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Atomic Weights of Zirconium, History

That the atomic weight of zirconium is approximately 90 is evident from a consideration of its specific heat, and of the isomorphism of its compounds with those of tin, silicon, and lead; and, finally, from a study of the Periodic Table.

Various attempts have been made to determine with accuracy the atomic weight of zirconium. Berzelius in 1825, by igniting the neutral sulphate Zr(SO4)2, obtained the following value for the ratio

ZrO2:SO3 = 0.75853:1,
whence
Zr = 89.46.

Hermann in 1844 analysed the tetrachloride ZrCl4, and the basic chloride 2ZrOCl2.9H2O, the mean of his results yielding the value 89.56 for the atomic weight of zirconium.

More important and accurate were the results of Marignac, published in 1860, This investigator analysed potassium zirconifluoride, K2ZrF6 and obtained the following values for the ratios:

ZrO2:K2ZrF6 = 0.43200:1, whence Zr = 90.03
K2SO4:K2ZrF6 = 0.61365:1, whence Zr = 91.54
ZrO2:K2SO4 = 0.70396:1 whence Zr = 90.68.

Assuming equality of accuracy for each of these values, the mean atomic weight of zirconium is 90.75, a result closely approximating to that accepted at the present time.

The above results are now, however, of purely historic interest. The most accurate researches are those of Weibull in 1881, and particularly those of Bailey in 1890 and Venable in 1898.

Weibull ignited the sulphate and selenate of zirconium, obtaining values for the ratios ZrO2:Zr(SO4)2 and ZrO2:Zr(SeO4)2. Bailey similarly ignited the sulphate, but in a double crucible to avoid the loss of traces of the light zirconia which in an ordinary crucible are apt to be carried away mechanically. He thus obtained eight highly concordant values for the ratio ZrO2:Zr(SO4)2. Venable prepared the pure basic chloride, ZrOCl2.3H2O, by crystallisation from hot hydrochloric acid, and dried it in a stream of hydrogen chloride at 100° to 125° C. Solution in water, followed by evaporation to dryness and subsequent prolonged ignition, converted the salt into the dioxide, thus enabling the ratio ZrO2: ZrOCl2.3H2O to be calculated.

The results of these investigators are given in the following table:

Authority*Ratio DeterminedNo. of Experiments.Mean Atomic Weight of Zirconium.
WeibullZrO2:Zr(SO4)2 = 0.43150:1789.54
ZrO2:Zr(SeO4)2 = 0.32558:1590.79
BaileyZrO2:Zr(SO4)2 = 0.43372:1890.65
VenableZrO2:ZrOCl2.3H2O = 0.52986:11090.81

* This and the succeeding values for the atomic weight of zirconium are not the figures given by the investigators in the original memoirs, but have been calculated from their experimental results, using the modern values for the fundamental atomic weights, namely, O = 16.000; S = 32.065; H = 1.00762; Cl = 35.457; K = 39.100; F = 19.015; and Se = 79.176.

Assuming that the mean values for the atomic weight of zirconium, as given in the last column of the table, are of equal importance, the general mean becomes Zr = 90.45. But this is unsatisfactory on account of the wide discrepancy between the two values found by Weibull. The results of Bailey and Venable appear to be the most reliable, the mean of these being Zr = 90.73. Nevertheless the figure accepted by the International Committee on Atomic Weights for 1917 is Zr = 90.6; a value that evidently needs revision.

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