Art. LII.—On the Electro-motive and Electrolytic Phenomena developed by Gold and Platina in Solutions of the Alkaline Sulphides.
[Read before the Wellington Philosophical Society, 29th January, 1871.]
In some former papers upon the absorption of sulphur by gold and platinum* I adduced evidence to show that this absorption was a chemical act; that a true chemical combination had been effected between the sulphur and the gold or platinum, as the case might be, the result being a sulphide of the metal used; and I stated that, so far as this evidence could be deemed worthy of acceptance, it impugned the correctness of the general belief that the absorption of certain gases by platinum was in every instance simply mechanical.
Irrespective, therefore, of the primary question raised, it became of some importance, upon general grounds, to obtain further and, if possible, decisive
[Footnote] * See Trans. N. Z. Inst., Vol. III., pp. 216 and 221.
evidence of the precise character of this absorption, and for this object I have continued my investigations, and have varied their nature so as to secure such evidence, quite independent of all that has been already advanced.
It will perhaps be remembered that the whole of my evidence upon this point, as at present given, is based upon certain chemical reactions which my processes obtained. That which I now beg to lay before this Society rests upon certain electrical reactions or manifestations which have discovered themselves to me; and, in anticipation of what should perhaps be left for disclosure as a resultant of particular experiments, I will state now that these electrical reactions are all extremely favourable, if not conclusive, as to the correctness of the opinion which ascribes this absorption to chemical action. I may observe here that this was the direction I contemplated taking for my experiments at the time (one which they would indeed naturally have taken), but that I had not the necessary apparatus to aid me in such a course.
It is well known that the exercise of chemical force, or, as it is named chemism, is always accompanied by a development of electricity, and this varies in its intensity with that of the chemical forces brought into play.
In cognisance of these facts, therefore, it occurred to me to test whether electricity is developed at all during the sulphurization of either of these metals.
Taking two plates of gold prepared chemically pure, I placed one of them in a cell charged with sea water, and the other in a porous cell charged with sulphide of ammonium, which cell I partially immersed in the former. These plates I connected voltaically at points quite clear of the liquid, and inserted a delicate galvanometer in the circuit, when I found the needle of this instrument was vigorously deflected over an arc of 20° to 30°, indicating, of course, that a strong current of electricity was being generated. The direction of this current, as shown by the needle, was from the inner to the outer cell. The gold in the sulphide solution, therefore, was the positive element of the pair.
Electric currents of equal strength were also developed by charging both cells with sea water, or with solution of potash or ammonia, and administering sulphuretted hydrogen to the gold plate in either cell. The direction of these currents was constantly from the cell to which the gas was applied. By charging one of the cells with potash or ammonia, and the other with sea water, a current of electricity was also produced, but this was of very feeble intensity, and might well be owing to traces of sulphur in the alkaline solutions.
To avoid any errors which might be caused by films of air or other gases adhering to the plates, and so getting in contact with the sulphide solutions, these plates, prior to their immersion, were always raised to a red heat, and
plunged while in this state into water or the solution about to be used, which liquids, moreover, had been previously boiled for many hours to expel the gases dissolved in them.
The same kind of electrical reactions were obtained by substituting platina for gold in these experiments.
It was further found that gold paired with platina in sulphide solutions is negative to it, but if paired with iron it is positive. It is necessary in order to obtain this latter result to use sulphide solutions quite clear from free alkalies, otherwise their electric order is reversed. For this purpose it is best to prepare such solutions by boiling the carbonate of the alkali with sulphur for some hours.
As appropriate to the subject, I will insert here a table of the electromotive order of the several metals I have tested in sulphide of sodium. The order observed is from negative to positive:—
The currents thus developed by gold and platinum in sulphide solutions soon ceased, but they were so well marked, and had such an apparent intensity, that I was induced to try if they had any effect upon certain metallic solutions, and on trial I found that, in the case of that obtained by the use of gold plates in contiguous solutions of potash and sulphide of potassium, a degree of intensity was reached sufficient to decompose solutions of copper, silver, and gold, and to deposit these several metals in adherent films upon proper electrodes. The same results were afterwards obtained with platinum.
This capability of gold and platinum to generate electrical currents under these circumstances—currents of such intensity as to exhibit true electrolytic effects—when taken along with the results of my former experiments on this subject, appears conclusive evidence in favour of the sulphurization of these metals being, as has been already argued, the result of chemical action.
It is very difficult to suppose that the mere absorption or condensation of sulphur from these various solutions by gold or platinum could ever produce electric currents having so great an intensity, since the sulphur, being in a liquid form, must already be condensed almost to a maximum.
I must apologise if I have appeared to dwell too long upon the subject of the nature of this absorption, but the exceedingly close structure of the metals experimented with, and their very high atomic equivalents, restrict this absorption to one of the most superficial character, and thus preclude us from
forming any opinion based upon those visible physical changes which are the usual concomitants of chemical action. Thus I am compelled to attack the subject by indirect methods, and these, being individually less decisive for the object in view, require a certain amount of variation to make up for their want of directness.
Judging from a variety of circumstances, which I need not here detail, I am inclined to believe a sub-sulphide of gold or platinum is produced in these experiments, and that in the case of the gold sulphide its colour is yellow, which, if true, would well account for the fact that no visible change is induced upon this metal by sulphurization.
It is very probable that were a bundle of light rays reflected from a series of gold plates in succession, and another from a series of sulphurized gold plates, a very manifest difference in the colour of the rays so reflected would be apparent.
It is only proper to state here that Professor Becquerel has just announced, in a series of papers, a certain electro-motive power of gold, platinum, etc., in pure water and also in neutral saline solutions, abstracts of which are given in the several numbers of the “Chemical News,” which I have placed upon the table for the reference of members. This is exceedingly relative to the subject of this paper, but it will be observed on reading these abstracts that the results of the author's researches are quite distinct from those here described, since the electric phenomena which he describes “are very feeble,” while those I have cited were strong enough to decompose metallic solutions. Moreover, these currents were produced by Professor Becquerel under such circumstances that he was driven to the conclusion “that capillary affinity plays a very important part in their production;” whereas the currents which I have described are so far superior to those producible by the exercise of capillary affinity that the influence of such affinities upon them would not be perceptible by aid of the instruments used. The sulphides do not appear, however, to have been experimented with by Professor Becquerel in the researches I have alluded to.