Part 4

After a careful examination of the methods available it becomes evident that no one of them was per se as accurate as the method employed by Morse and Burton, for the determination of the atomic weight of zinc, and more recently by Burton and Morse, for the determination of the atomic weight of magnesium. The method of work was to prepare pure metallic cadmium, to convert a weighed portion of the metal into nitrate by means of pure nitric acid, to decompose the nitrate completely to oxide and to weigh the oxide.

Amer. Chem. Journ. X, 311.

ib. XII, 219.

Preparation of Pure Cadmium.

The work of preparing pure cadmium was begun more than two years ago by Mr. W. V. Metcalf with Dr. H. N. Morse. I wish to express here my sincere thanks to him for the material with which the following determinations were made. The cadmium used by him was obtained from Schuchart and marked “Met. prss. (galv.) redus.”

The method of purification by fractional distillation in a vacuum, was essentially that employed by Morse and Burton for the purification of metallic zinc.

The distillation was carried out in hard glass tubes of the size of ordinary combustion tubing.

Fig. 1. represents such a tube. A hard glass tube, 600-700 mm. in length, was closed at one end and about 130 grams of cadmium introduced. The walls of the tube were heated and indented at the two points a, and b, with a red-hot file, dividing the tube into three sections marked A, B and C. The open end of the tube was drawn out, bent, and attached to a Sprengel air-pump by means of a rubber tube.

The joint was tied tightly with waxed cord and surrounded by mercury. When the manometer indicated that the tube was exhausted, it was gradually heated by the combustion furnace in which it rested. The metal in A melted and distilled slowly into the front portion of the tube. Most of it condensed in B, while a small part, together with any more volatile impurity, collected in C which was kept cooler than the remainder of the tube. When about four-fifths of the metal placed in A had distilled over, the tube was very slowly cooled. When cold, the tube was broken open, the portions in A and C being rejected in every case, while the metal was recovered from B in the form of a bar resting on the bottom of the tube, together with some crystal aggregates, suspended from the top and sides. A few crystal individuals were secured but the measurement of these will be considered later. The metal separated from the glass with a highly lustrous surface and did not attack the glass in the least.

The first distillation was effected in a tube bridged as represented in Fig. 1, but drawn out at each end. The original cadmium powder was heated in the tube in a stream of pure hydrogen gas, for the purpose of obtaining the metal in the form of bars, and to reduce any cadmium oxide contained in the powder.

Six distillations were made in a vacuum. In the first, 630 grams of metal were used being distilled in quantities of about 130 grams each. At the end of the sixth distillation, there were about 100 grams of pure cadmium at disposal. In the fifth and sixth distillations, the metal was heated just above the melting point for from twenty to twenty-four hours, before being forced over into the middle portion of the tube. By this means all the remaining traces of the more volatile arsenic were driven into the front part of the tube and separated from the cadmium.

The distillations.

The residue represents the undistilled portion remaining in A. The distillate, the material obtained from B after the distillation was completed. The coating, the substance which condensed in C.

Residue, Cd, Pt, Zn,? As?. Distillation I Distillate, Cd, Zn,? As? Coating, Cd, Zn,? As?.

Residue, Cd, Zn?, As?. Distillation II Distillate, Cd, Zn?, As?. Coating, Cd, Zn?, As?.

Residue, Cd, Zn?, As?. Distillation III Distillate, Cd, Zn?, As?. Coating, Cd, Zn?, As?.

Residue, Cd, Zn?, As?. Distillation IV Distillate, Cd, Zn?, As?. Coating, Cd, Zn?, As?.

Residue, Cd. Distillation V Distillate, Cd. Coating, Cd, As?.

Residue, Cd. Distillation VI Distillate, Cd. Coating, Cd.

The distillate from the last distillation was examined spectroscopically by Professor Rowland and found to be free from all traces of impurity which would be detected by that method. The chemical test for arsenic was more delicate than the spectroscopic and this failed to reveal a trace.

The preparation of pure nitric acid.

The method of preparing the pure acid and of preserving and transferring it was the same as adopted by Morse and Burton in their work on the atomic weight of zinc.

The simple form of apparatus is represented in fig. 2. A large platinum vessel containing fragments of ice was supported on a smaller platinum dish, from which it was separated by hooks of large platinum wire. The acid was distilled from a small flask as represented in the drawing.

The purest nitric acid which could be obtained was diluted with about an equal volume of water. The vessel containing the acid was heated very gently that the distillation might take place without boiling. The dilute acid condensed on the cold surface of the larger dish and collected in the smaller, in which it was preserved until used. This acid gave no residue on evaporation.

The arrangement of crucibles.

The arrangement of the crucibles in which the determinations were made is represented in fig. 3. 1 is a small porcelain crucible, (00) from the exterior and lid of which the glaze had been removed by hydrofluoric acid. The lid was separated from the crucible by hooks made from thick platinum wire, to allow free communication between the contents of the crucible and the external air. This would facilitate the outward diffusion of the oxides of nitrogen when liberated from the nitrate. 2 is an uncovered porcelain crucible (no. II) in which 1 was placed. From the exterior the glaze had been removed to prevent the crucible from adhering to the unglazed porcelain scorifier on which it rested. The exterior was carefully brushed after treatment with hydrofluoric acid to remove all loose particles adhering to its surface. Crucibles 1 and 2 were not separated during a determination.

3 is a nickel crucible about two and a half inches in diameter. The porcelain crucibles were not allowed to touch the nickel at any point. The nickel crucible was covered by a lid of nickel.

The mode of procedure.