Electrodes Clark

Assemble a Clark electrode (with a fresh membrane, if necessary), constant temperature bath, constant voltage source, and recorder following the manufacturer s instructions. In the subsequent discussion the Yellow Springs Instrument s Clark electrode and sample chamber are used. 

Since passage of oxygen through the Teflon membrane depends 

Place 5 ml of a freshly prepared lOmAf sodium dithionate solution in the sample chamber. 

Check to be sure that there are no bubbles in the sample chamber. Small bubbles often adhere to the wall of the sample chamber if it is not clean. If this occurs wash the glass chamber thoroughly with soap and water. 

Return the electrode assembly to the constant temperature bath. 1-44. Turn on the magnetic stirrer and recheck the sample chamber for bubbles. If any are present, remove them before proceeding. 

A major advance in the performance of amperometric oxygen sensors has been achieved by placing both the cathode and the anode behind the oxygen-permeable membrane (Fig. 7.4). This sensor is known as the Clark oxygen electrode. 

Several different membrane materials have been used, namely Teflon, polyethylene, and silicon rubber among others. It is possible to obtain some degree of selectivity by choosing the material of this membrane according to the conditions of the application. The diffusion through such a structure is more complicated. For radial geometry, the steady-state current is given as 

From this comparison we see that the depletion effect caused by the electrode itself is almost completely confined within the membrane. This is particularly important if the electrode is used in the medium which itself consumes (or produces) oxygen. Generally, for electrodes in which the depletion field extends beyond 

The flow sensitivity of the electrode has the same origin, as has been pointed out previously. A stagnant (Prandtl) boundary layer of thickness 5 forms around the spherical electrode (radius ro) placed in the liquid of kinematic viscosity v which is moving with linear velocity U. 

If the depletion layer is completely inside the stagnant layer, the current is not affected by the change of flow. From (7.18), we know that this happens when the electrode radius becomes small. For Clark-type electrodes, the flow insensitivity is obtained even for larger diameters of the electrode, because of the additional confining effect of the membrane which has lower oxygen transmissivity, DmSm, than that of the solution. 

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Gal-Or and Hoelscher (G5) have recently developed a fast and simple transient-response method for the measurement of concentration and volumetric mass-transfer coefficients in gas-liquid dispersions. The method involves the use of a transient response to a step change in the composition of the feed gas. The resulting change in the composition of the liquid phase of the dispersion is measured by means of a Clark electrode, which permits the rapid and accurate analysis of oxygen or carbon dioxide concentrations in a gas, in blood, or in any liquid mixture. 

FIGURE 6-18 Membrane-covered oxygen probe based on the Clark electrode. (Reproduced with permission from reference 60.)... 

Chronocoulometry, 62 Clark electrode, 190 Coated wire electrodes, 160 Cobalt, 82, 85 Cobalt phthalocyanine, 121 Collection efficiency, 113, 135 Collection experiments, 113 Combination electrode, 148 Compact layer, 19 Composite electrodes, 47, 114, 133 Computer control, 80, 106 Concentration profile, 7, 9, 11, 29, 36, 87, 132... 

Almost all of the methods described in Chapter 23 can be used for in vivo analyses, both voltammetric and potentiometric ones. The former are used primarily in the analysis of organic substances, which, within certain ranges of potential, can be either oxidized or reduced. Another popular method is the amperometric determination of oxygen in different biological media with the Clark electrode (Section 23.3). 

Many dehydrogenase enzymes catalyze oxidation/reduction reactions with the aid of nicotinamide cofactors. The electrochemical oxidation of nicotinamide adeniiw dinucleotide, NADH, has been studied in depthThe direct oxidation of NADH has been used to determine concentration of ethanol i s-isv, i62) lactate 157,160,162,163) pyTuvate 1 ), glucose-6-phosphate lactate dehydrogenase 159,161) alanine The direct oxidation often entails such complications as electrode surface pretreatment, interferences due to electrode operation at very positive potentials, and electrode fouling due to adsorption. Subsequent reaction of the NADH with peroxidase allows quantitation via the well established Clark electrode. 

In the absence of electron donors, complexes 1 display a catalase-like activity (Eq. (17)). Dioxygen evolution can be visually observed at [H202] >0.01M. This catalytic feature has been studied kinetically by monitoring the initial rates of 02 formation with a Clark electrode (53). 

The water-splitting activity of detergent-solubilized and AP-trapped PS2 reaction centers was measured using a Clark electrode. As shown in Figure 4, differences between the two types of samples were minimal, with a slight decrease of activity when the pH of the P-DM solution was shifted to pH 8 and a moderate increase following addition of A8-35, dilution below the CMC of p-DM and treatment with BioBeads. 

The dissolved oxygen content of a solution can be determined by measuring the diffusion current that results at a selected voltage. The Clark electrode was developed for this purpose and various modifications have subsequently been introduced. It consists basically of a platinum electrode separated from the sample by a membrane which is permeable to oxygen, e.g. Teflon or polyethylene. A reference electrode of silver/silver chloride in potassium chloride is used to complete the system (Figure 4.21). When a voltage that is sufficient to give the... 

Figure 13.1. A typical Clark electrode for oxygen measurement (Adapted from Ref. I.)...

The Clark electrode is widely used because it is the best available technique to date. There are a number of limitations and problems in its use that exist and it is important to be aware of these. We have briefly summarized the most critical ones and for greater details the reader is directed to the excellent review by Lee and Tsao.(l) 2 3 4 5... 

Long-term stability. The polarographic Clark electrode utilizes an electrolyte that is eventually consumed. Consequently, long-term use (several days) results in gradual drift in the electrode signal due tn electrolyte consumption. 

The problem with the Clark electrode is that some of these requirements have solutions that are opposing. For instance, flow dependence may be reduced by employing a thicker membrane but this would occur at the cost of increased response time. As a result, most commercially available systems are design compromises that sacrifice a part of some desirable feature. It should he noted that an optical measurement technique where oxygen and/or electrolyte is not consumed will be free of the drawbacks mentioned above. 

L. C. Clark first suggested in 1956 that the test solution be separated from an amperometric oxygen sensor by a hydrophobic porous membrane, permeable only for gases (for a review of the Clark electrode see [88]). The first potentiometric sensor of this type was the Severinghaus CO2 electrode [150], with a glass electrode placed in a dilute solution of sodium hydrogenocarbonate as the internal sensor (see fig. 4.10). As an equilibrium pressure of CO2, corresponding to the CO2 concentration in the test solution, is established in the... 

An elecrochemical device for detecting dissolved oxygen content. The well known Clark electrode consists of a platinum wire tip surrounded by a thin film of electrolyte solution that is shrouded by a plastic membrane. The membrane is permeable to oxygen, but impermeable to... 

Most of the amperometric flow-through biosensors based on commercially available enzymes are employed to measure consumed or released oxygen by using a Clark electrode or a solid-state electrode to monitor the hydrogen peroxide formed or an enzymatically reduced acceptor. 

The amperometric immunosensors reported so far rely on various methodological principles including use of a Clark electrode for detecting oxygen formation or depletion, an electrochemically active product yielded in an enzyme reaction or an antigen labelled with an electroactive species. 

In amperometry, we measure the electric current between a pair of electrodes that are driving an electrolysis reaction. One reactant is the intended analyte and the measured current is proportional to the concentration of analyte. The measurement of dissolved 02 with the Clark electrode in Box 17-1 is based on amperometry. Numerous biosensors also employ amperometry. Biosensors8-11 use biological components such as enzymes, antibodies, or DNA for highly selective response to one analyte. Biosensors can be based on any kind of analytical signal, but electrical and optical signals are most common. A different kind of sensor based on conductivity—the electronic nose —is described in Box 17-2 (page 360). 

Clark electrode One that measures the activity of dissolved oxygen by amperometry. 

We begin by pointing out that this concept of covering an electrode surface with a chemically selective layer predates chemically modified electrodes. For example, an electrode of this type, the Clark electrode for determination of 02, has been available commercially for about 30 years. The chemically selective layer in this sensor is simply a Teflon-type membrane. Such membranes will only transport small, nonpolar molecules. Since 02 is such a molecule, it is transported to an internal electrolyte solution where it is electrochemically reduced. The resulting current is proportional to the concentration of 02 in the contacting solution phase. Other small nonpolar molecules present in the solution phase (e.g., N2) are not electroactive. Hence, this device is quite selective. 

Fig. 7.5 Calculated concentration profiles for oxygen in Clark electrode consisting of (a) electrolyte (b) membrane and sample solution. The smooth solid curve is for bare Pt electrode of the same dimensions (adapted from Fatt, 1976)...

It would be expected that the speed of response scales again with the radius of the electrode. However, it has been found (Vacek et al., 1986) that the fastest speed of response for a hemicylindrical Clark electrode is obtained with an electrode radius of approximately 5-10 urn. This is due to the fact that as the radius decreases, the effect of the layers that are closer to the electrode surface becomes relatively more important than those that are farther away. In fact, with the further decrease of the radius the time response becomes longer than for the corresponding planar electrodes. 

The greatest impact of the Clark oxygen electrodes has been in medicine and physiology. (A schematic diagram of a catheter-size Clark electrode is shown in Fig. 7.7.) On the other hand, a temperature- and pressure-compensated Clark electrode for oceanographic measurements up to 600 ft has also been developed (Fatt, 1976). The normal temperature coefficient of the Clark electrode is 2%/°C... 

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