Physical Properties of Platinum

Platinum is a silver-white metal, capable of taking a high polish. It is slightly harder than silver, however, and does not tarnish. It is both malleable and ductile, admitting of being beaten out into thin foil and drawn into fine wire. Addition of a small quantity of iridium increases the hardness of the metal, but decreases its ductility.

Platinum is hardened by mechanical treatment, but is softened again on maintaining at bright red heat for several minutes.

The density of native platinum varies from 14 to 19, the variation being mainly due to impurities such as iron which may range from 5 to 16 per cent., and to smaller quantities of copper, iridium, osmium, rhodium, and palladium.

The density of pure platinum varies according to its physical condition, as is evident from the accompanying data which give the density calculated for a vacuum at 4° C.:

1	Annealed pure platinum wire	21.4403
	Same wire cold drawn, not annealed	21.4336
2	Annealed pure platinum wire	21.4312
	Same wire twisted, not annealed	21.3985

The mean coefficient of linear expansion of platinum with rise of temperature between 0° and 1000° C. is:

α = 0.0000105t.

For temperatures ranging from 0° to 100° C.:

α = 0.0000089t.

These values are not far removed from those observed for different kinds of glass. Thus, for temperatures round and about 20° C. the following values have been obtained:

	α
Soft glass	0.0000085
Hard glass	0.0000097
Flint glass	0.0000078

For this reason platinum wire is used in the construction of electrical and other apparatus in which it is necessary to pass wire through glass and leave a perfectly air-tight and hermetically sealed joint. Electric-light bulbs are an important example of this.

The specific heat of platinum between 0° and 100° C. has the mean value of 0.0323.

The heat liberated by one gram of platinum in cooling from t° to is given by the expression:

Quantity of heat = 0.031590(t - t₀) + 0.0₅29234(t² - t₀²); and the specific heat at any temperature t is given by:

Specific heat = 0.031590 + 0.0₅58468t.

It will be observed that the specific heat of the metal rises with the temperature, and experiment shows that this is the case also at very high temperatures such as those of the incandescent metal, although theory would require its constancy under these latter conditions.

The melting-point of platinum has been variously given, the most recent results being 1753° C., and 1755° C. For other data the reader is referred to the references given below. The lowest temperature at which the volatilisation of platinum can be detected in a complete vacuum is given by Knocke as 540° C.

The most intense lines in the arc and spark spectra of platinum are as follow:

Arc: 2659.60, 2702.50, 2705.99, 2719.12, 2830.40, 2929.90, 2998.07, 3042.75, 3064.82, 3408.27, 4442.75, 4498.93, 4521.10.

Spark: 2424.90, 2659.53, 2998.08, 3064.82, 3923.15, 4118.83, 4498.90, 4552.60.