Platinum-Iridium

The element is silvery white with a metallic luster its density is exceeded only by that of platinum, iridium, and osmium, and its melting point is exceeded only by that of tungsten and... 

Although its principal use is as a hardening agent for platinum, iridium is also used to make crucibles and devices requiring high temperatures. It is also used for electrical contacts. 

Mass kilogram kg Mass of a cylinder of platinum-iridium alloy kept at Paris. 

Analytical chemists make a distinction between calibration and standardization. Calibration ensures that the equipment or instrument used to measure the signal is operating correctly by using a standard known to produce an exact signal. Balances, for example, are calibrated using a standard weight whose mass can be traced to the internationally accepted platinum-iridium prototype kilogram. 

Variations in the Force Due to Gravity. The mass of an object is the quantity of matter ia the object. It is a fundamental quantity that is fixed, and does not change with time, temperature, location, etc. The standard for mass is a platinum—iridium cylinder, called the International Kilogram, maintained at the International Bureau of Weights and Measures, ia Snvres, France. The mass of this cylinder is 1 kg by definition (9). AH national mass standards are traceable to this artifact standard. 

Under severe conditions and at high temperatures, noble metal films may fail by oxidation of the substrate base metal through pores in the film. Improved life may be achieved by first imposing a harder noble metal film, eg, rhodium or platinum—iridium, on the substrate metal. For maximum adhesion, the metal of the intermediate film should ahoy both with the substrate metal and the soft noble-metal lubricating film. This sometimes requires more than one intermediate layer. For example, silver does not ahoy to steel and tends to lack adhesion. A flash of hard nickel bonds weh to the steel but the nickel tends to oxidize and should be coated with rhodium before applying shver of 1—5 p.m thickness. This triplex film then provides better adhesion and gready increased corrosion protection. 

Platinum—Iridium. There are two distinct forms of 70/30 wt % platinum—iridium coatings. The first, prepared as prescribed in British patents (3—5), consists of platinum and iridium metal. X-ray diffraction shows shifted Pt peaks and no oxide species. The iridium [7439-88-5] is thus present in its metallic form, either as a separate phase or as a platinum—iridium intermetallic. The surface morphology of a platinum—iridium metal coating shown in Figure 2 is cracked, but not in the regular networked pattern typical of the DSA oxide materials. 

Fig. 2. Scanning electron microscope photograph of platinum—iridium metal coating on titanium.

The second form consists of Pt metal but the iridium is present as iridium dioxide. Iridium metal may or may not be present, depending on the baking temperature (14). Titanium dioxide is present in amounts of only a few weight percent. The analysis of these coatings suggests that the platinum metal acts as a binder for the iridium oxide, which in turn acts as the electrocatalyst for chlorine discharge (14). In the case of thermally deposited platinum—iridium metal coatings, these may actually form an intermetallic. Both the electrocatalytic properties and wear rates are expected to differ for these two forms of platinum—iridium-coated anodes. 

P. S. S. Hayfteld and W. R. Jacob, "Platinum—Iridium-Coated Titanium Anodes ia Brine Electrolysis," paper presented 2i. A.dvances in Chlor-A.lkali Technology, London, 1979. 

Selected physical properties of rhenium are summarized ia Table 1. The metal is silvery-white and has a metallic luster. It has a high density (21.02 g/cm ). Only platinum, iridium, and osmium have higher densities. The melting poiat of rhenium is higher than that of all other elements except tungsten (mp 3410°C) and carbon (mp 3550°C). 

Sulfur combines direcdy and usually energetically with almost all of the elements. Exceptions include gold, platinum, iridium, and the hehum-group gases (19). In the presence of oxygen or dry air, sulfur is very slowly oxidized to sulfur dioxide. When burned in air, it forms predominantly sulfur dioxide with small amounts of sulfur trioxide. When burned in the presence of moist air, sulfurous acid and sulfuric acids are slowly generated. 

This international prototype, adopted by the 1st and 3rd CGPM in 1889 and 1901, is a particular cylinder of platinum—iridium kept at the International Bureau of Weights and Measures near Paris. It is the only base unit still defined by an artifact. 

Iridium on valve metals is suitable if the consumption rate of platinum is too high at elevated temperatures or critical composition of the medium. Mostly platinum-iridium alloys are used with about 30% Ir, because coating valve metals with pure iridium is somewhat complicated. For the same reason, other noble metals such as rhodium cannot be used [21]. At present there is little price difference between platinum and iridium. 

Rhodium and iridium have a resistance to anodic corrosion comparable with that of platinum, and are more resistant to the influence of alternating currents. A platinum-iridium alloy, in the form of a coating on titanium, is preferred to pure platinum for the production of chlorine from brine , due to its improved corrosion resistance and lower overvoltage. 

Iridium has been deposited from chloride-sulphamate and from bromide electrolytes , but coating characteristics have not been fully evaluated. The bromide electrolytes were further developed by Tyrrell for the deposition of a range of binary and some ternary alloys of the platinum metals, but, other than the platinum-iridium system, no commercial exploitation of these processes has yet been made. 

Platinum Platinum-coated titanium is the most important anode material for impressed-current cathodic protection in seawater. In electrolysis cells, platinum is attacked if the current waveform varies, if oxygen and chlorine are evolved simultaneously, or if some organic substances are present Nevertheless, platinised titanium is employed in tinplate production in Japan s. Although ruthenium dioxide is the most usual coating for dimensionally stable anodes, platinum/iridium, also deposited by thermal decomposition of a metallo-organic paint, is used in sodium chlorate manufacture. Platinum/ruthenium, applied by an immersion process, is recommended for the cathodes of membrane electrolysis cells. ... 

For scientific work the fundamental standard of mass is the international prototype kilogram, which is a mass of platinum-iridium alloy made in 1887 and deposited in the International Bureau of Weights and Measures near Paris. Authentic copies of the standard are kept by the appropriate responsible authorities in the various countries of the world these copies are employed for the comparison of secondary standards, which are used in the calibration of weights for scientific work. The unit of mass that is almost universally employed in laboratory work, however, is the gram, which may be defined as the one-thousandth part of the mass of the international prototype kilogram. 

The introduction of the sample into the adsorbent layer is a critical process in HPTLC. For most quantitative work a platinum-iridium capillary of fixed volume (100 or 200 nL), sealed into a glass support capillary of larger bore, provides a convenient spotting device. The capillary tip is polished to provide a smooth, planar surface of small area (ca 0.05 mm2), which when used with a mechanical applicator minimises damage to the surface of the plate spotting by manual procedures invariably damages the surface. 

Because of- the similarity in the backscattering properties of platinum and iridium, we were not able to distinguish between neighboring platinum and iridium atoms in the analysis of the EXAFS associated with either component of platinum-iridium alloys or clusters. In this respect, the situation is very different from that for systems like ruthenium-copper, osmium-copper, or rhodium-copper. Therefore, we concentrated on the determination of interatomic distances. To obtain accurate values of interatomic distances, it is necessary to have precise information on phase shifts. For the platinum-iridium system, there is no problem in this regard, since the phase shifts of platinum and iridium are not very different. Hence the uncertainty in the phase shift of a platinum-iridium atom pair is very small. 

Figure 5. X-ray absorption spectrum of a silica supported platinum-iridium catalyst at 100 K in the region of the L absorption edges of platinum and iridium. Reproduced with permission from Ref. 13. Copyright 1982, American Institute of Physics.

From results on interatomic distances derived from analysis of EXAFS data, one can draw some conclusions about the structure of platinum-iridium clusters (13,17). If the clusters were truly homogeneous, the interatomic distance characteristic of the platinum EXAFS should be identical to that characteristic of the iridium EXAFS. When we analyze EXAFS data on the clusters, however, we do not find this simple result. We find in general that the distances are not equal. The data indicate that the clusters are not homogeneous in other words,the environments about the platinum and iridium are different. We conclude that the platinum concentrates at the surface or boundary of the clusters. In the case of very highly dispersed platinum-iridium clusters on alumina, the clusters may well have "raft-like" two dimensional structures, with platinum... 

Recently we reported EXAFS results on bimetallic clusters of iridium and rhodium, supported on silica and on alumina (15). The components of this system both possess the fee structure in Efie metallic state, as do the components of the platinum-iridium system. The nearest neighbor interatomic distances in metallic iridium and rhodium are not very different (2.714A vs. 2.690A). From the results of the EXAFS measurements, we concluded that the interatomic distances corresponding to the various atomic pairs (i.e., iridium-iridium, rhodium-rhodium, and iridium-rhodium) in the clusters supported on either silica or alumina were equal within experimental error. Since the Interatomic distances of the pure metals differ by only 0.024A, the conclusion is not surprising. 

The unit of length, or distance, is the meter. Originally conceived of as one ten-millionth of the distance from the north pole to the equator through Paris, the meter is more accurately defined as the distance between two scratches on a platinum-iridium bar kept in Paris. The U.S. standard is the... 

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