« ZurückWeiter »
gas ceases also with the gold and platinum wires; and it ceases when, even as in the case of the other metals, the surface having become very much divided, oxidation, and, consequently, the reduction of the metals, is greatly facilitated; but it requires a longer time with platinum than for the other metals to arrive at the point where the gases are no longer visible.
3rd. When we continue the transmission of magneto-electric currents, even when the disengagement of gas no longer takes place, we perceive a rapid succession of changes of colour on the surface of the gold or platinum wires, similar to those which are seen on the surface of wires of oxidable metals, and particularly silver, in the same circumstances. The changes of colour correspond exactly to the alternate disengagement of oxygen and hydrogen produced by the currents, and, consequently, to the alternate oxidation and reduction of the metal.
The motives which precede, and which I have drawn from experiments contained in my preceding memoir, did not yet appear to me, as I have already stated, sufficient to warrant the admission as a principle acquired to science the fact of the oxidation of platinum under the immediate action of oxygen. I then took up the subject anew, with the view of studying it more closely, so as to prove in a decisive manner this new property, and to follow up some of its consequences.
I sought at first to show that platinum may become oxidized and deoxidized by the immediate action of oxygen and hydrogen disengaged at positive and negative poles of a pile. I shall show in the sequel that it is in the oxidation of which platinum is susceptible, that we may find the cause of electric currents which are produced in many cases by a pair of plates both of which are platinum ; in fine, I shall signalize some phenomena of another order, which seen to me to depend on the property of platinum to become oxidized, and, consequently, furnish new proofs of the existence of this property. II.-Action on Platinum of Oxygen and Hydrogen disengaged at
the Positive and Negative Poles of a Pile. In order to prove directly the oxidation of platinum, I endeavoured to expose a large surface to the action of the nascent oxygen, by making use of the positive pole of a pile in acidulated water, and then transporting it afterwards to the negative pole. I took for the second pole a very fine wire, very short, and likewise of platinum, in order to establish the greatest difference possible between the two surfaces. I collected with care, and separately in graduated tubes, the gas disengaged at each of the poles, and when the relation between the quantities of oxygen and hydrogen was not exactly that which constitutes water, I concluded that the gas which was not found in sufficient quantity for the proportion necessary, had been employed to oxidize or to deoxidize the great surface of platinum, accordingly as it was the oxygen or hydrogen that was in minority.*
• We should rather suppose that the excess of one or the other of these gases is intended to be understood. -EDIT.
These experiments have been made successively with nitric acid and with sulphuric acid, diluted with nine parts of water in volume; the two acids were perfectly pure as was the water likewise. The results have constantly appeared to me more conclusive with the solution of sulphuric acid; because in this solution, the water alone was decomposed, whilst in the nitric acid solution, a part of the acid itself being decomposed, the gases disengaged were not uniformly oxygen and hydrogen; there was found a little of the oxide of azote, and even pure azote. This inconvenience was afterwards avoided by employing nitric acid extremely diluted, but it then became necessary to have a very strong pile.
After having well cleansed a plate of platinum* of twenty centimetres in length by three in breadth, and washed it in distilled water, I rolled it into a spiral, and plunged it into water acidulated with sulphuric acid, under a graduated tube filled with the same liquid; a very fine and short platinum wire was plunged into the same acidulated water under a tube equally graduated. We could make the plate and the wire communicate with the poles of a pile by means of a platinum wire which was enclosed in tubes of glass, in such a manner that the gases proceeding from the decomposition of the liquid were entirely received in each of the graduated tubes or measures. The plate was first brought into communication with the pole , and the wire with the pole +: exactly 100 centimetres of hydrogen were obtained at the plate, and ifty of oxygen at the wire. The poles were changed: there was then sixteen cubic centimetres of oxygen at the plate, and forty-one of hydrogen at the wire; there was wanting then four and a half cubic centimetres of oxygen, which had been taken by the plate. This oxygen had not the power of being employed in combining with the hydrogen which should have been retained by the plate when it was in connexion with the negative pole. In fact, since hydrogen was not wanting in the former experiment, it did not remain adhering to the plate. Beside, in this experiment, as in the following, care was always taken to shake the plates very much, to detach all the gas from them which might remain adhering to their surface,
In another experiment, after carefully cleansing the plate as always before observed, it was first put in communication with the
+ pole, and the wire with the — pole : eight cubic centimetres of oxygen were obtained at the plate, and twenty of hydrogen at the wire; thus there was wanting two cubic centimetres of oxygen. The poles were reversed, and there was now obtained at the wire ten cubic centimetres of oxygen, and fifteen or sixteen of hydrogen at the plate; there was wanting from four to five cubic centimetres of hydrogen, almost double, and consequently the equivalent of the oxygen, which
• In cleansing plates of platinum, I have constantly followed the process indicated by Parady, which consists in beating them to redness, and rubbing them with a piece of potash whilst in a state of incandescence, then plunging them in boiling sulphuric acid, and afterwards washing them for a long time in distilled water frequently renewed.
had disappeared, and which had been employed to oxidize the of surface the plate.
I made the same experiments, making use of water very slightly acidulated with nitric acid for my liquid. The plate of platinum was cleansed as before, with much care, and rolled into a spiral; the platinum wire was only from four to five millimetres in length. Having brought the negative pole into connexion with the wire and the positive pole with the plate, I obtained eighty-seven cubic centimetres of hydrogen at the wire, and only forty of oxygen at the plate ; there wanted then three and a half cubic centimetres of oxygen. I left the apparatus for twenty-four hours without any change taking place, the plate and the wire soaking in the liquid. I now brought ihe + pole anew into connection with the plate, and the — pole to the wire: I obtained exactly forty cubic centimetres of oxygen at the plate, and eighty of hydrogen at the wire ; a proof that the coat of oxide formed on the surface of the platinum the day previous had continued, in spite of the contact of the slightly acidulated water. But having put the — pole to the plate, and the + pole to the wire, I had fifty cubic centimetres of hydrogen at the plate, and twenty-nine of oxygen at the wire; there was therefore wanting eight cubic centimetres of hydrogen, almost the equivalent of oxygen, which had rested on the plate. I took care, in each of these experiments, to change that part of the liquid which was in the measure (eprouvette), near the plate and the wire, in order that the conditions of the experiment might be the same in each case ; nevertheless, without exposing the plate and the wire to contact with the air.
I will now detail some experiments with water, holding in solution one-tenth of sulphuric acid by volume.
With a very feeble pile the + pole was connected with the plate, and the — pole to the wire; I obtained twenty measures of hydrogen, and only six of oxygen; there was wanting then four. I took a very powerful pile, the + pole of which was brought into contact with the wire, and the — pole in communication with the plate. Five measures of oxygen and six of hydrogen were now obtained; there was wanting of course four measures of hydrogen, which proves that there bad only been two measures of oxygen taken, or adhered to, by the plate, in the portion of which the hydrogen at least bad produced the reduction. The poles were again brought into the same position, and there was eight measures of oxygen at the wire, and sixteen of hydrogen at the plate. A plate of platinum which had remained exposed to the air was then taken, and being brought to the — pole, whilst the + pole communicated with the short wire, we only obtained five measures of hydrogen against five of oxygen; there then wanted five measures of hydrogen, which had been employed in reducing the slightly oxidized surface of the platinum.
A plate well cleansed was put to the - pole, and the wire was placed at the t pole: there was an almost inappreciable small quantity of hydrogen deficient. The poles were changed : there was at first ten of hydrogen, and three of oxygen only: after a second and similar operation, there was twenty of hydrogen and nine of oxygen, in all thirty of hydrogen and twelve of oxygen; there was wanting then three of oxygen. The experiment was continued, changing the poles from time to time, and there was wanting in all cases some measures of those gases which were disengaged on the plate; but this disappearance no longer took place when the plate had already been used for some time in the disengagement of the same gas.
I ought to remark, that when I alternately applied the + pole and the — pole on the plate, the quantity of oxygen and the quantity of hydrogen which disappeared was pot always an equivalent to the other. I attribute these differences, on one part, to the fact, that the liquid dissolved a little of the oxygen, on the other hand, to the circumstance, that the current is not uniformly distributed on all the points of a plate, especially when the surface is great; and consequently it may happen that all the points of the plate which have been covered by one of the gases, are not exactly covered again by the other. Perhaps also a part of the oxide formed becomes dissolved in the acidulated water, which explains why it is that there is always proportionally more oxygen than hydrogen disappearing. I shall give farther hints on another cause of this difference.
I have repeated the same experiments, substituting successively for the plate of platinum, spongy platinum and a wire of platinum, the surfaces of which had been rendered pulverulent by means of the alternate oxidations and reductions of the magneto-electrical currents to which it had been submitted. With the spongy platinum, which had been soaking several days in the acid, and had been purified, and which I placed at the negative pole, I obtained the desired proportions of the two gases. But having put it now in communication with the positive pole, I had only three measures of oxygen against ten of hydrogen. The remaining phenomena which were exhibited were the same as with the plate of platinum; though the morsel of sponge was very small, the very great number of points of contact which it presented to the liquid, from the fact of its nature, rendered it still more sensible than the plate to the successive absorptions of oxygen and hydrogen due to the alternate oxidations and reductious of the metal.
The wire, the surface of which was pulverulent, was about twelve centimetres long. Having been put in contact with the — pole, immediately after the operation which had thus modified its surface, an operation from which it always becomes a little oxidized, as I shall show farther on, it only gave five measures of hydrogen against five of oxygen, disengaged at the positive pole, which communicated with the ordinary platinum wire of five millimetres in length. Having changed the position of the poles, after this first operation, there were two measures of oxygen against eight of hydrogen; there still wanted two measures of oxygen; in all five, between the two experiments. Then, from thence the gases were disengaged in their proper proportions. Having once more changed the poles, I had only i welve of hydrogen at the long wire, against ten of oxygen at the short one; there was now eight measures of hydrogen minus, employed to reduce the oxide formed by the oxygen, which bad been wanting in the preceding experiments. Having again made a last experiment without changing the poles, I obtained exactly ten measures of hydrogen from the long wire against five of oxygen at the shorter one.
Finally, I placed in the same eudiometre, always filled with acidulated water, a long platinum wire, whose surface was pulverulent, and a short one, whose surface was smooth. I brought the long wire into communication with the + pole, and the short one with tie - pole; then when the gaseous mixture had filled the moiety of the eudiometre, I exploded it, and there remained an excess of hydrogen equal to a tenth of the whole volume. Having repeated the same experiment three or four times, I obtained almost constantly the same result; but at the close, the metallic powder which covered the long platinum wire, had almost totally disappeared. It is probable that the part of this very five powder which became oxidized, was dissolved in part in the acid, and thus laid bare the new particles, which, in their turn, became oxidized. This is what explains how it is that there was constantly an excess of hydrogen in the gaseous mixture. The same phenomenon was not exhibited with the piate, whose surface, which was not pulverulent, had not the power, even when it was oxidized, to dissolve in the acidulated water. It did not take place, either, with a long wire with a bright surface rolled helically, which I substituted for the wire with the pulverulent surface. In the latter case, the gaseous mixture which I obtained by putting the helical wire in communication with the + pole of the pile, presented an excess of three measures of hydrogen. By changing the position of the poles, and leaving the residual hydrogen in the eudiometer, I had again a gaseous mixture, which I detonated, but it presented scarcely any residue ; the excess of hydrogen proceeding from the first operation even, had totally disappeared. The excess was, then, due to this : that a part of the oxygen had been employed in oxidizing the surface of the long platinum wire, when that wire communicated with the positive pole; it disappeared because that, in making this same wire communicate with the negative pole, a part of the hydrogen which ought to have been found in the gaseous mixture, had been employed to deoxidize the wire.
The result of the preceding experiments appears to me to be:
1st. That platinum perfectly pure and cleansed may be oxidized at its surface under the action of the nascent oxygen.
2nd. That this oxide is not dissolved in water slightly acidulated, when the surface of the platinum on which it is formed is not pulverulent.*
• If the acid is concentrated or boiling, it is no longer the same. The plate which has been exposed to the action of an acid in these conditions, and which has afterwards been washed in distilled water, conducts itself as a perfectly unoxidized plate.