Luggin probe

Luff Schoorl Luggin probes Lulo Lumber... 

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.

Luggin probe phys chem A device which transmits a significant current density on the surface of an electrode to measure its potential. log-on, prob ... 

The variable-temperature cell in which reactive intermediates can be studied at temperatures down to — 130°C (Figure 6.18) is designed with opposed, coaxial working, and Luggin-probe electrodes surrounded by a platinum-coil counter electrode. The cylindrical symmetry provides uniform current and po-... 

Electro chemists are aware of the annoying residual uncompensated solution resistance Ru between the Luggin probe and the working electrode, see for example [74]. Although it is possible in principle to compensate fully for the iR error thus introduced [131,132], this is rarely done, as it introduces, in practice, undesirable instrumental oscillations or, in the case of damped feedback [132], sluggish potentiostat response. 

The anionic Ni(III) carborane, (Ni[C2B,H,(CH3)jJ2] , can be reversibly reduced at — 0.9 V in CHjCN to the dianion and cyclic voltammetry data at a hanging-Hg drop are given in Table 2. To minimize resistance losses, a luggin probe is used for the reference electrode and positive feedback iR compensation is employed. At each scan rate, the measured value of AEp is used to obtain >j/ from Table 1. Then, using a value of a calculated from the scan rate and Eq. (1), a value of kj/D is obtained at each V. The average of values at different scan rates is calculated D must be measured independently (from, e.g., the polarographic I value) and in the present example is 6.4 X 10 cm s . This yields ... 

This device is called a Luggin probe and its purpose is to minimise the ohmic drop (// ) This may become important for non-aqueous solutions. 

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. 

Apart from resistance, capacitive effects should also be taken into consideration. The best design of a Luggin probe is one with a narrow capillary at its tip with thin walls to prevent shielding, but with thick walls in the main body which widen rapidly away from the tip to reduce resistance in the control loop [21]. 

Positioning the reference electrode (or Luggin probe [34]) close to the working electrode further helps to minimise the IR drop between the reference and working... 

A problem that arises when the Luggin probe is positioned close to the metal/solution interface is the nonuniform current distribution (Fig. 4.3.4) near the... 

Current Interruption Technique. As discussed in the previous section, proper choice and positioning of the Luggin probe will minimize the contribution of the ohmic drop to the measured electrode potential. One can also correct the measured electrode... 

One can also measure the potential drop across the membrane in a cell [84] depicted in Fig. 4.3.25C, where current is passed through large electrodes and the ohmic drop is measured between the Luggin probes placed close to the membrane so that the IR drop between the Luggin tip and the membrane is negligible. From the measured drop, the resistance could be calculated using Ohm s law. 

The Luggin probe used to monitor the potential of the Hg indicator electrode is spaced close ( 1 mm) to the mercury droplet to reduce ohmic resistance. The top barrel is cleaned, wetted with concentrated NaNOj solution, and closed. Sufficient conductivity exists in this solution layer to permit the small currents necessary for potentiometric determination. When an excess of mercurous ion is generated, the potential of the Hg/Hg couple varies in accordance with the Nemst equation, and a voltage follower may be used to output the voltage curve to a computer data system. For precise work, dissolved oxygen should be removed from the supporting electrolyte and efficient stirring is needed. 

Fig. 11.3 — Schematic diagram of cell showing the principle of the Luggin probe. / s is the solution resistance between counter electrode and the tip of the probe, and / u the uncompensated resistance between the Luggin tip and the worldng electrode surface.

The majority of commercially available potentiostats have a facility for electronically compensating for the ohmic drop due to the solution resistance between the Luggin capillary and the electrode. The Luggin probe is placed far enough away into the solution to prevent shielding of the electrode, and part of the output signal from the current follower is fed back into the potentiostat to compensate for the resistance between the Luggin tip and the electrode. A typical circuit is shown in Fig. 11.12. 

---