729. Although not necessary for the practical use of the instrument I am describing, yet as connected with the important point of constant chemical action upon water, I now investigated the effects produced by an electro-electric current pa.s.sing through aqueous solutions of acids, salts, and compounds, exceedingly different from each other in their nature, and found them to yield astonishingly uniform results. But many of them which are connected with a secondary action will be more usefully described hereafter (778.).
730. When solutions of caustic pota.s.sa or soda, or sulphate of magnesia, or sulphate of soda, were acted upon by the electric current, just as much oxygen and hydrogen was evolved from them as from the diluted sulphuric acid, with which they were compared. When a solution of ammonia, rendered a better conductor by sulphate of ammonia (554.), or a solution of subcarbonate of pota.s.sa was experimented with, the _hydrogen_ evolved was in the same quant.i.ty as that set free from the diluted sulphuric acid with which they were compared. Hence _changes in the nature of the solution do not alter the constancy of electrolytic action upon water_.
731. I have already said, respecting large and small electrodes, that change of order caused no change in the general effect (715.). The same was the case with different solutions, or with different intensities; and however the circ.u.mstances of an experiment might be varied, the results came forth exceedingly consistent, and proved that the electro-chemical action was still the same.
732. I consider the foregoing investigation as sufficient to prove the very extraordinary and important principle with respect to WATER, _that when subjected to the influence of the electric current, a quant.i.ty of it is decomposed exactly proportionate to the quant.i.ty of electricity which has pa.s.sed_, notwithstanding the thousand variations in the conditions and circ.u.mstances under which it may at the time be placed; and further, that when the interference of certain secondary effects (742. &c.), together with the solution or recombination of the gas and the evolution of air, are guarded against, _the products of the decomposition may be collected with such accuracy, as to afford a very excellent and valuable measurer of the electricity concerned in their evolution_.
733. The forms of instrument which I have given, figg. 64, 65, 66. (709.
710. 711.), are probably those which will be found most useful, as they indicate the quant.i.ty of electricity by the largest volume of gases, and cause the least obstruction to the pa.s.sage of the current. The fluid which my present experience leads me to prefer, is a solution of sulphuric acid of specific gravity about 1.336, or from that to 1.25; but it is very essential that there should be no organic substance, nor any vegetable acid, nor other body, which, by being liable to the action of the oxygen or hydrogen evolved at the electrodes (773. &c.), shall diminish their quant.i.ty, or add other gases to them.
734. In many cases when the instrument is used as a _comparative standard_, or even as _a measurer_, it may be desirable to collect the hydrogen only, as being less liable to absorption or disappearance in other ways than the oxygen; whilst at the same time its volume is so large, as to render it a good and sensible indicator. In such cases the first and second form of apparatus have been used, figg. 62, 63. (707. 708.). The indications obtained were very constant, the variations being much smaller than in those forms of apparatus collecting both gases; and they can also be procured when solutions are used in comparative experiments, which, yielding no oxygen or only secondary results of its action, can give no indications if the educts at both electrodes be collected. Such is the case when solutions of ammonia, muriatic acid, chlorides, iodides, acetates or other vegetable salts, &c., are employed.
735. In a few cases, as where solutions of metallic salts liable to reduction at the negative electrode are acted upon, the oxygen may be advantageously used as the measuring substance. This is the case, for instance, with sulphate of copper.
736. There are therefore two general forms of the instrument which I submit as a measurer of electricity; one, in which both the gases of the water decomposed are collected (709. 710. 711.); and the other, in which a single gas, as the hydrogen only, is used (707. 708.). When referred to as a _comparative instrument_, (a use I shall now make of it very extensively,) it will not often require particular precaution in the observation; but when used as an _absolute measurer_, it will be needful that the barometric pressure and the temperature be taken into account, and that the graduation of the instruments should be to one scale; the hundredths and smaller divisions of a cubical inch are quite fit for this purpose, and the hundredth may be very conveniently taken as indicating a DEGREE of electricity.
737. It can scarcely be needful to point out further than has been done how this instrument is to be used. It is to be introduced into the course of the electric current, the action of which is to be exerted anywhere else, and if 60 or 70 of electricity are to be measured out, either in one or several portions, the current, whether strong or weak, is to be continued until the gas in the tube occupies that number of divisions or hundredths of a cubical inch. Or if a quant.i.ty competent to produce a certain effect is to be measured, the effect is to be obtained, and then the indication read off. In exact experiments it is necessary to correct the volume of gas for changes in temperature and pressure, and especially for moisture[A].
For the latter object the volta-electrometer (fig. 66.) is most accurate, as its gas can be measured over water, whilst the others retain it over acid or saline solutions.
[A] For a simple table of correction for moisture, I may take the liberty of referring to my Chemical Manipulation, edition of 1830, p. 376.
738. I have not hesitated to apply the term _degree_ (736.), in a.n.a.logy with the use made of it with respect to another most important imponderable agent, namely, heat; and as the definite expansion of air, water, mercury, &c., is there made use of to measure heat, so the equally definite evolution of gases is here turned to a similar use for electricity.
739. The instrument offers the only _actual measurer_ of voltaic electricity which we at present possess. For without being at all affected by variations in time or intensity, or alterations in the current itself, of any kind, or from any cause, or even of intermissions of action, it takes note with accuracy of the quant.i.ty of electricity which has pa.s.sed through it, and reveals that quant.i.ty by inspection; I have therefore named it a VOLTA-ELECTROMETER.
740. Another mode of measuring volta-electricity may be adopted with advantage in many cases, dependent on the quant.i.ties of metals or other substances evolved either as primary or as secondary results; but I refrain from enlarging on this use of the products, until the principles on which their constancy depends have been fully established (791. 848.);
741. By the aid of this instrument I have been able to establish the definite character of electro-chemical action in its most general sense; and I am persuaded it will become of the utmost use in the extensions of the science which these views afford. I do not pretend to have made its detail perfect, but to have demonstrated the truth of the principle, and the utility of the application[A].
[A] As early as the year 1811, Messrs. Gay-Lussac and Thenard employed chemical decomposition as a measure of the electricity of the voltaic pile. See _Recherches Physico-chymiques_, p. 12. The principles and precautions by which it becomes an exact measure were of course not then known.--_Dec. 1838._
-- vi. _On the primary or secondary character of the bodies evolved at the Electrodes._
742. Before the _volta-electrometer_ could be employed in determining, as a _general law_, the constancy of electro-decomposition, it became necessary to examine a distinction, already recognised among scientific men, relative to the products of that action, namely, their primary or secondary character; and, if possible, by some general rule or principle, to decide when they were of the one or the other kind. It will appear hereafter that great mistakes inspecting electro-chemical action and its consequences have arisen from confounding these two cla.s.ses of results together.
743. When a substance under decomposition yields at the electrodes those bodies uncombined and unaltered which the electric current has separated, then they may be considered as primary results, even though themselves compounds. Thus the oxygen and hydrogen from water are primary results; and so also are the acid and alkali (themselves compound bodies) evolved from sulphate of soda. But when the substances separated by the current are changed at the electrodes before their appearance, then they give rise to secondary results, although in many cases the bodies evolved are elementary.
744. These secondary results occur in two ways, being sometimes due to the mutual action of the evolved substance and the matter of the electrode, and sometimes to its action upon the substances contained in the body itself under decomposition. Thus, when carbon is made the positive electrode in dilute sulphuric acid, carbonic oxide and carbonic acid occasionally appear there instead of oxygen; for the latter, acting upon the matter of the electrode, produces these secondary results. Or if the positive electrode, in a solution of nitrate or acetate of lead, be platina, then peroxide of lead appears there, equally a secondary result with the former, but now depending upon an action of the oxygen on a substance in the solution.
Again, when ammonia is decomposed by platina electrodes, nitrogen appears at the _anode_[A]; but though an _elementary_ body, it is a _secondary_ result in this case, being derived from the chemical action of the oxygen electrically evolved there, upon the ammonia in the surrounding solution (554.). In the same manner when aqueous solutions of metallic salts are decomposed by the current, the metals evolved at the _cathode_, though elements, are _always_ secondary results, and not immediate consequences of the decomposing power of the electric current.
[A] Annales de Chimie, 1801, tom. li. p. 167.
745. Many of these secondary results are extremely valuable; for instance, all the interesting compounds which M. Becquerel has obtained by feeble electric currents are of this nature; but they are essentially chemical, and must, in the theory of electrolytic action, be carefully distinguished from those which are directly due to the action of the electric current.
746. The nature of the substances evolved will often lead to a correct judgement of their primary or secondary character, but is not sufficient alone to establish that point. Thus, nitrogen is said to be attracted sometimes by the positive and sometimes by the negative electrode, according to the bodies with which it may be combined (554. 555.), and it is on such occasions evidently viewed as a primary result[A]; but I think I shall show, that, when it appears at the positive electrode, or rather at the _anode_, it is a secondary result (748.). Thus, also, Sir Humphry Davy[B], and with him the great body of chemical philosophers, (including myself,) have given the appearance of copper, lead, tin, silver, gold, &c., at the negative electrode, when their aqueous solutions were acted upon by the voltaic current, as proofs that the metals, as a cla.s.s, were attracted to that surface; thus a.s.suming the metal, in each case, to be a primary result. These, however, I expect to prove, are all secondary results; the mere consequence of chemical action, and no proofs either of the attraction or of the law announced respecting their places[C].
[A] Annales de Chimie, 1804, tom. li. p. 172.
[B] Elements of Chemical Philosophy, pp. 144. 161.
[C] It is remarkable that up to 1804 it was the received opinion that the metals were reduced by the nascent hydrogen. At that date the general opinion was reversed by Hisinger and Berzelius (Annales de Chimie, 1804, tom. li. p. 174,), who stated that the metals were evolved directly by the electricity: in which opinion it appears, from that time, Davy coincided (Philosophical Transactions, 1826, p. 388).
747. But when we take to our a.s.sistance the law of _constant electro-chemical action_ already proved with regard to water (732.), and which I hope to extend satisfactorily to all bodies (821.), and consider the _quant.i.ties_ as well as the _nature_ of the substances set free, a generally accurate judgement of the primary or secondary character of the results may be formed: and this important point, so essential to the theory of electrolyzation, since it decides what are the particles directly under the influence of the current, (distinguishing them from such as are not affected,) and what are the results to be expected, may be established with such degree of certainty as to remove innumerable ambiguities and doubtful considerations from this branch of the science.
748. Let us apply these principles to the case of ammonia, and the supposed determination of nitrogen to one or the other _electrode_ (554. 555,). A pure strong solution of ammonia is as bad a conductor, and therefore as little liable to electrolyzation, as pure water; but when sulphate of ammonia is dissolved in it, the whole becomes a conductor; nitrogen _almost_ and occasionally _quite_ pure is evolved at the _anode_, and hydrogen at the _cathode_; the ratio of the volume of the former to that of the latter varying, but being as 1 to about 3 or 4. This result would seem at first to imply that the electric current had decomposed ammonia, and that the nitrogen had been determined towards the positive electrode. But when the electricity used was measured out by the volta-electrometer (707.
736.), it was found that the hydrogen obtained was exactly in the proportion which would have been supplied by decomposed water, whilst the nitrogen had no certain or constant relation whatever. When, upon multiplying experiments, it was found that, by using a stronger or weaker solution, or a more or less powerful battery, the gas evolved at the _anode_ was a mixture of oxygen and nitrogen, varying both in proportion and absolute quant.i.ty, whilst the hydrogen at the _cathode_ remained constant, no doubt could be entertained that the nitrogen at the _anode_ was a secondary result, depending upon the chemical action of the nascent oxygen, determined to that surface by the electric current, upon the ammonia in solution. It was the water, therefore, which was electrolyzed, not the ammonia. Further, the experiment gives no real indication of the tendency of the element nitrogen to either one electrode or the other; nor do I know of any experiment with nitric acid, or other compounds of nitrogen, which shows the tendency of this element, under the influence of the electric current, to pa.s.s in either direction along its course.
749. As another ill.u.s.tration of secondary results, the effects on a solution of acetate of pota.s.sa, may be quoted. When a very strong solution was used, more gas was evolved at the _anode_ than at the _cathode_, in the proportion of 4 to 3 nearly: that from the _anode_ was a mixture of carbonic oxide and carbonic acid; that from the _cathode_ pure hydrogen.
When a much weaker solution was used, less gas was evolved at the _anode_ than at the _cathode_; and it now contained carburetted hydrogen, as well as carbonic oxide and carbonic acid. This result of carburetted hydrogen at the positive electrode has a very anomalous appearance, if considered as an immediate consequence of the decomposing power of the current. It, however, as well as the carbonic oxide and acid, is only a _secondary result_; for it is the water alone which suffers electro-decomposition, and it is the oxygen eliminated at the _anode_ which, reacting on the acetic acid, in the midst of which it is evolved, produces those substances that finally appear there. This is fully proved by experiments with the volta-electrometer (707.); for then the hydrogen evolved from the acetate at the _cathode_ is always found to be definite, being exactly proportionate to the electricity which has pa.s.sed through the solution, and, in quant.i.ty, the same as the hydrogen evolved in the volta-electrometer itself. The appearance of the carbon in combination with the hydrogen at the positive electrode, and its non-appearance at the negative electrode, are in curious contrast with the results which might have been expected from the law usually accepted respecting the final places of the elements.
750. If the salt in solution be an acetate of lead, then the results at both electrodes are secondary, and cannot be used to estimate or express the amount of electro-chemical action, except by a circuitous process (843.). In place of oxygen or even the gases already described (749.), peroxide of lead now appears at the positive, and lead itself at the negative electrode. When other metallic solutions are used, containing, for instance, peroxides, as that of copper, combined with this or any other decomposable acid, still more complicated results will be obtained; which, viewed as direct results of the electro-chemical action, will, in their proportions, present nothing but confusion, but will appear perfectly harmonious and simple if they be considered as secondary results, and will accord in their proportions with the oxygen and hydrogen evolved from water by the action of a definite quant.i.ty of electricity.
751. I have experimented upon many bodies, with a view to determine whether the results were primary or secondary. I have been surprised to find how many of them, in ordinary cases, are of the latter cla.s.s, and how frequently water is the only body electrolyzed in instances where other substances have been supposed to give way. Some of these results I will give in as few words as possible.
752. _Nitric acid._--When very strong, it conducted well, and yielded oxygen at the positive electrode. No gas appeared at the negative electrode; but nitrous acid, and apparently nitric oxide, were formed there, which, dissolving, rendered the acid yellow or red, and at last even effervescent, from the spontaneous separation of nitric oxide. Upon diluting the acid with its bulk or more of water, gas appeared at the negative electrode. Its quant.i.ty could be varied by variations, either in the strength of the acid or of the voltaic current: for that acid from which no gas separated at the _cathode_, with a weak voltaic battery, did evolve gas there with a stronger; and that battery which evolved no gas there with a strong acid, did cause its evolution with an acid more dilute.
The gas at the _anode_ was always oxygen; that at the _cathode_ hydrogen.
When the quant.i.ty of products was examined by the volta-electrometer (707.), the oxygen, whether from strong or weak acid, proved to be in the same proportion as from water. When the acid was diluted to specific gravity 1.24, or less, the hydrogen also proved to be the same in quant.i.ty as from water. Hence I conclude that the nitric acid does not undergo electrolyzation, but the water only; that the oxygen at the _anode_ is always a primary result, but that the products at the _cathode_ are often secondary, and due to the reaction of the hydrogen upon the nitric acid.
753. _Nitre._--A solution of this salt yields very variable results, according as one or other form of tube is used, or as the electrodes are large or small. Sometimes the whole of the hydrogen of the water decomposed may be obtained at the negative electrode; at other times, only a part of it, because of the ready formation of secondary results. The solution is a very excellent conductor of electricity.
754. _Nitrate of ammonia_, in aqueous solution, gives rise to secondary results very varied and uncertain in their proportions.
755. _Sulphurous acid._--Pure liquid sulphurous acid does not conduct nor suffer decomposition by the voltaic current[A], but, when dissolved in water, the solution acquires conducting power, and is decomposed, yielding oxygen at the _anode_, and hydrogen and sulphur at the _cathode_.
[A] See also De la Rive, Bibliotheque Universelle, tom. xl. p. 205; or Quarterly Journal of Science, vol. xxvii. p, 407.
756. A solution containing sulphuric acid in addition to the sulphurous acid, was a better conductor. It gave very little gas at either electrode: that at the _anode_ was oxygen, that at the _cathode_ pure hydrogen. From the _cathode_ also rose a white turbid stream, consisting of diffused sulphur, which soon rendered the whole solution milky. The volumes of gases were in no regular proportion to the quant.i.ties evolved from water in the voltameter. I conclude that the sulphurous acid was not at all affected by the electric current in any of these cases, and that the water present was the only body electro-chemically decomposed; that, at the _anode_, the oxygen from the water converted the sulphurous acid into sulphuric acid, and, at the _cathode_, the hydrogen electrically evolved decomposed the sulphurous acid, combining with its oxygen, and setting its sulphur free. I conclude that the sulphur at the negative electrode was only a secondary result; and, in fact, no part of it was found combined with the small portion of hydrogen which escaped when weak solutions of sulphurous acid were used.
757. _Sulphuric acid._--I have already given my reasons for concluding that sulphuric acid is not electrolyzable, i.e. not decomposable directly by the electric current, but occasionally suffering by a secondary action at the _cathode_ from the hydrogen evolved there (681.). In the year 1800, Davy considered the sulphur from sulphuric acid as the result of the action of the nascent hydrogen[A]. In 1804, Hisinger and Berzelius stated that it was the direct result of the action of the voltaic pile[B], an opinion which from that time Davy seems to have adopted, and which has since been commonly received by all. The change of my own opinion requires that I should correct what I have already said of the decomposition of sulphuric acid in a former series of these Researches (552.): I do not now think that the appearance of the sulphur at the negative electrode is an immediate consequence of electrolytic action.
[A] Nicholson"s Quarterly Journal, vol. iv. pp. 280, 281.
[B] Annales de Chimie, 1804, tom. li. p. 173.
758. _Muriatic acid._--A strong solution gave hydrogen at the negative electrode, and chlorine only at the positive electrode; of the latter, a part acted on the platina and a part was dissolved. A minute bubble of gas remained; it was not oxygen, but probably air previously held in solution.
759. It was an important matter to determine whether the chlorine was a primary result, or only a secondary product, due to the action of the oxygen evolved from water at the _anode_ upon the muriatic acid; i.e.
whether the muriatic acid was electrolyzable, and if so, whether the decomposition was _definite_.
760. The muriatic acid was gradually diluted. One part with six of water gave only chlorine at the _anode_. One part with eight of water gave only chlorine; with nine of water, a little oxygen appeared with the chlorine; but the occurrence or non-occurrence of oxygen at these strengths depended, in part, on the strength of the voltaic battery used. With fifteen parts of water, a little oxygen, with much chlorine, was evolved at the _anode_. As the solution was now becoming a bad conductor of electricity, sulphuric acid was added to it: this caused more ready decomposition, but did not sensibly alter the proportion of chlorine and oxygen.
761. The muriatic acid was now diluted with 100 times its volume of dilute sulphuric acid. It still gave a large proportion of chlorine at the _anode_, mingled with oxygen; and the result was the same, whether a voltaic battery of 40 pairs of plates or one containing only 5 pairs were used. With acid of this strength, the oxygen evolved at the _anode_ was to the hydrogen at the _cathode_, in volume, as 17 is to 64; and therefore the chlorine would have been 30 volumes, had it not been dissolved by the fluid.
762. Next with respect to the quant.i.ty of elements evolved. On using the volta-electrometer, it was found that, whether the strongest or the weakest muriatic acid were used, whether chlorine alone or chlorine mingled with oxygen appeared at the _anode_, still the hydrogen evolved at the _cathode_ was a constant quant.i.ty, i.e. exactly the same as the hydrogen which the _same quant.i.ty of electricity_ could evolve from water.
763. This constancy does not decide whether the muriatic acid is electrolyzed or not, although it proves that if so, it must be in definite proportions to the quant.i.ty of electricity used. Other considerations may, however, be allowed to decide the point. The a.n.a.logy between chlorine and oxygen, in their relations to hydrogen, is so strong, as to lead almost to the certainty, that, when combined with that element, they would perform similar parts in the process of electro-decomposition. They both unite with it in single proportional or equivalent quant.i.ties; and the number of proportionals appearing to have an intimate and important relation to the decomposability of a body (697.), those in muriatic acid, as well as in water, are the most favourable, or those perhaps even necessary, to decomposition. In other binary compounds of chlorine also, where nothing equivocal depending on the simultaneous presence of it and oxygen is involved, the chlorine is directly eliminated at the _anode_ by the electric current. Such is the case with the chloride of lead (395.), which may be justly compared with protoxide of lead (402.), and stands in the same relation to it as muriatic acid to water. The chlorides of pota.s.sium, sodium, barium, &c., are in the same relation to the protoxides of the same metals and present the same results under the influence of the electric current (402.).