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Art. XLII.—On the Oxidation of Silver and Platinum by Oxygen in presence of Water.
[Read before the Wellington Philosophical Society, 29th January, 1876.]
I Shall confine myself in this paper to a statement of results, and the considerations which led me to seek them, as I intend leaving the discussion of these in their various relations to certain debateable subjects for another opportunity, my investigations upon this matter being as yet incomplete.
A knowledge of the fact that gold and platinum readily combine with sulphur at a common temperature, and that the compounds thus formed cannot be detected by mere physical tests, suggested to me that oxygen may also combine with these metals under conditions somewhat similar, and in this manifesting none of the more distinguishing signs of chemical action, has consequently to this time been overlooked.
Acting at once upon this suggestion, I fortunately made a series of experiments to test the correctness or otherwise of my suspicion, and the results of these experiments, showing them in the main, I believe, to be correct, I submit to your notice.
I should premise my statement of these results by informing you, in anticipation of what will in due course appear, that one of my principal tests for the oxidation of these metals is that known as the “mercury test,” by which it will perhaps be remembered I had the honour of demonstrating before you experimentally the sulphurization of gold by sulphuretted hydrogen; and that this test is based upon the fact that mercury readily amalgamates with silver or platinum when in contact with them, but that if the minutest film of any substance intervenes between the two metals, amalgamation is either retarded or altogether prevented; thus, by the aid of this test, minute quantities of a substance enfilming either of these metals may readily be detected.
Commencing with silver, I ascertained the following facts regarding it:
| 1. |
That pure silver immersed for a few hours in distilled water or in the purest water I could obtain, has its surface so modified that it will not amalgamate immediately afterwards. |
| 2. |
That such an effect is not produced when either rain or spring water is used. |
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| 3. |
That silver thus modified by distilled water is brought back to the amalgamable state by contact for a short time with rain or spring water, also with acetic acid or ferrous sulphate, also by raising its temperature to about 500° F. |
| 4. |
That electric currents are generated by this metal in saline water free from chlorides, iodides, or bromides, also in water charged with any of these salts. |
| 5. |
That in dry air silver does not pass into this non-amalgamable state. |
| 6. |
That spongy silver immersed in an aqueous solution of sodic chloride (in an agate vessel) soon renders it very alkaline. |
These results, taken conjointly, signify, I think, undoubtedly that silver is a metal which oxidizes in a superficial way with far greater facility than we have heretofore considered possible.
Thus, in experiment 1, I hold this metal is oxidized by atmospheric oxygen contained in the distilled water used, and the oxides of silver not being reducible or at least readily reducible by mercury, amalgamation is prevented or greatly retarded with water containing chlorides. In experiment 2, we must suppose the silver has also oxidized, but the oxide thus formed has been decomposed by the alkaline chlorides present, argentiferous chloride thus resulting as a secondary product, and this compound, being, as we know, readily decomposable by mercury, amalgamation proceeds with rapidity. However, in regard to silver thus acted upon by chlorides, I always noticed that amalgamation did not appear to proceed instantly when it was placed in contact with the mercury as clean silver does; there was, as it were, a momentary hesitation manifested by the mercury before amalgamation proceeded.
The effect of acetic acid and of ferrous sulphate in experiment 3 is perhaps referable to a solution of argentiferous oxide in the one case, and to its reduction in the other. At the same time, however, we must consider that basic and insoluble silver salts may form here, and these, being readily decomposable by mercury, amalgamation is not retarded. The facts, Nos. 4, 5, 6, I think will be seen corroborative of the correctness of the conclusions I have above drawn.
I may state in further support of this conclusion that I have observed silver, as precipitated from its nitrate, darken near the surface of the solution, and it is only colourless and lustrous where distant therefrom, or when overhung by masses of silver. This darkening I attribute to a superficial oxidation of the silver by the atmospheric oxygen which has permeated the solution used. This metal also, contrary to general belief in regard to it, decomposes mercuric chloride. All these results were in the first place obtained from the metal electro-plated from its pure cyanide upon silver
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wire; but afterwards, for greater certainty in the matter, I employed silver most carefully prepared, and by approved processes for pure silver. As electro-plated upon lengths of surgical wire, it is most easily worked, and, being thus in a spongy form, its behaviour with the mercury test can be minutely and readily observed. It is necessary, of course, to well wash this silver from alkaline cyanide by distilled water before using it in these experiments. I should inform you I could not observe that sun-light exerted an effect in any of these reactions, whether accelerating or retarding.
In regard now to the metal platinum, I ascertained that it is also passed to a condition in which it will not amalgamate, by giving it contact for a short time with distilled or ammoniated water, also with aqueous solutions of the alkalies, their carbonates or chlorides, while acids generally put it quickly back into an amalgamable condition; an elevation of its temperature to about 400° F. will also accomplish this.
Platinum also generates electric currents when paired with graphite in saline or alkaline solutions.
These facts, I think, show undeniably that platinum not only absorbs oxygen, as is already known, but that this absorption is, in the cases cited, a chemical one, an oxide or a suboxide of this metal being formed. That in the so-termed mechanical absorption of certain gases by platinum, platinic compounds are produced, is an idea which I have long since entertained. *
In conclusion, I would beg to inform you that, from a partial investigation of the behaviour of gold in certain liquids, I believe this metal is also oxidizable under conditions somewhat similar to those under which I have stated silver to be, but the results of this investigation I will endeavour to communicate at our next meeting.
[Footnote] * Trans. N. Z. Inst.,” Vol. III., Art. XXXVIII.