Isolation of Argon
|Argon is always obtained from atmospheric nitrogen. By the removal of oxygen and nitrogen from air freed from carbon dioxide, etc., we obtain "atmospheric argon," in which the only impurities are small traces of the other inert gases, helium, neon, krypton, and xenon, amounting in all to about 0.25 per cent, by volume. |
The chief methods used for "atmospheric argon " are as follows: -
Cavendish's original experiment has been Fig. - Rayleigh and Ramsay's developed and modified by Rayleigh and Ramsay, modification of Cavendish's whose apparatus is represented diagrammatically apparatus, in fig.
|Rayleigh and Ramsay's modification of Cavendish apparatus. |
The glass flask A, having a capacity of about 50 litres, is supported mouth downwards in a suitable stand. It is closed by a rubber bung through which pass five tubes. Two of these carry heavy copper leads which terminate above in the electrodes B, C, constructed of heavy platinum rod. The bent tube D delivers soda solution against the top of the flask so that it runs down over the whole interior surface of the flask and away through the exit pipe E: the liquid is kept in circulation by a small iron centrifugal pump, and at one point passes through a worm cooled externally with water. The mixture of 11 volumes of oxygen and 9 volumes of air is supplied through the tube F.
In order to maintain between the electrodes the potential difference required to give a steady flame, it is necessary to use a transformer or induction coil which will give a P.D. of 6000 to 8000 volts on open secondary circuit. This falls to about 2000 in working, but a transformer giving only this voltage, or even 4000-5000 volts, on open secondary circuit will give unsteady conditions in use and probably lead to overheating and fusion of the electrodes. It is, however, necessary to the efficiency of the process that the electrodes should be kept red hot.
The primary current is regulated by means of a choking-coil with a movable iron armature, and is about 40 amps, at 30 volts. The rate of consumption of energy is about 800 watts, i.e. 1 h.p. roughly; and the rate of absorption of gas is about 20 litres per hour (25 c.c. per watt-hour), which may, by paying careful attention to the composition of the gases, be maintained until the amount,of argon in the globe is sufficient to dilute the gases considerably.
Contrary to expectation, increase of pressure does not materially increase the rate of combination; but it has been found that with approximately the same size of flame the absorption increases rapidly with increase in the size of the globe.
This process can be pushed to completion and a mixture of argon and oxygen obtained, from which the latter gas is removed by alkaline pyrogallol, cuprammonium solution, or red-hot copper; but it is found better to use it only for the preliminary concentration of argon and to complete the removal of nitrogen by one of the other methods.
- When nitrogen is passed over red-hot magnesium it is absorbed with formation of magnesium nitride, Mg3N2, and it was by systematic repetition of this process that Ramsay first obtained argon from atmospheric "nitrogen." It has the disadvantage, however, of being extremely slow, and it was early suggested that other metals which more readily form nitrides should be substituted for the magnesium; chief of these were lithium, obtained by electrolysis of the chloride; and barium, prepared by the action of sodium on barium fluoride or BaNa2F4.
A very rapid method and the best available for some years is also a modification of the original magnesium method, due to Maquenne. The absorbent is a mixture of 5 parts of lime, which should be free from carbonate and hydroxide, and 3 parts of magnesium powder; and the addition of a small proportion of metallic sodium is stated to increase the rate of absorption.
The mixture is heated to bright redness in a glass tube, any hydrogen or carbon monoxide evolved being pumped off. Atmospheric "nitrogen" is then admitted, and the absorption of the nitrogen is evidenced by a glow which passes along the tube as the mixture becomes converted into calcium nitride. The gas issuing from the tube is freed from traces of hydrogen and carbon monoxide by passage over red-hot copper oxide and solid caustic potash, and is finally dried by means of phosphorus pentoxide.
For the removal of the last traces of nitrogen, red-hot crystallised calcium has been used; and the same end is attained by maintaining an electric arc between calcium electrodes in the gas for twenty-four hours.
- A method for preparing argon which appears to be cheap and effective is that due to Fischer and Ringe. A mixture of 90 per cent, of calcium carbide and 10 per cent, of calcium chloride, heated to redness in an iron retort, having the open end water-jacketed and fitted with a water-cooled stopper, is used to absorb the oxygen and nitrogen from air.
In an experiment the retort is first warmed and the contained gases are pumped out: the temperature is then raised to 800°, air is admitted, and when the oxygen and nitrogen have been absorbed, more air is admitted from time to time to compensate for this absorption. When the whole apparatus is full of impure argon the gas is circulated through the system until no further absorption is indicated by the manometer.
The chemical changes which take place are as follows: - nitrogen acts upon calcium carbide to form calcium cyanamide and carbon, the carbon unites with oxygen to form carbon dioxide (which is absorbed by the calcium carbide with formation of calcium carbonate and carbon), and some carbon monoxide which in turn is oxidised by the copper oxide to carbon dioxide and absorbed by the potash.
Eleven litres of fairly pure argon were thus obtained in two days by the use of 7 kilos of calcium carbide.
- A good method for the laboratory preparation of small amounts of argon depends on the fact that commercial compressed oxygen, prepared by fractional distillation of liquid air, contains over 3 per cent, of argon and but little nitrogen. If the oxygen be removed by means of red-hot iron or copper and the residual gas freed from nitrogen by one of the above methods, a fairly pure sample of argon is readily obtained.
The argon obtained by any of the above methods always contains a small proportion of the other inert gases (0.25 per cent., of which the greater part is neon) which cannot, of course, be removed by chemical methods. The isolation of pure argon from the mixture may be carried out by one of the following methods: -
- Fractional diffusion.
- Fractional liquefaction of the gas or fractional distillation of the liquid.
- Fractional crystallisation of the liquid.
- Fractional absorption in cooled charcoal.
Of these (1) is too slow and troublesome to be of much practical use, and (3) presents great experimental difficulties, and has only been carried out for the accurate determination of the atomic weight (q.v.). Both the other methods, (2) and (4), are good, but (2) is perhaps the simpler: it is quite easy to separate argon from helium and neon, on the one hand, and from xenon and krypton, on the other, by fractional distillation at the temperature of liquid oxygen.