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form it out of. There are various devices, however, by which anions can be formed. Thus, if a platinum plate be surrounded by an electrolyte kept saturated with chlorine gas, and used as cathode, some of the chlorine will be converted into chlorions (p. 238).
(iii.) Diminution of the charge on a cation. -- (iv.) Increase of the charge on an anion.
It is worth while to consider the view taken of these processes on the electron theory. As regards convection in the interior of the liquid, it is not necessary to postulate free electrons, for the evidence all goes to show the existence of positive and negative ions formed out of ordinary chemical atoms. Let us turn, then, to the transfer of electricity between the electrode and electrolyte, taking first the simple case of solution of zinc at the anode. We have represented it above as involving the addition of two positive charges to an atom of zinc, the ion thus formed being, unlike the neutral zinc atom, soluble in water. Now, free positive electric charges are unknown : the electrons are always negative. It would be more exact, therefore, to say that two negative charges are removed from a zinc atom, leaving a zinc ion. That is
Zn = Zn" + 20 This leads to a brief consideration of the theory of conduction in metals, which, though not strictly a part of our subject, throws light on it. It is now supposed that the atoms of metals are to a certain extent dissociated into free electrons and the residua, i.e. the positively charged metal ion. The metal ions are probably, like the atoms, more or less fixed in position ; but the electrons, on account of their small size, are capable of threading their way amongst the atoms of the metal : hence, when an electric field is applied to the metal, electrons will be driven in the direction opposite to the nominal direction of the field, and a current will be set up.
Thus, even metallic conduction is now regarded as convective; and in particular, current in a metal is mainly, if not exclusively, “anionic."
There exists, therefore, normally, in a metal dissociation of
the kind expressed in the above equation for zinc. Suppose, now, the zinc to be placed in an electrolyte, add a circuit to be completed. It is found that zinc ions have a strong tendency to come out of the metal into the liquid, a process which has been compared with ordinary solution of a solid. If this takes place, there will be left an excess of electrons in the metal, i.e. the zinc electrode will be negatively charged. Thus the reaction involves an electric current round the circuit ; between the electrode and electrolyte it is carried by zinc ions dissolving, while along the metallic conductor it is carried by electrons, diffusing from the place where they are in excess to the other parts of the conductor. The current is therefore exclusively anionic in the metal, exclusively cationic in passing from metal to liquid, partly one, partly the other, in the mass of the liquid.
In case (ii.) at an anode it is to be supposed that each hydroxyl ion, on reaching the electrode, gives up its superfluous electron, which is carried by the action of the electric field into the metal. The atoms of oxygen and hydrogen are not capable of following it, since platinum (or nickel) only absorbs gases to a minute extent. They are therefore left at the boundary of the liquid, where they rearrange themselves in the most stable form, viz. by forming liquid water and gaseous oxygen. The current in passing between the metal and solution is in this case exclusively anionic.
Similarly, when a ferrous salt is oxidised at the anode, this means that the Fe" ion gives up one more electron to the electrode. The reaction ought, therefore, to be written
Fe* = Fe*"+ ☺ We shall in future commonly use the negative symbol by preference, as being, so far as known, in closer harmony with the facts.
Nothing has been said, so far, as to the causes that regulate which out of several possible reactions will actually take place at an electrode. This subject can only be completely discussed with the aid of the conception of potential, which will be introduced later ; but we may go over it here in a
qualitative manner. As guiding principles it may be said that (i) of two possible reactions at an electrode, that which is the more easily effected will occur; (ii) each kind of ion is the more easily discharged, the more of it there is in solution. For elucidation of these rules, certain cases will be considered in detail.
(A) Eletrolysis of Copper Sulphate between Copper Plates (copper voltameter). The ions in solution are Cu" H (from the water), so," OH' (from the water). At the cathode the possible reactions are—
(a) Discharge of Cu"
(c) Reduction of Cu" to Cu Formation of negative ions is impossible, since there is no material in the electrode to make them out of. Of the three reactions (a) is the easiest, and is that which occurs, for the most part. It is found, however, that under normal conditions (c) also takes place to a small extent, so that the solution comes to contain a little cuprous sulphate. The formation of it does not go very far, because, if much cuprous ions were present, they, too, would be discharged.
Cu += Cu Hence the two processes—reduction of Cu" and discharge of Cu—amount to a discharge of Cu" in two stages. The effect at first is that the deposition of copper is less than that calculated from Faraday's law, but after a time a state of equilibrium is set up, with a small quantity of cupro-ions present, and then the behaviour of the voltameter is normal. As to reaction (6) discharge of hydrogen is, ceteris paribus, more difficult than of copper ; and, further, the conditions are not equal, for while in a strong solution of CuSO, there is abundance of copper ions, there are very few of hydrogen, so that evolution of hydrogen is rendered still more difficult. Nevertheless, if too strong a current is used, some hydrogen will be evolved, and the explanation of this is an important illustration of theory. Deposition of copper on the cathode naturally tends to deplete the solution in the immediate neighbourhood of the cathode, of
copper ions. The supply is made up by diffusion from other parts of the liquid. If the current is strong (in proportion to the area of the cathode) it may very well happen that the cathode layer of liquid falls very far below the rest of the electrolytes in its content of copper ions, so much so, that on account of their scarcity it will no longer be easier to deposit them than hydrogen, so that some hydrogen will come down as well. Evolution of hydrogen is therefore favoured by a high current density, and opposed by anything that improves the interchanges between different parts of the liquid, such as stirring. The most effective kind of stirring is moving the cathode itself, since it is the layer in immediate contact with it that needs replenishing. Accordingly it has been found that whilst o'o2 amp. per sq. cm. is about the limit that can be safely used with a stationary cathode, oʻ2 amp. per sq. cm, can be successfully applied to produce copper tubes on a rapidly rotating mandril.
Thus both the irregularities of the copper voltameter mentioned on p. Io are explained satisfactorily by the theory.
At the anode in the copper voltameter the possible processes are
(a) Formation of copper ions (cations) from the electrode.
It is found that the facility of the three processes is in the order stated. Under ordinary circumstances, a (solution of copper) is the main reaction. No free oxygen is formed, but (6) must occur to a small extent, as the copper plates turn black from formation of copper oxide. An exact investigation of the reaction does not appear to have been made.
(B) Electrolysis of Copper Sulphate between Platinum Plates.—The phenomena at the cathode are the same as in the preceding case. At the anode reaction (a) is no longer possible. Instead of it we have
(a') Formation of platinum ions.
This is far more difficult, more so than processes (6) and (c); consequently the platinum plates remain untouched (at
least, under ordinary conditions), and discharge of anions takes place. Accordingly, as OH' or so," is discharged we should have
2OH' = H,0 + 0 + 2 © SO," + H2O = H2SO4 + 0 + 2
0 e so that in either case gaseous oxygen would be liberated.
The exact course which this second reaction takes has been the subject of much discussion. We shall return to it when considering the relation of electrode potentials to the discharge ; for our immediate purposes it is immaterial which of the two we suppose to be the actual reaction.
(C) Electrolysis of Sulphuric Acid between Platinum Plates (Hofmann voltameter).
At the cathode the only possible reaction is discharge of hydrogen ions: the process is therefore quantitative, and excellently adapted for voltametric purposes. The only exception that can be taken to it is this : hydrogen and oxygen are both slightly soluble in water; they will therefore dissolve at the electrodes, and diffuse across the liquid ; but the presence of hydrogen at the anode, or oxygen at the cathode, will give opportunity for recombination with the nascent gases there, in which case a trifle of the current will be wasted in reforming water. It is probable that this happens, but only to a minute extent, on account of the slight solubility of the gases. It may be minimised by keeping the anode and cathode well apart; and even when this is not done the effect seems to be of small consequence, as it does not appreciably affect the exactness of Oettel's form of voltameter, in which the gases are evolved close side by side.
At the anode the circumstances are practically the same as in the electrolysis of copper sulphate, but they have been more closely studied.
The anions available for discharge are those of water and of sulphuric acid. So far we have tacitly assumed that the latter are SO," only; but there is good reason to believe that
1 Very possibly H,O, may be formed in the first instance ; but that is immaterial.