Platinum is the best catalyst for the manufacture of sulphuric acid by the contact process ; at a temperature below a red heat it is capable of causing the reaction 2S02+0,=250, to be nearly completed. The resulting SO, is dissolved in water till an acid of the required strength is obtained (see SULPHURIC ACID). Its use for this purpose has been partly given up, as some cheaper catalysts such as the oxides of iron and chromium, although less efficient, have been found more economical. In this process platinum is always used as platinum black. The material to be platinized (asbestos, pumice, brick, barium sulphate, etc.) is dipped in a dilute solution of platinum chloride, dried and ignited, an enormous catalytic surface of platinum black being thus pro duced. Another large and rapidly extending use for platinum is in the catalytic production of nitric acid from ammonia by oxida tion. A mixture of ammonia gas and air (or oxygen) in the right proportion is passed in a very rapid stream over a network of heated platinum gauze. Either ammonium nitrate or nitric acid can be produced according to the proportions of oxygen and ammonia in the reaction mixture. The reaction is exothermic, and by adjusting the rate of flow of the mixed gases the reaction temperature can be kept constant.
Platinum or, better, iridium-platinum alloy is the most suitable anode material for electrolysis of all kinds. It remains unattacked under practically all conditions, but owing to the initial expense, some form of carbon, especially Acheson graphite, is now gener ally used for this purpose. Two methods of measuring high temperatures depend upon the use of platinum ; the first is the platinum resistance thermometer, which depends upon the change of resistance of a platinum coil with temperature when a constant current is passing through it; the second is the use of a platinum and rhodium–platinum thermocouple. By careful calibration either of these instruments is capable of giving a very exact measurement of high temperatures. As platinum is not oxidized by air at the temperature of the electric spark, it is very suitable in electro-technics for contact-points of magnetos and induction coils and the armatures of electric bells: it can also be used when caused to glow by the passage of an electric current as a heating-element ; for some of these uses it has now been displaced by cheaper materials, one of the best being a chromium–nickel alloy known as nichrome.
Platinum is extensively used for some surgical instruments such as needles for hypodermic syringes, for the metal can be sterilized even in the flame of a match; it is also used in the incandescent state as an electric cautery. Unfortunately it can not be hardened sufficiently to make a good cutting edge. In dentistry platinum was formerly used extensively for making dental plates, etc., as an alloy of 2 parts of silver to one of platinum possesses the same coefficient of expansion as the hard porcelain-like material of which artificial teeth are composed. Owing to the rise in the price of platinum, gold or gold alloys have now almost entirely displaced it for this purpose. The use of platinum in jewellery is considerable; during the World War palladium replaced it to a large extent and now an alloy called white gold, which consists chiefly of gold alloyed with a small quantity of either palladium or platinum, is largely displacing it. Another use for platinum is the production of photographic prints by the platinotype process. These prints are of great beauty owing to the variety of tones that can be thus produced, and have the advantage over silver prints of being absolutely permanent.
Platinum Compounds.—Platinum does not combine directly with oxygen at any temperature, although in the molten state it absorbs several times its volume of this gas, which, however, is released when the metal cools. Four oxides of platinum are
obtainable by indirect methods, viz., PtO, and Of these, Pt0 and are important as they are the bases of the platinous and platinic salts. Platinous oxide, PtO, is obtained by gentle ignition of the corresponding hydrate, PtO, which is obtainable by precipitating either or with caustic alkali. It is a black powder, soluble in acids when freshly prepared. Platinic oxide, Pt0,, is known in the hydrated form Pt(OH)„ but only a portion of the combined water can be removed before deeper decomposition begins to take place. The hydroxide is obtained by adding excess of caustic soda to a boiling solution of which gives a yellow solution of sodium platinate, and then precipitating the cold solution with acetic acid. It is a white or yellowish solid soluble both in dilute acids and in alkalis when freshly prepared, but after losing water on heating it becomes black and insoluble. The peroxide, is obtainable by elec trolyzing an alkaline solution of at o° C. It is a red brown unstable solid which readily parts with some of its oxygen at ordinary temperatures.
Two well-defined chlorides of platinum are known—platinous chloride, and platinic chloride, PtCl4. Platinous chloride is formed by heating chloroplatinic acid to 300° C or evaporating a solution of the same substance with absolute alcohol to dryness several times. It is a greenish or sometimes brownish substance insoluble in water but soluble in hydrochloric acid or alkali chloride solutions, with which it forms double chlorides of the form these are the alkali salts (platinochlorides) of chloro platinous acid, H,PtC1,. This acid, which is known only in solution, can be obtained also from either its barium or silver salt by exact precipitation by means of sulphuric or hydrochloric acid respec tively. The best known salt of the acid is the potassium com pound, which is easily produced by reducing a boiling solution of potassium chloroplatinate with sulphur dioxide. It crystallizes in dark red prisms easily soluble in water, and in the case of certain unfortunate persons, traces of it act as a violent irritant to the mucous membrane of the eyes and nose. Platinous chloride com bines with various unsaturated substances to form double com pounds; it combines directly with carbonic oxide at 25o° C to form three compounds, (PtC12)2,3C0; and with phosgene to form with ethylene to produce and with phosphorus trichloride to form compounds and Platinic chloride, is formed by the action of chlorine on platinum at a temperature below 500°. If platinum is dissolved in aqua regia or in hydrochloric acid in a stream of chlorine and the solution is evaporated' with hydrochloric acid till free from ' nitric acid, reddish-brown crystals of chloroplatinic (platini chloric) acid, are produced: these if ignited in a stream of hydrogen chloride gas at 165° give a residue of as a reddish-brown, very hygroscopic, crystalline mass. The pure chloride, however, is rarely met with, and the platinic chloride of commerce is the compound formed by heating the hexa-hydrated compound to oo° C. Chloroplatinic (platini chloric) acid is a weak acid, it reddens litmus paper and decom poses metallic carbonates producing the corresponding platini chlorides. It forms potassium and ammonium compounds which are nearly insoluble in water and quite insoluble in a mixture of alcohol and ether; the corresponding sodium compound is soluble in this mixture, and this circumstance affords the best method of separating potassium from sodium quantitatively. Chloroplatinic (platinichloric) acid forms compounds with organic bases which are usually difficultly soluble in water.