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when a divalent substance dissociates, it usually does so in two stages; thus, sulphuric acid would break up according to the equations
H,SO, ŽH + HSO,
HSO) ZH + SO." The presence of these intermediate ions HSO, offers the simplest explanation of the phenomena actually found, of which the most noteworthy is formation of persulphuric acid. We may suppose that discharge is partly by the intermediate ions, and that these, when neutral, unite to form the new acid
2 HSO. = 20 + H.S.O. Such a view of the reaction is strengthened by considering the conditions that favour formation of persulphates. It is to be expected that fairly strong sulphuric acid should contain a large proportion of HSO, since dissociation is always favoured by increase of volume, and it is only on proceeding to considerable dilution that there is room for both stages of this process to be carried out. Accordingly, if 50 to 60 per cent. acid be electrolysed, only a small amount of oxygen is produced, and a considerable proportion of persulphuric acid remains in the liquid. As that acid breaks up easily on rise of temperature, the most favourable yield is at low temperatures; at – 2° 64 per cent. has been obtained. Again, acid potassium sulphate might be expected to dissociate in the two stages
KHSO, = K + HSO"
HSOZ = H + SO." Accordingly, on electrolysing a solution (preferably saturated) of KHSO., potassium persulphate is obtained in considerable quantities.
In acid as weak as that used in accumulators, or voltameters (usually about 1'2 density), only a small amount of persulphuric acid is formed ; the readings of a voltameter may, however, be rendered slightly inaccurate from this cause.
(D) Electrolysis of Caustic Soda between Nickel Plates (Oettel voltameter).
· F. M. Perkin, Electrochemist., 1. 189 (1901).
The cations present are Na'and H:. Of these, hydrogen is much the more easily discharged; and even if sodium be discharged primarily, it reacts with water and forms more hydrogen. Thus the reaction at the cathode is exclusively formation of gaseous hydrogen.
The only anion present is hydroxyl. Of this there is a large quantity, since it is formed not only from water, which is very slightly dissociated, but from soda, which is largely so. We have, therefore, as alternatives
(a) Formation of nickel ions.
Now, if an experiment were made using nickel as anode in dilute acid, it would be found that the metal dissolved, for it is much more ready to form ions than platinum, and the conditions are unfavourable to discharge of hydroxyl ions, as they are very scarce. But in a strong alkali the circumstances are different; there are plenty of hydroxyl ions, consequently they are discharged, and the nickel remains untouched. The reaction is then, as in the acid voltameter
20H' = H,0 +0+20 the oxygen produced being strictly proportional to the quantity of electricity. If, from any cause, the voltameter, after use, should show the green colour of nickel ions, this is an indication that the reaction is not proceeding as it should.
(E) Electrolysis of Hydrochloric Acid between Platinum Plates.—The only cation present is hydrogen, consequently the discharge at the cathode consists exclusively of that substance. Of anions, there are chlorine from the acid and hydroxyl from the water; either of these may be discharged, and as a rule appreciable amounts of each are produced. There is also, at the anode, the third possibility of solution of platinum, and this very commonly occurs to a small extent. The most interesting question is as to the relative proportions of chlorine and oxygen in the anode gas; it depends essentially on the relative proportions of chlorions and hydroxyl ions in the liquid. The former, being derived exclusively from the acid, are roughly proportional in number to the strength of the acid ; the hydroxyl
ions, on the other hand, are very few in number in any acid solution ; indeed, it is found that the number of hydrogen and hydroxyl ions vary inversely to each other, so that in a strong acid (i.e. strong solution of hydrogen ions) the amount of hydroxyl is much smaller than in pure water, even ; and conversely, in a strong alkali the amount of hydrogen ions is proportionately reduced. Hence with increase in the strength of acid, not only are the chlorions increased, but the hydroxyl ion unt. Aved, and so in both ways production of chlorine favfall from the expense of oxygen. Haber and Grindberg giuches a limiting numbers :
reckoned on the oxygen proHCI, 95%
duced in a voltameter. CurHCI, 30%
rent density o'02 ampere HCI, 40%
per square centimetre. There are several secondary effects in this case. The loss due to diffusion and recombination, referred to on p. 28, in the case of oxygen and hydrogen, is much exaggerated here on account of the large solubility of chlorine, especially in weak acids. If the gases are produced side by side, à large loss of chlorine will occur.
It can be minimised by placing a porous diaphragm between the anode and cathode.
Chlorine and hydroxyl produced together at the anode combine to produce several reactions that are not possible when either is produced separately. Thus
Cl + 5H' = HCIO + 2 อ This hypochlorous acid is decomposed by the hydrochloric present, but may be demonstrated by passing a rapid current of CO, through the liquid.
Cl + 5OH HCIO; + 2H,0 +6 0 From 1 to 30 per cent. of chloric acid has been found. Since it requires five hydroxyl ions to one chlorine, it is naturally favoured by dilution of the acid.
| This is an instance of the “law of mass action.” See further below, p. 174. See also Donnan, Thermodynamics, in this series.
? Zeitschr, anorg. Chem., 16. 198.
Again, the ion of chloric acid CIO: combines with another hydroxyl to form perchloric acid.
CIO; + OH' = HCIO, + 20 As this reaction depends on the previous formation of chloric acid, it occurs mainly when the electrolysis has been proceeding some time.
(F) Electrolysis of Potassium Chloride betwnno D . Electrodes.—The process at the anode in ' n preceding case. There are two catio that at the cathode discharge m these. Hydrogen is the more r
anlal O., anu potassions are discharged, the mo w ed will react with water, generating hydrogen : hence, regards the substance liberated from the electrode, it is immaterial which ion actually carries the current. The secondary reaction of the potassium on the water, however, produces hydroxyl ions
K + H,0 = K + OH' + H and the liquid round the cathode therefore turns alkaline. Further details will be considered later in dealing with the practical application of electrolysis for manufacture of caustic potash.
$ 4. MIGRATION OF IONS. In the interior of an electrolyte anions and cations are always present in equal measures; they therefore both participate in carrying the current, in the way described in § 2. According to the formula there given the cationic current density is 96600 ynuk, the anionic 96600 yqua; n, the concentration, refers to the dissolved salt as a whole, and y, the degree of dissociation, is, of course, the same for both components of the dissociating substance, so the two parts of the current are in proportion to the velocities of the cations and anions respectively.
With regard to the velocity of an ion, however, some preliminary remarks are necessary. If a charged particle be placed in an electric field, there is a definite force on it, just as
when a heavy body is placed near the earth there is a force (of gravity) on it tending to make it move vertically downwards. If the field is uniform, the force is (like gravity) constant. Such a force will not cause the particle to move with uniform speed, but, like a falling body, with uniformly accelerated speed. But an ion moving in an electrolyte is not so much to be compared with a heavy mass falling freely, as with a raindrop falling through the air and suffering considerable resistance on that account. A raindrop does not increase constantly in speed in its fall from the clouds, but, on account of air-friction soon reaches a limiting veloci£t with which it will continue to move however far it falls. Tha,, an ion moving through a liquid experiences a very la resistance, and cannot travel any appreciable distance wit constant acceleration; rather, owing to repeated collisions with molecules of solvent, its average speed is brought down to a fixed, and—as experiment showsvery moderate amount.
The limiting velocity that a raindrop reaches will depend on the intensity of the force of gravity that draws it down ; and in the same way the velocity of an ion depends on the intensity of the electric field (known also as the potential gradient) driving it. We shall adopt the practical unit, the volt, as measure of the difference of potential between two points, and therefore express the gradient of potential in volts per centimetre length.
The precise way in which the velocity of ions depends on potential gradient is shown by the experimental result known as Ohm's laie, which is that in any given conductor the electric current is proportional to the difference of potential driving it. This, of course, does not mean that other circumstances-temperature, concentration, etc.—may not affect the current, but merely that under similar conditions the potential difference affects it in the manner stated.
Ohm's law may, of course, be stated in the form that the current density is proportional to the potential gradient (this is equivalent to considering its application to a conductor i cm. long and of 1 sq. cm. cross-section).
Now, as the number of ions in a solution and the charges T. P. C.