The early determinations of the atomic weight of lithium by Arfvedson, Vauquelin, Gmelin, and Kralovansky were very erroneous, owing to the presence of sodium in their calcium salts. The results obtained by Hermann, Berzelius, and Hagen, 6.1, 6.7, and 6.6, were also incorrect, and need not be discussed.
Better determinations were made by Mallet, Troost, Diehl, Stas, and Dittmar, whose results are tabulated below. The atomic-weight values are calculated from the data.
O=16.000; Cl=35.457; C=12.003; Ag=107.880; N=14.008
The manipulation of Stas was much superior to that of the other investigators, and his ratio LiCl:LiNO3 is very nearly correct. His analysis of lithium chloride is very erroneous, as was shown by Richards and Willard in their classic researches on the atomic weight of lithium.
The results of their analyses of lithium chloride are
AgCl:LiCl=100:29.5786, whence Li = 6.940;
Ag:LiCl=100:39.2992, whence Li = 6.939.
A description of the determination of the first of these ratios is given in Volume I. of this series of text-books, as an example of the refinements employed in modern atomic-weight research.
Further evidence concerning the atomic weight of lithium was furnished by Richards and Willard in their synthesis of lithium perchlorate by evaporating lithium chloride with perchloric acid. The result obtained was
LiCl:40=100:150.968, whence Li = 6.936.
In the foregoing calculations the modern values for the atomic weights of silver and chlorine have been employed, but the following calculation indicates the assumption to be unnecessary. Taking the composition of silver chloride to be that given by Richards and Wells,
Ag:AgCl=100:132.867,
it follows that
(LiCl/AgCl)×(AgCl/Ag) = LiCl/Ag = 0.295786 × 1.32867 = 0.393002.
The mean of this result and that found directly (0.392992) is 0.392997. Hence
(40/LiCl)×(LiCl/Ag) = 40/Ag=1.50968 × 0.392997 = 0.593300 =64:107.871.
Accordingly, Ag=107.871, and Cl=107.871×0.32867=35.454. But LiCl = 64/1.50968 =42.393.
Hence
Li = 42.393 - 35.454, = 6.939.
The current table of the International Committee on Atomic Weights gives
Li = 6.94. |