Platinum oxide quinones

Catalytic hydrogenation of thebaine-quinone over platinum oxide in glacial acetic acid proceeds with the rapid absorption of one mole of hydrogen and the somewhat slower absorption of a second mole to give tetrahydro-thebaine-quinone [xvm]. If the reduction is stopped after the first mole of hydrogen has been absorbed, a product that is probably mainly [xix] is obtained it is believed to be mainly [xrx] as mainly non-phenolic matter is obtained after one minute s boiling with concentrated acid, under which conditions [xvi] is isomerized to [xvh] [3]. Attempted demethylation of [xvni] with hydriodic acid affords only tars [3]. 

A wide variety of quinones spontaneously adsorb onto various electrodes, including gold, platinum, carbon, and especially mercury. On mercury electrodes, these quinonoid monolayers often exhibit nearly ideal electrochemical responses in low-pH electrolytes, so making them attractive model systems for probing the thermodynamics of adsorption. In low-pH electrolytes, both the oxidized and... 

On the contrary, no oxidation or reduction current peaks were observed on the sp3-carbon-comprising wide-gap DLC (Eg 1.7 eV) electrodes in Ce3+/4+, Fe(CN)63 /4. and quinone/hydroquinone redox systems, as already mentioned in Section 6.3. Thus, we conclude that DLC is electrochemically inactive in itself. It gains electrochemical activity upon introducing a significant (ca. 10 %) admixture of platinum to the film bulk. Figure 34 shows the dependence of the Fe(CN)63 reduc-... 

Electrochemical (EC) techniques provide an alternative way to detect sulfur containing molecules. Earlier methods of EC detection involve the application of a gold/mercury electrode.15 Platinum and gold electrodes have also been used for anodic detection of thiols,16 but this requires high oxidation potentials, which complicates analytical applications. Thus, chemically modified electrodes with inorganic or organic mediators have been employed to facilitate electron-transfer between the electrode and the analyte, and therefore reduce the oxidation potential. Recently, pyrroloquinoline quinone (PQQ) modified electrodes have been developed for detection of endo- and exogenous thiols.17... 

E. Katz, T. Lotzbeyer, D. Schlereth, W. Schuhmann, and H.-L. Schmidt, Electrocatalytic oxidation of reduced nicotinamide coenzymes at gold and platinum electrode surfaces modified with a monolayer of pyrroloquinoline quinone. Effect of Ca2+ cations, J. Electroanal. Chem. 373, 189-200 (1994). 

The process very likely occurs in. the same way at a platinum anode. Hydroquinone itself, when oxidized electrolyfinally, yields only traces of quinone (Liebermann4), quinhydrone being the chief product. 

The reversible quinone-hydroquinone system also behaves in a somewhat unusual manner at a platinized platinum electrode there is little polarization other than that due to concentration changes at the electrode, both for oxidation and reduction. With other electrode materials, however, there is marked polarization, especially as the c.d. is increased at sufficiently high currents diffusion becomes rate determining, but at lower values the nature of the slow process is not at all clearly understood. The polarization appears to be influenced in an unexpected and complex manner by the hydrogen ion concentration of the electrolyte. ... 

Electrochemical detectors are based upon the volta-metric oxidation or reduction of separated analytes at a micro- or thin-film electrode. A number of pharmacologically active compounds that are aldehydes, ketones, or quinones (such as doxorubicin), or nitro compounds (such as nitrofurantoin) are amenable to reduction at a mercury or platinum electrode electron-rich indole derivatives and catecholamines can be oxidized at these electrodes. An important condition that must be fulfilled for electrochemical detection to be practicable is that the mobile phase must be capable of conducting an electrical current. This makes electrochemical detection particularly useful in reversed-phase liquid chromatography, where buffered water mixed with one or more organic cosolvents is usually the mobile phase. 

More properly, the above remark refers to the initial step of this reaction. Studies performed using platinum and Sn02 electrodes indicate that the quinone/hydroquinone redox reaction involves two distinct, consecutive charge transfer steps. Also the hydroquinone oxidation at the Ti02 photoanode follows presumably a two-step mechanism. 

Modified electrodes for this analytical purpose have mostly been formed by electrode adsorption of the mediator systems on the electrode surface or by electropolymerization [24,116]. Recently, for example, NAD(P)H oxidations have been performed on platinum or gold electrodes modified with a monolayer of pyrroloquinoline quinone (PQQ) [117] or on poly(methylene blue)-modified electrodes with different dehydrogenases entrapped in a Nafion film for the amperometric detection of glucose, lactate, malate, or ethanol [118]. In another approach, carbon paste electrodes doped with methylene green or meldola blue together with diaphorase were used for the NADH oxidation [119]. A poly(3-methylthio-phene) conducting polymer electrode was efficient for the oxidation of NADH [120]. By electropolymerization of poly(aniline) in the presence of poly(vinylsulfonate) counterions. 

BZ was al so partially oxidized by Ag(II) in a small H-cell with stationary platinum electrodes. Compounds identified in anolyte extracts included phenol, hydroquinone, benzoquinone, benzaldehyde, benzoic acid, methyl benzoate, benzonitrile, benzonitrile aldehyde, and 4-nitro butylnitrile. The yellow color of the anolyte was probably due to benzoquinone, which had a relatively high concentration. A compound which was tentatively identified as benzoquinone epoxide ( 11403) was present at the highest concentration and is believed to be a product of the oxidation of benzoquinone. Numerous nitrated aromatics were also detected and include nitrobenzene, dinitrobenzene isomers, nitrophenol isomers, and dinitnophenol isomers. Intermediates are summarized in Table 3 and classified as I. BZ substrate II. nitrated BZs HI. phenols, quinones, and epoxides IV. nitrated phenols V. BZ substituted with aliphatic and aromatic... 

LB chlorophyll monolayers (25 mN/m 1.4 nm2/molecule) on Pt electrodes showed low photoactivity, possibly caused by a quenching of excited states by the metal electrode or by total reversibility of electron exchange. Addition of electron acceptors, e.g., quinones, had no effect. The optically transparent tin oxide semiconductor electrode proves to be a much better subphase for the generation of photocurrents. Chlorophyll-coated Sn02 combined with a platinum electrode gave approximately 100 nA/cm. Similar results were obtained with photovoltaic systems of the form mercury droplet/buffer solution/chlorophyll a monolayer/electron acceptor monolayer/aluminum (Fig. 6.9.3). The quantum yield of such monolayer arrangements never exceeded 10" in any of these systems and is thus far away from competitive inorganic semiconductor cells (Norris and Meisel, 1989). 

In this medium, longer times are necessary to obtain a full coloration. In Figure 14.8, which shows the evolution with the polarization potential of the stabilized reflectance of a film standardized by the reflectance of bare platinum, the polarization time was 2,23 s. The IR absorption has quite disappeared, the main absorption at 2.16 eV is due to the charge transfer between benzenoid and quinoid rings and is characteristic of oxidized, unprotonated PANI forms. However, this band does not exist at potentials lower than 300 mV, which once more sustain the idea that C=N type sites can exist which are not directly associated to the presence of real quinone rings , whose presence is necessarily connected to the appearance of excitonic absorption. 

Glucose oxidase is immobilized in a polyacrylamide matrix. Oxygen and quinone which serve as mediators are reduced to hydrogen peroxide and hydroquinone, respectively. Since their oxidation starts at E, > 0.7-0.9 V, it is possible to displace the initial potential for the oxidation of glucose by 0.6-0.8 V towards positive values compared to its stationary electrochemical oxidation to gluconic acid on platinum. ... 

EC is most often used in the analysis of catecholamines and aromatic amines, since these compounds are easily oxidized at low potentials (Table 1). However, most alkaloids also contain oxidizable functional groups, and are well suited for oxidative EC detection. Many contain either a phenol group or an indole nucleus, and even more contain a tertiary aliphatic amine. In addition, many aliphatic alcohols and amines, which are oxidized at high potentials on carbon electrodes, can be detected at much lower potentials with gold or platinum electrodes (Sect. 2.4). Alkaloids, however, do not usually contain easily reducible groups like quinones or aromatic nitro groups. 

An alternative is the quinhydrone electrode, which is based on the electro-chemically reversible oxidation-reduction system of p-benzoquinone (quinone) and hydroquinone in which hydrogen ions participate (see also Chap. II.9). The construction is very simple, in that a noble metal wire, usually platinum, is introduced into a solution containing some crystals of quinhydrone. For pH < 9, the potential of this electrode depends on the pH of the adjacent solution according to... 

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