Platinum isolation

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... 

Another problem in the construction of tlrese devices, is that materials which do not play a direct part in the operation of the microchip must be introduced to ensure electrical contact between the elecuonic components, and to reduce the possibility of chemical interactions between the device components. The introduction of such materials usually requires an annealing phase in the construction of die device at a temperature as high as 600 K. As a result it is also most probable, especially in the case of the aluminium-silicon interface, that thin films of oxide exist between the various deposited films. Such a layer will act as a banier to inter-diffusion between the layers, and the transport of atoms from one layer to the next will be less than would be indicated by the chemical potential driving force. At pinholes in the AI2O3 layer, aluminium metal can reduce SiOa at isolated spots, and form the pits into the silicon which were observed in early devices. The introduction of a tlrin layer of platinum silicide between the silicon and aluminium layers reduces the pit formation. However, aluminium has a strong affinity for platinum, and so a layer of clrromium is placed between the silicide and aluminium to reduce the invasive interaction of aluminium. 

As already noted (p. 1073), the platinum metals are all isolated from concentrates obtained as anode slimes or converter matte. In the classical process, after ruthenium and osmium have been removed, excess oxidants are removed by boiling, iridium is precipitated as (NH4)2lrCl6 and rhodium as [Rh(NH3)5Cl]Cl2. In alternative solvent extraction processes (p. 1147) [IrClg] " is extracted in organic amines leaving rhodium in the aqueous phase to be precipitated, again, as [Rh(NH3)5Cl]Cl2. In all cases ignition in H2... 

Frenchman P. F. Chabeneau, and subsequently in London by W. H. Wollaston/ who in the years 1800-21 produced well over 1 tonne of malleable platinum. These techniques were developed because the chemical methods used to isolate the metal produced an easily powdered spongy precipitate. Not until the availability, half a century later, of furnaces capable of sustaining sufficiently high temperatures was easily workable, fused platinum commercially available. 

In 1803, in the course of his study of platinum, Wollaston isolated and identified palladium from the mother liquor remaining after platinum had been precipitated as (NH4)2PtCl6 from its solution in aqua regia. He named it after the newly discovered asteroid, Pallas, itself named after the Greek goddess of wisdom (ytaXXd iov, palladion, of Pallas). 

Ciamician and Dennstedt reacted the potassium salt of pyrrole with chloroform in ether and isolated, after much purification, 3-chloropyridine, which was confirmed by crystallization with platinum. While the pyrrole salt can be used as the base, the chloroform carbene is typically formed with an alkali alcohol. Forty years later, Robinson and co-workers made 3-chloroquinolines from indoles using the Ciamician-Dennstedt reaction. ... 

The reaction product (1-carbethoxymethyM-carbomethoxy-pyridinium bromide) was obtained in crystalline form. (It formed prisms melting at 166°-169°C after recrystallization from a mixture of isopropanol and acetone.) It was not necessary to isolate it. For the following reduction step, the reaction mixture was brought into solution by the addition of about 1 liter of warm ethyl alcohol. It was then hydrogenated at about 30 atm pressure in the presence of 2 g of platinum oxide. The temperature rose during this reaction to about 40°C. 

The excellent resistance of platinum, rhodium and iridium to oxidation at high temperatures finds numerous applications in technology, in particular in the form of platinum-based alloys. Osmium and ruthenium form volatile oxides which may be isolated (OSO4 and RujOj), and they are not widely used. 

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

Until about 40 years ago, these elements were referred to as "inert gases" they were believed to be entirely unreactive toward other substances. In 1962 Neil Bartlett, a 29-year-old chemist at the University of British Columbia, shook up the world of chemistry by preparing the first noble-gas compound. In the course of his research on platinum-fluorine compounds, he isolated a reddish solid that he showed to be 02+(PtFB-). Bartlett realized that the ionization energy of Xe (1170 kJ/mol) is virtually identical to that of the 02 molecule (1165 kJ/mol). This encouraged him to attempt to make the analogous compound XePtF6. His success opened up a new era in noble-gas chemistry. 

Apparatus. Use the apparatus shown in Figs. 14.2(a) and (b). The generator cathode (isolated auxiliary electrode) consists of platinum foil (4 cm x 2.5 cm, bent into a half cylinder) and the generator anode (working electrode) is a rectangular platinum foil (4 cm x 2.5 cm). For potentiometric end point detection, use a platinum-foil electrode 1.25 cm x 1.25 cm (or a silver-rod electrode) in combination with an S.C.E. connected to the cell by a potassium chloride- or potassium nitrate-agar bridge. 

Apparatus. Use the apparatus of Section 14.7. The generator anode is of pure silver foil (3 cm x 3 cm) the cathode in the isolated compartment is a platinum foil (3 cm x 3 cm) bent into a half-cylinder. For the potentiometric end point detection, use a short length of silver wire as the indicator electrode the electrical connection to the saturated calomel reference electrode is made by means of an agar-potassium nitrate bridge. 

Palladium and platinum are the longest known and most studied of the six platinum metals [1-11], a reflection of their abundance and consequent availability. Platinum occurs naturally as the element, generally with small amounts of the other platinum metals. It was used as a silver substitute by Colombian Indians and first observed there by Ulloa (1736), who called it platina del Pinto ( little silver of the Pinto river ) but the first sample was actually brought to Europe in 1741 by Charles Wood, Assay Master of Jamaica. Palladium was isolated in 1803 by W.H. Wollaston, who was studying the aqua regia-soluble portion of platinum ores (he announced his discovery by an anonymous leaflet advertising its sale through a shop in Soho) and named it after the newly discovered asteroid Pallas [12],... 

Compared with the plethora of platinum(IV) compounds, the palladium(IV) complexes are as yet relatively few in number [10, 11]. When isolable, they tend to resemble the corresponding platinum compounds. 

Measurements of the true specific heats at low temperatures have been carried out by Eucken, who worked in such a way that to a weighed quantity of the substance in the form of a block, or in a proper isolated vessel, a known quantity of heat was added by means of an electrically heated platinum spiral, the resistance of wThich at the same time served to measure the temperature. The quotient of the electrical energy spent by the rise of temperature gives the specific heat. The correction for cooling (in vacuum of 5 Jo mm.) amounted to 20 per cent., and for heat capacity of the apparatus 5 per cent., yet the results are stated to be accurate to 1 per cent. 

Recently it was demonstrated that a platinum black-PTFE electrode, originally designed for a fuel cell, is excellent for the chlorination of double bonds and, depending on the other electrolysis conditions, it was possible to isolate the dichlorocompound or the chlorohydrin (Danger and Yurchak, 1970). Moreover, if a chlorine cathode is used, the overall process occurs with a net output of energy, i.e. the cell may do external work and the procedure has been named electrogenerative chlorination . 

---