Atomic Weight of Iridium

That the atomic weight of iridium is of the order of 193, and not a multiple or submultiple of this amount, may be gathered from several lines of argument. The more important of these may be briefly summarised as follows:

1. The specific heat of iridium between 0° and 100° C. is 0.0323. Application of the Law of Dulong and Petit leads to an atomic weight of 198, the atomic heat being taken as 6.4.
2. A study of the chemical properties of iridium and its compounds shows that, whilst closely resembling platinum in many respects, it forms a fitting link between that element and osmium. With an atomic weight intermediate in value between 190.9 (at. wt. of osmium) and 195.2 (at. wt. of platinum), iridium falls into a suitable position in the Periodic Table where these analogies are recognised.
3. Iridium sulphate unites with the sulphates of the alkali metals to yield a series of well-defined crystalline salts known as alums. These are isomorphous with the better known alums of aluminium, chromium, and iron. Application of Mitscherlich's Law, therefore, points to their having a composition represented by the general formula:
   
   M₂SO₄.Ir₂(SO₄)₃.24H₂O,
   
   that is, a molecule of the salt contains two atoms of iridium. Analyses of these substances prove the atomic weight of iridium, therefore, is 193.1.

The first determination of the atomic weight of iridium is due to Berzelius. His single analysis of potassium chlor-iridate leads to the very high value Ir = 196.7.

This value was not supplanted until Seubert's analyses of ammonium and potassium chloriridate were published in 1878. The salts were ignited in a stream of hydrogen, and the residual metal weighed; in the case of the potassium salt, of course, the metal was obtained mixed with potassium chloride, which was dissolved out, recovered by evaporation, and weighed. The results were as follow:

(NH₄)₂IrCl₆: Ir:: 100.000: 43.732; whence Ir = 193.38
K₂IrCl₆: Ir:: 100.000: 39.880; whence Ir = 192.99
K₂IrCl₆: 2KCl:: 100.000: 30.820; whence Ir = 192.88
K₂IrCl₆: 4Cl:: 100.000: 29.291; whence Ir = 193.26

In 1890, Joly determined the percentages of potassium chloride and iridium in the salt IrCl₃.3KCl.3H₂O, and the percentage of iridium in the salt IrCl₃.3NH₄Cl:
Ir: 3KCl:: 100.000: 115.848; whence Ir = 193.07
(NH₄)₃IrCl₆: Ir:: 100.000: 42.003; whence Ir = 193.27

Determinations of the percentage of iridium in ammonium chlor-iridate were made by Hoyermann in 1911:
(NH₄)₂IrCl₆: Ir:: 100.000: 43.630 whence Ir = 192.59

and by Holzmann in 1912:
(NH₄)₂IrCl₆: Ir:: 100.000: 43.734 whence Ir = 193.40

The preceding results fall considerably short of the modern standard of atomic weight work, and further determinations of the atomic weight of iridium are desirable. Archibald, in 1909, announced that he had determined the value Ir = 192.90 from analyses of potassium iridi-chloride, but no details of the work have yet been published.

The International Committee's Table for 1918 gives

Ir = 193.1.