomel electrode (c) frontal types of capillaries and positions (r/) rearside capillaries (after von Fraunhofer and Banks )...

Measurements of the corrosion potential of a single metal corroding uniformly do not involve an IR drop, but similar considerations do not apply when the metal is polarised by an external e.m.f., and under these circumstances the IR drop must be minimised by using a Luggin capillary placed close to the surface of the electrode (see Fig. 1.22, Section 1.4). Even so, the IR drop is not completely eliminated by this method, and a further error is introduced by the capillary shielding the surface from the current flow... 

Accurate control of potential, stability, frequency response and uniform current distribution required the following low resistance of the cell and reference electrode small stray capacitances small working electrode area small solution resistance between specimen and point at which potential is measured and a symmetrical electrode arrangement. Their design appears to have eliminated the need for the usual Luggin capillary probe. 

The experimental setup is shown in Figure 9.23. The Pt-black catalyst film also served as the working electrode in a Nafion 117 solid polymer electrolyte cell. The Pt-covered side of the Nafion 117 membrane was exposed to the flowing H2-02 mixture and the other side was in contact with a 0.1 M KOH aqueous solution with an immersed Pt counterelectrode. The Pt catalyst-working electrode potential, Urhe (=Uwr)> was measured with respect to a reversible reference H2 electrode (RHE) via a Luggin capillary in contact with the Pt-free side of the Nafion membrane. 

Fig. 16. Small-scalo laboratory cell for preparative electrolysis. A, Pt gauze working electrode. B, Pt sheet secondary electrode. C, Reference electrode. D, Luggin capillary on a syringe barrel so that the position of the tip of the Luggin probe relative to the working electrode is readily adjustable. E, Glass sinter to separate anode and cathode compartments. F, Gas inlet to allow stirring with inert gas or the continuous introduction of reactant. G, Three-way tap where a boundary between the reference electrode and the working solutions may be formed.

A version of the galvanostatic method is that where the current is turned off (or a current f = 0 is applied ) and the polarization decay curve is measured. Consider an electrode which up to the time t = 0, when the current was turned off, had the potentiaf F at the net current density When the current is turned off, the ohmic voftage drop in the electrolyte gap between the electrode and the tip of the Luggin capillary vanishes, so that the potential instantaneously shifts to the value F (Fig. 12.11). After that the electrode potential returns (falls) relatively slowly to its open-circuit value, for which a certain nonfaradaic charging current is required. Since ip + =... 

FIGURE 32.2 Scheme of a four-electrode system for polarization measurements at an ITIES comprising a potentiostat (POT), two reference electrodes connected to the cell by means of Luggin capillaries (REl, RE2), and two counter electrodes (CEl, CE2). The planar ITIES is formed at the edge of a round hole in a glass barrier between the spaces for the aqueous (water) and the organic (org) phases. 

Fig. 5.15 Basic circuit for the electrode potential measurement during current flow A is working (indicator, test), B, auxiliary and R, reference electrode connected by means of the Luggin capillary (arrow) and P, potentiometer. 

Fig. 5.17 A four-electrode potentiostatic circuit (voltage clamp). and R2 are reference electrodes with Luggin capillaries (arrows) attached as close as possible to the membrane or ITIES (dashed line), B, and B2 are auxiliary electrodes, P and P2 potentio-stats, G programmed voltage generator and Z recorder... 

In principle, different reference electrodes may be used if the cell is provided with a separate compartment and a Luggin capillary. But if the flow cell technique is to be applied, it is more convenient to avoid the use of capillaries where the solution cannot be easily exchanged. Active bulk components could diffuse through the capillary and give rise to erroneous responses. A small palladium gauze charged with hydrogen directly immersed in the solution can be used as the reference electrode (PdH ) [18]. 

There are two difficulties with this method. The first one is due to the fact that in reality the potentiostat keeps the potential between the working and the reference electrode constant there is an ohmic resistance Rq between the tip of the Luggin capillary (see Chapter 2) and the working electrode, giving rise to a potential drop I R-n (7 is the current). Since I varies in time, so does the potential drop by which ry is in error. However, modern potentiostats can correct for this to some extent. The second difficulty is more serious. Immediately after the... 

The solution used for all experimentation was 5 M NaCl, made up using triply distilled water. Electrolysis experiments were carried out in a one-compartment cell at either room temperature or at 90°C. All electrochemical experiments were carried out using a two-compartment cell, with the RE compartment connected to the WE and CE compartments via a Luggin capillary. 

Nevertheless, even with this experimental set-up, the iR drop is not completely eliminated. The situation can be improved if the reference electrode is placed very close to the working electrode through a Luggin capillary, see Figure 9. 

The ideal positioning for the Luggin capillary is at a distance 2d from the surface of the working electrode, where d is the outlet diameter of the capillary. 

The reference electrode (commonly a saturated calomel electrode or a silver/silver chloride electrode) is, ideally, placed in close proximity to the working electrode via a Luggin capillary. 

Luggin Capillaries The location, where the potential of the electrolyte should be measured, is connected to the RE by the Luggin (Haber-Luggin) capillary. Its design can significantly influence the accuracy of the acquired potential values. Requirements are... 

Different positions of the Luggin capillary are shown in Eig. 6 [4]. In example 1, a too large distance between the working... 

Fig. 6 (From Ref. [4]) Positioning of the Luggin capillary, problematic 1-3 suitable 4-6 W working electrode L Luggin capillary C counter electrode I cell current, R electrolyte resistance.

Penetration of gas bubbles into the Luggin capillary can disturb the steady operation. In the worst case, the control circuit of the potentiostat will be interrupted and the cell current can increase... 

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