Atomic Weights of Germanium, History

The order of magnitude of the atomic weight of germanium has never been in question, since before the discovery of this metal a space with a corresponding atomic weight was allotted to it by Mendeleeff in his Periodic Table. "We must expect the discovery of many yet unknown elements," said Mendeleeff in his Faraday Lecture in 1889; "for example, elements analogous to aluminium and silicon, whose atomic weights would be between 65 and 75." The elements referred to are gallium, Ga (atomic weight = 69.6), and germanium, Ge (atomic weight = 72.5). That the atomic weight of germanium is about 72 is shown as follows. The specific heat of the metal between 0° C. and 440° C. is 0.0737-0.0757, which gives an atomic heat of 5.34-5.47 if the atomic weight is 72.5. This value is further supported by the isomorphism of potassium germanifluoride, K₂GeF₆, with the corresponding silicifluoride, K₂SiF₆. The atomic weight of germanium was determined by Winkler in 1886, and no more recent determination has been made. A weighed quantity of purified germanium tetrachloride was added to excess of standardised silver nitrate solution, and the silver remaining in solution after precipitation of the chlorine was titrated with ammonium thiocyanate solution, according to Volhard's method. As the mean of four closely agreeing results, germanium tetrachloride, GeCl₄, was found to contain 66.173 per cent, of chlorine; and from this percentage the atomic weight Ge = 72.32 ± 0.045 was calculated on the basis H = 1, O = 15.96, Ag = 107.66. This value was recalculated by Clarke in 1910 on the basis O = 16.000, Ag = 107.880 and Cl = 35.458, so that Ge = 72.504.

The value adopted by the International Atomic Weights Committee for 1917 is Ge = 72.5.