Determination of the Atomic Weight of Cadmium and the Preparation of Certain of Its Sub-Compounds

Part 1

Transcriber’s Notes:

Underscores “” before and after a word or phrase indicate italics_ in the original text. Small capitals have been converted to SOLID capitals. Typographical and punctuation errors have been silently corrected. The symbol “ī” (small i with macron)was used in place of the numeral 1 (one) with macron. The symbol “̅2” (overline + 2) was used in place of numeral 2 with macron.

Determination of The Atomic Weight of Cadmium and The Preparation of Certain Of Its Sub-Compounds.

Dissertation,

Presented to The Board of University Studies of The Johns Hopkins University,

For The Degree of Doctor of Philosophy,

By Harry C. Jones

1892.

Contents.

Page Determination of the Atomic Weight of Cadmium 1 Introduction and Historical Statement 2 Preparation of Pure Cadmium 22 The Preparation of Pure Nitric Acid 28 The Arrangement of Crucibles 30 The Mode of Procedure 32 The Weighing 37 Taring the Crucibles 40 The Results 42 Objections to the Method 45 Advantages of the Method 48 The Oxalate Method 50 Preparation of Pure Oxalic Acid 51 Preparation of Cadmium Oxalate 52 Mode of Procedure 53 The Drying and Weighing of the Oxalate 55 The Results 58 Advantages of the Method 60 Disadvantages of the Method 61 Preparation of Certain Sub-compounds of Cadmium 63 Historical 64 The Preparation of Cd₄Cl₇ 66 The Preparation of Cd₄Br₇ 78 The Preparation of Cd₁₂I₂₃ 82 The Preparation of Cadmium Hydroxide and Oxide 82 Notes on Crystals of Metallic Cadmium 97 The Cohesion Phenomena of Cadmium 103 Biographical Sketch 106

Acknowledgment.

It affords me great pleasure to express my sincere thanks to Professor Remsen for his instruction and personal supervision during my entire connection with the University; to Dr. Morse, under whose immediate guidance the work described in this dissertation was completed; to Dr. Renouf for valuable assistance in qualitative chemistry and to Dr. Williams, with whom the branches of mineralogy and geology were followed as subordinate subjects.

Determination of the Atomic Weight of Cadmium.

Introduction and Historical statement.

A careful examination of the literature on the atomic weight of cadmium will convince any one that considerable uncertainty yet remains in reference to this constant. Six experimenters have worked on this problem but the results of no one of them can be accepted as being more accurate than those of all others. The value assigned to cadmium varies from 111.48 to 112.32 on the basis of oxygen = 16. The best work has apparently been done by von Hauer, Lenssen and Huntington. The results of these three seem entitled to about equal confidence, yet the figure obtained by von Hauer differs from that of Huntington by three tenths of a unit.

The more prominent difficulties which have been encountered were:

First. The preparation of cadmium compounds free from all impurities, and which at the same time were well adapted to weighing.

Second. The lack of a thoroughly simple and exact method for the analysis of cadmium compounds.

Third. Insufficient care in weighing in many cases whereby small errors were introduced into the results.

The methods which have been employed are:

1 Conversion of the metal into the oxide. (Stromeyer).

2 Conversion of the sulphate into the sulphide. (von Hauer and Partridge).

3 Decomposition of the oxalate to the oxide. (Lenssen and Partridge).

4 Determination of the chlorine in cadmium chloride, by which the relation between the chloride and metallic silver was established. (Dumas.)

5 Precipitation of the bromine in cadmium bromide as silver bromide. (Huntington.)

6 The conversion of the oxalate into the sulphide. (Partridge.)

The different pieces of work will be taken up in chronological order and briefly considered.

Stromeyer, Schurigg Journ. 22, 366. 1818, determined the atomic weight of cadmium a short time after the discovery of the element. He does not describe his method in detail but established the relation between cadmium and oxygen to be:

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.

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