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for instance, in the electrolytic cell of Fig. I by the appearance of 8 grms. of oxygen at the anode and 1'0075 grms. of hydrogen at the cathode. But it often happens that more than one reaction occur simultaneously : in this case the laws must be taken as meaning that the total amount of material deposited at the anode makes up one gram-equivalent, and the same at the cathode. Thus, if current were passed through a solution containing copper and nickel under such conditions that they come down together, one faraday might deposit (on the cathode) x equivalents of copper and y of nickel; but it would be found that x + y = 1.
Substances deposited on the anode are called anions, those deposited on the cathode cations, the term ion being given by Faraday to the particles that he assumed to travel through the electrolyte. Accordingly, a cation travels in the nominal direction of the electric current (Fig. 1), while an anion travels against it. Cations include all the metals and hydrogen; anions, chlorine, bromine, iodine, fluorine, groups such as NO3, SO4, and acid radicles generally (as well as OH).
It will be seen from the above classification that electrolytes are very commonly salts. Solutions of acids, bases, and salts in water are, indeed, the electrolytes most commonly dealt with ; but similar solutions in pyridine, liquid ammonia, and various other solvents are also electrolytes; so are fused salts, and even a certain number of solids, to a slight degree; while electrolytic decomposition of gases has also been observed.
When a process of electrolysis does occur in an unambiguous way, it may be used to determine the quantity of electricity flowing. A cell arranged for this purpose is known as a voltameter. The chief forms of voltameter are the following :
1. The Water Voltameter.-In nearly all cases it is the volume of gas evolved that is measured. The oxygen and hydrogen may be collected separately, but that is unnecessary for voltametric purposes, and the instrument takes a simpler form when they are allowed to mix. As electrolyte, dilute sulphuric acid has been much used, but caustic soda is better." A very practical form of the apparatus is that of Oettel, shown in Fig. 2. It consists of a glass jar, some 15 cm. high by 5 in diameter, containing two cylindrical nickel electrodes. The leads to these are passed airtight through an indiarubber stopper, which also carries the gas delivery tube; the latter, conveniently with a rubber joint in it, is bent down to deliver into a gas-measuring tube standing over water. The solution used is 15 per cent. NaOH free from chlorine, and should nearly fill the jar.
The reaction at both anode and cathode is quantitatively exact, so that one faraday evolves 8 grms. of O and
Fig. 2. 1'0075 of H. In order to calculate the weight of gas from the volume, corrections must be made for
(i.) Barometric pressure b (in millimetres).
(iii.) Difference in pressure between the gas and the external air, due to the column of water (height h) left in the gas tube at the time of measuring. This is equivalent to a mercury
column of height 12:6.
(iv.) The aqueous vapour with which the gas is saturated. The pressure of this p can be found from table, p. 255.
The actual pressure in the gas tube is b
13:6; that of the
dry gas contained b - - P; and one-third of this is oxygen. Hence, according to the laws of gases, if v is the measured volume of the mixed gas, the volume of oxygen reduced to normal temperature and pressure is
760 273 tt The specific volume of oxygen is 700'3 c.c. per gram (at N.T.P.) and o'00008283 gm. corresponds to one coulomb, so that the quantity of electricity is
v(6 - 12:6-P)
— X 18373 coulombs
273 + ť The calculation may, of course, be made in a similar way by means of the hydrogen. At the average temperature and pressure of the laboratory
this comes to about 55 coulombs per cubic centimetre of the mixed gases. Hence a 100 c.c. gas tube does conveniently for an experiment in which 500 coulombs of electricity are used; and further, it is easy to reckon roughly the strength of current fiowing by observing how much gas is given off in 20 or 30 seconds.
Another common form of the water voltameter is Hofmann's apparatus for the decomposition of water (Fig. 3). In this the electrodes are of platinum, and are sealed through the glass ; carbon is not available on account of its power of absorbing gases largely. The gases are collected separately in the graduated tubes at the sides, and may be run off
from time to time by the taps at the top. Fig. 3. The volume of oxygen and hydrogen
collected may be compared as a test of the accuracy of the voltameter. The mode of reduction of the .
gas volume is similar to that with Oettel's form, but it must be remembered (i.) that as the liquid stands higher in the middle than the side tubes, the pressure in the latter will be greater than atmospheric; (ii.) the height h, representing this difference of level, is no longer of water—if the electrolyte be of density d the equivalent column of mercury is 106; (iii.) the saturation pressure of water vapour is less over a solution than over pure water : as a rough rule in the matter, it may be taken that over 15 per cent. NaOH it is 82 per cent., over 30 per cent. H,SO., 84 per cent. of the values given in table, p. 255.
Since the electrodes are of platinum, sulphuric acid may be used instead of soda. It will be found, however, that while the volume of hydrogen is exact, that of the oxygen is too low especially if the acid be strong, on account of formation of ozone and persulphuric acid.
The chief disadvantage of Hofmann's form is its considerable electrical resistance.
Both forms of voltameter work better after they have been running for a short time, as the electrolyte is then saturated with the gases.
For large currents the weight voltameter (Fig. 4) may be used. The mixed gases are passed through a small chamber full of strong sulphuric acid, to dry them, and allowed to escape. The apparatus is weighed before and after.
The water voltameter has been used as a meter for domestic electric supply. In this case, as readings are only taken at comparatively long intervals, the mea
Fig. 4. surement is by the volume of liquid electrolysed away. As a meter, the instrument has the serious
disadvantage of requiring nearly two volts to work it, and so absorbing an appreciable fraction of the electric energy it measures.
If a water voltameter be provided with a capillary tube, through which the gases have to escape, the pressure in the tube will be roughly proportional to the rate at which the gas is flowing, and may therefore be taken as a measure of the rate of flow of the electricity, i.e, of the current. Such an instrument, called an ampere-manometer,' has occasionally been used instead of an ordinary ampere-meter.
The electrolytic process is used for the preparation of oxygen and hydrogen commercially. The vessels holding the electrolytes are of iron, as also the electrodes; 15 per cent. soda solution is employed, and as the water is electrolyzed away it must be replaced from time to time, distilled water being used to prevent accumulation of chlorides. The gases are collected separately in domes, under a pressure of about 60 mm. of water-greater pressure causes a risk of mixing. In order to reduce the resistance of the cell, it is packed in a wooden box with sand, so that the heat developed by the current keeps the temperature up to about 70°C. The voltage required is then only about 2.8 volts for each cell. The cells are constructed to take 600 amperes, and yield 220 litres of hydrogen and IIo of oxygen per hour, the purity of the gas being about 97 per cent.
2. Silver Voltameter. — This is undoubtedly the most accurate of all. The weight of silver deposited from a solution by a measured current in a measured time has been determined several times, the most important determinations being those of F. and W. Kohlrausch 3 and Lord Rayleigh. The former found o‘0011183 gm. per coulomb, the latter o‘0011180. The greatest difficulty in such experiments is the measurement of the current in electromagnetic measure, i.e. in accordance with the definition of the ampere. Many subsequent experimenters
"Ostwald, Zeitschr. fhys. Chem., 35. 36 (1900).