Air gap electrode

G. B. Marshall and D. Midgley, Potentiometric Determination of Sulphite by Use of Mercury(I) Chloride-Mercury(II) Sulphide Electrodes in Flow Injection Analysis and in Air-Gap Electrodes. Analyst, 108 (1983) 701. 

Nitrogen occurs in various forms in biological fluids and certain electrodes are suitable for at least three of these, namely, ammonium, ammonia, proteins (including enzymes) and urea. By way of illustration of possible applications, direct measurement of ammonia in serum and blood is clinically interesting since high levels are encountered in hepatic coma and may also indicate the nature of gastro-intestinal haemorrhage in accident cases. The ammonia gas electrode [149] and the air-gap electrode [150] are suitable for this purpose. 

The air-gap electrode [ 1 SO] may be used in the secondary quantitative stage just as conveniently as other designs of gas electrode and with the added advantage that the ion-selective electrode sensing surface does not make contact with the fouling proteins present in many clinical samples. The rapid determination of urea in blood over the range 10" —10" mol dm" has been reported for this electrode [150] and values were within 2.2 per cent of AutoAnalyzer values [379]. The excellent stability of the air-gap electrode facilitated the assay of 460 samples over 4 weeks, the cost per assay being just 2 per cent of that of the AutoAnalyzer method, but the response time per sample rose from an initial 3 min to about 5—6 min over the period [379]. 

Gas sensors and biosensors are obtained by fixing an auxiliary chemical or biochemical system over the ISE membrane. The analyte reacts with the auxiliary system with production or consumption of the ion that is sensed by the ISE. Two basic types of gas sensor, the Severinghaus electrode and the air-gap electrode, are described elsewhere in this encyclopedia. 

Figure 2 Air-gap detectors. (A) Detector with an electrolyte film. 1, detector body 2, sample vessel 3, indicator (pH) electrode 4, electrolyte film 5, reference electrode 6, salt bridge 7, sample 8, stirring bar and 9, seal. (From Ruzicka J and Hansen EH (1974) A new potentiometric gas sensor - the air-gap electrode. Analytica Chimica Acta 69 129-141.) (B) Detector with an electrolyte drop. 1, Conical flask 2, stopper with electrodes 3, indicator electrode (Ag wire) 4, reference electrode (SCE) 5, electrolyte drop 6, sample and 7, stirring bar. (After Fligier J, Czichon P, and Gregorowicz Z (1980) A very simple air-gap cyanide sensor. Analytica Chimica Acta 118 145-148.)...

With air-gap electrodes an additional 0.1% of a non-ionic wetting agent is added (such as Victarwet (Stauffer Chemical Cor.) [167]) ) such as Fluoropore with 1 ju pore size and a polyethylene backing (Millipore) [171]... 

U. of immobilized urease (Boehringer-Enzygel-Urease-650 units/g Coming glass-bound urease-300 units/g) is coated onto the teflon stirrer of an air-gap electrode, and a nylon net is placed over it and tied at the end. Thanks to the favorable hydrodynamics of this constmction, an analysis requires only about two minutes, even though the 2-amino-2-hydroxymethyl-1,3-propandiol buffer used (0.5 M, pH 8.5) results in a pH value at which the enzyme cannot display its optimal activity. 

Figure 1. Sketch of an electrochemical cell whose equilibrium (open circuit) potential difference is AE. (a) Conventional configuration and (b) short-circuited configuration with an air gap. M and R are the electrodes, S is the solvent (electrolyte solution). Cu indicates the cables connecting the two electrodes to a measuring instrument (or to each other).

Initial resistance caused due to air-gap is very high to allow conduction of current. Hence, the arc is first initiated by narrowing its gap momentarily while 110-220 V DC is applied. Once the current picks up flow, the temperature across the arc-gap shoots up promptly. The electrodes are pulled apart leaving a gap of 20 mm to 1 cm, thereby establishing the electric arc whose temperature varies from 4000 to 8000° K. 

The air electrode consists of an insulated metal wire with its tip 1-2 mm above the liquid surface and with polonium deposited on the tip to make the air gap conducting. 

Figure 57. In the case of current constriction induced by a partially contacted metal electrode (shown by electrical potential lines in the inset, contact in the center, separation by an air gap otherwise) the impedance response ideally consists of two semicircles. At high frequencies the air gap (cf. distance between curved electrode and plane surface) becomes dielectrically permeable.286 Reprinted from J. Fleig and J. Maier, Electrochim. Acta, 41 (1996), 1003-1009. Copyright 1996 with permission from Elsevier.

Another alternative, an ionized air reference electrode (IAE), has been described by Foulkes et al. [66] for use with solid polymer electrolytes, in particular Nafion 425. Use of the IAE is achieved by establishing electrical contact with the solid electrolyte from a weak source of ionizing radiation to ionize an air gap. In contrast to conventional reference electrodes, the IAE is a liquid-free device and can make electrical contact with a solid polymer electrolyte without a salt bridge. It is also temperature independent. In view of these special characteristics, the IAE might provide a useful alternative to conventional reference electrodes for use with solid polymer electrolytes. 

Any air gaps between the cell electrodes and specimen surfaces will introduce a series capacitance. Accidental gaps are usually avoided by applying thin foil electrodes, e.g. tin foil applied with a minimum quantity of pure vaseline or... 

The condition to obtain electromodulation (electro- and photo-reflectance) spectra is the existence of a built-in electric field in a structure under investigations. This condition is usually fulfilled in majority structures. Typical CER method utilizes a capacitor-like system with one top semitransparent electrode and one bottom copper-block electrode. The sample is glued to the bottom electrode by using a silver pasta. The front electrode is separated from the sample surface by a spacer (e.g. 0.1 mm). Thus there is nothing in direct contact with the sample. It means that the sample does not conduct any currents and the external electric field is able to change the carriers distribution inside it. Note that the voltage drop appears mainly in the air gap between the front electrode and the sample. The limit for the applied voltage is the electric breakdown in this air gap. It means that the maximal amplitude of EM in the CER technique usually is more limited than the EM amplitude in ER or PR techniques. 

Fifty-Microliter Samples. For small sample volumes. Durst (23) showed that with a gelled fluoride electrode in an inverted position and a conventional calomel reference electrode, the fluoride electrode could be made the sample container, and measurements can be made in samples as small as 50 juL. This concept was applied here with the chloride electrode in the inverted position because all connections in the electrode are solid state no liquids are inside the body. The fluoride electrode (Orion Model 94-09), used here as the reference, was directly opposed to the chloride electrode with an air gap of about I mm (Figure 4). Both flat membranes were lightly coated with silicone oil to make them hydrophobic, and the test solution was pipetted into the gap. 

Another limitation that is specific to the parallel plate electrospinning technique is the collection of extremely thin nanofibers, which have been observed to break because they were unable to sustain the forces of their own weight and of the repulsive charges from other fibers [62]. An electrically resistive substrate inserted into the gap between the plates can provide support to fibers suspended between the plates without influencing fiber quality [62], and may also help to shield any conductive materials below the air gap, which may attract unwanted non-aligned nanofibers. Substrates with bulk resistivity greater than 10 Q cm, such as quartz and polystyrene, are suitable for placement between parallel electrodes, while materials with bulk resistivity of less than 10 Q cm, such as glass, may result in random fiber orientations [67, 68]. 

Another important experiment used to measure the Volta potential difference between two liquids was described by Kenrick [9] and is illustrated here for the mercury electrolyte solution interface (see fig. 8.12). An air gap between the two liquids is established in a cylindrical tube T. The mercury emerges from a central reservoir in a stream of small droplets which flow down the center of the tube. On the other hand, the solution, namely 0.1 M HCl, flows down its walls. The experiment is designed so that the two liquids flow sufficiently rapidly that no charge can be built up on their surfaces. As a result the Volta potential difference across the air gap is zero. The HCl solution is part of an electrochemical half-cell connected to a calomel electrode. The total cell may be described as... 

Trasatti [2] has described the methods used to estimate the absolute electrode potential on the basis of suitable extrathermodynamic assumptions. The method presented here is the one which gives an estimate which can be related to the potential scale used by physicists. Moreover, the resulting estimates of the absolute values of the standard electrochemical potential are based on experimentally measured quantities. The analysis is illustrated here for cell (9.3.30), which contains a hydrogen electrode. An air gap is introduced into the cell, so that the solutions surrounding each electrode are separated. The resulting cell is... 

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