Preparation of CertainSub-compounds of Cadmium.
Cadmium acts so generally as a bivalent element that it is usually regarded as entering into combination only where it can play this rôle. The only compound described, in which it has apparently a lower valence than two, was prepared by Marchand[10]. It was obtained by heating cadmium oxalate to the melting point of lead when a green powder remained behind which resembled chromium oxide. When heated on the air it appeared to be decomposed into metal and oxide. When treated with mercury the compound was not altered. An analysis showed it to have the composition represented by the formula Cd₂O.
A. Vogel[11]has shown that the green powder described by Marchand consists of a mixture of the metal and oxide. When this mixture is treated with dilute acetic acid the metal remains behind as microscopic glistening globules. The lower the temperature at which the oxalate is decomposed the more oxide and the less metal were found in the product.
There was then no compound known in which cadmium acted as if its valence was less than two when this work was undertaken.
That it may act with a greater valence was shown by R. Haafs[12]. He found that when zinc hydroxide was treated with hydrogen dioxidecertain compounds of zinc and oxygen were formed containing more oxygen than the normal oxide ZnO. The close resemblance between zinc and cadmium led him to try the same reaction with cadmium. Hydrogen dioxide was accordingly allowed to act on cadmium hydroxide and the resulting product analyzed. There were formed Cd₅O₈, Cd₃O₅ and Cd₄O₇. In no case was the compound CdO₂ obtained. These compounds are described as fairly stable even at a hundred degrees.
When anhydrous cadmium chloride is heated with metallic cadmium in a vacuum, or in an atmosphere of nitrogen, to the fusing point of thechloride, the molten chloride quickly assumes a garnet red color. In order to investigate this phenomenon a quantity of the chloride was prepared by dissolving the redistilled metal in an excess of hydrochloric acid, evaporating the chloride to dryness on a water bath, and finally removing the water of crystallization by heating in a current of dry hydrochloric acid gas. The heating was effected by placing the chloride in a long platinum boat, which was shoved into a large glass tube, through which was passed a current of the acid gas. The tube was heated by means of a combustion furnace and the chloride kept in the molten condition for two or three hours. By this means a perfectly white crystalline chloride of the composition CdCl₂ was obtained, free from water or oxychloride.
The chloride and an excess of metal were placed in a long-necked flask of hard glass and after the displacement of the air by nitrogen, heated to the melting point of the chloride. The liquid chloride attained its maximum depth of color in a few minutes, nevertheless the heating was continued for five hours. When the temperature was allowed to rise much above the melting point of the chloride the red substance underwent decomposition and globules of metal collected upon the walls of the flask. For this reason no more heat was applied than was just necessary to keep the contents of the flask in a liquid condition. During the very gradual cooling of the flask it was shaken gently in order tofacilitate the sinking of any metal, which might be mechanically retained by the chloride.
On cooling, the solidified mass possesses a slightly greenish tint which disappeared when cold, the substance having then a grayish white color and a cleavage resembling that of talc or brucite. When examined under the microscope it was found to be perfectly homogeneous and free from metal. It gave no metallic streak when rubbed between agate surfaces.
An analysis of the first preparation showed the following composition;
These proportions are nearly those of a compound having the composition Cd₄Cl₇, in which the calculated percentages are:
(Foot note). In the paper in the American Chemical Journal XII, 488, which records this work the analyses and percentages were calculated on the basis of the atomic weight of cadmium = 111.7. Although my work since this date has shown that 112.07 is the true value, yet I think it preferable to use the old number here since the changes to be introduced would be very slight and the same results are thereby kept uniform in the two publications.
In order to determine whether the close approximation to definite atomic proportions might not be accidental, the material was reheated with an excess of the metal for twenty hours. The product was analyzed.
A second preparation of the substance was made in all respects like the first. Two analyses were made.
A third preparation was made like the first and second and analyzed.
When the new substance is heated it fuses to a red liquid and then breaks up into metal and the chloride of cadmium. Its reactions are in general those of a strong reducing agent. Treated with nitric acid, oxides of nitrogen are liberated. With dilute hydrochloric, sulphuric and acetic acids it gives free hydrogen. In the presence of dilute acids it reduces mercuric to mercurous chloride, or to metallic mercury.
Three determinations of the reducing power of the substance were made with a freshly prepared specimen, by dissolving weighed portions in hydrochloric acid and measuring the hydrogen liberated.
The following results were obtained:
An examination of the analyses shows beyond question that the substance formed by the action of metallic cadmium on the molten anhydrous chloride is of definite composition. The proportion of cadmium to chlorine could not be changed even when the substance was heated with the metal for twenty hours, while a very short time was sufficient for its formation when the metal and chloride were melted together.
It may be possible that a substance possessing these properties is not a definite chemical compound but a mixture of cadmous and cadmic chlorides or a solution of one in the other.
If it were a solution it is difficult to see why the composition of the solution should be so constant, since the solubility of a substance is generally altered by a change in temperature. The differentpreparations were not made at exactly the same temperature yet the composition of the different preparations was the same.
If the substance was a mixture of the two chlorides, when treated with water the cadmic chloride would most probably dissolve directly leaving the cadmous chloride to be acted upon by the water. The decomposition by water will however be seen not to be as simple as would be expected under these conditions.
From the above considerations it appears highly probable that the substance is a definite chemical compound of cadmic and cadmous chlorides. If cadmic chloride can form a chemical compound with the chloride of another element there appears to be no reason why it should not form a compound with another chloride of cadmium, as with cadmous chloride.
The anhydrous bromide of cadmium was prepared by dissolving the carbonate in an aqueous solution of hydrobromic acid, evaporating the bromide to dryness on the water bath and heating the residue in a current of dry hydrobromic acid gas. When the bromide was heated with an excess of the metal in an atmosphere of nitrogen it conducted itself in general like the chloride. When the molten bromide and the metal came in contact the salt quickly became deep red in color. After heating for some time considerable dissociation was produced by raising the temperature. This was more apparent in the preparation of thebromide than with the chloride. On cooling, the mass possessed a greenish tint which disappeared when cold, the bromide then being very nearly the same color as the corresponding chloride. Also like the chloride it appeared to be homogeneous and free from metal. Two determinations of cadmium and two of bromine were made, using the product as soon as prepared.
The percentage of cadmium and bromine found agrees very closely with that of a compound of the formula Cd₄Br₇. The relation of cadmium to bromine in this would be:
When this compound was heated for a long time with an excess of the metal its composition was not appreciably changed.
The compound Cd₄Br₇ is a strong reducing agent: giving with nitric acid oxides of nitrogen, with dilute hydrochloric, sulphuric or acetic acid, free hydrogen, and with mercuric chloride, mercurous chloride or metallic mercury. The action of water on the bromide by means of which cadmous hydroxide was formed, was not studied as carefully as with the chloride but appeared to be essentially the same.
Cadmic iodide was prepared in the same manner as the bromide. It was dried in a stream of hydriodic acid gas at as low temperature as possible to lessen the decomposition of the hydriodic acid. When the anhydrous iodide was heated with an excess of metal in an atmosphere of nitrogen the red color of the iodide became intensified. Heating was continued until there was evidence of dissociation, which, under the same conditions, was less marked than with the chloride and much less than with the bromide. Owing to the high specific gravity of the iodine compound some difficulty was experienced in obtaining a preparationfree from metal. This difficulty was finally overcome by keeping the material just above its melting temperature for a long time and constantly jarring the flask. During the process of cooling a decidedly greenish tint was observed which disappeared as the process was continued. When cold the substance resembled the chloride and bromide. Two determinations of cadmium were made in the first preparation.
As these results did not correspond to the composition represented by the formula Cd₄I₇, which our experience with the chloride and bromide had led us to expect, we reheated the material for several hours with an excess of the metal. Two analyses of the product gave:
showing that the iodide had taken up during the first heating all the metal which it could retain. The analytical results suggest the formula Cd₁₂I₂₃, in which the calculated percentages are:
In its conduct towards dilute hydrochloric and acetic acids and water the substance behaves like the corresponding chloride and bromide.
When the substance Cd₄I₇ is treated with water a complicated reaction takes place. The general character of the reaction appears to be the same with the chloride, bromide and iodide. The decomposition of the chloride was studied more thoroughly than that of the other compounds.
When the finely powdered chloride is treated with water it yields cadmic chloride which passes into solution, a small quantity of a white flocculent material which may be cadmic hydroxide but which in no case could be entirely freed from traces of chlorine, and a highly lustrous crystalline substance which rapidly lost its crystalline appearance andpassed over into a grayish white amorphous compound, which when freed from chlorine was found to be cadmous hydroxide, of the formula Cd(OH). The separate products resulting from the treatment with water were analyzed.
Approximately seven-eighths of the total cadmium dissolved as cadmic chloride while the remainder was contained in the flocculent precipitate and in the gray crystalline compound.
The percentage of cadmium in the white precipitate is less in this analysis than in the former. The cadmium in solution is again about seven-eighths of the total and the chlorine present in the same solution shows that the cadmium was all combined as cadmic chloride.
All attempts to determine the composition of the gray crystalline compound failed, owing to the rapidity with which it decomposed with water. Even with the most rapid work it could not be isolated in the undecomposed condition.
Analyses of the partially decomposed crystals gave variable proportions of metal and halogen but never less than eight equivalents of the former to one of the latter.
While the decomposition of Cd₄Cl₇ with water cannot at present be fully explained, yet it is clear from the analyses that one eighth of the total cadmium is thrown down as a white precipitate and a crystallinecompound which as will be seen passes over into cadmous hydroxide. One half of the cadmous chloride is oxidized to cadmic chloride taking the chlorine from the other half.
The compound Cd₄Cl₇ was treated directly with absolute alcohol with the hope of obtaining the crystalline substance in an undecomposed condition. Although a substance of the same general appearance as that formed in the presence of water was obtained yet it decomposed so readily that a satisfactory analysis could not be made.
Notwithstanding the rapidity with which the decomposition of the crystalline compound begins, long continued washing was necessary inorder to completely remove the chlorine. The extraction of the last traces of the halogen is hastened by the use of warm instead of cold water. The temperature of the water must not exceed 50°C. In water whose temperature approaches the boiling point the hydroxide is slowly decomposed with liberation of metal.
The new hydroxide is a strong reducing agent. It dissolves in dilute acids; yielding with nitric acid oxides of nitrogen, with hydrochloric or sulphuric acid free hydrogen. After washing with warm water until all the chlorine had disappeared, it was dried over phosphorus pentoxide and analyzed.
The calculated percentage of cadmium in Cd(OH) is:
The determination of water in cadmous hydroxide was made by placing a small specimen tube containing the hydroxide in a Kjeldahl flask which was heated in a bath of concentrated sulphuric acid. During the heating a slow current of dry nitrogen was passed over the substance.
Average amount of water = 7.07 per cent.
The calculated percentage of water in Cd(OH) is, 6.99.
At the temperature at which concentrated sulphuric acid gives off dense white fumes cadmous hydroxide gives off all its water and passes over into a heavy yellow powder. At 150°C not a trace of water was liberated. Under the microscope the yellow powder was found to consist of minute translucent crystals.
The calculated percentage of metal in Cd₂O is 93.32 per cent.
If water of too high temperature is employed in washing the subhydroxide, the presence of free metal in it can be detected underthe microscope and by rubbing between agate surfaces. If the yellow suboxide is strongly heated it breaks up into a mixture of oxide and metal which possesses a distinctly green color. Towards acids the suboxide conducts itself like the subhydroxide.
It is a fact of some interest in connection with the periodic arrangement of the elements, that the tendency toward the formation of a lower series of compounds which becomes so strongly developed in mercury begins to exhibit itself in some slight degree in cadmium.