Oliloroplatinic acid, H,PtCI., is obtained by dissolving metallic platinum in aqua regia and then evaporating the solution. If this chloro platinic acid is stood over caustic potash for a time and the residue is heated in chlorine gas to a temperature of about 350° Centigrade, the tetrachloride of platinum (PtC14) is formed, one of the most important salts derived from the metal. The addition of sodium or potas sium to the tetrachloride gives, respectively, the chloroplatinate of sodium or of potassium. The salts of platinum are usefully applied as chemical reagents and in photography.
The finely divided form of platinum, known as "platinum black,o was discovered by Liebig. It possesses the striking property of absorbing and occluding oxygen in great quantity, taking up as much as 800 times its own bulk of the gas. This quality is also possessed to a marked but somewhat lesser degree by platinum sponge. Melted platinum, on cooling, emits the oxygen it has taken up with an intermittent crepitation. Ammonium platinum chloride, when heated to redness, becomes finely divided. A character istic of the so-called "platinum-sponge," which also absorbs and condenses gases, is that when brought into contact with hydrogen or coal gas, some of the occluded gas will be emitted as a flame. Many years ago Dobereiner utilized this property for a self-lighting lamp.
In the Bureau of Standards the purity of platinum ware is now tested by a simple ther moelectric method which leaves the object that has been tested absolutely unharmed. As the thermoelectric motive force of platinum against many of its alloys has been almost exactly de termined, this is used as a test of purity. In testing a crucible, for example, two wires of pure platinum only 0.1 millimeter or 0.2 milli meter in diameter, are arc-soldered to the rim and are connected with an ordinary pyrometer galvanometer or millivoltmeter. The junction point of one of these wires with the crucible is heated by a small oxyhydrogen flame or other blast flame, while the corresponding junction point of the other wire is kept cool by an air blast. An interposed sheet of asbestos prevents radiated heat passing from the heated to the cool part of the crucible; the temperatures are measured by a platinum thermocouple. The significance of the result is shown by reference to a chart giving the isothermal curves for the iridium or rhodium content of platinum.
While during the Great War the use of platinum for catalyzing processes in the pro duction of concentrated sulphuric acid for the manufacture of explosives, for dehydrating nitric acid and for aeroplane and other war machinery, has been made especially prominent, the wide employment of it in electric appliances of various kinds is of at least equal importance. Large amounts of it have been used for dental work, because of its durability and its resist ance to the action of acids. The latter reason has made it of the greatest possible value for various chemical vessels, such as crucibles, re torts, etc. Within the past 10 or 15 years it has been employed by manufacturing jewelers in many ways, for watch cases and for others of the smaller ornamental pieces, and for precious stone settings, more especially for diamond settings. Because of the imperative
demand for platinum in the great war indus tries, the War Industries Board decreed that after 1 Oct. 1918 the use of this metal in the manufacture of new stocks of jewelry or for other non-essential purposes should be pro hibited. The same law applied to iridium, palladium, osmium, ruthenium, rhodium and compounds thereof. It was said that the gov ernment had a sufficient supply on hand after commandeering the stock of unmanufactured metal, and the prohibition, therefore, did not extend to platinum jewelry already made up, which could be sold as heretofore, that is, by manufacturers or dealers licensed by the War Industries Board. The decree was the result of a special emergency act to effect government control over platinum, iridium and palladium, passed by Congress 6 Oct. 1917 and amended 1 July 1918. The situation at that time, after a careful survey, was recognized as serious. Platinum in the United States sold in 1890 at $16 per troy ounce and is selling now at $105 per troy ounce, five times the cost of gold. This situation arose directly from the em bargoes of 1915 put upon the exportation from the countries at war, England, France, Germany and Russia, and from the practical cessation of the Russian supply, followed by increased de mands after our entry into the war. Prior to the war Russia had furnished 95 per cent of the world's entire stock. All this has induced an active search for substitutes which might prove more or less satisfactory, and alloys of gold and palladium, or of gold and osmium, or gold and nickel, have been used with fairly good results in many cases. The shortage has also stimulated the search for platinum, not only in the metallic deposits of various parts of the United States, but throughout the entire world, and several new, minor sources have come to light. It is ppssible that a change in metallurgi cal methods may furnish a quantity of platinum and palladium from certain Canadian sources that is not now recovered, and that United States and Colombian resources' may be developed to a greater extent. Of the 5,000,000 or more ounces of platinum already in existence, it has been estimated very conservatively that catalyz ing processes have claimed 500,000 ounces, dental uses 1,000,000, chemical apparatus 1,000, 000 ounces, electrical devices 500,000 ounces and jewelry 500,000. These estimates are probably set rather too low in view of the fact that the total production of the metal has certainly been as much as 5,000,000 ounces; indeed some au thorities have placed the total Russian production alone as high as 7,000,000 ounces. Of course, this was crude platinum and would furnish only about 5,800,000 ounces of the refined metal. In the period from 1900 to 1917 the imports of un manufactured platinum, and of bars and ingots, into the United. States, have had a total weight of 52,767.2 kilos, or 1,696,711 troy ounces and a total value of $48,981,879. To this must be added imports of crucibles, retorts, etc., worth $2, 302,236, giving a total value of platinum in all forms of $51,284,115. Of course, a good pro portion of the import was crude platinum.