PHILOSOPHICAL TRANSACTIONS. I. Additional Researches on the Electrolysis of Secondary Compounds. By J. FREDERIC DANIELL, Esq., D.C.L, For. Sec. R.S., Prof. Chem. in King's College, London, and W. A. MILLER, Esq., M.D., Dem. of Chem. in King's College, London. Received February 15,-Read February 25, 1844. THE authors of the following paper having agreed to work together upon the subject of the electrolysis of secondary compounds as opened by one of them, in two letters addressed to Dr. FARADAY, and honoured with a place in the Philosophical Transactions for 1839 and 1840, have arrived at some results, which probably will not be without interest to the Royal Society, and which they have now the honour to communicate. In the two papers just alluded to, the following points were established. First. When aqueous solutions of the neutral metallic salts are exposed to the action of the voltaic current, they are invariably decomposed. When the metal is one of that class which does not decompose water at ordinary temperatures, it is precipitated in the metallic state at the platinode: when it is of the class which does decompose water, its oxide, with an equivalent proportion of hydrogen, appears at the same electrode. The acid at the same time is set free at the zincode, accompanied by an equivalent proportion of oxygen. Second. When these results are compared with those of an independent voltameter in the same circuit, it is found that a certain proportion of the force which resolves a single equivalent of a simple electrolyte into its anion and cation, produces the resolution of a full equivalent of the complex electrolyte into a simple metallic cation and a compound anion. Third. When aqueous solutions of ammoniacal salts are electrolysed, similar results are obtained; but instead of a simple metal being disengaged at the platinode, an equivalent of ammonia, accompanied in all cases by an equivalent of hydrogen, appears at the platinode. Fourth. We are thus entitled to conclude that in the electrolysis of complex electrolytes, different * By a simple electrolyte is meant one containing only two elementary substances, being the simplest form of matter capable of electrolysis. elements travel together under the influence of the voltaic current as compound anions and cations, or as iso-electric bodies; that is, groups which are equivalent to each other, and to simple ions, in their relations to the electric forces. One of the most interesting results of this investigation, was the independent confirmation which it afforded of two celebrated hypotheses, viz. that of DAVY regarding the constitution of aqueo-acids, and the general analogy in the constitution of salts, whether derived from oxyacids or hydro-acids; and that of BERZELIUS concerning the constitution of ammonium. It was with a view to extend our knowledge of such iso-electric groups, and to trace their connexion with chemical radicles, that we commenced our experiments. Before we enter upon their detail, it will be best to make a few remarks upon the mode of measuring their results, which has been adopted. It is founded upon the undoubted law of definite electric action; and we have indifferently taken the amount of the ions disengaged at either or both the electrodes by the primary action of the current, or the secondary action of the elements. But there is another mode of effecting this purpose, by a diaphragm cell, in which the products of electrolysis may be kept separate; a method founded upon the hypothesis, that the voltaic decomposition of an electrolyte is not only effected by the disengagement of its anion and cation at their respective electrodes, but by the equivalent transfer of each to the electrodes ; so that the measure of the quantity of matter translated to either side of the diaphragm might be taken as the measure of the electrolysis. This was the mode which was necessarily adopted in the investigation before adverted to, where attempts were made to compare the results of the electrolysis of aqueous solutions of acids and alkalies with the simultaneous decomposition of saline solutions. One circumstance, however, was then overlooked, which it is necessary to attend to in making the comparison, viz. that the disengagement of whole equivalents of the ions at the electrodes is only accompanied, upon the hypothesis assumed, by the actual transfer of half an equivalent to either side. This will clearly appear from the following diagram. Let A, B, C, &c. in the preceding diagram represent a series of particles of chlorine, a, b, c, &c. a series of particles of potassium in combination with the particles of chlorine in the row above them, X, Y a central line or diaphragm, and Z and P the electrodes. If 1 represent the arrangement before the current passes, the particles A and a, B and b, &c. being combined to form compound particles of chloride of potassium, 2 would represent the arrangement after a single equivalent of each ion had been disengaged at the electrodes. Each particle would thus necessarily have moved forward half a step, combining with the next adjacent particle, so that Ba, Cb, Dc, &c. would now form the chain between the electrodes. If another equivalent be now supposed to be set free at each electrode, one particle of each ion would have passed the central line, one equivalent would thus be transferred, whilst two had been disengaged, as in fig. 3. In the instances above referred to, the transfer of one quarter of an equivalent represented half of an equivalent of the hydrates respectively electrolysed. Little stress however need be laid upon this correction, inasmuch as we shall presently show that the hypothesis upon which it is founded, although generally received, is itself destitute of foundation. 1. Our first object was to ascertain more clearly than had yet been done, the influence of water in the aqueous solution of an electrolyte, by comparison with the results of the electrolysis of the same compound when in the state of igneous fusion. It had been already determined, that with regard to the chlorides no difference occurred, and that the amounts of chlorine evolved in the same circuit from fused chloride of lead*, and from dissolved chloride of sodium and muriate of ammonia, were the same. In the last case, ammonia and hydrogen were evolved at the platinode in equivalent proportions to the chlorine at the zincode. Here a compound cation (NH) was separated from an elementary anion. When nitrate of silver in solution is subjected to electrolysis, the simple cation, silver, separates from the compound anion (NO), and upon substituting the salt in a state of fusion, for the solution, we obtained the same result. No gas was evolved, but crystallized metallic silver was deposited upon a silver platinode, which gradually increased in length, as it was slowly withdrawn from the liquid salt, just as in the analogous experiment with fused chloride of silver, devised by Dr. FARADAY. Nitrous fumes were at the same time given off from the platinum zincode in abundance. From these experiments it is evident that neither the grouping of a compound cation, nor of a compound anion, is necessarily altered by water in the transit to their respective electrodes. 2. We now turned our attention to that most interesting group of salts, the phosphates, which has been so ably discussed by Professor GRAHAM; not without hopes of confirming by electrolysis the beautiful theory of their constitution, which he has + Ibid. p. 110. * First Letter, p. 108. derived from considerations purely chemical. The double diaphragm cell which we chiefly employed in these experiments, is represented in the annexed wood-cut. screws. A and B are the two halves of a stout glass cylinder, accurately ground so as to fit with shoulders liquid tight. C is a hollow ring of glass, also ground on either side, with a flat shoulder to fit against the two half cylinders, which are pressed home by the Each side of the ring is furnished with a rim, which is grooved to admit of a thin piece of bladder being tied over it to form a kind of drum; at K is a small hole to admit of the cavity being filled with a liquid. D and E are two bent tubes, fitted to the two half cylinders for collecting the gases evolved in the experiments. g and h are two circular platinum electrodes connected by wires, i, f (passing through corks in the necks of the half cylinders), with the battery. The apparatus when adjusted forms three compartments, each of which may be filled with the same or a different liquid, and the whole may be supported on a light frame of wood. We will not attempt to describe the particulars of every experiment referred to in the following pages, for their number is very great, and their details would be both tedious and useless; we will only select some of the principal, in the results of which we can trace no ambiguity; and have no doubt we shall obtain credit for every care in determining the purity of the substances which we employed, and in making the various analyses which were required. (a.) A strong solution of tribasic phosphate of soda and water (2NaO, HO, P2 O5) (rhombic phosphate) was placed in the platinode cell of the diaphragm apparatus ; the centre cell and the zincode cell were both charged with a dilute solution of soda (4). The power of twenty cells of the small constant battery was transmitted through it, oxygen was evolved at the zincode, and in thirty-seven minutes 48 cubic inches of hydrogen were collected from the platinode; the experiment was then stopped and the solutions examined. The liquid from the zincode cell was carefully neutralized with nitric acid, and then gave a copious yellow precipitate with nitrate of silver, soluble in nitric acid, and in ammonia, the well-known characters of the tribasic phosphate of silver (3AgO, P2O). The solution in the platinode cell had become much more alkaline than at first, but when neutralized with nitric acid, gave a similar yellow precipitate with nitrate of silver. There could be no doubt therefore that tritoxyphosphion (or the radicle of the tritohydrate of phosphoric acid) had travelled to the zincode. The experiment was more than once repeated with unvarying results. (b.) A solution of tribasic phosphate of soda (3NaO, P2 O5) (sub-phosphate) was substituted in the same arrangement for the rhombic phosphate, and, after the action of the battery, on neutralizing the liquid in the zincode cell, a similar copious yellow precipitate was obtained from nitrate of silver. (c.) To complete the series of tribasic phosphates in which different bases are substituted for each other, a solution of the phosphate of soda, ammonia and water NaO NHO P2O (microcosmic salt), was subjected in the same manner to electroно lysis, the zincode cell being charged with solution of soda; the solution in the zincode cell became acid to the test of litmus, and produced an abundant yellow precipitate in solution of nitrate of silver; proving that it was the tribasic acid which had travelled as before. (d.) A similar arrangement was made with a solution of dibasic phosphate (pyrophosphate) of soda (2NaO, P2 O5). Hydrogen and oxygen in equivalent proportions were respectively given out at the platinode and zincode, and the process was carried on to the same extent as before. The solution at the zincode after electrolysis was neutralized by nitric acid. When tested with solution of nitrate of silver, a copious white precipitate was formed, indicating that in this case deutoxyphosphion (or the radicle of the deuto-hydrate of phosphoric acid) had been transferred to the zincode. (e.) A solution of monobasic phosphate (NaO, P2 O.) (metaphosphate) was subjected to electrolysis under the same circumstances. The solution in the zincode cell was now found to be strongly acid instead of alkaline; when added to a solution of albumen it immediately coagulated it; and with nitrate of silver and nitrate of baryta it produced the characteristic gelatinous white precipitates, which are distinctive of the monobasic phosphates of those bases. It was thus fully established by these experiments, and confirmed by their repetition, that the three hydrates of the phosphoric acid constitute three essentially distinct acids, which may be expressed as follows: H +P2O proto-hydrate, and that when disengaged from their corresponding saline compounds by the power of the voltaic current, they travel as three distinct oxyphosphions in the circuit. This view, which cannot be doubted as arising from the properties of the acids so disengaged, derives confirmation from the observation that the quantity of acid transferred from the monobasic phosphate of soda was so much greater for the same amount of hydrogen disengaged, than from the dibasic and tribasic salts of the same base. |