Work platinum

Figure 7 Typical cell for electrolysis with a working platinum gauze electrode...

Carbonaceous Overlayers What is the composition of the working platinum catalyst surface When the surface is examined after carrying out any one of the hydrocarbon conversion reactions, it is always covered by a near-monolayer amount of carbonaceous deposit. 

One other type of chemically modified electrode has been used in the study of cytochrome c, as described by Lewis and Wrighton. In this work platinum, gold, and p-type silicon substrates were modified with a polymeric overlayer which contained bipyridinium-type one-electron redox centers. These immobilized mediators transfer electrons between the electrode surface and diffusing ferricytochrome c at the potential of the mediators which is ca. 0.59 V more negative than the formal potential of cytochrome c. Oxidation of ferrocytochrome c is precluded because of the difference in the formal potentials of cytochrome c and the immobilized mediator. This same group has recently immobilized 2,3,4,5-tetramethyl-l-(dichlorosilylmethyl)-[2]-ferrocenophane on platinum. The juxtaposition of the formal potentials... 

A DC potential may develop at the electrode metal/solution interphase. The absolute potential of this interphase (half-cell electrode potential) cannot be measured—it must be considered unknown. However, the potential difference between two electrodes can be measured with an ordinary voltmeter connected to the two metal wires from the electrodes. If file metals were different, then they could generate a potential difference of 1 V or more. However, here we presume that the same electrode material is used and that the measured potential difference is small. We will discuss the case for three different electrode materials important in biological work platinum, silver coated with silver chloride (AgCl), and carbon. To the extent that both electrodes are equal, we have a symmetrical (bipolar) system, and the voltage—current dependence should not be dependent on polarity. 

There actually are such membranes in common use in experimental work. Platinum at high temperatures is permeable to hydrogen but not to other substances, and this fact is extensively used to control the oxidation state of experiments. This is discussed further in 11.5.2. 

In general, the same precautions regarding crucible materials must be observed for the dihalides as for the trihalides, cf. ch. 32, section 2.1. Although noble metal containers were employed in much of the early work, platinum is particularly unsuited because of the formation of stable R-Pt alloys (Brewer et al., 1950 Bedford and Catalano, 1970). Several of the dihalides melt and vaporize congruently, cf. ch. 32, section 4.4.3, and can be purified by sublimation or distillation and single crystals can be grown from the melts. The majority of the reduced halides are thermally unstable and preparation of high purity samples is difficult. 

It is advisable to test a small portion of the filtrate for platinum by acidifying with hydrochloric acid and adding a few drops of stannous chloride solution a yellow or brown colour develops according to the quantity of platinum pVesent. The yellow colour is soluble in ether, thus rendering the t t more sensitive. If platinum is found, treat the filtrate with excess of formaldehyde and sodium iQrdroxide solution and heat,- platinum black septarates on standing and may be filtered and worked up with other platinum residues (see Method 3). 

The element is a steel-white metal, it does not tarnish in air, and it is the least dense and lowest melting of the platinum group of metals. When annealed, it is soft and ductile cold-working greatly increases its strength and hardness. Palladium is attacked by nitric and sulfuric acid. 

L. Rhenus, Rhine) Discovery of rhenium is generally attributed to Noddack, Tacke, and Berg, who announced in 1925 they had detected the element in platinum ore and columbite. They also found the element in gadolinite and molybdenite. By working up 660 kg of molybdenite in 1928 they were able to extract 1 g of rhenium. 

Iridium, a metal of the platinum family, is white, similar to platinum, but with a slight yellowish cast. Because iridium is very hard and brittle, it is hard to machine, form, or work. 

Does the platinum working electrode serve as the cathode or the anode in this analysis ... 

At the start of the nineteenth century, platinum was refined in a scientific manner by William Hyde WoUaston, resulting in the successful production of malleable platinum on a commercial scale. During the course of the analytical work, WoUaston discovered paUadium, rhodium, indium, and osmium. Ruthenium was not discovered until 1844, when work was conducted on the composition of platinum ores from the Ural Mountains. 

There are several exceUent sources of information about the platinum-group metals. The exceUent reference work G. Wilkinson, R. D. GiUard, and J. A. McCleverty, eds.. Comprehensive Coordination Chemistry Pergamon Press, Oxford, U.K., 1987, contains iadividual chapters devoted to descriptive chemistry of each element. 

Two works provide exceUent coUections of articles describiag important modem appfications of PGMs. These are E. H. Hardey, ed.. The Chemistry of Platinum Group Metals, Elsevier, 1991, and Gmelin Handbook of Inorganic Chemistry Platinum Suppl Vol. Al, Sptinger-Vedag, New York, 1986. 

The formation of acids from heteroatoms creates a corrosion problem. At the working temperatures, stainless steels are easily corroded by the acids. Even platinum and gold are not immune to corrosion. One solution is to add sodium hydroxide to the reactant mixture to neutralize the acids as they form. However, because the dielectric constant of water is low at the temperatures and pressure in use, the salts formed have low solubiHty at the supercritical temperatures and tend to precipitate and plug reaction tubes. Most hydrothermal processing is oxidation, and has been called supercritical water oxidation. 

Whereas it is no longer an iaterpolation standard of the scale, the thermoelectric principle is one of the most common ways to transduce temperature, although it is challenged ia some disciplines by small iadustrial platinum resistance thermometers (PRTs) and thermistors. Thermocouple junctions can be made very small and ia almost infinite variety, and for base metal thermocouples the component materials are very cheap. Properties of various types of working thermocouple are shown in Table 3 additional properties are given in Reference 5. 

Diacetone-L-sorbose (DAS) is oxidized at elevated temperatures in dilute sodium hydroxide in the presence of a catalyst (nickel chloride for bleach or palladium on carbon for air) or by electrolytic methods. After completion of the reaction, the mixture is worked up by acidification to 2,3 4,6-bis-0-isoptopyhdene-2-oxo-L-gulonic acid (2,3 4,6-diacetone-2-keto-L-gulonic acid) (DAG), which is isolated through filtration, washing, and drying. With sodium hypochlorite/nickel chloride, the reported DAG yields ate >90% (65). The oxidation with air has been reported, and a practical process was developed with palladium—carbon or platinum—carbon as catalyst (66,67). The electrolytic oxidation with nickel salts as the catalyst has also... 

Potentiometric Titrations. If one wishes to analyze electroactive analytes that are not ions or for which ion-selective electrodes are not available, two problems arise. First, the working electrodes, such as silver, platinum, mercury, etc, are not selective. Second, metallic electrodes may exhibit mixed potentials, which may arise from a variety of causes. For example, silver may exchange electrons with redox couples in solution, sense Ag" via electron exchange with the external circuit, or tarnish to produce pH-sensitive oxide sites or Ag2S sites that are sensitive to sulfide and haUde. On the other... 

The Kad Fischer jack on the back of most pH meters, used to monitor Kad Fischer titrations, suppHes a constant regulated current to the cell, which can consist of two identical (platinum) working electrodes. The voltammograms shown in Figure 9 illustrate the essential features of this technique. The initial potential difference, AH, is small because both redox forms of the sample coexist to depolarize the electrodes. The sample corresponds to the wave on the right-hand (cathodic) side of each figure and is therefore easily oxidized. The titrant is represented by the wave on the left-hand (anodic) side and is therefore easily reduced. Halfway to the end point the potential difference,, remains small, but at the end point the potential difference,... 

Since NO production depends on the flame temperature and quantity of excess air, achieving required limits may not be possible through burner design alone. Therefore, many new designs incorporate DENOX units that employ catalytic methods to reduce the NO limit. Platinum-containing monolithic catalysts are used (36). Each catalyst performs optimally for a specific temperature range, and most of them work properly around 400°C. 

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