Iridium arsenide

Iridium Arsenide, IrAs2, has been obtained in a pure form by heating iridium chloride with an excess of arsenic in a current of hydrogen.4 It is also formed when an intimate mixture of the finely divided metal and excess of arsenic is heated in an indifferent atmosphere. The arsenide may be analysed by the method described under palladium arsenide (p. 73). 

More than 200 ores are known to contain cobalt but only a few are of commercial value. The more important are arsenides and sulfides such as smaltite, C0AS2, cobaltite (or cobalt glance), CoAsS, and linnaeite, C03S4. These are invariably associated with nickel, and often also with copper and lead, and it is usually obtained as a byproduct or coproduct in the recovery of these metals. The world s major sources of cobalt are the African continent and Canada with smaller reserves in Australia and the former USSR. All the platinum metals are generally associated with each other and rhodium and iridium therefore occur wherever the other platinum metals are found. However, the relative proportions of the individual metals are by no means constant and the more important sources of rhodium are the nickel-copper-sulfide ores found in South Africa and in Sudbury, Canada, which contain about 0.1% Rh. Iridium is usually obtained from native osmiridium (Ir 50%) or iridiosmium (Ir 70%) found chiefiy in Alaska as well as South Africa. 

Ruthenium, osmium, rhodium, iridium, palladium and platinum are the six heaviest members of Group VII1. They are rare elements platinum itself is the commonest with an abundance of about 10-6% whereas the others have abundances of the order of 10"7 % of the earth s crust. They occur in Nature as metals, often as alloys such as osmiridium, and in arsenide, sulfide and other ores. The elements are usually associated not only with one another but also with the coinage metals copper, silver and gold. The main suppliers are South Africa, Canada and the USSR. 

Wohler obtained phosphorus by strongly heating a mixture of sand and bone-black (calcium phosphate and carbon) — the modern process, now carried out in the electric furnace. He prepared artificial nickel arsenide and worked on the separation of iridium and osmium in a pound of platinum residues given him by Dumas on a visit to Paris in 1833. A supposed silver suboxide, Ag40, prepared by Wohler by the action of caustic potash solution on a supposed subcitrate made by heating silver citrate in hydrogen at 100°, was not analysed and was probably a mixture of silver and silver oxide AggO the red colour of the solution of the subcitrate was probably due to colloidal silver. ... 

Platinum has a certain tendency to form sulfides and arsenides, as for instance in the minerals cooperite PtS and sperrylite PtASj. The incHnation of platinum to form sulfides has the consequence that platinum often occurs with copper and nickel ores, especially pendandite. The metals are, however, mainly found as metallic phases, alloys with each other, with gold and with iron. The important alloy osmiridium with high contents of osmium and iridium also occurs in nature. 

The development of a new generation of catalysts and semiconductors will be needed if a photochemical process (using solar radiation) is to be viable, but research and development in this area is growing rapidly. The sources of hydrogen include hydrocarbons, alcohols, and organic acids. Ruthenium, rhodium, and iridium catalysts, and various arsenide, selenide, and telluride semiconductors (see Chapter 15), may lead the way to an efficient system. 

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