Electrodes Peroxide function

Fig. 2.4.1. -pO plots for gas oxygen electrode 1, according to equation (2.4.1) 2, peroxide function 3, thermodynamically favourable dependence 4, experimental... 

The calibration of a potentiometric cell with the Pt(02) membrane electrode with the use of KOH, in the range of pO values from 1 to 4, results in a slope value of 0.0995 V [236]. However, a Pt(02)IZr02(Ca0) gas membrane oxygen electrode should exhibit the peroxide function in the stated pO range, and, therefore, the correctness of the data presented is very doubtful. 

The potentiometric cell construction was similar to that in equation (2.4.28) NaOH and KOH were used as Lux bases for obtaining calibration plots. The behaviour of the membrane oxygen electrode agrees qualitatively with the results obtained in studies of other alkali metal halide melts (Fig. 2.4.13). All the calibration plots are characterized by an inflection point whose position shifts to higher pO values as the melt temperature lowers. At 700 and 800 °C changes in the character of the potential-determining process are observed at pO = 3, at 600 °C the inflection takes place at pO = 4 (Table 2.4.5). It is obvious that such a shift is caused by the peroxide function of the gas membrane oxygen electrode. 

It is very interesting to examine the behaviour of the membrane oxygen electrode with a lowered partial pressure of gaseous oxygen in the inner space of the electrode (inside the YSZ test-tube) in the melts. The peroxide function of gas oxygen electrodes (and of the gas membrane oxygen electrode, as well) is caused by the formation of stable peroxide ions at the interface boundary. Therefore, the reduction of the partial pressure of 02 in the inner electrode space should result in a shift of the inflection point of the E-pO calibration plot to lower pO values. It follows from the above-considered investigations... 

The anomalous behavior of the electrode Pt(02) has been noted and explained by peroxide function of the gas oxygen electrodes (of the first type). On the contrary to gas electrodes, metal-oxide electrodes (of the second type) were reversible with the slope corresponding to the reaction ... 

Nature uses an array of enzymes to carry out a large number of organic reactions. In a sense, a redox enzyme is a surface which provides or accepts electrons, much like an electrode surface (Fig. 1), The most striking similarity between the two is that electrons are transferred one at a time in both systems. This has been pointed out by a number of workers, most recently by Guengerich and MacDonald (8) in a study of cytochrome P-450 systems. The extent to which an electrode actually functions as surface will be discussed in the latter part of this paper. The biggest difference between the two systems is that an enzyme surface has a more sophisticated structure which holds substrate molecules in certain ways or conformations so that unique reactions may take place. A second major difference is that an enzyme is part of a system to transfer electrons between a substrate and a secondary redox reagent such as air or peroxide (in oxidations). In an electrochemical system, the electrons go into the circuit. 

Redox-based biosensors. Noble metals (platinum and gold) and carbon electrodes may be functionalized by oxidation procedures leaving oxidized surfaces. In fact, the potentiometric response of solid electrodes is strongly determined by the surface state [147]. Various enzymes have been attached (whether physically or chemically) to these pretreated electrodes and the biocatalytic reaction that takes place at the sensor tip may create potential shifts proportional to the amount of reactant present. Some products of the enzyme reaction that may alter the redox state of the surface e.g. hydrogen peroxide and protons) are suspected to play a major role in the observed potential shifts [147]. 

Kemer W, Kiwit M, Linke B, Keck FS, Zier H, Pfeiffer EF. The function of hydrogen-peroxide-detecting electroenzymatic glucose electrode is markedly impaired in human subcutaneous tissue and plasma. Biosensors Bioelectronics 1993, 8, 473. 

MWCNTs were functionalized with iron phthalocyanines (FePc) to improve the sensitivity towards hydrogen peroxide. A highly sensitive glucose sensor with an FePc-MWCNT electrode based on the immobilization of GOx on poly(o-amino-phenol) (POAP)-electropolymerized electrode surface [219]. A hemin-modified MWCNT electrode to be used as a novel 02 sensor was obtained by adsorption of hemin at MWCNTs and the electrochemical properties of the electrode were characterized by cyclic voltammetry [220]. 

Acetylcholineesterase and choline oxidase A Cross-linkable polymer, poly (vinyl pyridine) derivatized at the N atoms with a combination of iron-linking and redox functionalities was used to immobilize the enzymes and to shuttle electrons. Enzymes were deposited with the polymer and deposited onto C electrode. For peroxide selectivity over ascorbate is achieved by incorporation of Nafion. The microsensors if they can be successfully used in vivo will provide valuable information for brain diseases (Parkinsion s and Alzheimer s). [88]... 

Tarasevich et al. [417, 420] employed the rotating disk electrode with an oxide disk electrode to study the electrochemical reactions of peroxide in conjunction with a gasometric method, by means of which the rate of the peroxide decomposition via a purely chemical pathway [eqn. (72)] could be followed independently. The authors compared the rate of gas evolution and peroxide electroreduction and oxidation, respectively, as a function of electrode potential and attributed the difference of these rates to chemical... 

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