Atomic Weight of Boron, History

It follows from the analyses and vapour densities of numerous boron compounds that the atomic weight of boron is three times its combining weight.

The determinations of the combining weight leave much to be desired. Various attempts have been made to measure the ratio Na₂B₄O₇.10H₂O:Na₂B₄O₇, but the results of different experimenters do not agree, and the method, involving as it does the use of a hydrated salt, is a bad one. The best determinations of the combining weight are probably those due to Gautier, whose values for the atomic weight of boron, derived by four methods, are as follows: -

B₂S₃:3BaSO₄::16.8855:100 ⇒ B = 11.028
B₆C:CO₂::177.258:100 ⇒ B = 10.999
BBr₃:3AgBr::44.512:100 ⇒ B = 11.027
BCl₃:3AgCl::27.2845:100 ⇒ B = 10.955

The first two ratios are only based upon a few experiments, and from the nature of the analytical operations involved the results cannot be regarded as possessing much more than corroborative value. From a chemical point of view the other two ratios would be expected to yield more reliable results, but Gautier's method of determining them is open to serious criticism.

Mention should be made of Ramsay and Aston's determination of the ratio Na₂B₄O₇:2NaCl, effected by distilling anhydrous borax with hydrochloric acid and methyl alcohol. The results obtained lead to the value B = 10.951. The other determinations are of little importance.

The value at present adopted for the atomic weight of boron is B = 11.0.