Atomic Weight of Aluminium, History

From the analyses and molecular weights of aluminium chloride, bromide, and iodide, various double compounds that the preceding salts form with organic substances, aluminium methyl, aluminium ethyl, and aluminium acetylacetonate, it follows that the atomic weight of aluminium is approximately 27, or three times its combining weight. This conclusion is in harmony with Dulong and Petit's Law, the isomorphism of (i.) alumina, ferric and chromic oxides, (ii.) the aluminium, gallium, indium, chromium, iron, vanadium, cobalt, and rhodium alums, (iii.) the aluminium, iron, chromium, and cobalt double oxalates, etc.

The atomic weight was determined by Mallet in 1880 by four distinct methods, the results of which are appended: -

(NH₄)₂SO₄.Al₂(SO₄)₃.24H₂O:Al₂O₃::100:11.2793 ⇒ Al = 27.152
3Ag:AlBr₃::100:82.455 ⇒ Al = 27.109
3H₂:2Al::100:896.33 ⇒ Al = 27.095
2Al:3H₂O::100:99.818 ⇒ Al = 27.072

The first ratio was determined by calcining ammonium alum and measuring the loss in weight; the second, by titrating aluminium bromide against silver according to the procedure of Stas. The third was determined by dissolving aluminium in sodium hydroxide and measuring the hydrogen evolved, and the fourth was carried out like the third, the hydrogen, however, being burnt to water. Other determinations are of less importance. The atomic weight of aluminium is at present taken as Al = 27.1.

From the freezing-point determinations made by Heycock and Neville, it appears that aluminium in dilute solution in tin has the molecular formula Al₂; but it is not certain that pure tin separates on freezing. Aluminium likewise appears to be present largely as diatomic molecules in solution in mercury.