Chemical Properties of Platinum

Platinum is one of the most permanent of metals, remaining untarnished in air at high and low temperatures, even in the presence of moisture. It is attacked both by fluorine and by chlorine when heated in these gases; in fluorine at 500° to 600° C., yielding the tetrafluoride, and if in the form of sponge, in chlorine at about 350° C., yielding the dichloride.

When heated in a current of chlorine at about 1400° C. platinum apparently volatilises, condensing on cooler parts of the apparatus in crystalline form. This is attributable to the formation of a volatile chloride.

When heated in dry oxygen, either in the form of thin foil or as sponge, it becomes superficially blackened in consequence of oxidation to the monoxide, PtO. This compound, however, decomposes at higher temperatures.

Platinum is very slowly attacked by hot concentrated sulphuric acid, particularly at about 250° to 280° C., but the introduction of sulphur dioxide into the liquid, by addition of a piece of sulphur or carbon, completely prevents the solution of the metal. Carbon dioxide and nitrogen are without influence, but arsenious and antimonious oxides exert a marked protective effect upon the metal. It is suggested that in the case of the pure acid, dissociation of sulphur trioxide into dioxide and oxygen takes place, the last named attacking the platinum. The sulphur dioxide, when added from an external source, owes its protective influence to the fact that it represses the dissociation, whilst the arsenious and antimonious oxides act by absorbing the oxygen. In attacking platinum with sulphuric acid, the presence of oxygen, although facilitating solution of the platinum, is not absolutely essential. The oxygen plays a secondary part in that it modifies the equilibrium of the initial action of the acid upon the metal, as represented by the equation:

2Pt + 7H₂SO₄ ⇔ 2Pt(OH)(HSO₄)₂ + 3SO₂ + 4H₂O,

the reaction being regarded as reversible.

From the solution obtained by dissolving platinum in concentrated sulphuric acid two substances have been isolated, namely, a yellow hydroxy hydrogen sulphate, Pt(OH)(HSO₄)₂, and a reddish brown compound, the composition of which is probably represented by the formula Pt(OH)₂(HSO₄)H₂O. This last-named substance is only formed after prolonged boiling of platinum with concentrated acid for some twenty hours. It crystallises in rectangular prisms, which are very soluble in water, in concentrated sulphuric acid, and also in alcohol.

Dilute sulphuric acid does not attack platinum, not even if hydrogen peroxide is added; neither does a mixture of glacial acetic acid and hydrogen peroxide.

Nitric acid is without action on pure platinum, and the hot concentrated acid should extract practically nothing from a well-cleaned crucible of the pure metal.

When alloyed with certain other metals such as silver, for example, platinum dissolves to a considerable extent in nitric acid.

Aqua regia dissolves platinum with ease, yielding the hexachlor-platinic acid. This is mainly due to the chlorine liberated, since boiling hydrochloric acid, like boiling nitric acid, alone is without action on the metal.

As a solvent for platinum a mixture of concentrated hydrochloric acid with chloric acid has been recommended, its activity being due to the chlorine liberated by the interaction:

5HCl + HClO₃ = 3Cl₂ + 3H₂O.

Platinum is slightly attacked by fused alkali carbonate, more so by the fused nitrate or hydrogen sulphate, and strongly attacked by fused hydroxide or peroxide.

When platinum wire is heated in ammonia gas at 800° C., its surface becomes dull, and shows a more or less blistered appearance under the microscope. A fine deposit of platinum black also gradually collects on the surface of the metal due to disintegration of the compact platinum.

When platinum is heated in a luminous coal-gas flame a black layer is formed. If the carbon is now burnt off in air the metal is left in a rough and brittle condition, but without having undergone any loss in weight. The action is considerably enhanced by the presence of alloyed rhodium or iron, whilst iridium has a less marked effect.

In the case of pure platinum the action is due to the presence of carbon disulphide in the coal gas.

A mixture of methane, ethylene, carbon monoxide, and hydrogen is without action upon the pure metal, although commercial platinum is blackened by it.

When heated in direct contact with different forms of carbonaceous material such as coal, coke, charcoal, etc., platinum is liable to become brittle and to fracture easily.

At 1600° C. platinum effects the reduction of both ferric oxide and ferrosoferric oxide in air, oxygen being evolved and a solid solution of iron in platinum remaining as residue. The same reaction obtains at lower temperatures, namely, at 1200° C., if the oxygen pressure is very small. This explains why platinum crucibles sometimes increase in weight when used to heat oxides of iron at high temperatures.

Sulphur alone has no action upon platinum, but metallic sulphides are liable to attack it. Phosphorus, phosphides, and phosphates under reducing conditions attack the metal, so that these and the aforesaid sulphides should not be ignited in platinum crucibles in quantitative analysis. Ferric chloride solution is reduced to ferrous chloride when evaporated in a platinum dish, platinum passing into solution.

When plates of platinum are kept in an alkaline solution of potassium permanganate for some twenty-four hours at the ordinary temperature they become superficially attacked, and on treatment with dilute potassium iodide solution and hydrochloric acid, red solutions are obtained, from which platinum sulphide may be precipitated on passage of hydrogen sulphide.

Platinum readily alloys with many metals. The ease with which it alloys with lead is a property made use of in assaying the metal.

The only suitable solder for platinum and iridio-platinum is pure metallic gold.

A pure white deposit of platinum may be obtained by using a boiling solution consisting of:

4 grams platinum tetrachloride,
20 grams ammonium phosphate,
90 grams sodium phosphate, and
5 grams sodium chloride

per litre. The article to be plated should be kept in motion, and a potential difference of from 6 to 8 volts maintained.