Atomic Weights of Tin, History

That the atomic weight of tin is approximately 119 is indicated by the following considerations:

1. The specific heat of the cast metal between 0° C. and 100° C. is 0.0559. Assuming, in accordance with Dulong and Petit's law, a mean atomic heat of 6.4, the atomic weight of tin must be about 115.
2. Stannic oxide is isomorphous with titanic oxide, whilst fluostannates are isomorphous with fluotitanates, etc. Analysis of these compounds and the application of Mitscherlich's law indicate an atomic weight of 119 for tin.
3. There is a space in the Periodic Table for an element of atomic weight lying between 114.8 and 120.2, and the properties of tin indicate that it would fill that space very satisfactorily.

Numerous attempts have been made to determine with accuracy the atomic weight of tin, but it was not until the beginning of the year 1915 that a thoroughly satisfactory result was arrived at. The results obtained by Gay Lussac, Berzelius, Mulder, Vlaanderen, Dumas, and van der Plaats are now merely of historical interest, and are briefly summarised in the table. The value for the atomic weight of tin that was accepted by the International Atomic Weights Committee from 1903 to 1915, namely 119.0, was based solely on the work of Bongartz and Classen; but it has now been replaced by Briscoe's value, namely 118.7.

Bongartz and Classen carried out five series of experiments involving (a) the oxidation of electrolytic tin to stannic oxide; (b) electrolysis of ammonium stannichloride; (c) electrolysis of potassium stannichloride; (d) electrolysis of stannic bromide; and (e) the preparation of stannic sulphide from electrolytic tin and the estimation of its sulphur by oxidising to sulphate and precipitating as barium sulphate. The mean result deducible from the ratios found by these investigators is Sn = 119.05.

Six years later Schmidt1 made a single determination of the atomic weight of tin, which yielded the value 118.49. Meyer and Kerstein as the result of two determinations of the ratio Sn:SnO₂ found the value 117.54.

Undoubtedly the most accurate work is that recently published by Briscoe, who determined the value for the ratio SnCl₄:4Ag. Stannic chloride was prepared by direct union of its elements, purified by fractional distillation, and collected and sealed in glass bulbs. The chlorine was estimated by breaking one of the bulbs in a solution of silver in nitric acid, whereby silver chloride was precipitated - the process being completed by subsequent addition of silver nitrate solution, and finally by titration with standard silver nitrate or sodium chloride solution, according to circumstances. The process of the titration and the attainment of the end-point were ascertained nephelometrically. From the mean of fifteen highly concordant experiments the atomic weight of tin was found to be 118.698.

The following table summarises the results of the different investigators referred to above:

Authority	Ratio Measured.	No. of Experiments	Atomic Weight of Tin
Gay Lussac, 1811	Sn:SnO2 = 100:127.2	1	117.6
Berzelius, 1812.	Sn:SnO2 = 100:127.2	1	117.6
Mulder, 1849.	Sn:SnO2 = 100:127.517	3	116.3
Vlaanderen, 1858	Sn:SnO2 = 100:127.082	2	118.2
Dumas, 1859	Sn:SnO2 = 100:127.105	2	118.06
	SnCl4:4Ag = 60.207:127.100	2	117.98
Van der Plaats, 1885	Sn:SnO2 = 100:127.102	7	118.07
Bongartz and Classen, 1888.	Sn:SnO2 = 100:126.877	11	119.06
	Sn:(NH4)2SnCl6 = 32.369:100	16	119.09
	Sn:K2SnCl6 = 29.040:100	10	119.07
	SnBr4:Sn = 100:27.123	10	118.97
	Sn:2BaSO4 = 100:392.056	8	119.08
Schmidt, 1894.	Sn:SnO2 = 0.5243:0.6659	1	118.49
Meyer and Kerstein, 1913	Sn:SnO2 = 1.9719:2.5087	2	117.54
Briscoe, 1915	SnCl4:4Ag = 60.3742:100	15	118.698

The figure 118.70, which appears to approach very closely to the true value, was in 1916 accepted by the Atomic Weights Committee.