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Atomic Weight of Selenium






The first determination of the atomic weight of selenium was due to Berzelius, who in 1818 synthesised the tetrachloride, with the following result:

Se:4Cl::100:179, ⇒ Se = 79.23.

In 1847 Sacc determined the atomic weight by several methods, of which only two proved of value, namely: (i) pure selenium dioxide was reduced with ammonium hydrogen sulphite and the resulting selenium weighed; (ii) barium selenite was calcined with sulphuric acid and the resulting barium sulphate weighed. The results were as follows:

3 experiments - SeO2:Se::100.000:71.088, ⇒ Se = 78.68.

4 experiments - BaSeO3:BaSO4::100.000:88.437, ⇒ Se = 78.58.

A totally different procedure was followed in 1852 by Erdmann and Marchand, who analysed mercury selenide with the following result:

3 experiments - HgSe:Hg::100.000:71.7327, ⇒ Se = 79.05.

In 1860 Dumas reverted to the original method of Berzelius, but in a series of not very concordant experiments obtained a decidedly higher value for the atomic weight:

7 experiments - Se:4Cl::100.000:178.652, ⇒ Se = 79.39.

Sixteen years later, Ekmann and Pettersson estimated the silver in pure silver selenite by ignition and the selenium in the pure dioxide by reduction with sulphurous acid from its solution in dilute hydrochloric acid. The results were:

7 experiments - Ag2SeO3:2Ag::100.000:62.957, ⇒ Se = 78.95.

5 experiments - SeO2:Se::100.000:71.191, ⇒ Se = 79.08.

In 1898 Lenher made a careful study of the atomic weight of selenium by new methods. Firstly, pure silver selenite was transformed into silver chloride by heating in a stream of hydrogen chloride; the silver chloride was weighed and then reduced in hydrogen, the resulting silver also being weighed:

11 experiments - Ag2SeO3:2AgCl::100.000:83.558, ⇒ Se = 79.32.

8 experiments - Ag2SeO3:2Ag::100.000:62.895, ⇒ Se = 79.29.

Secondly, ammonium bromoselenate was reduced with hydroxyl-amine hydrochloride and the resulting selenium weighed:

8 experiments - (NH4)2SeBr6:Se::100.000:13.3224, ⇒ Se = 79.24.

In 1901 Steiner published the results of two analyses of selenium diphenyl which were effected by combustion, the resulting carbon dioxide being absorbed and weighed:

2 experiments - (C6H5)2Se:12CO2::100.000:226.536, ⇒ Se = 78.98.

The following year Julius Meyer reverted to the analysis of silver selenite. The analysis, however, was made by electrolytic deposition of the silver from a solution of the salt in potassium cyanide:

5 experiments - Ag2SeO3:2Ag::100.000:62.9193, ⇒ Se = 79.16.

In 1910 Kuzma and Krehlik made a careful re-determination of the atomic weight of the element, the method used being reduction of the dioxide with sulphurous acid. The result was as follows:

10 experiments - SeO2:Se::100.000:71.2385, ⇒ Se = 79.26.

The value adopted by the Chemical Society on the recommendation of the Sub-Committee on Atomic Weights (1929) is Se = 79.2.

According to Aston selenium is not a simple element, but consists of a mixture of no fewer than six isotopes, the atomic masses of which are, in order of intensity, 80, 78, 76, 82, 77, 74. The atomic number of selenium is 34.


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