282.]

AMOUNT OF ELECTROLYSIS DEFINITE.

575

iv. The amount as well as the direction of electrolysis is definite, and it is dependent upon the action in the battery; being directly proportionate to the quantity of electricity in circulation. It has been amply proved by experiment that for every 65 milligrammes of zinc which is dissolved in any one cell of the battery, provided local action be prevented, 18 milligrammes of water are decomposed in the voltameter; or if, as in the preceding experiment, several electrolytes be arranged in succession, each compound will experience a decomposition proportioned to its chemical equivalent. For instance-if the current be made to pass first through fused plumbic iodide (PbI), and then through fused stannous chloride (SnCl,)-for each 65 milligrammes of zinc dissolved in any one cell of the battery, 207 milligrammes of lead, and 118 milligrammes of tin will be separated on the respective platinodes, whilst 254 (or 2 × 127) milligrammes of iodine, and 71 milligrammes or 2 × 35 ̊5 milligrammes of chlorine will be evolved on the respective zincodes. These numbers correspond with the chemical equivalents (not the atomic weights) of the several elements named.

Variations in the intensity of the current (i.e., variations in the quantity of electricity which passes through a given transverse section of the conductor in unit time) produce no variation in the amount of chemical decomposition which is effected by the arrangement. For example: if three similar voltameters, provided with plates of equal area, be arranged as at a, b, c, fig, 235, the first will transmit twice as much electricity in a given time as either of the

others. The current will therefore have twice the intensity in a; but the total quantity of gas collected from b and c together will be exactly equal to the total amount yielded

FIG. 235. 6

by a in the course of the experiment. Hence it follows that the quantity of electricity which is separated from a given quantity of matter in the act of combination is able, irrespective of its intensity, when thrown into the current form, to produce the decomposition of an equivalent quantity of any compound body which is susceptible of electrolysis; and hence it has been concluded that the equivalent weights of the simple bodies are those weights of

576

LAWS OF ELECTROLYSIS.

[282.

each substance which are associated with equal quantities of electricity, and have naturally equal electric powers.

To these laws may be added a fifth-viz.:

v. Those bodies only are electrolytes which are composed of a conductor and a non-conductor. The conductors accumulate on the platinode, the non-conductors on the zincode. For example, plumbic iodide when melted conducts the current; metallic lead, which is a conductor, accumulates at the platinode; whilst iodine, which is a non-conductor even when melted, collects at the zincode. On the other hand, red chloride of sulphur (S′′Cl,) is not an electrolyte, although composed of single equivalents of its components; and melted sulphur, and chlorine, when the latter is liquefied by pressure, are both insulators of electricity. A compound composed of two conductors is equally unfit for electrolysis. For instance, a metallic alloy, such as plumber's solder, composed of two parts, or one equivalent, of lead, and one part, or one equivalent, of tin, when melted, conducts the current perfectly, but no separation of its constituents is effected.

(283) Relative Decomposability of Electrolytes.-Every elec trolyte, since it can transmit a current, is also capable of generating a current if it be employed to excite action in the battery itself. Comparatively few electrolytes, however, are practically available for this purpose. It is necessary that the deposited compounds be dissolved as fast as they are produced; otherwise the crust of insoluble matter which is formed upon the plates of the battery introduces a mechanical obstacle by which the action is speedily checked.

Great differences occur in the facility with which different electrolytes yield to the decomposing power of the voltaic current. Generally speaking, the greater the chemical opposition between the elements of a compound the more readily it yields to electrolysis. The following table exhibits the order in which, according to Faraday, the different compounds which are enumerated yield to electrolysis; those which are most readily decomposed standing first on the list :

Solution of potassic iodide

Fused argentic chloride

Fused zincic chloride

Fused plumbic chloride

Fused plumbic iodide
Hydrochloric acid

Diluted sulphuric acid.

(284) Electro-chemical Actions.-The suspension of chemical action which occurs under the influence of electrical induction is

284.]

SUSPENSION OF CHEMICAL BY VOLTAIC ACTION.

577

one of the most interesting illustrations of the correlation of physical forces. This suspension is well shown in the way in which zinc when placed in contact with copper beneath the surface of sea-water, acts in preventing the corrosion of copper, and transfers to itself the chemical energy which would otherwise be manifested upon the copper (261). A similar suspension of chemical action is produced in the ordinary case of the decomposition of water between two platinum electrodes by the voltaic current; here the electricity appears to act by weakening, or rather by partly neutralizing, ordinary chemical attraction in one direction, whilst it strengthens or adds to it in the opposite, and hence the partieles which were previously in combination with each other lose their attraction one for the other, and acquire it for those particles which are next adjacent to them in the liquid; if, for instance, the brackets above the subjoined formulæ indicate the state of combination of the elements of hydrochloric acid before the passage of the current, the brackets below would indicate the effect produced after its transmission, thus :—

Here z c is supposed to represent the battery, and + and the terminal wires of the arrangement: the positive electricity seems to detach the chlorine adjacent to it from the hydrogen with which it was previously in combination, whilst the negative electricity produces a similar effect upon the contiguous particles of hydrogen, and the intermediate portions are polarized in the nanner above represented.

The following modification of this experiment also shows in a striking manner the remarkable influence of electric polarity upon chemical attraction:-If two separate glasses filled with diluted sulphuric acid be placed side by side, and into one glass the negative wire of the battery is plunged, whilst the positive wire dips into the other cell, no decomposition will ensue; but if a connexion be established between the two glasses by means of a slip of platinum foil, one end of which is made to dip into each, the current will be immediately transmitted: hydrogen will be evolved upon the platinode in one glass, and oxygen upon the zincode in the other glass; whilst owing to the polar condition into which the connecting slip of platinum is thrown, hydrogen will be given off from one end of the slip, and oxygen will be

578

ELECTROLYSIS OF SALTS.

[284

evolved upon the other extremity of it, although the metal itself experiences no sensible change beyond a slight rise of temperature.

(285) Electrolysis of Salts.-It has already been stated (282, iv.) that when a binary compound, such as a fused chloride or an iodide, is submitted to electrolysis, the ions or components of the compound are separated at the respective electrodes in equivalent proportions; the metal appearing at the platinode, whilst the chlorine, or corresponding element, is deposited at the zincode. If the zincode of the battery be formed of a substance capable of combining with the chlorine or corresponding element, an equivalent amount of the chloride or other compound of this metal will be formed there; and when the metal of the zincode is the same as that contained in the compound which is undergoing decomposition, the original compound is reproduced. For instance, if a quantity of fused argentic chloride (AgCl) be decomposed by a current which is conducted into it by means of silver wires, the quantity of the chloride will undergo no alteration; for in this experiment, as fast as the silver is deposited upon the negative wire, a corresponding quantity of silver will be dissolved from the positive wire, since the latter wire combines with the equivalent quantity of chlorine which is liberated at this point. Let Ag+ represent the positive silver wire, or zincode, by which the current is conveyed into the melted chloride, and - Ag the negative wire: if the brackets in the upper row of symbols which follow indicate the combination before the passage of the current, the lower ones will show the arrangement after the occurrence of the decomposition :

-Ag|AgCl AgCl AgCl | Ag+

AgAg | CIAg ClAg CIAg |

An examination of the products furnished by the electrolytic decomposition of aqueous solutions of the oxysalts (or salts formed from acids which contain oxygen), exhibits results which appear to be at variance with the statement that the components of an electrolyte are separated in equivalent proportions—but further investigation shows that they are strictly in accordance with it; these experiments also lead to the very interesting conclusions which have an important bearing upon the theory of salts in general.

When a solution of an oxysalt such as sodic sulphate is sub

285.]

ELECTROLYSIS OF OXYSALTS.

579

whilst at The pro

mitted to electrolysis, a quantity of acid accumulates around the positive plate, and of alkali around the negative plate the same time both oxygen and hydrogen are set free. portions of each may be determined by means of a diaphragm apparatus, in which the products of decomposition can be kept separate from each other, and the gases which are evolved can be separately collected. Such an apparatus was contrived by Daniell, and is represented in fig. 236. A and B are the two halves of a

FIG. 236.

stout glass cylinder, which are fitted by grinding upon a hollow ring of glass, c; the two rims of this ring are ground down to a grooved shoulder, so as to allow a thin piece of bladder to be tied over each end of the ring, which is thereby converted into a kind of drum; at K is a small hole through which the cavity thus formed can be filled with liquid; D and E are two bent glass tubes for carrying off the gases evolved during electrolysis; g and h are two large platinum electrodes, which pass through corks in the necks of the cylinder, and can be connected with the battery by means of the wires, f, i. The apparatus thus forms three compartments, which may be filled with the liquid for experiment, and the whole may be supported in a frame of wood. By the employment of this apparatus, it is found that while a quantity of acid accumulates at the zincode, an equivalent quantity of alkali is set free at the platinode. At the same time, a quantity of gas is also emitted from each electrode, that from the zincode being oxygen, and that from the platinode hydrogen. Upon placing a voltameter in the course of the circuit, it is found that a quantity of gas is emitted from the saline liquid, exactly equal to that obtained from the voltameter; and upon neutralizing the acid and alkali, they likewise are in equivalent ratios to the gas which is emitted (Daniell, Phil. Trans. 1839, 97, and