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Atomistry » Titanium » Physical Properties » Atomic Weight, History | ||||||||||||||||||
Atomistry » Titanium » Physical Properties » Atomic Weight, History » |
Atomic Weights of Titanium, History
The earliest accepted values for the atomic weight of titanium were inaccurate on account of imperfect analytical methods. Thus Dalton, in 1808, gave the value Ti = 40 (O = 7), whilst Berzelius in 1813 suggested the number 1801.0 (O = 100) or 288.16 (O = 16). That the atomic weight of titanium must be of the order of 48 is evident from the following considerations:
The analytical investigations on which the atomic weight of titanium is based fall into two categories: (i) the work of Berzelius, H. Rose, Mosander, Pierre, and Demoly from 1813 to 1849; (ii) the work of Thorpe, 1883-1885. Except for the early work of Berzelius, and that of H. Rose in 1823, who prepared the disulphide TiS2, and converted it into the dioxide TiO2, the processes adopted have consisted in hydrolysing the pure tetrachloride or tetrabromide with water or alkali, and estimating the titanic oxide and the hydrochloric or hydrobromic acid produced. The titanic oxide was estimated by Thorpe by decomposing the tetra-halide with water in a platinum or porcelain dish, according to the reaction TiX4 + 3H2O ⇔ H2TiO3 + 4HX, evaporating with ammonia, and then igniting and weighing the residue. The halogen hydracid was estimated by hydrolysing the titanium halide, allowing the solution to stand until it was clear, then either precipitating nearly all the halogen with a known weight of pure silver dissolved in nitric acid, and finishing the estimation by titrating with centinormal silver solution, or by adding excess of silver solution and weighing the silver halide precipitated. The results of Thorpe alone are accepted in computing the atomic weight of titanium. They are as follows:
The mean value adopted by the International Committee on Atomic Weights for 1917 is 48.1. |
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