Part 2

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

Four different specimens of cadmium chloride were used in this work and from these specimens portions were taken for analysis. These portions were treated differently in different analyses, therefore it will be necessary to give a brief descriptions of them and mention the number of the determinations, in which each one was used. Chloride of cadmium was prepared in the following manner. A solution of pure hydrochloric acid was prepared by passing a current of hydrochloric acid gas into pure water which was contained in a porcelain crucible until no more was absolved. The water used had been purified by distilling against a platinum dish and the hydrochloric acid gas was obtained by heating ordinary concentrated chemically pure hydrochloric acid in a distilling bulb whose neck had been closed by fusion in order to avoid the use of a cork or rubber stopper. Hydrochloric acid thus prepared will leave no residue on evaporation when air is excluded (Stas, Aronstein’s German translation, p. 111). A piece of platinum foil freed from iron by heating in the vapors of ammonium chloride as recommended by Stas (Aronstein’s translation, p. 112) was introduced and a piece of cadmium laid on it. Solution begins at once, the hydrogen being liberated on the platinum foil. During the later part of the process, heat was applied. After all of the metal had dissolved, the solution was evaporated, the platinum foil having previously been removed. The crystals of cadmium chloride which separated were not dried but allowed to remain slightly moist with hydrochloric acid. If no platinum foil is used, the solution of the pure metal becomes exceedingly difficult, unless a very large excess of acid is used. No objection can be raised to the use of platinum foil for in making fifty grammes of cadmium chloride it cost less than a tenth of a milligramme and even this could probably have been avoided by using a somewhat larger amount of hydrochloric acid. The foil was always kept submerged in the acid liquid. The moist crystals of cadmium chloride were transferred to a combustion tube passing through an asbestus covered air-bath, and dried in a current of hydrochloric acid gas for several hours at 300°C. The hydrochloric acid gas had been passed through a long calcium chloride tube to dry it, although calcium chloride probably does not do this very thoroughly. The hydrochloric acid gas was then replaced by a current of nitrogen prepared as has already been described under the sulphide method. After the current of nitrogen had been passing for about half an hour, the tube was allowed to cool, and the chloride transferred to another combustion tube, one end of which had been sealed in the flame of a blast lamp. The other end was drawn out and attached to a Sprengel mercury pump. After exhausting, the chloride was sublimed in the vacuum. This takes place at a moderate temperature and the sublimate has a beautiful crystalline structure and is perfectly white. The crystalline mass exposes so much surface that water is taken up very rapidly when exposed to the air. This action is so rapid that the crystals cannot be transferred to a weighing-glass without introducing an appreciable error. The whole sample was accordingly transferred to a stoppered glass bottle which was kept under a bell jar with sticks of caustic potash. Three samples were prepared in this manner, the first being used in determination one, the second in determinations two to seven inclusive, and the third in determinations eight to nineteen inclusive. The samples used in determinations twenty and twenty-one were prepared in the following manner: About three grammes of cadmium were placed in a combustion tube in which three bridges (as in the distillation of pure cadmium) had been made. A section may be represented thus

The metal was placed in cavity A and a stream of chlorine passed through the tube. The chlorine was prepared from potassium bichromate and hydrochloric acid and dried by passing it through a long tube containing calcium chloride. When the air had been displaced, the cadmium was heated. It fused and began to burn to the chloride which partly flowed over the bridge into cavity B and partly distilled over into this cavity. When the reaction had ended, the current of chlorine was replaced by one of dry nitrogen, and the tube was allowed to cool and the chloride taken for analysis XX. The specimen used in analysis twenty-one was prepared in exactly the same way, only the chlorine used was obtained from manganese dioxide, sodium chloride and sulphuric acid, and was dried with phosphorous pentoxide instead of calcium chloride.

The special treatment of the portions taken for analysis was as follows: Those taken for determinations I, II and from XI to XIX inclusive were placed in a platinum boat and put into combustion tube. A current of hydrochloric acid gas obtained by heating the aqueous acid was passed through the tube. The gas had been dried by calcium chloride. When the air was displaced, the chloride was heated somewhat higher than its fusing-point i.e. to incipient redness, and maintained there for a length of time varying from a few minutes to more than an hour. The hydrochloric acid was then displaced by a current of nitrogen, and the chloride allowed to cool. The boat with the chloride, while still slightly warm, was placed in a weighing-tube, cooled in a dedicator containing caustic potash and weighed. The chloride thus prepared is transparent and presents only a small surface to the air. It takes water up so slowly that no error is introduced from this source. This was tested in one case by allowing a boat containing some chloride thus prepared to stand in the air for a certain length of time and noting the increase in weight. It was quite slow. In several cases specimens of chloride were tested for hydrochloric acid using tropaeolin as an indicator. It was always found neutral. The portions used for determinations III and VI to X inclusive were prepared in exactly the same manner as the preceding ones except that the hydrochloric acid gas in which they were fused was not dried but used just as it came from the aqueous acid. In some cases the platinum boat in which the chloride was fused was weighed before and after the fusion. The weight remained unchanged.

For determinations IV and V, about six grammes of cadmium chloride were placed in a platinum boat, and more than two-thirds of it distilled out in a current of hydrochloric acid gas which had not been dried. Part of the distillate was collected after cooling in nitrogen and used in determination IV while the residue remaining in the boat was used for determination V. The method of preparing the chloride used in determinations XX and XXI has already been described.

The Filters.