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Atomic Weight of Barrium, History





Approximate Atomic Weight of Barrium

Early determinations gave to barium an equivalent of approximately 68.5, and its properties indicate a close connection with the divalent alkaline earth group, so that its atomic weight must be in the neighbourhood of 137.


Exact Atomic Weight of Barrium

The earliest determinations were made by Berzelius from the analysis of barium carbonate. He also found the ratio of barium chloride to silver chloride precipitated from it, and of barium chloride to barium sulphate, but his values varied through a wide range and have only a historical significance.

In 1893 Richards published an extensive series of carefully conducted determinations of the ratio of anhydrous barium bromide to the amount of silver required to precipitate the bromine in it, and of barium bromide to the amount of silver bromide precipitated by it. The mean value of the former was found to be 137.747: 100, and of the latter 137.749: 174.080, giving atomic weights 137.38 and 137.37 respectively.

Later he used barium chloride instead of the bromide, and obtained as the mean of ten determinations of the ratio 2AgCl: BaCl2, 100: 72.654, and as the mean of fourteen determinations of the ratio 2Ag: BaCl2, 100: 96.522. The atomic weights calculated from these values are 137.36 and 137.34 respectively.

The International Commission on Atomic Weights for 1909 adopted the value

Ba = 137.37, which is still retained (1925).
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