Carbon vitreous

Glassy, or vitreous, carbon is a black, shiny, dense, brittle material with a vitreous or glasslike appearance (10,11). It is produced by the controUed pyrolysis of thermosetting resins phenol—formaldehyde and polyurethanes are among the most common precursors. Unlike conventional artificial graphites, glassy carbon has no filler material. The Hquid resin itself becomes the binder. 

Of recent years the use of mercury film electrodes based on substrates other than platinum has been explored, and increased sensitivity is claimed for electrodes based on wax-impregnated graphite, on carbon paste and on vitreous carbon a technique of simultaneous deposition of mercury and of the metals to be determined has also been developed. 

A similar, but highly porous, vitreous carbon material—reticulated vitreous carbon (RVC)—has found widespread application for flow analysis and spectro-electrochemistry (25). As shown in Figure 4-10, RVC is an open-pore ( spongelike ) material such a network combines the electrochemical properties of glassy carbon with many structural and hydrodynamic advantages. These include a very high surface area ( 66 cm2 cm-3 for the 100-ppi grade), 90-97% void volume, and a low resistance to fluid flow. 

FIGURE 4-10 The open-pore structure of reticulated vitreous carbon. 

Redox switching, 126 Reference electrodes, 100, 105, 142 Reflectance spectroscopy, 44 Resistance, 22, 105 Resolution 50, 71 Reverse pulse polarography, 68 Reversible systems, 4, 31 Reticulated vitreous carbon, 114, 115 Riboflavin, 37... 

The redox properties of a series of heterometal clusters were assessed by electrochemical and FPR measurements. The redox potentials of derivatives formed in D. gigas Fdll were measured by direct square wave voltammetry promoted by Mg(II) at a vitreous carbon electrode, and the following values were determined 495, 420,... 

If the electrochemical sensor does not require a permselective membrane, immobilization of the enzyme onto the surface of the electrode is possible. Glassy carbon graphite reticulated vitreous carbon and carbon paste electrodes... 

Kirkpatrick, M.J., Finney, W.C. and Locke, B.R. (2003) Chlorinated organic compound removal by gas phase pulsed streamer corona electrical discharge with reticulated vitreous carbon electrodes, Plasmas and Polymers 8, 165-77. 

These authors have also developed a cell98 (Fig. 26) that employs reticulated vitreous carbon as a working electrode and they find that such a design allows for much faster electrolysis. Using such a cell, they have studied the [Ru(NH3)6]3+/2+ couple... 

Shaw et al. [64] described a (D)-penicillamine detection method in blood samples that had been treated with EDTA, deproteinized with trichloroacetic acid, and analyzed within 1 h. Penicillamine was detected at a vitreous-carbon electrode operated at +800 mV after HPLC separation. A linear calibration graph was obtained, and the method had a limit of detection equal to 5-20 ng. The method was useful in clinical and in pharmacokinetic studies. 

Yoshida has studied anodic oxidations in methanol containing cyanide to elucidate the electrode processes themselves.288 He finds that, under controlled potential ( 1.2 V), 2,5-dimethylfuran gives a methoxynitrile as well as a dimethoxy compound (Scheme 57). Cyanide competes for the primary cation radical but not for the secondary cations so that the product always contains at least one methoxy group. On a platinum electrode the cis-trans ratio in the methoxynitrile fraction is affected by the substrate concentration and by the addition of aromatic substances suggesting that adsorption on the electrode helps determine the stereochemistry. On a vitreous carbon electrode, which does not strongly adsorb aromatic species, the ratio always approaches the equilibrium value. 

Florence [98] carried out anodic stripping voltammetry of bismuth in a weakly acidic medium, with a polished vitreous carbon electrode mercury-... 

A method described by Florence and Farrer [584] separated tin from its associated lead by distillation from an aqueous sulfuric acid medium into which the vapour from boiling 50% hydrobromic acid is passed. The distillate provides an ideal supporting electrolyte for the determination of tin (II) (produced by reduction with hydrazinium hydroxide) by anodic stripping at a rotating vitreous-carbon electrode in the presence of codeposited mercury [585,586]. The tin is deposited at -0.70 V versus the SCE for 5 minutes, and then stripped at -0.50 V during a sweep from -0.70 V to -0.45 V at 5 V per minute. Tin in seawater is coprecipitated on ferric hydroxide, and the precipitate is then dissolved in the aqueous sulfuric acid, and subjected to the above procedure. The average content for Pacific coastal waters was found to be 0.58 xg/l. 

A variety of electrodes have been used in this technique, including rotating mercury coated vitreous carbon [756], wax impregnated graphite cylinders [758], and hanging mercury drops [753-755]. 

Several approaches have been undertaken to construct redox active polymermodified electrodes containing such rhodium complexes as mediators. Beley [70] and Cosnier [71] used the electropolymerization of pyrrole-linked rhodium complexes for their fixation at the electrode surface. An effective system for the formation of 1,4-NADH from NAD+ applied a poly-Rh(terpy-py)2 + (terpy = terpyridine py = pyrrole) modified reticulated vitreous carbon electrode [70]. In the presence of liver alcohol dehydrogenase as production enzyme, cyclohexanone was transformed to cyclohexanol with a turnover number of 113 in 31 h. However, the current efficiency was rather small. The films which are obtained by electropolymerization of the pyrrole-linked rhodium complexes do not swell. Therefore, the reaction between the substrate, for example NAD+, and the reduced redox catalyst mostly takes place at the film/solution interface. To obtain a water-swellable film, which allows the easy penetration of the substrate into the film and thus renders the reaction layer larger, we used a different approach. Water-soluble copolymers of substituted vinylbipyridine rhodium complexes with N-vinylpyrrolidone, like 11 and 12, were synthesized chemically and then fixed to the surface of a graphite electrode by /-irradiation. The polymer films obtained swell very well in aqueous... 

Cyclic voltammetry experiments were controlled using a Powerlab 4/20 interface and PAR model 362 scanning potentiostat with EChem software (v 1.5.2, ADlnstruments) and were carried out using a 1 mm diameter vitreous carbon working electrode, platinum counter electrode, and 2 mm silver wire reference electrode. The potential of the reference electrode was determined using the ferrocenium/ ferrocene (Fc+/Fc) couple, and all potentials are quoted relative to the SCE reference electrode. Against this reference, the Fc /Fc couple occrus at 0.38 V in acetonitrile and 0.53 V in THF [30, 31]. 

Glassy carbon (vitreous carbon) is a further modification. It is a smooth and tight material of high corrosion resistance and for many reactions it may be a suitable alternative of platinum. It is relatively expensive, but much cheaper than noble metals. It is very hard and brittle and allows only limited possibilities for... 

The oxidation of enol ethers at a reticulated vitreous carbon anode [2, 3] (Scheme 1) in a mixture of methanol/THF containing tetraethylammonium tosylate as the electrolyte and 2,6-lutidine as the base leads to substituted tetrahydrofu-ran and tetrahydropyran rings in good yields (51-96%). The major product obtained had a trans-stereochemistry. The cyclization failed to make seven-membered ring products. In order to determine the... 

The direct reduction of haloalkynes using either mercury or vitreous carbon as the cathode has been examined in considerable detail [80-84] one example is portrayed in Eq (77). The influence of reduction potential, current consumption, proton donor, electrode, and substrate concentration on the course of the process has been examined. Vitreous carbon electrodes are preferred, though mercury has been used in many instances. Unfortunately, these reactions suffer from the formation of diorganomercurials. While both alkyl iodides and bromides can be used, the former is generally preferred. Because of their higher reduction potential, alkyl chlorides react via a different mechanism, one involving isomerization to an allene followed by cyclization [83]. 

The first coupling reaction of this type studied utilized a 3-methoxyphenyl ring as the aryl coupling partner (Scheme 36) [47a, c]. The reaction employed constant current electrolysis conditions and a reticulated vitreous carbon anode (RVC). A good yield of cyclized material was obtained. However, the reaction was plagued by the formation of secondary products derived from over-oxidation (35 and 36) of the initially formed cyclization products (33 and 34). The amount of over-oxidized material could be greatly reduced with the use of controlled potential electrolysis conditions. 

Glassy carbon. We have not really mentioned glassy carbon (GC) before since so few analysts use it. GC is a conductive form of carbon made by pyrolysing carbon or graphite. For this reason, it is sometimes called vitreous carbon or pyrolytic carbon . GC is very hard - as diamond is - thus making it difficult to machine and hence to make electrodes. This hardness also explains why glassy carbon is relatively expensive. 

Glassy carbon (GC) A form of carbon that is extremely hard and highly conductive to electrons, and is hence a good choice of material for constructing inert electrodes (also known as vitreous carbon). 

Figure 3. Electrolyses of 50 mM Na2S04 adjusted to pH 2 continuously saturated with O2 at a reticulated vitreous carbon cathode in a flow-cell. Plots of H2O2 formed versus electrolysis time for...

In a similar electrolysis (Alvarez-Gallegos and Fletcher 1998), hydrogen peroxide was electrogenerated in the presence of 1 mM Fe and during the electrolysis, aliquots of the catholyte were taken and voltammograms were recorded for each one at a rotating vitreous carbon disc electrode between the potential limits, + 1.100 V and -0.400 V (SCE). The oxidation... 

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