Oxygen-indicating electrodes

To measure Eh in field conditions hermetic flow-through vessels made of inert material are used, which isolate tested water from the air oxygen. Indicator electrodes are placed into the vessel (no fewer than 3-4 electrodes are recommended), and a thermometer. Ihe vessel is washed before the action. 

The Essential Steps of the Development of Reference Electrodes for Use with Oxygen-Indicating Electrodes... 

Indicator electrodes for anions may take the form of a gas electrode (e.g. oxygen electrode for OH- chlorine electrode for Cl-), but in many instances consist of an appropriate electrode of the second kind thus as shown in Section 15.1, the potential of a silver-silver chloride electrode is governed by the chloride-ion activity of the solution. Selective-ion electrodes are also available for many anions. 

An example of amperometric methods used for analytical purposes is the sensor proposed in 1953 by Leland C. Clark, Jr. for determining the concentration of dissolved molecular oxygen in aqueous solutions (chiefly biological fluids). A schematic of the sensor is shown in Fig. 23.1. A cylindrical cap (1) houses the platinum or other indicator electrode (2), the cylindrical auxiliary electrode (3), and an electrolyte (e.g., KCl) solution (4). The internal solution is separated by the polymer... 

The partial oxygen pressure, p02, is particularly significant in metabolic processes of cells, and its variation from normal values often indicates pathologies (ischemic diseases, strokes, tumors). Accurate and localized measurements of the oxygen concentration are also desirable for differentiation between venous and arterial blood, or for cerebral mapping of task activation. In the past, invasive methods were used involving oxygen-sensitive electrodes which had to be placed directly in the blood or tissue and could only offer p02 from a few body points. 

The cosubstrate, oxygen, and the product, hydrogen peroxide, of the GOD-catalyzed glucose oxidation are electrochemically active. Therefore, gas sensitive and redox electrodes can be used as indicator electrodes (see Fig. 38). 

Flow systems with oxygen-indicating GOD electrodes have been studied by Macholan et al. (1981) and Pacakovd et al. (1984). The enzyme was fixed to polyamide and gelatin, respectively. With aqueous sample solutions a measuring frequency of 30/h and 60/h has been obtained. Biological samples, like blood, serum or plasma, have not been processed. 

We have repeatedly emphasized that the most reliable data on Lux acid-base interactions taking place in ionic melts can be obtained using the potentiometric method, which provides detection of the equilibrium oxide-ion concentrations over wide pO ranges. These investigations imply the use of indicator electrodes reversible to O2- in cells with the liquid junction. Literature sources contain much information devoted to studies of the features of reversibility of the following kinds of oxygen electrodes ... 

As repeatedly mentioned in the previous parts, the correctness of the constants obtained for the Lux acid-base equilibria depends essentially on the correctness of the assumption about the completeness of dissociation under the experimental conditions of the strong Lux bases, which are used for potentiometric titration of the studied acids and for calibration of the potentiometric cell with the indicator oxygen electrode. For the latter factor, the accordance to the corresponding Nernst equation of the potentialdetermining process at the indicator electrode is less important than the completeness of the oxide-ion donor s dissociation with the formation of O2-. 

All the routines described for the determination of the thermodynamic (concentration) parameters in metal oxide solutions include some indirectly obtained values. For example, the equilibrium concentration of metal cations is calculated proceeding from the quantity of the oxide-ion donor consumed for titration (precipitation). Direct determination of the concentration of metal cations in the melt (if it is possible) allows one to obtain more correctly the obtained solubility product values. Our paper [332] reports a method for correction of the solubility product values for oxides on the basis of the potentiometric titration data. The modification of the standard routine consists of the simultaneous use of two indicator electrodes, one of which is the membrane oxygen electrode and the other is a metal electrode, reversible to the cations the oxide consists of. This routine was used to estimate the solubility products of copper(I) and nickel(II) oxides in the molten KCl-NaCl equimolar mixture at 700 °C. Investigation of Cu20 by the proposed method is of considerable importance since, as will be shown further, the process of dissociation/dissolution of copper(I) oxide in molten alkali-metal halides differs from the generally accepted one which was considered, e.g. in Ref. [119]. 

The choice of the indicator electrode is largely determined by the species involved in the sensing reaction. Oxygen and H2O2, which are the cosubstrate and product of oxidases, as well as NAD(P)H—the cosubstrates of about 300 pyridine nucleotide-dependent dehydrogenases—can be determined amperometrically. Based on this principle, many enzyme sensors of the first generation have been developed and commercialized (see section 17.4 and table 17.2). 

In the dissolution and deposition of most metals, the activation energy is small and hence the transfer overpotential is low. In transition metals such as iron and nickel, it is much higher. Conspicuously high overpotentials indicate electrode processes in which hydrogen and oxygen are moved. 

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. 

Indicator electrode 1 is coated with the enzyme glucose oxidase and a mediator, described below. The enzyme is a protein that catalyzes the reaction of glucose with oxygen ... 

PSA is relatively scarcely diffused, but offers an excellent option in the analysis in complex matrices, as it can be the case of envirramiental samples. PSA is similar to ASV, but differs in the way how the deposited metals are stripped from the electrode. After the preconcentration step at a controlled potential, the potentiostatic control is disconnected and the metal is reoxidized by an oxidizing agent present in solution, such as Hg(II) or oxygen, or by the application of a constant anodic current to the electrode. The potential spontaneously assumed by the indicator electrode— WE in the previous step—is monitored as a function of time. The experimental conditions are set in a way that the rate of reoxidation of the metal is constant during the stripping process and is determined either by the constant anodic current imposed or by the diffusion of the oxidant from the solutimi to the electrode surface. In the latter case, the mass transport of the oxidant is... 

Laccase was immobilized alone or co-immobilized with GOD or cytochrome b2 by entrapment in gelatin. The layer had a thickness of 20-30 The enzyme membranes contained 120 U/cm laccase, 56 U/cm GOD and 30 U/cm of cytochrome b2, respectively . For preparation of the sensor, the mono-or bi-enzyme layer was fixed on the dialysis membrane (17 pm thick) of the modified oxygen electrode, which comprised a platinum indicator electrode of 0.5 mm diameter covered by a dialysis membrane held by an 0-ring. 

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