The Preparation of Cd₄Cl₇.
When anhydrous cadmium chloride is heated with metallic cadmium in a vacuum, or in an atmosphere of nitrogen, to the fusing point of the chloride, 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 to facilitate 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;
| Amount | of | chloride | used | .33541 | gr. |
| ” | ” | cadmium | found | .21559 | ” |
| ” | ” | chlorine | ” | .11943 | ” |
| Cadmium. | Chlorine. | ||||
| 64.27 per cent. | 35.61 per cent. | ||||
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.
| Amount | of | chloride | used | 1.45970 | gr. |
| ” | ” | cadmium | found | .93904 | ” |
| ” | ” | chlorine | ” | .52329 | ” |
| Cadmium. | Chlorine. | ||||
| 64.33 per cent. | 35.85 per cent. | ||||
A second preparation of the substance was made in all respects like the first. Two analyses were made.
First Analysis:
| Amount | of | chloride | used | .61010 | gr. |
| ” | ” | cadmium | found | .39235 | ” |
| ” | ” | chlorine | ” | .21725 | ” |
| Cadmium. | Chlorine. | ||||
| 64.31 per cent. | 35.61 per cent. | ||||
Second Analysis:
| Amount | of | chloride | used | .20616 | gr. |
| ” | ” | cadmium | found | .13266 | ” |
| ” | ” | chlorine | ” | .07352 | ” |
| Cadmium. | Chlorine. | ||||
| 64.35 per cent. | 35.66 per cent. | ||||
A third preparation was made like the first and second and analyzed.
Analysis:
| Amount | of | chloride | used | .2832 | gr. |
| ” | ” | cadmium | found | .18244 | ” |
| ” | ” | chlorine | ” | .10123 | ” |
| Cadmium. | Chlorine. | ||||
| 64.42 per cent. | 35.74 per cent. | ||||
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:
| Hydrogen found. | Hydrogen calculated for Cd₄Cl₇. | ||
|---|---|---|---|
| 1ˢᵗ | determination | 15.67 c.c. | 15.65 c.c. |
| 2ⁿᵈ | ” | 11.80 c.c. | 11.82 c.c. |
| 3ʳᵈ | ” | 23.00 c.c. | 23.03 c.c. |
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 different preparations 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.