Platinum wire microelectrode

Fig. 1 Comparison between electrolysis and limiting currents. (A) A faradaic electrolysis current between platinum plate electrodes at an applied voltage beyond the threshold potential of the electroactive species. (B) Effect of replacing the platinum plate electrodes with platinum wire microelectrodes. In the absence of stirring, the current reaches a limiting value rather than increasing linearly with an increasing applied voltage.

Amperometric titrations may also be carried out with twin polarised electrodes. Commonly used are a pair of platinum wire microelectrodes. It is important to remember that, in such a situation, whatever charge is apphed to one electrode will induce the opposite charge in the other. This may at times result in undesirable side reactions. Minimum effective potentials should always be used. In some cases, both the reactant (analyte) in the solution under analysis and the reactant in the titrant may behave reversibly at the electrodes. In some others, only the reactant (analyte) in the solution may act reversibly in others again, only the titrant reactant may act. 

The rotating platinum microelectrode was first introduced by Laitinen and Kolthoff in 1941. Figure 17.3 (a) depicts a simple rotating platinum microelectrode which is made out from an usual standard mercury seal . A platinum wire (length 5.0 mm diameter 0.5 mm) protrudes from the lower end wall of a 21... 

Laitinen, H.A., and I. M. Kolthoff Voltammetric Determinations Amperometric Titrations with a Rotating Microelectrode of Platinum Wire. J. Phys. Chem. 45, 1079 (1941). 

Voltammetry. The voltammetric techniques are based on the current-voltagetime relationship at microelectrodes. To perform voltammetry, the oil/antioxidant sample is dissolved in a solvent containing an electrolyte and a three-electrode system (glassy carbon working electrode, a platinum wire reference electrode, and platinum wire auxiliary electrode) is inserted into an oil/solvent solution. A fresh oil typical of the application (100% standard) and the solvent system (0 % standard) is used to calibrate the voltammetric instrument for % remaining antioxidant determination (Kauffman, 1989 and 1991). 

A mercury-supported tBLM was formed at the tip of a microelectrode for measuring single-channel activity [90]. To this end, use was made of a platinum wire embedded in a thin glass capillary and terminated with a platinum microdisc. 

For the flow around a cylinder, a platinum wire ( ( > = 50 (jm) constituted the cylinder and acted as a microelectrode. The wire was placed along a diameter in a tube and was covered by a varnish except for a small portion around the tube axis. 

Electrochemical methods for NO determination offer several features that are not available with spectroscopic approaches. Perhaps the most important is the capability of microelectrodes to directly measure NO in single cells in situ, in close proximity to the source of NO generation. Figure 2 shows sensors that have been developed for the electrochemical measurement of NO. One is based on the electrochemical oxidation of NO on a platinum electrode (the classical Clark probe for detection of oxygen) and operates in the amperometric mode [17]. The other is based on the electrochemical oxidation of NO on conductive polymeric porphyrin (porphyrinic sensor) [24]. The Clark probe uses a platinum wire as a working electrode (anode) and a silver wire serves as the counterelectrode (cathode). The electrodes are mounted in a capillary tube filled with a sodium chlo-ride/hydrochloric acid solution separated from the analyte by a gas-permeable membrane. A constant potential of 0.9 V is applied, and direct current (analytical signal) is measured from the electrochemical oxidation of NO on the platinum anode. In the porphyrinic sensor, NO is catalytically oxidized on a polymeric metalloporphyrin... 

A microelectrode was made of a Teflon-coated platinum wire having a diameter of 50 pm. The microelectrode was installed with a micro-manipulator. An Ag/AgCl reference electrode (1 mm (dia) x 10 mm) and... 

For more than 300 platinum microelectrodes produced, the average metallic wire radius is 30 + 3 pm (i.e. an accuracy of 10%). This value is coherent with the wire radius commercially indicated and shows the good repeatability of microelectrodes fabrication. In addition, more than 100 reproducible cyclic voltammograms can be recorded successively in the ferricyanide solution without modification of the curve shape. 

Fig. 11.8 Cyclic voltammograms for the oxidation of 5 mM ferrocene in [C4mim][PF6] on a platinum microelectrode (diameter 10 j.m) at lOOmVs 1. Reference electrode was a Pt wire inserted into [C4mim][PF6]...

Stabilization of BLMs at the surface of electrodes has been reported by a number of groups [28-30]. For example, the tip of a Teflon-coated platinum microelectrode was cut in situ with a scalpel while immersed in a lipid solution (lipid in a hydrocarbon solvent). Upon immersion of the wire into an aqueous solution of 0.1 M KCl, the phospholipid coating adhering to the metal surface spontaneously thinned to form a BLM directly adjacent to the electrode surface... 

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