Experimental Researches in Chemistry and Physics/Oxide of silver in ammonia
On the Solution of Silver in Ammonia[1].
The ease with which the compounds of silver are dissolved by ammonia, and the frequent formation of powerfully detonating and dangerous substances in these solutions, are well known. I have been induced to examine some of the phenomena presented by these bodies, and perhaps an account of what is, I believe, original, may not be unacceptable as an addition to the scanty stock of information published on this subject.
When the oxide of silver, precipitated either by the alkalies or alkaline earths, is put into solution of ammonia, it is entirely dissolved, producing a pale brownish solution. If this solution be exposed in an open vessel, a brilliant pellicle forms on its surface, which, when removed, is succeeded by another and another until most of the metal is separated.
This, which is an oxide of silver, was noticed long ago by Berthollet in the ‘Annales de Chimie,’ tome i., and he there states its production to be dependent on the abstraction of ammonia by the atmosphere.
From some difference which exists between this solution of silver and that of the nitrate when treated by precipitants, and from other circumstances, I was induced to collect and analyse some of the oxide, to ascertain its identity with the common oxide, or that previously dissolved. 20 grains that had been dried for some hours on the sand-bath, were put into a small glass retort, they were decomposed by heat, and the gas liberated received over water; it equalled 2·75 cubical inches. 18 grains of silver remained in the retort, and the 2·75 of oxygen being equivalent to ·935 grain, we have those numbers as the proportions of the elements in the oxide, the loss being supposed to be water, some of which had condensed in the neck of the retort. Now—
| Oxygen. | Silver. | Oxygen. | Silver. |
| ·935 | 18 | 7·5 | 1·444 |
The same method of analysis was applied to oxide of silver, precipitated by potash from nitrate of silver, it having been well washed and dried: 40 grains gave 7·9 cubical inches, and 36·4 grains of silver remained. The 7·9 cubical inches = 2·686 grains, and
| Oxygen. | Silver. | Oxygen. | Silver. |
| 2·686 | 36·4 | 7·5 | 101·6, |
the number for silver very nearly as given in the most correct elementary treatises on chemistry. There appears, therefore, to be no error in the mode of analysis, and the oxide by ammonia seems to contain less oxygen than that precipitated by alkalies. Again,—
30 grains of the oxide of silver were put into a retort, and decomposed with every precaution as before; the silver left weighed 27·4 grains, and the quantity of gas given off was 4·125 cubical inches. I suspected that a small portion of carbonate of silver had been mixed with the oxide, for when the ammoniacal solution has been long exposed to the air, much carbonate of ammonia and of silver is formed in it; the gases were therefore placed over solution of potash, and were reduced in bulk to 3·625, which was pure oxygen. This volume is equivalent to 1·2325 gr., and 1·2325∶27·4 ∷ 7·5∶166·7, a proportion of silver still higher than in the first experiment, but which may be accounted for by the purification of gas and the small quantity of oxygen that remained in the retort.
In a third experiment, 24 grains of silver were left; 4·25 cubical inches of gas were given off, which decreased over potash to 4. In order to estimate the proportion of azote arising from the air in the retort, the 4 were treated with nitrous gas of known purity, and gave results equal to 3·475 of pure oxygen. This is equal to 1·1815 gr., and 1·1815∶24 ∷ 7·5∶152·3.
One or two other experiments varied considerably from this, giving a greater proportion of silver, but the mean of many gave the oxygen to the silver as 7·5 to 157·4.
There is every reason, therefore, to believe this a protoxide of silver, containing about two-thirds the quantity of oxygen found in the common oxide, or that obtained by precipitation from the nitrate; and there are also other circumstances observable in its solution and during its formation which favour this notion.
When this oxide forms on the surface of an ammoniacal solution by slow spontaneous evaporation, it takes a crystalline form, which, however, is quickly lost by its covering the whole surface of the liquor. It is of a grey colour by reflected light, and highly resplendent; the light transmitted through thin films is of a bright yellow colour. When heated gradually it is reduced, giving off oxygen without change of form; but heated suddenly, it fuses first, and leaves a solid button of silver: under pressure, it perhaps might be fused without decomposition.
Potash precipitates the solution of oxide of silver in ammonia white; carbonate, or subcarbonate more abundantly, and white; alcohol and æther throw down precipitates, at first white, but rapidly changing colour; when dry, they detonate by heat or friction. Chromate of ammonia does not precipitate until nitric acid be added. Tincture of galls gives a very copious black precipitate, different in appearance to that obtained from the nitrate of silver by adding ammonia after the tincture. Solution of iodine in water gives a brown curdy precipitate, but with nitrate of silver a yellow turbidness. Muriatic acid or muriates always form chloride of silver.
It is probable, from these circumstances, that part of the silver exists in the solution in the state of protoxide, and as no gas is given off during the solution of the original oxide, that a portion of nitric acid and water have been formed.
M. Berthollet has in the paper before referred to, described a fulminating compound of silver and ammonia, obtained from solutions similar to those from which the above oxide had been obtained, and has stated it to be his opinion that it is a compound of protoxide of silver and ammonia. As it is frequently left in the form of a black powder when oxide of silver is dissolved in ammonia, I imagined it might be a compound of the peroxide with the alkali, as protoxide was formed and held in solution; and that the circumstance of the liberation of azote, which gave rise to the idea of its being a combined protoxide, might be explained by the further formation of a portion of oxide similar to that already described.
The method of obtaining this compound has been to precipitate oxide of silver from the nitrate by alkalies, or better, by lime·water, to wash and dry it well, and then to leave it in contact with liquid ammonia for ten or twelve hours; the greater part is dissolved, but a black powder remains, which is fulminating silver; if the solution be heated, note is given off} and a further quantity of fulminating silver is obtained (Annales de Chimie, tome i.).
I find that fulminating silver may he formed from any precipitated oxide of silver, whether moist or dry, recent or old. Boil the oxide carefully in a tube with a mixed solution of potash and ammonia for a few moments; the potash absorbs, all the carbonic acid that may have been united to the oxide, and to a certain degree prevents its solution in the ammonia; a black powder, similar to that procured by the other process, results.
In order to gain some evidence respecting the nature of the oxide combined with the ammonia in fislminating silver, I endeavoured to ascertain the mode of formation of that compound. It appears to be formed in every case where common oxide of silver is dissolved in ammonia, and the entire solution of all solid matter is no evidence of its nomexistenee, for the compound is itself soluble in ammonia, though not so much so as the oxide. When there is an excess of oxide, unless it predominate in a great degree, the undissolved portion will be found to contain fulminating silver, and when the whole is dissolved, by heating the solution, it is thrown down.
To ascertain whether the liberation of azote depended upon the formation of the fulminating compound, I boiled, for a few moments, a solution of the oxide in ammonia; the solution became highly coloured, azote was given 06] and a black curdy precipitate formed, which left the liquid colourless; separatml by a filter, the precipitate proved to be fiilminating silver. The solution was again heated, it again blackened, gave of azote, and again a precipitate formed; this was not fuhninating silver, but merely oxide: filtered and again heated, it gave off note, and more of the oxide was formed; and this occurred with the same solution a fourth and fifth time. The liberation of the azote, therefore, does not belong exclusively to the formation of fulminating silver, but seems rather to depend on the production of protoxide.
I endeavoured to form fulminating silver by using the protoxide described in the first part of this paper, but could not succeed: I got nothing but a black powder from it, which appeared to be the same oxide in another form. I endeavoured also to form fulminating silver from those portions of oxide given off by the further boiling of solutions which had previously yielded the detonating compound, but failed; I presume from its being also a protoxide. When the fulminating compound is dissolved in the acids, it gives off a gas which I believe to be oxygen, but I could not work with quantities sufficient to ascertain this point. Perhaps to these reasons for supposing fulminating silver to be a compound rather of the peroxide than the protoxide, may be added the easy solubility of the protoxide in ammonia, and the difficult solubility of the detonating compound.
The oxide which is obtained by boiling solution of silver in ammonia, I have supposed to be a protoxide similar to the one obtained by spontaneous evaporation. This opinion is founded on the liberation of azote during its formation in consequence of the decomposition of ammonia by oxygen, and on its apparent incapability of forming fulminating compounds: the idea is supported by the following circumstance. A tube, in which solutions of silver in ammonia had been repeatedly boiled, became coated on the inside with the oxide, so as to be perfectly opake; on pouring dilute nitric acid into it to remove the oxide, the tube became lined with brilliant metallic silver, which, however, was soon dissolved by the continued action of the acid. I attribute this phenomenon to the reduction of one part of the oxide by another, which was thus rendered soluble in the acid.
When a portion of the ammoniacal solution is evaporated to dryness in a platinum capsule, it leaves a film of oxide, which, when decomposed by heat, gives a perfectly continuous and smooth coat of silver to the vessel. I have also covered other metals, as iron and copper, with silver in the same way, and found that the burnisher might he applied without any injury to the coating. It is probable that a solution of silver of this kind might be applied in some cases in the arts, to the purposes of ornament and utility.
Combination of Ammonia with Chlorides[2].
It has been already shown, particularly by Sir H. Davy, that several of the binary compounds of chlorine, as those of phosphorus, tin, &c., exert a strong affinity for ammonia, condensing it when in the gaseous state, and neutralizing its alkaline properties. The combinations which will here be offered to notice are of a different kind, and if they deserve any attention, it will be in consequence of the weakness of the power which is exerted in their formation, and the slight change of properties induced on the substances by union.
It has been frequently observed by chemists, that if well-fused chloride of lime be placed in ammoniacal gas, there is a rapid absorption of the gas, and the chloride becomes covered with a white powder. If ammonia be repeatedly added until the absorption ceases, the mass of chloride swells, cracks, splits in all directions, and at last forms a white pulverulent substance.
Exposed to the atmosphere, it deliquesces, but not so rapidly as chloride of lime. Thrown into water it dissolves, forming a strong alkaline solution. Heated, it gives off ammonia, and the chloride remains unchanged. Placed in chlorine it inflames spontaneously, and burns with a pale yellow flame.
The fused chlorides of barium and strontium suffer a very slight change in ammoniacal gas in many days; after more than a fortnight, the chloride of strontium, weighing about 30 grains, had absorbed only a cubical inch of gas, and a slight efflorescent appearance was seen on the broken edge.
A piece of fused chloride of silver, weighing about 80 grains, placed in ammoniacal gas, gradually absorbed more than 40 cubical inches. The action took place over the whole surface of the mass, but most speedily at the fractured edges. The chloride swelled considerably, and crumbled into powder. The substance formed was at first white, but it