Platinum Alloys

Aluminium. - Platinum readily alloys with aluminium, and a definite compound, PtAl₃, containing 70.4 per cent, of platinum is formed. It is violet-black in colour, and yields dendritic and octahedral crystals. It occurs in increasing quantities in the alloys containing from 10 to 70 per cent, of platinum, and may be isolated from them by treatment with 5 per cent, hydrochloric acid, in which the compound is insoluble. Alloys containing less than 10 per cent, of platinum are white, malleable, and soft, consisting of aluminium and eutectic containing 9 per cent, of platinum. Alloys containing from 10 to 70 per cent, of platinum are also white, but hard and brittle; those with a platinum content of 70 to 90 per cent, are yellow and brittle, becoming white and malleable as the platinum increased beyond 90 per cent.

Cadmium. - By heating platinum in hydrogen laden with cadmium vapour obtained from boiling metal, a white, crystalline, and very brittle compound, of formula PtCd₂, is obtained.

Copper. - With copper, platinum yields a series of very tenacious alloys. Those containing over 40 per cent, of platinum are white. The freezing-point curve of the alloys, containing from 80 to 100 per cent, of copper, falls continuously to the melting-point of copper. The metals yield a continuous series of mixed crystals.

Gold. - Platinum and gold form a continuous series of mixed crystals, at any rate up to 60 per cent, of the former metal. The alloys containing over 40 per cent, of platinum are white.

Iridium is usually alloyed with platinum in order to increase its hardness, although it reduces its ductility. The presence of iridium in platinum crucibles renders them subject to proportionately greater losses on heating at temperatures above 900° C. Below this temperature, however, and up to a content of at least 3 per cent, of iridium the loss on heating is negligible.

An alloy of platinum and iridium, containing 10 per cent, of the latter metal, is attacked by boiling concentrated sulphuric acid to the extent of 0.10 gram per sq. dcm. per hour at 365° C. On boiling the resulting solution with ammonia the platinum is deposited as sponge, whilst the solution becomes green, changing to deep violet upon addition of nitric acid in consequence of the presence of iridium salts.

This 10 per cent, alloy is recommended for the construction of apparatus used in the preparation of fluorine. The same alloy was employed in the manufacture of the prototype metres and the geodesic rules, which latter are four metres in length, for the Comite Internationale des Poids et Mesures, and the Association Geodesique Internationale. The density of this alloy is high, namely, about 21.52 at 20° C., which renders it useful for standard weights, particularly in view of its incorrodibility and power of taking a high polish.

For the manufacture of standard rules, Matthey recommends an alloy containing not less than 85 per cent, of platinum and some 15 per cent, of iridium. For standard weights the same investigator suggests 20 per cent, of iridium, the remainder being platinum. This latter alloy is, like the previous one, both malleable and ductile, and has a density of 21.61. Alloys containing 25 per cent, of iridium are more difficult to work into sheet and wire, whilst alloys with 30 per cent, and 40 per cent, iridium can only be worked at a temperature just below the melting-point. When cold the last-named alloys are brittle.

Alloys of platinum and iridium are sometimes used in dentistry.

Lead. - Platinum readily alloys with lead, yielding three compounds, namely, Pb₂Pt, PbPt, and one the composition of which has not been determined. The alloys gradually increase in hardness with rise of platinum content from 0 to 45 per cent. Those containing up to 30 per cent, of platinum are readily fractured, and the fresh surfaces oxidise readily in air. The fact that platinum so readily alloys with lead is made use of in assaying the metal, since its alloys can be cupelled.

Magnesium unites with platinum to yield a crystalline compound, PtMg₂.

Osmium yields alloys with platinum characterised by hardness and tensile strength. They are very resistant to acid attack, and offer high electrical resistance. The alloys usually contain from 1 up to 10 per cent, of osmium.

Potassium readily unites with platinum on warming, yielding a hard but brilliant alloy, which is attacked by water.

Silver. - Alloys of platinum and silver have received a considerable amount of attention owing to their importance in assaying. No definite compound of the two metals appears to be formed.

Pure platinum is only very slowly attacked by boiling with concentrated sulphuric acid, and when its alloy with silver is submitted to similar treatment, the silver is more or less completely dissolved out, leaving the platinum as a black residue. Complete removal of the silver, however, is only effected when the initial proportion of that metal in the alloy is some 90 per cent, or more. With smaller quantities of silver some is left behind in the platinum residue. This is well illustrated by the following data obtained by Thompson and Miller:

Percentage Composition of Alloy.		Silver in residue (per cent.).
Platinum.	Silver.
10.39	89.61	Trace
20.59	79.41	0.59
31.46	68.54	0.98
37.89	62.11	2.24
57.05	42.95	2.70

A portion of the platinum tends to pass into solution with the silver, but by slightly diluting the sulphuric acid this may be.prevented. According to Steinmann the most satisfactory results are obtained with a dilution of 100 volumes of acid to 22 volumes of water, the alloy being heated in this mixture, twice repeated, at about 240° C.

Although platinum is not attacked by nitric acid, yet it dissolves slightly when its alloys with silver are so treated, yielding a colloidal solution which is dark-brown in colour. This is particularly the case with alloys containing about 20 per cent, of platinum. On standing for several days the platinum flocculates, and deposits as a black powder in a high state of subdivision, the solution becoming decolorised.

According to Spiller, nitric acid of density 1.42 will dissolve 0.75 to 1.25 per cent, of platinum from its alloy with 12 times its weight of silver, whilst a more concentrated acid leads to the separation of platinum black. A less concentrated acid dissolves less platinum.

The following data, taken from the results of Thompson and Miller, are interesting in that they show how imperfect is the separation of platinum from silver when alloys of the two metals are attacked by nitric acid of density 1.10:

Composition of Alloy. Percentage.		Composition of Residue from 100 parts of Alloy.		Dissolved Platinum from 100 parts of Alloy.
Platinum	Silver	Platinum.	Silver
10.39	89.61	3.59	0.27	6.80
20.59	79.41	6.77	1.81	13.82
31.46	68.54	24.50	12.09	6.96
37.89	62.11	35.49	13.64	2.40
57.05	42.95	52.97	12.19	4.08

More recently somewhat similar results have been obtained by Koifman working with alloys containing from 0.219 to 5.162 per cent, of platinum.

Selenic acid dissolves out the silver from platinum alloys, leaving a residue of undissolved platinum.

Alloys of silver and platinum are used by jewellers and by English dentists under the name of dental alloy, in the form of wire and sheet. Two qualities of these are recognised, the compositions of which vary somewhat with the makers, but approximate to the following:

	Silver	Platinum.
First quality	66	33
Second quality	75	25

These alloys are more durable and more resistant to corrosion than silver.

Platinum occurs in the following alloys used in the preparation of dental amalgams:

Name.	Tin	Silver	Gold	Platinum
Fletcher's perfected standard alloy	62.30	30.30	5.80	1.60
S. S. White's globe.	53.36	44.74	1.50	0.40
Welch's gold and platinum alloy:
Old	54.00	44.00	1.30	0.70
New	51.90	46.00	1.70	0.40

Sodium and platinum alloy when brought together at a high temperature or pressure.

Thallium. - Platinum alloys with thallium, yielding a compound, PtTl, which resembles, at any rate in its physical properties, the compound PtPb. The alloy is conveniently formed by throwing platinum sponge upon the surface of fused thallium. The compound yields steel-grey, prismatic needles, and is extracted by treating an alloy containing less than 10 per cent, of platinum with dilute nitric acid. Its density is 15.65 at 14° C.; hardness 3; melting-point 685° C.; and specific heat 0.0450. When subjected to continued heating above its melting- point, a little thallium is lost, but pure platinum is not obtained even on prolonged fusion in the oxyhydrogen flame. When boiled with aqua regia, thallium chlor-platinate is obtained as an insoluble compound.

Insoluble in hydrochloric acid, the alloy is only superficially attacked by sulphuric or nitric acid, and by fusion with potassium hydrogen sulphate. It readily dissolves in molten zinc, lead, or silver, and amalgamates with mercury.

Tin. - Platinum readily alloys with tin. The complicated freezing-point curve for Pt-Sn is indicative of the formation of several compounds, namely, Pt₃Sn, which is only stable below 1370° C. and decomposes above that temperature into crystals of platinum and a fused mixture containing 80 per cent, of platinum; PtSn; Pt₂Sn₃, and possibly a polymorphous form of this alloy; and a fourth compound PtSn₄ (Podkopeeff) or Pt₃Sn₈ (Doerinckel).

The alloys containing up to 30 per cent, of platinum are scarcely any harder than their constituents; above 30 per cent, the hardness rapidly increases with the platinum, reaching a maximum with 80 per cent, of that metal.

The tetrastannide, PtSn₄, was first isolated by Deville and Debray by treating an alloy containing 2 per cent, of platinum with dilute hydrochloric acid. It remained behind as brilliant, insoluble lamellae.

The sesquistannide, Pt₂Sn₃, was described by Schutzenberger in 1884. The existence of a compound of formula Pt₄Sn₃ has also been assumed, but this is not supported by an examination of the freezing-point curve.

Zinc. - When platinum is heated in hydrogen laden with zinc vapour from the boiling metal, the compound PtZn₂ is formed. Another compound, PtZn, has also been obtained.

Platinum Amalgam is obtained by trituration of platinum sponge with mercury in a warm mortar; it cannot be obtained by direct union of platinum foil and mercury.

The amalgam has a silvery appearance, and with 12 per cent, of platinum is soft and greasy to the touch, but higher percentages of platinum increase its stiffness. When heated strongly the mercury is volatilised and platinum remains as a grey residue.

When shaken with water for some fifteen seconds, platinum amalgam is converted into an emulsion of butter-like consistency, and having a volume some five times that of the original amalgam. It is quite stable between 100° C. and - 80° C. in air, but in vacuo it diminishes somewhat in volume, a little water and gas being liberated. When cooled to -80° C. its appearance under the microscope is cellular, water being disseminated throughout the interstices of the amalgam.

Platinum amalgam emulsifies in a similar manner when shaken with dilute sulphuric acid; with aqueous ammonia, ammonium chloride, or sodium chloride; and in contact with several organic solvents, such as alcohol, acetone, etc.

Microscopic examination proves that an amalgam is formed during the simultaneous reduction of platinum and mercury from aqueous solutions of their salts.