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Atomic Weight of Tantalum






The earliest investigations into the atomic weight of tantalum were carried out by Berzelius, Rose, Hermann, and Blomstrand, but the various values they obtained are now only of historical interest, as the materials used were not pure. In 1866, Marignac made four analyses of pure potassium tantalum fluoride, K2TaF7, by heating the salt with concentrated sulphuric acid to remove hydrofluoric acid; the potassium sulphate thus formed was extracted with water and the residue ignited and weighed as tantalum pentoxide, with the following mean results:

; hence Ta = 183.3.

; hence Ta = 182.1.

; hence Ta = 183.7.

Four analyses of the corresponding ammonium salt, (NH4)2TaF7, were also made, with the result:

; hence Ta = 182.3.

Despite the facts (a) that the figures for individual determinations differed by several whole units and (b) that the results as a whole were obviously discordant, Marignac's work formed the basis for the accepted atomic weight of tantalum (namely, 183) for forty years. The suitability of the double fluorides for use in the determination of the atomic weight has been questioned.

In 1906, Hinrichsen and Sahlbom used a very simple method. Metallic tantalum was converted directly into the oxide by heating in oxygen. The mean of five experiments gave the following ratio:

; hence Ta = 182.3.

The extreme values of the individual determinations still differed by more than a unit, and it is doubtful whether metallic tantalum can be obtained sufficiently pure for atomic weight determination.

Balke in 1910 hydrolysed tantalum pentachloride, TaCl5, to the pentoxide, Ta2O5, with water and a small quantity of nitric acid. The mean of eight experiments gave:

; hence Ta = 181.49.

The difference between the extreme values was 0.14. In the following year hydrolysis of the pentabromide with water and nitric acid was used by Chapin and Smith, who from eight experiments obtained the ratio:

; hence Ta = 181.80.

The difference between the extreme values was 0.23. It will be observed that the value given by the pentabromide is appreciably higher than that given by the pentachloride, although the figures for individual determinations by each method were reasonably concordant. Sears and Balke subsequently found that tantalum pentoxide is slightly volatile at ignition temperatures, and since it occludes nitric and other acids it is useless for work requiring great accuracy. In a fresh attempt to establish the atomic weight, Sears and Balke treated the pentachloride with silver in the presence of hydrofluoric acid. Five experiments gave the mean ratio:

; hence Ta = 181.05.

But the extreme values showed a difference of 0.46, from which it was concluded that the tantalum pentachloride used varied slightly in its composition, and that this salt is also unsuitable for use in the determination of the atomic weight. This conclusion has been confirmed.

The International Committee on Atomic Weights adopted the value 181.5 in 1912, and this figure was altered to 181.3 in 1929. The decimal place is, however, obviously uncertain.


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