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. [3], 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.[3], 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

A glance at these results shows that there is a variation of .36 of a unit and that the atomic weight in general increases with the number of determinations. In the first four determinations, there may have been loss of cadmium by reduction and subsequent volatilization, but in the later determinations this is not probable. It is believed that the greater part of the variation was due to imperfect dehydration of the oxalate. This and other sources of error in this method will be referred to later. The nickel crucible used gave a slight sublimate on heating, even after fifteen hours’ blasting. This condensed on the porcelain crucible as a brownish coating but, as both the crucible and its tare were blasted for the same length of time, it did not seem to change the difference of their weights. More than a dozen nickel crucibles were tried but none was found not to give a sublimate. The amount was so slight that no attempt was made to determine its nature.

The Sulphide Method.

This method is based on the conversion of cadmium oxalate into cadmium sulphide by heating in a current of hydrogen sulphide. The method has been used by Partridge. This result was 111.61 for the atomic weight of cadmium.

Preparation of Hydrogen Sulphide

In the present work this gas was always prepared from potassium hydrosulphide which was made from barium sulphide (commercial). Barium sulphide was treated with dilute hydrochloric acid and the resulting hydrogen sulphide washed thoroughly with a solution of potassium hydrosulphide and then with pure water. It was then passed into a strong solution of potassium hydroxide until the later was saturated. When it was required, it was set free from this solution by adding dilute sulphuric acid and again washing the resulting gas with a strong solution of potassium hydrosulphide.

Preparation of Nitrogen.

Whenever a current of nitrogen was required, it was prepared by passing air over a layer of hot copper gauze in a combustion tube. A short layer of copper oxide was first introduced, then the copper gauze and finally another layer of copper oxide. The air was dried with caustic potash before entering the tube and the nitrogen obtained was also passed through a long tube filled with lumps of this substance before being used.

Mode of Procedure.

A number of weighing tubes 140 millimetres long and 13 millimetres internal diameter were made especially for this work. They were always used in pairs, one being kept as a counterpoise. A porcelain boat of such dimensions as just to slide into the tube was placed in each one. For a determination, a tube and its boat were tared with another tube and boat, glass against glass and porcelain against porcelain until the difference in weight was less than two tenths of a milligramme. Both boats were heated in a current of hydrogen sulphide to incipient redness for about one hour. The current of hydrogen sulphide was then replaced by one of nitrogen, in which the boats were cooled, but while still warm they were transferred to their weighing tubes and allowed to cool in a dessiccator containing caustic potash and weighed. Before weighing, the stoppers of the weighing tubes were loosened for a moment in order to equalize the internal and external pressure. This treatment was usually repeated two or three times and the difference in weight remained perfectly constant. A portion of cadmium oxalate sufficient for a determination was placed in the weighed boat and dried at 150°C. The oxalate had been prepared exactly like that used in the oxalate method which has already been described. The gas pressure in the laboratory varied very much while this method was under investigation and great difficulty was experienced in maintaining a constant temperature although a thermoregulator was used. Sometimes a specimen of oxalate which was supposed to be dry would lose several tenths of a milligramme when the thermometer would only have gone up to 160°C or 165°C for an hour by accident. Under these conditions the drying was so uncertain that only four determinations were completed although many were started. The boat containing the oxalate which had been dried and weighed was placed on supports of unglazed porcelain in a combustion tube and a current of dry hydrogen sulphide passed over it. As soon as the air was expelled, the tube, which was in a combustion furnace, was slowly heated until all the oxalate seemed to be decomposed and then raised to dull redness. After this temperature had been maintained for about an hour, the sulphide was allowed to cool to a temperature of about 200°C. and the current of hydrogen sulphide replaced by dry nitrogen, using a three-way stopcock. When nearly cold the boat was slipped into its weighing-tube and weighed, the same precautions being used as when weighing the empty boat.

At this stage the sulphide was always from one to two milligrammes lighter than at the end of the determination. It was reheated for periods of one hour until the weight remained constant. This generally required from three to five hours. All weighings were reduced to the vacuum standard on the basis of 4.5 for the Sp. Gr. of cadmium sulphide, 3.31 for the Sp. Gr. of cadmium oxalate, 8.4 for the Sp. Gr. of brass weights and 21 for the Sp. Gr. of platinum weights.

The results are as follows:

Cd C_{2}O_{4} CdS At. Wt. Cd. I. 2.56319 1.84716 112.25 II. 2.18364 1.57341 112.17 III. 2.11643 1.52462 112.05 IV. 3.13105 2.25582 112.12

The first three determinations were made exactly as above described, the heating in hydrogen sulphide being done in a Bohemian glass combustion tube. The hydrogen sulphide was dried with calcium chloride.

The fourth determination was made under somewhat different conditions. The boat containing the weighed oxalate was placed in a combustion tube which passed through an asbestus covered air-bath. The air was displaced by a current of dry hydrogen sulphide and the bath slowly heated. When the temperature had risen to 210°C. it was maintained there for three hours, and then raised to 250°C. for three hours. The sulphide then weighed 2.27 grammes, being 14 milligrammes heavier than when the determination was finished. It was replaced in the tube and reheated in a current of hydrogen sulphide at a temperature of 300°C. for four hours. It was then transferred to a porcelain tube and heated to redness for one hour. It then weighed 2.25437 grammes, being 1.45 milligrammes lighter than at the end of the determination. The weight did not become constant until it had been heated six hours more to redness in a current of hydrogen sulphide. When this oxalate was slowly heated in H_{2}S, a small amount of oxalic acid sublimed to the colder part of the tube, but, in the other cases where the heating was more rapid, only carbon monoxide, carbon dioxide, and water were observed.

Discussion of the Method.

When hydrogen sulphide is passed through a red-hot tube, sulphur is deposited on the colder parts because at this temperature hydrogen sulphide dissociates and the elements do not recombine on cooling. In this work, a faint sublimate was noticed before coming to the zone of sulphur deposit. On exposure to air, it deliquesced in a few minutes forming small yellow drops which had a saline taste, and gave tests for potassium and sulphur. The sublimate had a yellow color and was evidently formed by the action of sulphur on glass. It seemed to do no harm, but in the fourth determination an effort was made to avoid it by using a porcelain tube instead of a glass combustion tube for heating to redness in a current of hydrogen sulphide.

The fact that sulphide of cadmium was always too light after the first hour’s heating in hydrogen sulphide proves that it must have contained some oxide of cadmium even after this heating. Oxide of cadmium is readily absorbed by the glaze on porcelain, and some error must have been introduced in this way because it would not be converted into sulphide after forming a silicate.

The effect of this would be to give a low result for the atomic weight of cadmium. To get some idea of the magnitude of this error, the sulphide was poured out of the boats used in the first and second determinations. They were then warmed with nitric acid for a few moments, washed in water, and heated over the blast lamp for a few minutes. The boats used as tares were treated in exactly the same manner. On weighing, the boats in which the oxalate in determinations I and II had been decomposed, were found to be 1.12 milligrammes and .82 milligrammes heavier respectively than at the beginning of the determinations. This would only introduce an error of .03 of a unit in the atomic weight on account of the small difference in weight between these amounts of oxide and equivalent amounts of sulphide. The boats were warmed, as above mentioned, with nitric acid to remove any adhering sulphide. This might have decomposed some cadmium silicate at the same time, and the error due to cadmium oxide thus be found smaller than it really is.

The following experiment was made in the hope of avoiding the formation of cadmium silicate. The glaze was removed from the inside of a porcelain boat by hydrofluoric acid followed by a thorough scouring with sand and water. The boat was then heated in the flame of a blast lamp for several minutes, tared against another boat which was not treated with hydrofluoric acid. Both were heated to redness in a current of hydrogen sulphide for an hour, cooled, weighed, and then heated in hydrogen sulphide for another hour, and weighed again. The boat gained 1.7 milligrammes during this second heating, showing that a boat whose glaze has been removed by hydrofluoric acid could not be used in this method. Throughout this work, great care was taken to exclude the oxygen of the air from the cadmium sulphide, while hot. The current of hydrogen sulphide in which the cadmium sulphide is heated must not be too slow, otherwise the sulphur in the dissociated gas will diffuse to the colder parts of the tube and condense, the residual gas becoming very rich in hydrogen. The hydrogen will then reduce some of the sulphide to metal, causing loss by volatilization. One determination was lost in this way, over two milligrammes of the sulphide being sublimed out, and it could easily be detected on the side of the tube. It is believed that the cause of the variations in the four determinations made by this method, is due to imperfect dehydration of the oxalate. It did not seem advisable to continue this part of the work any farther; therefore the chloride method was taken up.

The Chloride Method.

Huntington had determined the ratios of CdBr_{2} to AgBr and also CdBr_{2} to Ag very carefully, obtaining the result 112.24 for the atomic weight of cadmium. Morse and Jones had obtained 112.07 for this constant by the oxide method. The object of the work about to be described was to find the cause of this discrepancy if possible. It was thought advisable however to make some determinations of the ratio of CdCl_{2} to AgCl before beginning the bromide method.

Dumas, in 1859, used cadmium chloride to determine the atomic weight of the metal. He did not establish its ratio to silver chloride but to silver by titration. He prepared cadmium chloride by dissolving the metal in hydrochloric acid and melting the resulting product in a platinum capsule for five or six hours. He made two series of three determinations. The chloride used in the first series was yellow in places and not completely soluble. The result was 112.476. The second series was made with chloride which was perfectly white and soluble and gave 112.007 for the atomic weight of cadmium. It is evidently more reliable than the first series and Dumas himself concluded that the atomic weight is very near 112.01.

Preparation of Cadmium Chloride