Part 2

Cd : O = 100 : 14.352.

If the atomic weight of oxygen = 16, ” ” ” ” cadmium = 111.483.

The very low result as compared with all subsequent work was probably due to the presence of a small amount of zinc, since the cadmium used was obtained from zinc ores and no adequate means of separation from the zinc is described.

von Hauer, Journ. f. prakt. Chem. 72, 338. 1857. His method consisted in reducing a weighed amount of cadmium sulphate to the sulphide in a stream of hydrogen sulphide, under pressure, at an elevated temperature, and weighing the sulphide. The reduction was shown to be complete by proving the absence of sulphate in the sulphide.

64.2051 grams of cadmium sulphate gave 44.4491 ” ” ” sulphide.

If the atomic weight of oxygen = 16, ” ” ” ” ” sulphur = 32.059, ” ” ” ” cadmium = 111.935.

The atomic weight of cadmium calculated as an average of the nine determinations made using the above values for oxygen and sulphur = 111.94.

Maximum, 112.121. Minimum, 111.796. Mean, 111.940.

The work of von Hauer is greatly to be preferred to that of Stromeyer. The large amount of material used in each determination tended to lessen any experimental error. A very considerable degree of care seems to have been exercised in purifying the cadmium sulphate. In determinations 1-5 a different specimen of sulphate was employed from that in determinations 6-9. The average value found in the first five determinations = 111.910, in the last four = 111.977. The close agreement between the results obtained from the different preparations of the sulphate argues in favor of a fair degree of purity for all the material.

The method of weighing the more or less hygroscopic cadmium sulphate is open to criticism when employed in accurate work. The cadmium sulphate was placed in an open boat, dried, cooled over sulphuric acid, and weighed. It was again dried, cooled as before, and weighed. The second weighing could be quickly accomplished since the approximate weight was known. The two weighings agreed to within less than a milligram or a third drying and weighing were made. An error of a milligram in the weight of the sulphate produced an average error in the atomic weight of cadmium of about .06. That a discrepancy of greater or less magnitude was introduced from this source will be readily seen.

Dumas Ann. Chim. Phys. 55, 158. 1859, determined the relation between cadmium chloride and the metallic silver required to precipitate the chlorine. Metallic cadmium was dissolved in boiling hydrochloric acid and the solution evaporated. The cadmium chloride was fused for five or six hours in a stream of hydrochloric acid gas. Six determinations were made. 23.0645 grams of cadmium chloride were equivalent to 27.173 grams of metallic silver.

If the atomic weight of silver = 107.93. ” ” ” ” ” chlorine = 35.45. ” ” ” ” cadmium = 112.322.

The atomic weight of cadmium calculated as the average of the six determinations made, using the above values for silver and chlorine = 112.241.

Maximum, 112.759. Minimum, 111.756. Mean, 112.241.

The large difference between the results would indicate some considerable source of error in part or all of the determinations. The first three determinations were made from a different specimen of cadmium from the last three.

In the first three the cadmium used does not seem to have been purified and the cadmium chloride prepared from it was more or less tinted brown. In the last three a new specimen of metal was used which in Dumas’ words could reasonably be considered to be absolutely pure. The chloride prepared from it was colorless, well crystallized and perfectly soluble in water. In order to show clearly the wide discrepancy between the results obtained from the two specimens of cadmium which were used, the separate determinations are given in detail.

At. Wt. CdCl₂ Ag. Cadmium. 1 2.369 2.791 112.322 2 4.540 5.348 112.347 3 6.177 7.260 112.759 4 2.404 2.841 111.756 5 3.5325 4.166 112.135 6 4.042 4.767 112.130

The average result of the first three determinations = 112.476. The average result of the last three determinations = 112.007. From Dumas’ own statement concerning the purity of the cadmium chloride analyzed, determinations 4-6 are much to be preferred to determinations 1-3 and the most probable value from Dumas’ work would be very nearly 112.

Lenssen Journ. f. prakt. Chem. 79, 281. 1860, regarded the oxalate of cadmium as well adapted to the determination of the atomic weight of cadmium. A solution of cadmium chloride which had been purified by repeated crystallization was treated with an excess of a solution of pure oxalic acid. The cadmium oxalate formed was filtered off, washed, and carefully dried in the air at 150° C. until the last trace of water was removed. 1.5697 grams cadmium oxalate gave 1.0047 grams cadmium oxide.

If the atomic weight of oxygen = 16, ” ” ” ” carbon = 12.003, ” ” ” cadmium = 112.043.

The average of the three determinations using the above values for oxygen and carbon is 112.067.

Maximum, 112.304. Minimum, 111.911. Mean, 112.067.

The small amount of material used in each determination, the small number of determinations made, and the rather large difference between the highest and lowest result are objectionable. There are certain weak points in the method but to these reference will be made later.

Huntington, Proc. Amer. Acad. 17, 28. 1882, working with Cooke, made two series of determinations of the atomic weight of cadmium. In the first series the relation between cadmium bromide and the silver bromide formed from it was determined. In the second, the relation between cadmium bromide and the silver required to precipitate the bromine.

The cadmium bromide was prepared by dissolving the carbonate in hydrobromic acid and subliming the product in a stream of carbon dioxide.

In the first series of eight determinations 23.3275 grams of cadmium bromide were equivalent to 32.2098 grams of silver bromide.

If the atomic weight of silver = 107.93. ” ” ” ” ” bromine = 79.95. ” ” ” ” cadmium = 122.239.

Maximum, 112.290. Minimum, 112.169.