Imágenes de páginas
PDF
EPUB

be

an

round. As it is, to attempt such a change would only cause confusion, and we may, in electro-chemistry at any rate, be content to adhere to the current usage. The “direction of the current” will therefore mean the direction in which cations (H: Na', etc.), and the electricity associated with them, move.

It may be objected that these explanations drawn from the electron theory are too hypothetical. They are put forward in the belief that any tolerably consistent picture of the actions to be studied is better than none; provided, of course, it be recognised that if new facts are discovered which the explanation does not fit, the picture must be modified accordingly. We may, however, leave hypothesis for undoubted fact in considering how a convective current of electricity is to be measured.

Let a pair of parallel metal plates be connected to the poles of a battery or other generator of electricity: then between them there will “ electric field,” i.e. a space in which electrical actions are exerted. Such a field will really arise, round the connecting wires, etc., and whether plates or any other conductors be used, but the circumstances are simpler and more marked in the case imagined. To make illustrative experiments we

FIG. 8. may suppose a pair of brass plates with a few centimetres of air-space between (Fig. 8). If now a gilt pith-ball suspended by a silk thread and positively charged be lowered into the space between the plates, it will be repelled by the positive, attracted by the negative, plate. The direction in which it will move is that of the electric field. If a negatively charged ball be used, it will be repelled by the negative, attracted by the positive, plate; and will consequently move in the direction opposite to that of the electric field. If a positively charged ball start from the positive plate and be

+

driven by the electric field across to the negative, and there gives up its charge, it has carried so much electricity across the intervening space; its motion therefore constitutes an electric current, directed from the positive to the negative plate, i.e. in the direction of the field. If a negatively charged ball starts from the negative plate and is driven by the forces of the field to the positive, and there gives up its charge, this is also a current; but as positive and negative electricity are identically opposite in their effects, the carrying of positive electricity in one sense is the same as the carrying of negative in the opposite sense.

Hence the movement of the negative charges will constitute a current in the same direction as that of the positive, i.e. the direction of the electric field; and the total electric current is the sum of the currents due to the motion of positive charges in one sense and negative in the other : we shall call these two currents cationic and anionic respectively.

All this is applicable to the electric field between the two 'plates of an electrolytic ceil ; when a battery is joined up, so as to make one plate positive and the other negative, a stream of cations is set up in the direction of the cathode, where they give up their charges; and one of anions towards the anode, where they give up their charges. Both these streams constitute electric current in the same direction, i.e. by convention, the direction from anode to cathode, and the total current is the sum of the two.

Now the current, being the rate of flow of electricity, is measured by the amount of electricity conveyed across section of the conductor in a second; or, on the ionic theory, it is the number of ions traversing a section of the conductor in a second, multiplied by the charge on each.

In an electrolyte, since under ordinary circumstances there is no perceptible electrification, the amount of positive and negative electricity contained must be the same. The anions and cations must precisely neutralise each other : they are not necessarily equal to each other in number, for an ion may carry two, three, or more charges; thus in NaCl the number of sodions would be equal to that of chlorions; but in Na.SO, the number of sodions would be double that of sulphions. But if

a

we express the quantity of ions in gram-equivalents, we may say simply that there must be as many equivalents of positive as of negative ions.

Let n be the concentration of a solution, expressed in gramequivalents per cubic centimetre, i.e. of the total salt (acid, or base) weighed out in making up the solution. This salt is not all ionised. Let y be the fraction that is ; then yn is-also in gram-equivalents—the amount of ions, positive or negative, formed. y is known as the degree of dissociation. But since a gram-equivalent of any ion carries the same charge, one faraday, there exists in the solution

96600 yn coulombs per cubic centimetre of free positive electricity, and the same amount of negative. If the cations are moving with an average velocity uc cms. per second, all those that exist in a length of uc cms, will cross a given section of the electrolyte in a second, i.e. all those existing in Uc C.C. will cross one square centimetre of section in a second. This number is, of course, ucyn, and the quantity of electricity conveyed is 96600 ucyn per second : this is the current per square centimetre, or current density conveyed by the positive ions. Similarly if the anions are moving with an average velocity ua in the opposite .sense, they produce a current 96600 usyn, and the total current density is

96600 yn (ua + uc) It will be shown subsequently how by the aid of measurements of the current the various quantities in this expression may be determined.

§ 3. PHENOMENA AT THE ELECTRODES. It is necessary to distinguish between the mode in which the current is conveyed in the interior of the electrolyte, on the one hand, and between the electrodes and electrolyte on the

1 In order to adhere to the C.G.S. system, we shall (following Kohlrausch) express concentration in gram-equivalents for c.c., although it is not actually possible to put so much as a gram-equivalent of salt in that volume.

other. In accordance with the fundamental laws of electricity, the total current is the same across any section of the conducting circuit; and in any case the total current is the sum of the parts conveyed by positive and negative charges. But the ratio in which the current is shared between positive and negative carriers need not be the same throughout, and usually is not. Either the cationic or anionic current may, under some circumstances, vanish; in which case the total current conveyed is of the other kind. The proportion in which the current is shared between the available ions depends on local circumstances, and is usually different in the interior of a liquid conductor to what it is at the solid boundaries. We shall begin by considering the boundary conditions.

The most important ways in which current can be led in at the anode—from electrode to electrolyte, are

(i) Formation of a cation.
(ii) Discharge of an anion.

Since electrodes are usually simple pieces of metal, they are obviously capable of furnishing cations : usually only one kind of cations; but if an alloy or amalgam be used, more than one is available. Thus, a brass electrode might, according to circumstances, yield copper or zinc ions. When a cation is formed from a metal electrode, a positive charge is thereby conveyed into the liquid, i.e. in the nominal direction of the current. Sometimes the amount of metal dissolved exactly corresponds to the current according to Faraday's laws: in that case the whole current is carried in this way (the whole current is therefore a “ cationic” one) and no other process can occur at the anode. Examples of this are to be found in the solution of zinc in an ordinary voltaic cell, and the solution of mercury at the anode of a mercury voltameter. These reactions may be written

Zn + 2

2 Hg + 2 = Hg." (the latter because it was shown by Ogg? that mercurous ions are really diatomic).

· Zeitschr. phys. Chem., 27. 285, žu (1898).

+

= Zn:

On the other hand, there are always anions in the solution, and they may be discharged. This happens, e.g., at the anode of the water voltameter. Here current could only be conveyed into the electrolyte by cations, if platinum were to dissolve (in the acid voltameter) or nickel (in the alkaline). But formation of these cations, though not impossible, is difficult, and actually the reaction occurring is an evolution of oxygen :

2 OH' = H,0 +0+2 0 There are other ways in which current may be taken into a solution. We may mention, for completeness

(iii) Oxydation of a positive ion. If a platinum anode be immersed in a ferrous salt, on leading in current the ferrous salt will (under certain circumstances) be converted to ferric. 1.e. the reaction

Fe" + = Fe

. will take place. The formation of a more highly charged cation, here, is obviously analogous to the formation of a new cation that occurs in case (i) and equally involves a transport of positive electricity from electrode to electrolyte.

(iv) Similarly a negative ion may be converted into another with a lower charge. If a platinum plate be immersed in a ferrocyanide solution, and it be used as anode, ferricyanide may be formed. Here the reaction is :

FeC,NG" = Fe C,Ng" +It appears therefore that oxidation is an essentially anodic process, reduction (in the chemical sense) a cathodic one.

There are other anodic reactions that cannot be so simply expressed. These will be considered in dealing with particular cases later.

At the cathode the possible reactions are the converse of those at the anode, viz.

(i.) Discharge of a cation. The usual process : e.g. deposition of hydrogen, copper, silver, mercury, in the voltameters described in $ 1.

(ii.) Formation of an anion. This is not possible when an ordinary metallic cathode is used, for there is no material to

« AnteriorContinuar »