Preliminary:

Definition of Electrodes

Anode and Cathode.

CONTENTS

Electrolytes.

Ions, Anions, Cations.

On Some General Conditions of Electrochemical Decomposition (Omitted).

On a New Measurer of Volta-electricity:

Restatement of Faraday's Law...

Forms of Gas Voltameter..

Effect of Size of Electrodes.

Solubility of Gases Evolved

Effect of Variation of Intensity of Current..

Effect of Concentration

Experiments with other Electrolytes.

On the Primary or Secondary Character of the Bodies Evolved at the

Electrodes (Omitted).

On the Definite Nature and Extent of Electrochemical Decomposition :
Recapitulation and Restatement of Faraday's Law.

Experimental Verification:

With Various Salts..

With Various Electrodes in Solutions..

With Various Electrodes in Fused Salts..

26

27

32

34

Preliminary Table of Ions and their Electrochemical Equivalents..
Conclusion..

42

43

ON ELECTROCHEMICAL DECOMPOSITION

BY

MICHAEL FARADAY

PRELIMINARY

THE theory which I believe to be a true expression of the facts of electrochemical decomposition, and which I have therefore detailed in a former series of these Researches, is so much at variance with those previously advanced that I find the greatest difficulty in stating results, as I think, correctly, whilst limited to the use of terms which are current with a certain accepted meaning. Of this kind is the term pole, with its prefixes of positive and negative, and the attached ideas of attraction and repulsion. The general phraseology is that the positive pole attracts oxygen, acids, etc., or more cautiously, that it determines their evolution upon its surface; and that the negative pole acts in an equal manner upon hydrogen, combustibles, metals, and bases. According to my view, the determining force is not at the poles, but within the body under decomposition; and the oxygen and acids are rendered at the negative extremity of that body, whilst hydrogen, metals, etc., are evolved at the positive extremity.

To avoid, therefore, confusion and circumlocution, and for the sake of greater precision of expression than I can otherwise obtain, I have deliberately considered the subject with two friends, and with their assistance and concurrence in framing them, I purpose henceforward using certain other terms, which I will now define. The poles, as they are usually called, are only the doors or ways by which the electric current passes into and out of the decomposing body; and they of course, when in contact with that body, are the limits of its extent in the direction of the current. The term has been generally applied

to the metal surfaces in contact with the decomposing substance; but whether philosophers generally would also apply it to the surfaces of air and water, against which I have effected electrochemical decomposition, is subject to doubt. In place of the term pole, I propose using that of electrode,* and I mean thereby that substance, or rather surface, whether of air, water, metal, or any other body, which bounds the extent of the decomposing matter in the direction of the electric current.

The surfaces at which, according to common phraseology, the electric current enters and leaves a decomposing body are most important places of action, and require to be distinguished apart from the poles, with which they are mostly, and the electrodes, with which they are always, in contact. Wishing for a natural standard of electric direction to which I might refer these, expressive of their difference and at the same time free from all theory, I have thought it might be found in the earth. If the magnetism of the earth be due to electric currents passing round it, the latter must be in constant direction, which, according to the present usage of speech, would be from east to west, or, which will strengthen this to help the memory, that in which the sun appears to move. If in any case of electro - decomposition we consider the decomposing body as placed so that the current passing through it shall be in the same direction, and parallel to that supposed to exist in the earth, then the surfaces at which the electricity is passing into and out of the substance would have an invariable reference, and exhibit constantly the same relations of powers. Upon this notion we purpose calling that towards the east the anode, and that towards the west the cathode; and whatever changes may take place in our views of the nature of electricity and electrical action, as they must affect the natural standard referred to, in the same direction, and to an equal amount with any decomposing substances to which these terms may at any time be applied, there seems no reason to expect that they will lead to confusion or tend in any way to support false views. The anode is therefore that surface at which the electric current, according to our present expression, enters: it is the negative extremity of the decomposing body; is where oxygen,

* ἤλεκτρον, and ὁδὸς, a way.

tavo, upwards, and idòc, a way: the way which the sun rises.

Karà, downwards, and idòç, a way: the way which the sun sets.

chlorine, acids, etc., are evolved; and is against or opposite the positive electrode. The cathode is that surface at which the current leaves the decomposing body, and is its positive extremity; the combustible bodies, metals, alkalies, and bases, are evolved there, and it is contact with the negative electrode. I shall have occasion in these Researches, also, to class bodies together according to certain relations derived from their electrical actions; and wishing to express those relations without at the same time involving the expression of any hypothetical views, I intend using the following names and terms. Many bodies are decomposed directly by the electric current, their elements being set free; these I propose to call electrolytes.* Water, therefore, is an electrolyte. The bodies which, like nitric or sulphuric acids, are decomposed in a secondary manner are not included under this term. Then for electrochemically decomposed, I shall often use the term electrolyzed, derived in the same way, and implying that the body spoken of is separated into its components under the influence of electricity: it is analogous in its sense and sound to analyze, which is derived in a similar manner. The term electrolytical will be understood at once: muriatic acid is electrolytical, boracic acid is not.

Finally, I require a term to express those bodies which can pass to the electrodes, or, as they are usually called, the poles. Substances are frequently spoken of as being electro-negative or electro-positive, according as they go under the supposed influence of a direct attraction to the positive or negative pole. But these terms are much too significant for the use to which I should have to put them; for, though the meanings are perhaps right, they are only hypothetical, and may be wrong; and then, through a very imperceptible, but still very dangerous, because continual, influence, they do great injury to science by contracting and limiting the habitual views of those engaged in pursuing it. I propose to distinguish such bodies by calling those anionst which go to the anode of the decomposing body; and those passing to the cathode, cations; and when I have occasion to speak of these together, I shall call them ions. Thus, the chloride of lead is an electrolyte, and when electrolyzed λεктρov, and λúw, solvo. Noun, electrolyte; verb, electrolyze.

*

† ȧvior, that which goes up. (Neuter participle.)

Kariv, that which goes down.

evolves the two ions, chlorine and lead, the former being an anion, and the latter a cation.

These terms, being once well defined, will, I hope, in their use enable me to avoid much periphrasis and ambiguity of expression. I do not mean to press them into service more frequently than will be required, for I am fully aware that names are one thing and science another.*

It will be well understood that I am giving no opinion respecting the nature of the electric current now, beyond what I have done on former occasions; and that though I speak of the current as proceeding from the parts which are positive to those which are negative, it is merely in accordance with the conventional, though in some degree tacit, agreement entered into by scientific men, that they may have a constant, certain, and definite means of referring to the direction of the forces of that

current.

[Section IV., including eight pages "On Some General Conditions of Electrochemical Decomposition," is here omitted.]

ON A NEW MEASURER OF VOLTA-ELECTRICITY

I have already said, when engaged in reducing common and voltaic electricity to one standard of measurement, and again when introducing my theory of electrochemical decomposition, that the chemical decomposing action of a current is constant for a constant quantity of electricity, notwithstanding the greatest variations in its sources, in its intensity, in the size of the electrodes used, in the nature of the conductors (or non-conductors) through which it is passed, or in other circumstances. The conclusive proofs of the truth of these statements shall be given almost immediately.

I endeavored upon this law to construct an instrument which should measure out the electricity passing through it, and which, being interposed in the course of the current used in any particular experiment, should serve at pleasure, either as a comparative standard of effect or as a positive measurer of this subtile agent.

Since this paper was read, I have changed some of the terms which were first proposed, that I might employ only such as were at the same time simple in their nature, clear in their reference, and free from hypothesis. + [See page 7.]