Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/119

Rh ELECTROLYSIS 101) fleet ro- apillary &amp;gt;heno- nena. The moleculiir state of the deposit varies very much with the density of the current, i.e., the current strength per unit area of elec trode (Bunsen, Pogg. Ann., xci. 619). With small current density the metals are deposited as well-shaped crystals ; on increasing the density, reguline metal (similar to the metal when smelted) is obtained, but with great density the deposit is amorphous, botry- oidal, or pulverulent. With some metals, the molecular state differs with the solutions from which they are deposited. Thus silver from dilute solution of the nitrate, with great current density, appeal s as a black powder, becoming grey- white and crystalline when the cur rent ceases (Wied., Galv., Bd. 1. 336a) but from solution of potas sium silver cyanide it is electrolysed as reguline metal. Gold and platinum exhibit a similar behaviour. For a good instance of amorphous deposit, see the account of Gore s explosive antimonv in his Electrometallurgy, p. 103. (3.) The ions very frequently react upon the electrodes and produce in some cases very interesting chemical actions. If the cation and cathode are both metals, an alloy of the two is the usual if not universal result. This is well known in the case of the electrolysis of many metals and salts with mercury electrodes, and the combina tion of the hydrogen set free by electrolysis with electrodes of pal ladium, nickel, and iron may be similarly regarded ; and perhaps the compounds derived when ammonium salts are decomposed with a mercury cathode. Copper, when deposited on platinum, alloys with it to a certain extent, the alloy penetrating to a considerable depth (Gore, Electro-metallurgy, p. 47). Faraday noticed the com bination of tin and lead with platinum electrodes in the electrolysis of the fused salts of those metals. The action of the anion upon the anode furnished Faraday with nn accurate and convenient means of estimating the amount of chemical decomposition jiroduced by a definite quantity of elec tricity, and thereby of confirming the law given by equation (1) (Exp. Res., 807-822). Thus by varying the anodes, while the cathode remained the same, in the decomposition of acidulated water he found the amount of hydrogen liberated at the cathode, and there fore the chemical decomposition, independent of the nature of the electrodes ; and by electrolysing various chlorides, as of silver, tin, lead, with an anode of the same metals respectively, he was enabled to determine very accurately the amount of chlorine separated. &quot;We shall have more to say on the bearing of this hereafter. The oxygen liberated by the electrolysis of acidulated water frequently unites with the anode; even if this is of carbon it becomes oxidized to CO and C0 2 ; this was noticed by Faraday (Exp. cs., 744), and is interesting as showing the active state of the oxygen when separated. But perhaps the most interesting examples of the action of the ions on the electrodes are furnished by the capillary phenomena exhibited by mercury in contact with dilute acid, on the passage of the current. If we have a drop of water upon a surface of Hg, and the water be connected with the positive, while the Hg is con nected with the negative pole of a battery, the water will gather itself up into a spherical drop, and on reversing the current will spread itself overthe metal. This phenomenon is supposed by Wiede- mann to be due, in the former case, to the reduction of a film of oxide on the surface of the Hg by the liberated H, thereby giving a cleaner surface with a higher capillary constant, and, in the latter, to the oxidation of the surface by the liberated oxygen, and this view is borne out by numerous experiments. Thus a reducing agent, such as crystal of sodium thio-sulphate (Na 2 S 2 3 ), intro duced into the drop of water produces similar contraction of the drop, while an oxidizing agent, as K 2 Cr 2 7, produces on the con trary a similar dispersion. A drop of Hg in dilute sulphuric acid, connected with the positive pole of a battery, while the negative electrode is near it, extends toward that electrode on the passage of the current, becoming covered with a film of suboxide, which then dissolves in the H.jS0 4, and leaves again a bright surface, when the drop returns to its original position, and a series of oscillations are thus set up (see Wied. Galv., i. 368 sqq.). With solutions of alka line cyanides containing mercury Gore obtained oscillations pro ducing sounds (Elcc.-Mctall., p. &quot;197; Proc. Roy. Soc., 1862). It was observed by Erman that a drop of mercury in a horizontal tube, with dilute acid on both sides, moved at the passage of the electric current through the tube towards the negative electrode. These phenomena have been investigated further by Lippmann (Pogg. Ann., cxlix. 547, trans, in Phil. Mag. [4] xlvii. 281). One of the forms of his apparatus is as follows. A glass tube A, drawn out to a short capillary point of about ^$ mm. radius, con tains mercury which penetrates into the fine point and partly fills it, the remainder being filled with dilute H.-S0 4, into which the capillary opening dips; below the electrolyte is a surface of mercury, serving sis the positive electrode, sufficiently bro.id for the capillary effects there to be neglected. The negative electrode i^ the mercury in the tube A. Lippmann showed by this apparatus that, in order to compensate the change in the capillary constant of the mercury produced by a definite electromotive force of polarization, a definite increase of pressure on the mercury in A is required. As for an electromotive force of polarization equivalent to a Daniell cell the compensating pressure was 260 mm., and as the quantity of elec tricity required to polarize the electrodes is very small, this apparatus, when once it has been graduated by observing the compensating pressure for known electromotive forces, may evidently be employed as a sensitive and convenient electrometer for electromotive forces less than the maximum of polarization of the electrodes. We may mention one other example of the action of the ions Pasciv- upon the electrodes. An iron wire is usually attacked by dilute ity. HiS Oj (sp. gr. 1 3) ; but if previously to its being immersed in that liquid it is employed as the anode in the electrolysis of diluted oxygen acids, the nitric acid has no longer any effect upon it, not even tarnishing the surface, and the wire differs from ordinary iron in being strongly electro-negative to it, and indeed to copper, in dilute acids (Martens, Pogg. Ami., Ixi. 121). It is then said to be in the passive state, and is considered to be covered with a film of oxide which is strongly electro-negative, and insoluble in dilute nitric acid (Faraday, Phil. Mag., ix. p. 60, 1836, x. p. 175, 1837; Beetz, Pogg. Ann., Ixii. 234, Ixiii. 415). De Regnon, however (Cornptes Rcndus, Ixxix. 299), attributes the phenomena to polariza tion. This peculiar state may be induced by various processes; Keir (Phil. Trans., 1790) observed it when an iron wire was dipped into strong nitric acid (sp. gr. 1 5), by which its surface is not attacked. A more dilute solution has the same, effect (Schonbein, Pogg. Ann., xxxviii. 444), if the wire be immersed several times, or if the solution contain chromic or sulphuric and permanganic acids (Boutmy and Chateau, Cosmos, xix. 117). Iron when dipped in very strong solution of AgN0 3 does not precipitate the silver, and is electro-negative even to that metal. Another method of render ing iron passive, evidently the same in principle as the one first mentioned, is to touch the iron wire immersed in dilute nitric acid, by carbon, platinum, or other electro-negative element itself in contact with the liquid ; and on the contrary, passive iron becomes active if it be touched by a body electro-positive to it, as copper or zinc. If a passive wire be partly immersed in the dilute acid, and an active wire in contact with it be slowly introduced into the liquid, the latter becomes passive too ; but if they touch under the surface, both are rendered active. Iron is rendered passive also by heating in a current of oxygen or an oxidizing flame until it is tarnished On the other hand, the passive metal becomes active under the influence of any reducing action upon its surface, whether by deposition of H upon it by electrolysis, by heat ing the metal in a reducing flame, or by abrading the surface. One modification of the electrolytic method is to touch the metal in dilute nitric acid, for a moment, with a copper wire. The point touched becomes immediately active, and therefore electro-positive to the rest, and so currents are set up from active to passive metal through the acid, which accordingly reverse the state of both parts, and a curious series of oscillations result, ending in the whole becoming active. (Schonbein, I.e. Compare these with the phenomena of alternation of passive and active states of iron, and of the oxidized and bright surfaces of amalgamated zinc described by Joule, Phil. Mag., 1844, i. 106). Iron is not the only metal which behaves thus. Nickel, cobalt, tin, bismuth, and even copper, all exhibit similar phenomena in strongHN0 3 and as positive electrodes ; and aluminium thus treated is electro-negative even to passive iron (see Wiedemann, Galv., Bd. i. 539-542). (4.) The ions act upon the fluid surrounding the electrodes. Second- Actions of this kind in both fused and dissolved electrolytes nearly ary ac- always occur unless the ions combine with the electrodes ; thus per- tions. chlorides, if such exist, are formed from the chlorides, and per- chlorates from chlorates at the anode (Kolbe). At the cathode the secondary actions are cases of reduction ; thus if solution of potassio iodide be electrolysed, corresponding to 1 equivalent of iodine at the anode, there will appear not only 1 equivalent of H, at the cathode, but an equivalent of KHO as well, so that the potassium liberated from the iodine must have acted upon the water and formed KHO. If ammonium chloride be electrolysed, the chlorine at the anode reacts upon the NH 4 C1, giving free nitrogen and nitrogen-chloride. The electrolysis of ammonium nitrate is still more interesting, as NIL, and H are separated at the cathode, where the hydrogen reduces the nitric acid of the nitrate, and nitrogen is evolved, while at the anode NO 3 is deposited, which forms with the water nitric acid and oxygen, the latter reacting upon the ammonia of the nitrate, again evolving nitrogen, so that that element appears at both poles, at one mixed with ammonia, at the other with oxygen (Miller). Some of the reactions investigated by Kolbe and Burgoin with organic salts are very interesting, but more exclusively to the chemist. The oxidizing and reducing actions are very powerful, as the bodies probably act in the &quot; nascent state.&quot; Solutions of acetate and nitrate of&quot; lead, when electrolysed by currents of small density, deposit at the positive electmle hy dra ted peroxide of lead as a black powder. If a polished iron plate be used as the anode, the deposit shows prismatic colours depending on the thickness, and the process has been applied in the arts to colour metallic toys, under the name of metallochromy. If a fine wire as cathode be placed vertica ly above the anode plate.