An Examination of Some Methods Employed in Determining the Atomic Weight of Cadmium

Part 1

An Examination of some Methods Employed in Determining the Atomic Weight of Cadmium.

A Thesis

Presented to the Board of University Studies of the Johns Hopkins University for the Degree of Doctor of Philosophy.

by John E. Bucher.

1894

Contents.

I. Introduction and Historical Statement 1 II. The Oxalate Method 3 Preparation of Pure Cadmium 3 Preparation of Nitric Acid 4 Purification of Water 4 Purification of Oxalic Acid 5 Preparation of Cadmium Oxalate 7 Procedure 8 Results 13 III. The Sulphide Method 16 Preparation of Hydrogen Sulphide 16 Preparation of Nitrogen 17 Mode of Procedure 18 Results 24 Discussion of the Results 24 Discussion of the Method 26 IV. The Chloride Method 33 Preparation of Cadmium Chloride 35 The Filters 48 Analytical Process 52 Results 57 Discussion of the Results 58 V. The Bromide Method 69 Preparation of Cadmium Bromide and Hydrobromic Acid 70 Method of Analysis 78 Results 80 Discussion of the Results 80 VI. Syntheses of Cadmium Sulphate 82 Results 90 Discussion of the Results 91 VII. The Oxide Method 94 Results 96 Discussion of the Results 97 Determination of Error 104 Discussion of the Oxalate Method 114 VIII. Other Methods 119 IX. Conclusion 122

Acknowledgement.

The author wishes to acknowledge his indebtedness for advice and instruction to Professor Morse at whose suggestion and under whose guidance this work has been carried on. He also wishes to express his thanks for instruction to Professor Remsen in Chemistry, Professor Williams in Mineralogy, Dr. Ames in Physics and Mr. Hulburt in Mathematics.

Introduction and Historical Statement.

The atomic weight of cadmium has been investigated by a number of chemists but the results obtained vary between wide limits. The work described in this paper was undertaken with the object of finding the cause of the discrepancy in some of the methods employed. A complete historical statement has been given by Morse and Jones, (Amer. Chem. Jour., 14. 261.) and it is only necessary, here, to give a summary for the purpose of reference:

Ratio. At. Wt. Cd. 1818, Stromeyer, Cd : CdO 111.483 (Schweiggers Jour. 22, 336.) 1857, Von Hauer, CdSO{4} : CdS 111.935 (Jour. f. Prakt. Chemie 72, 338.) 1859, Dumas, 1st series. CdCl{2} : Ag 112.416 2d „ CdCl{2} : Ag 112.007 (Ann. Chim. Phys. , 55, 158.) 1860, Lenssen, CdC{2}O{4} : CdO 112.043 (Jour. f. Prakt. Chem. 79, 281) 1882, Huntington and Cooke, CdBr{2} : AgBr 112.239 „ „ CdBr{2} : Ag 112.245 (Proceedings Amer. Acad. 17, 28) 1890, Partridge, 1st Series CdC{2}O{4} : CdO 111.816 „ 2d „ CdSO{4} : CdS 111.727 „ 3d „ CdC{2}O{4} : CdS 111.616 (Amer. Jour. Sci., 40, 377) 1892, Morse and Jones, 1st Method, Cd : CdO 112.0766 2d „ CdC{2}O{4} : CdO 112.632 1892, Torimer and Smithy CdO : Cd 112.055 (Zeit. f. Anorg. Chem. I. 364)

In this summary as well as in the rest of this paper the following atomic weights are used:

Oxygen = 16.00 Sulphur = 32.059 Carbon = 12.003 Chlorine = 35.45 Bromine = 79.95 Silver = 107.93

The Oxalate Method

Preparation of Pure Cadmium.

“Cadmium met. puriss. galv. reduc”, obtained from Schuchardt, was used for preparing pure cadmium. It was heated to redness in a current of hydrogen which had been purified by washing with both acid and alkaline solutions of potassium permanganate. This treatment converted the metallic powder into a bar which could be distilled in a vacuum. The metal was then distilled nine times in the same manner that Morse and Burton, Amer. Chem. Jour. 12, 219, had distilled zinc. All distillations were made slowly except the last one, which was made quite rapidly.

Preparation of Nitric Acid.

Whenever pure nitric acid was required, it was purified by distilling against a platinum dish and collecting the distillate in a smaller one of the same metal. The nitric acid used was dilute and free from chlorine.

Purification of Water.

The water used in this work was purified by distilling twice from an alkaline solution of potassium permanganate, always rejecting the first part of the distillate. Whenever water was needed in the preparation of a pure compound e.g. cadmium oxalate, oxalic acid, cadmium nitrate, etc., it was subjected to the additional process of being distilled against a large platinum dish which was kept cool by placing ice inside it.

Purification of Oxalic Acid.

Commercial oxalic acid was heated with a fifteen percent solution of hydrochloric acid until all was dissolved. The solution was then warmed for twenty four hours. On cooling, crystals of oxalic acid separated out and these were washed with a little cold water to remove the mother liquor. They were then dissolved in hot ninety-five percent alcohol and allowed to crystallize slowly on cooling. The acid was next crystallized from ether in which it is only sparingly soluble. After this it was boiled with water until the odor of ethyl acetate had disappeared. Finally it was recrystallized three times from water and dried in the air at ordinary temperatures.

Preparation of Cadmium Oxalate.

A weighed piece of cadmium was dissolved in nitric acid and the excess of acid evaporated off. The nitrite was then dissolved in a large quantity of water and an equivalent amount of oxalic acid in solution added. The oxalate separated in a few moments as a crystalline precipitate. It was collected on a porcelain filter and washed thoroughly to remove nitric acid and ammonium nitrate. A considerable amount of ammonium nitrate was formed during the solution of the cadmium in nitric acid. The oxalate was finally dried in an air-bath for fifty hours, at 150°C.

Procedure

Enough cadmium oxalate for a determination was placed in a weighing tube which had been tared against a similar vessel and dried at 150°C. until the weight remained constant. It was then poured into a weighed porcelain crucible. The tube and its tare were now dried again at the same temperature to constant weight in order to avoid any error resulting from moisture being absorbed by the cadmium oxalate which adhered to the weighing-glass. The crucibles used in these determinations were arranged in the same manner as those employed by Morse and Jones in their work on this method. A small porcelain crucible on whose edge were placed three short platinum wires bent in the shape of the letter U, was placed in a larger porcelain crucible. The platinum wires prevented the lid from sticking to the crucible after heating and also allowed the products of decomposition to escape. The glaze was removed from the outside of the larger crucible with hydrofluoric acid to avoid sticking when heated to a high temperature. A second pair of crucibles arranged in the same manner was tared against the first one and in all cases treated like it. After the oxalate had been poured into the weighed crucible, it was decomposed by placing the crucible with its contents in a cylindrical asbestus covered air-bath, and slowly raising the temperature until the mass beams uniformly brown in color. In the last five determinations, the temperature was not allowed to exceed 300°C and after from forty to eighty hours the loss in weight was about ninety percent of the amount calculated for complete decomposition. In the first four the temperature was much higher and the time employed shorter. After the oxalate had been thus treated nitric acid was added and the contents of the crucible dissolved completely. The crucible was then transferred to a bath constructed by placing a larger porcelain crucible in a still larger one of iron and filling the intervening space with sand. It was slowly heated until the nitric acid had all evaporated and the dry nitrate began to give off red fumes. The crucibles were then removed to a similar bath containing iron filings instead of sand. This bath was heated by means of a single burner as long as red fumes were observed, and then for about five hours with a triple burner. Finally, the crucibles were transferred to a nickel crucible in the bottom of which a plate of unglazed porcelain was placed. The nickel crucible which had previously been set tightly into a hole cut in an asbestos board was then heated over the blast lamp for two hours. After this the porcelain crucible and contents were weighed and then reheated for half hour periods as before until three successive weighings remained constant. This usually required from three to four hours of blasting. In all determinations, the resulting product was tested for oxides of nitrogen with potassium iodide, starch and hydrochloric acid, but none was found. All weighings were reduced to the vacuum standard on the assumption of 8.4 for the Sp. Gr. of brass, 21. for platinum, 3.31 for the oxalate and 8.15 for cadmium oxide. The results are:

Cadmium Oxalate. Cadmium Oxide. At. Wt. Cd. I 1.97674 1.26414 111.73 II 1.94912 1.24682 111.82 III 1.96786 1.25886 111.77 IV 1.87099 1.19675 111.77 V 1.98941 1.27242 111.79 VI 1.37550 .87994 111.85 VII 1.33313 .85308 111.95 VIII 1.94450 1.24452 112.04 IX 2.01846 1.29210 112.09

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