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days, to contain all the sulphuric acid, and the negative cup all the potash. (H. Davy.)
Place of Decomposition.
The separation of the ions takes place only in the immediate neighbourhood of the electrodes, not at any part of the liquid at a distance from them. (H. Davy, De la Rive.) If a solution of common salt, coloured with infusion of violets, be divided into three portions by two membranous partitions—the electrodes dipping into the outermost divisions—the colour of the liquid changes in these divisions alone, not in the middle. (De la Rive.)
When a number of currents, either equally or unequally strong, pass simultaneously through the same liquid, either in the same or in opposite directions, neither of them is disturbed by the rest. (Marianini.)
Relation between the Quantity of the Electric Current and the Quantity of
A battery which retains a platinum wire to of an inch thick in a state of constant ignition during the whole time occupied by the decomposition, decomposes one grain of water in 3 minutes: this quantity of electricity is perhaps equal to that of a powerful stroke of lightning. (Faraday.)
The quantity of electricity which enters the liquid is directly proportional to the quantity of liquid decomposed. Hence, the quantity of electricity in the current may be determined from the quantity of the products of decomposition. (Faraday, De la Rive.) The greater therefore the quantity of electricity which the apparatus employed yields in a given time, the greater will be the quantity of liquid decomposed, provided that the electricity possesses the requisite tension. Hence the batteries of Grove, Daniell, Sturgeon, Smee, and others, have the strongest decomposing action, the electrical machine the weakest.,
Faraday's Volta-electrometer or Voltameter. Into the lower part of a graduated tube closed at the top (App. 28) are inserted, opposite to each other, two platinum wires, to the ends of which are attached two small plates of platinum placed upright in the tube. The open end of the tube is inserted into one aperture of a vessel, two-thirds filled with dilute sulphuric acid of sp. gr. from 1.25 to 1.336, the other aperture being closed with a stopper. The tube is filled with liquid by inverting the apparatus. It is then placed upright, and the two platinum wires connected with the poles of the battery, in order to determine the quantity of detonating gas evolved in a given time. The decomposition must not be allowed to go on long enough to bring the gas in contact with the platinum plates, because these plates would give rise to a slow re-union of the gases.-In many liquids, as in hydrochloric acid, only hydrogen gas has to be collected, in others only oxygen, as in the case of sulphate of copper. In such cases, the voltameter may’be formed of a graduated tube, having a platinum wire inserted into its upper and closed end, filled with the liquid, and inverted in a glass vessel into which the other electrode is introduced (App. 29). Other arrangements are likewise described by Faraday.
Whether the quantity of electricity in the current be measured by the voltameter or by the galvanometer (in proportion to the tangent of the deflection, and therefore by means of Nervander's compass) or by the electromagnetic balance, the proportion obtained is invariably the same. (Jacobi.)
An atom of any one electrolyte requires for its decomposition the same quantity of electricity as an atom of any other, whether the combination be held together by strong or by feeble affinity.
If the current of a battery be passed through the voltameter, and thence -by means of a platinum wire entering at the upper end and conveying positive electricity_into a glass tube containing fused protochloride of tin, and having inserted into its lower end, a platinum wire, which serves as the negative electrode,—then, for every 9 parts of water decomposed in the voltameter, 58-53 parts of tin are deposited on the last-mentioned wire (the atomic weight of tin is 59).—When fused chloride, iodide, oxide, and borate of lead, were treated in a similar manner, the quantity of lead obtained was too small in proportion to the water decomposed, viz. : to 9 parts of water, 100-8, 89, 93-2 and 101:3 lead, whereas the atomic weight of lead is 103.8. The cause of the deficiency is probably that a portion of the precipitated lead was redissolved by the anion. When two silver wires are introduced as electrodes into fused chloride of silver, the weight of the positive electrode diminishes almost exactly by 108·1 parts of silver for every 9 parts of water decomposed in the voltameter, whilst that of the negative electrode increases by the same quantity. Chloride and iodide of lead treated in the same manner, lead being used as the positive electrode, give 101.5 and 103.5 lead for every 9 parts of water. (Faraday.)
If the same current be made to pass through fused chloride of lead and solution of Glauber's salts, one atom of Glauber's salt is decomposed for every atom of lead reduced. If instead of Glauber's salt a solution of common salt be used, the electricity being conducted into it by means of a weighed tin plate, then for every atom of lead reduced, one atom of soda is separated, and one atom of tin dissolved. (Daniell.)
If the current of a battery be passed through a number of metallic solutions connected by platinum wires, the metals are precipitated in the ratio of their atomic weights,—e. g., about four times as much silver as copper. (Matteucci.)
For Berzelius's objections to Faraday's law—that equal numbers of atoms of liquid require equal quantities of electricity to decompose them-vid. Berzelius' Jahresb., 15, 34.
The quantity of electricity which an atom of liquid requires to decompose it, is equal to the quantity which an atom of the same liquid evolves during its electro-chemical decomposition by ponderable bodies. (Faraday.)
If an atom of hydrogen has given up a quantity of negative electricity, denoted by x, and an atom of oxygen the same quantity of positive electricity, in combining to form water, this same quantity of electricity must be restored to each of them by the current.—When an atom of water is decomposed by an atom of zinc which combines with its oxygen, x units of negative electricity are set free from the zinc,—while the atom of hydrogen evolved on the copper, takes x units of negative electricity from it, and sets free x units of positive electricity, which flow through the connecting wire to the x units of negative electricity liberated from the zinc).
The following experiments afford an approximate demonstration of this law; but the quantity dissolved in each cell of the battery in proportion to 9 parts (1 At.) of water in the voltameter, was always greater than
32.2 parts (1 At.), on account of pure chemical action, which could not be altogether prevented.
When a Daniell’s constant battery, with amalgamated zinc was used, 33.6 parts of zinc were dissolved in each cell of the battery for every 9 parts of water decomposed. (Jacobi.)
With batteries containing only one liquid, the loss of zinc is greater. When the form of the battery and the nature of the liquid are varied, the following differences are observed,-F. denoting Faraday's apparatus, with double copper surface (p. 424); Tg. the common trough-battery, with single copper surface; Q Z. the surface of the plates in square inches, PZ. the number of pairs; 1 Tr, the number of atoms of zinc dissolved in one trough during the decomposition of one atom of water; and Tot. the number of atoms of zinc dissolved in all the troughs taken together. In both batteries, the liquid used was a mixture of 200 measures of water, 4.5 of oil of vitriol, and 4 of strong nitric acid.
Q Z. PZ. 1 Tr. Tot. F... 3 40 2.25 88.4 Tg.. 4
40 3.54 141.6
QZ, PZ. 1 Tr. Tot.
QZ. PZ. 1 Tr.
Tot. 67.6 155.0
When different liquids are used in a Faraday's battery of forty pairs, the quantities of zinc dissolved in each cell forone atom of water decomposed are as follows: With 200 measures of water mixed with 8 of strong nitric acid, 1.85 At.; the same quantity of water with 16 nitric acid, 1.82; with 32 nitric acid, 2.1 At; with 16 measures of strong hydrochloric acid, 3.8; with 9 measures of oil of vitriol, 4:66; with 16 measures of strong hydrochloric and 6 of nitric acid, 2:11; with 4:5 measures of oil of vitriol and 4 of nitric acid, 2.26; with 9 measures of oil of vitriol and 4 of nitric acid, 2.79; and with 9 measures of oil of vitriol and 8 of nitric acid, 2.26 At. zinc. Nitric acid is therefore the best for this battery; and different degrees of dilution of this acid do not affect, to any considerable extent, the proportion between water decomposed and zinc dissolved. (Faraday.)
İf the current from about four pairs of zinc and copper be made to pass into a solution of nitrate of silver, and the quantity of zinc dissolved be the same one time as another, the quantity of silver separated will likewise be constant,—whether the zinc be quickly dissolved by the use of strong and warm acid, and the galvanometer strongly deflected by the current-or the zinc be slowly dissolved by cold, weak acid, and the galvanometer feebly deflected. The same quantity of electricity passes through the liquid in both cases, though in different times: hence the quantity of silver precipitated is likewise the same. Similar results are obtained with a battery of copper, platinum, and nitric acid.—If a pile a be constructed of lead and platinum plates, and a pile b of copper and platinum, the weight of the lead plates being to that of the copper plates in the ratio of the atomic weights of the metals, viz., as 103.8 : 32,—and the currents of both batteries be passed through solution of nitrate of silver contained in separate vessels,—then, when all the copper and lead are dissolved, the quantities of silver separated in the two vessels will be found to be equal. (Matteucci.)
Since an electrical machine developes much less electricity in a given time than a galvanic battery, even with very small plates, it does not readily produce decomposition, notwithstanding its high tension. When the machine is employed for this purpose, the two electrodes are conuected with the two coatings of the electrical battery, or one with the conductors and the other with the rubber or the ground. The excessive tension of the electricity must be weakened by giving a large surface to the battery or the conductor, or else by passing the charge through a wet string, so that it shall no longer burst violently through the electrolyte; and its deficiency in quantity must be compensated by properly enveloping the electrode, so as to concentrate the electricity at its entrance into the liquid upon a small point.
Wollaston fused fine gold or silver wires into glass or sealing-wax, so that only the extreme points of the wires came in contact with the liquid. In this manner he decomposed water, and sulphate of copper and corrosive sublimate dissolved in water, by about 100 turns of the machine. But in this decomposition of water, both oxygen and hydrogen were evolved at each electrode—a result probably arising from the great intensity of the electricity, which Wollaston took no means to counteract.—Sir H. Davy effected in two hours, by Wollaston's method, the decomposition of aqueous solution of sulphate of potash contained in two cups connected by asbestus. (Comp. Gahn. and Hisinger, Gilb. 27, 311.)—Bonnijol (Bill. univ. 45, 213) decomposed water by friction electricity, and likewise by atmospheric electricity, which he caused to enter the liquid by wires half a millimetre in diameter.
When only one wire is surrounded with sealing-wax, detonating gas is evolved only on this one in pure water; none on the other. The stronger the sparks, the greater is the quantity of detonating gas evolved; and when the sparks cease, the gas produced is diminished to a mere trace. A solution of sulphate of soda yields but a trace of gas, even when strong sparks are passed. Hence the decomposition of water by machine electricity is different from that produced by galvanic electricity. (Faraday.)
When moistened litmus paper is connected with the wire of the conductor of the electrical machine, and moistened turmeric paper with the wire of the rubber, and the two papers united by means of a thread seventy feet long moistened with solution of turmeric paper,-decomposition takes place and therefore reddening of the litmus paper by evolved acid and browning of the turmeric paper by liberated alkali) as strongly as when the two papers are united by a shorter connection. This is due to the high tension of the machine electricity. Similar remarks apply to the decomposition of iodide of potassium. (Faraday.)
It is not even necessary to have the circuit regularly closed. If moistened litmus paper be connected with the conductor (or turmeric paper with the rubber) and likewise with a string saturated with sulphate of soda, and communicating with the ground (or the gas-pipes of London), or even if the paper connected with the conductor or the rubber be insulated,—decomposition ensues, in consequence of the combination of the highly intense electricity of the machine with the opposite electricity proceeding from the latent electric fluid of the earth or the air. Hence the opposite reaction shows itself at the part of the paper farthest from the point at which the electricity of the machine enters. If a thread, wetted with solution of sulphate of soda and attached to the conductor, be brought in contact with turmeric paper connected by a wire with the gas-pipes under ground, the paper becomes reddened by the negative electricity which flows from the earth into the liquid. A similar effect is produced upon litmus paper connected in like manner with the rubber. If a piece of litmus paper and a piece of turmeric paper in the form of acute triangles, be moistened with solution of sulphate of soda, laid together by their shortest sides, and placed between conducting wires attached to the conductor and rubber of the machine-the ends of the wires being about half an ineh distant from the points of the papers-reddening takes place at both points, but disappears again when the points are turned round, so as to bring the turmeric paper opposite to the positive conductor. The two paper triangles may also be laid upon glass, and connected with the two conductors by strings, six inches long, moistened with solution of sul. phate of soda.—Solutions of iodide of potassium and acetate of lead give corresponding results. (Faraday.)
If the electricity from the machine be communicated through the medium of a wet string or a wire, or in small sparks, to an electrode dipping into a liquid, -a solution of iodide of potassium mixed with starch is turned blue by half a turn of the machine-solution of sulphate of soda reddens litmus and turmeric paper after two or three turns hydrochloric acid evolves chlorine—and sulphate of copper deposits copper on the silver cathode after twenty turns. (Faraday.)
Aqueous solution of iodide of potassium, contained in a small U-tube, into which are immersed platinum wires proceeding from the conductor and the rubber, acquires a yellow colour in the positive arm. (Henrici, Pogg. 47, 431.)
Influence of the Intensity of the Current on Decomposition. [The more readily the atoms of a liquid are transposed-whether from its peculiar nature or from the thinness of the film of liquid between the electrodes,—the smaller is the tension which the electric current requires in order to penetrate it, the greater the quantity of electricity which flows into the liquid from a given source, and the greater the quantity of liquid decomposed in a given time. If, however, by increasing the number of pairs, or otherwise, the tension has been raised to such a degree, that all the electricity evolved from the source in a given time is able to enter the liquid—so that the galvanometer shows the same deflection whether the circuit be closed by a metallic conductor or by a liquid—a further rise of intensity does not accelerate the decomposition.]
Aqueous solution of iodide of potassiuin may be decomposed by a current of the smallest tension; next in order of facility come fused chloride of silver, fused protochloride of tin, fused chloride of lead, fused iodide of lead, water containing hydrochloric acid, water containing sulphuric acid. That the chloride of lead is more easily decomposed than the iodide, perhaps arises from the platinum electrode having a greater affinity for chlorine than for iodine. (Faraday.)
If the cells a and b (App. 30) contain dilute sulphuric acid of 1.25 sp. gr.,-0, P, and i being platinum, g amalgamated zinc, and h a platinum dlate, on which is laid a piece of paper moistened with solution of iodide of potassium, and having the end of the platinum wire placed in contact with it,-continued decomposition of the iodide of potassium takes place, whilst on the two platinum plates immersed in 6 not a single gas-bubble appears, even in the course of several days. Even when the paper at h is removed and metallic contact established, not a particle of gas is evolved in b; neither does any evolution of gas ensue, when the cell b contains aqueous solution of potash instead of sulphuric acid. But when to the sulphuric acid in a a little nitric acid is added, an evolution of gas takes place on the platinum plates in the course of twelve seconds, because the nitric acid increases the intensity of the electric current. (Faraday.)
If two pieces of paper lying on platinum plates are placed in the current of a single pair of amalgamated zinc and platinum immersed in