1272 MELLOR AND RUSSELL: THE PREPARATION OF PURE

CXXIX.-T%e Preparation of Puw Chlorine and i ts Behaviour to wards Hydrogen.

By J. W. MELLOE and EDWARD JOHN RUSSELL. IT is not yet known who first discovered the curious suspension of chemical action often noticed between carefully dried substances which, in ordinary circumstances, react with one another. So far back as 1802, Mrs. Fulhame * pointed out that gold salts are not reduced either

* Quoted in Thomson, A System of Chenzislry, vol. ii, p. 454, 1802.

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CHLORINE AND ITS BEEAVIOUR TOWARDS HYDROGEN. 1273

by hydrogen or by (‘ phosphorated ether ” unless first moistened with water, and she suggested a theory to account for the influence of water in chemical reactions. Higgins,” in 1814, observed that “ dry muri- atic acid has no action on dry calcareous earth, while these sub- stances readily unite if moisture is present.” I n 1837, BonsdorfE (Ann. Phys, Chern., 1837, 41, 293; 42, 325) demonstrated that atmo- spheric air freed from carbon dioxide and moisture does not tarnish clean surfaces of metallic potassium, arsenic, +f bismuth, lead (com- mercial or pure), zinc, cadmium, iron, or copper ; in 1838, Regnsult (Ann. Chim. Phys., 1838, [ii], 60, 176) failed to induce dry olefiant gas to combine with chlorine in diffused daylight ; E. A. Parnell (B.A. Reports, 1841, 51) drew attention t o the important part pIayed by water in chemical reactions and showed that whilst moist hydrogen sulphide acts vigorously on papers impregnated with salts of lead, mercury, and copper, there is no action i f the hydrogen sulphide is well dried; and Andrews, in 1842, stated in a foot-note to one of his papers (Trans. Roy. Irish. Acad., 1842, 19, 398; or S c i e n t 9 c Memoirs, 1889, p. 90) that although moist chlorine combines energetic- ally with zinc, copper, and iron-filings, perfectly dry chlorine ‘‘ has no action whatever a t ordinary temperatures . . . . the same remarks may be applied to the behaviour of dry bromine in contact with dry metals.” Kolb (Compt. rend., 1867, 64, 861; see also Debray, ibid., 1848, 26, 603) showed, in 1867, that dried oxides and hydroxides of calcium, barium, magnesium, sodium, and potassium do not increase in weight in an atmosphere of dry carbon dioxide. I n 1869, came Wanklyn’s observations (Chern. News, 1869, 20, 271) that sodium and chlorine do not unite; this is sometimes said to be the earliest record of the influence of small quantities of water in promoting chemical action, although, as Dixon (Trans., 1896, 69, 775) has pointed out, Wanklyn does not state whether moisture has any effect or not on the combination. Dubrunfaut (Compt. rend., 1871, 73, 1395) was under the impression that water assists the combustion of carbon, but his experiments were far from being conclusive, and he did not reply to Dumas’ statement (Compt. rend., 1872, 74, 13) that pure graphite burns completely in oxygen dried by sulphuric acid. I n 1880, Dison (B.A. Reports, 1880, 593) opened up the way for systematic research on this subject. Baker (Trans., 1894, 65, 611) has compiled a list of papers published between that date and 1894.

W e have made some experiments with the object of finding whether pure chlorine will combine with dry hydrogen. Armstrong, in his

* Higgins’ Experimem% and Observations on the Atomic Theory, 1814 ; we are in- debted to the kindness and courtesy of Dlr. V. H. Veley for these two references.

t Bonsdorff states that Bergmann had previously demonstrated that dry air had no action on metallic arsenic.

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1274 MELLOR AND RUSSELL: THE PREPARATION Ol? PURE

Presidential address to the Eiitish Association at Aberdeen in 1885, expressed the belief that there would be no action between the pure gases. Dixon and Harker (Maizchester Memoirs, 1889, [iv], 3, 118 ; 1890, [iv], 4, 1) found that the more carefully the mixed gases were dried, the greater the intensity of light required to cause detonation, but an electric spark always caused an explosim a t once, no matter how carefully tho gases were dried, and measurements of the rate of pro- pagation of the detonation wave showed that this rate was rather greater in dry gases thanin moist.* I n other words, rnoisture retards rather than accelerates reaction under these conditions. Pringsheim (Ann. PAYS. Chem., lSS7, [iii], 32, 422) published an investigation on the union of hydrogen and chlorine, and showed that when heated the dried gases exploded a little later than the moist gases, but just as violently. H e also found that the dry gases explode in sunlight with a feeble clicking sound. I n 1894, Baker confirmed Dixon and Harker’s observation on the effect of light, and prepared a mixture which did not explode on exposure to sunlight, and of which only 75 per cent. combined in fnur days.

I n most of the investigations mentioned above, the mixture of gases was obtained by electrolysis of hydrochloric acid in aqueous solution, but oneof us has shown (Mellor, Trans., 1901, 79, 225) that traces of oxygen are always present in the electrolytic gases, and, although it might be possible to remove this by subsequent treatment, we considered it safer to adopt a method for preparing the gases which would preclude the possibility of oxygen or a chlorine oxide being present.

Preparc6 tiom of Pure Chlorine.

Shenstone has shown (Trans., 1897, 71, 471) that the most trustworthy method by which pure chlorine can be obtained is the electrolysis of fused silver chloride. I n our first experiments, we closely followed Shenstone’s instructions, and in fact he kindly supplied us with a number of carbon electrodes ; these mere made from carbon filaments used for 250 c.p. incandescent lamps flashed on to stout platinum wires. Although we obtained a small quantity of pure chlorine in this way, we experienced two difficulties. I n the first place, we were unable to obtain a supply of electrodes which worked as well as those Mr. Shenstone kindly gave us; those sent by the same makers were very apt to disintegrate under the conditions of our experiments and were altogether untrustworthy in protracted operations at high temperatures. In the second place, w0 found it very difficult to prevent contact between the silver chloride and the platinum wires.

* The mean rate of explosion in dried mixtures of hydrogen and chlorine was 1795 metres per see. In moist mixtures, 1770 metres per sec.

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CRLORINE AND ITS BERAVIOUR TOWARDS HYDROGEN. 1275

Even a small quantity of silver chloride on the wire at-tacks it to such an extent as to ruin the experiment.

W e therefore adopted the following modification and find it to be a fairly easy and practical method for preparing large quantities of pure chlorine.

The Silver Chloride.-About 600 grams of purified silver chloride were fused in a basin and kept in that state for four or five hours to drive off all the water. After cooling, the horny mass was cut up into small pieces and placed in the electrolytic vessel.

The electyolytic vessel was a V-shaped tube of the hardest Jena glass, 3 cm. in diameter. One arm is shown in section in Fig. 2 (p. 1276) ; a side tube was sealed into each limb.

The electrodes consisted of stout carbon rods, 30 em. long and 5 mm. in diameter, specially prepared by Conradty of Nuremberg at a high temperature for the electrolysis of fused salts. The method by which

FIQ. 1.

Apparatus for th,c preparation, &e., of pzcre chlorine.

these were fixed into the electrolytic vessel will be readily understood on reference to Fig. 2. A piece of hard Jena glass tubing about 100 cm. long and of just suEcient diameter to allow the carbon rods t o pass through was sealed on to a short piece of tubing of about 1 cm. diameter ; a constriction mas then made in each limb of the V-tube, leaving just sufficient space for the narrower part of the tubes just mentioned to pass freely in and out.

The carbon rods were then passed through the inner tube, thick plaster of Paris was poured in and over tha t sodium silicate which was drawn in to the plaster of Paris by pumping out air from the other side. After a little time, mercury was also added to surround the projecting part of the electrode and allow electrical contact to be made.

I n Fig. 2, E represents the carbon rod, D the glass tube surround- ing it, C the plaster of Paris plug, B the layer of mercury into which

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1216 MELLOR AND RUSSELL: THE PREPARATION b~ PURE

the rod E projects, and ,4 a lnper of sodium silicate. When D is carefully chosen and the constriction sufficiently well made, the surface of plaster exposed to the interior of the vessel is extremely small.

This joint is perfectlyair-tight and is capable of retaining a vacuum for weeks. It has to be protected by asbestos paper from heat radi- ated from the flame or it tends to crack, In order to connect the hard Jena glass of the electrolytic vessel with the soft glass of the rest of the apparatus, we made similar joints at b (Fig. I, p. 1275), one of which is also shown in section in Fig. 2.

Section of one limb of electrolytic vessel.

The electrolysis being started in a vacuum, it is obviously necessary to connect each limb of the electrolytic vessel with the apparatus in order to keep the level of the molten silver chloride the same in both. The V-tube was then placed in a copper bath, packed with asbestos, and heated by means of two blow-pipes until the silver chloride had fused. An automatic Sprengel pump, working continu- ously for two days, exhausted the apparatus and removed the greater part of the moisture given off at this high temperature by the plaster of Paris and any gases that might be occluded in the electrodes.

A current of 2.8 amperes (10 volts) was now passed, and chlorine

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CHLORINE AND ITS BEHAVIOUR TOWARDS HYDROGEN. 1217

was liberated in a fairly steady stream; the current mas sent some- times in one direction and sometimes in the other to sweep out com- pletely any trace of air or moisture from both limbs and electrodes. This was continued at intervals for two or three days; during the whole time the temperature was kept as high as was consistent with the safety of the vessel, and the pump remained working.

A metre column of fragments of solid potash (e, Fig. 1) not being sufficient to protect the pump from chlorine, we devised the mercury wash-bottle shown a t C . By adjusting the height of the reservoir d, the gas can be made either to pass over the surface or bubble through a column of mercury as may be required. A fairly clean surface can always be obtained. A jar of mercury was put round the junction of the rubber and the glass to prevent air leaking in. The mercury washer and the potash column effectually prevent any chlorine from reaching the pump.

The silver tree has been a fertile source of trouble in preparing pure chlorine by the electrolysis of silver chloride. Our electrodes being very stout, we were able, without injuring the apparatus, to adopt drastic measures, which proved entirely successful in dealing with this trouble. When the ammeter showed that some irregularities were beginning:in the electrolytic cell, we raised the temperature almost to the softening point of the glass and cut out all the external resistance so as to simultaneously increase the current. The increased current at the elevated temperature melted or shattered the silver tree, and electrolysis was discontinued for a short time until the former temperature was restored. When electrolysis started again, it was found to proceed quite normally.

When it was thought tha t all foreign gases had been removed, the pump was shut off and the apparatus allowed to fill with chlorine. One of the test-tubes (Fig. 1) was sealed off and its point broken under mercury. Perfect absorption took place ; we could detect no trace of residual gas. Nevertheless, we exhausted the apparatus and filled again, repeating the operation four times in all. Each time, samples were taken and absorption of chlorine by the mercury was invariably complete.

The pressure of chlorine in the apparatus during the last filling was determined by one of us momentarily opening the mercury gauge f, which had previously been exhausted with the rest of the apparatus but shut off during the evolution of chlorine, while the other read the depression produced. As the mercury was covered with a thin film of glycerine, a reading, probably correct to 1 or 2 mm., was obtained before appreciable absorption began.

The Preparation of Pure Hydrogen.-Here we followed Scott’s directions (Phil. T!*anns., 1893, 184, 548) in which hydrogen was

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1278 MELLOR AND RUSSELL: THE PREPARATION OF PURE

prepared i n the first instance by the action of steam on sodium, and subsequently absorbed by palladium to allow gaseous impurities to be removed by the pump. The apparatus (Fig. 3) having been exhausted, the water in A was gently heated, the steam in contact with the sodium in B generated hydrogen which passed through the phosphoric oxide contained in C and was absorbed by the palladium foil * in the tube D. The experimental vessels were, of course, shut off. D was surrounded by a bath of paraffin, which, during the preliminary exhaustion, was kept at a temperature of 1SO'. While hydrogen was being generated, the temperature was allowed to fall to tha t of the room ; the absorption was rapid bet ween 120' and 80'. When D was cold, the flasks A and B were shut off and the pump kept working to exhaust the tube D and the experimental vessels. After exhaustion, D was heated until sufficient hydrogen was obtained to fi11 the apparatus to the same pressure as the chloriue.

FIG. 3.

Apparatus for the preparcbtion of pure hydroyea.

The Experimental Vessels.-We have adopted an old method of Professor Dixon's for mixing the gases by the use of the double-bulb condensers employed with Soxhlet extractors. Out of a large number, those were selected which had approximately equal capacities for the inner and outer bulbs. After sealing platinum wires to the tube leading to the outer bulb in each condenser, they were connected in the manner shown in Fig. 1 and carefully cleaned by boiling with concentrated nitric acid, well washed in distilled water, and dried at a high tempera- ture in a current of dry, dust-free air.

A quantity of phosphoric oxide, carefully purified by slow distillation in a current of oxygen over red-hot, spongy platinum, was introduced into each of the inner and outer bulbs. A small piece of glass rod with sharp edges was also placed in each inner bulb. The inner bulbs

* We wish to thank Messrs. Johnson and Matthey for kindly lending us the palladium used in this research.

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CHLORINE AND ITS BEHAVIOUR TOWARDS HYDROGEN. 1279

being all in communication with each other were filled with chlorine, the outer ones with hydrogen. The glass springs shown in the figure were used i n order t o prevent any danger of snapping through too great a rigidity.

After the vessels had been filled with the gases, they mere sealed off and left to dry in the dark for nine months. Immediately before an experiment, the bulbs were shaken in a dark place so as to break the inner one, and a sufficient length of time allowed to elapse to ensure complete diffusion.

Behaviour of the Mixture of Chloride and liydrogen.

(1 ) Action of the Electric Xpark.-A small spark a t once caused a violent explosion, shattering the whole apparatus. The bulbs had been surrounded by a stout bell-jar so that the gases could be drawn over heated copper oxide to estimate the amount of hydrogen left over, but so little was found tha t we mere forced to the conclusion that combination had been complete,

(2) Action of Heat.-We found tha t mixtures of moist hydrogen and chlorine in similar bulbs explode in the neighbourhood of 260". One of the bulbs was therefore heated rather above that temperature (270') for some minutes, but no explosion took place. The bulb was allowed to cool and the gases pumped out and analysed. Very little combination could be detected, practically the whole of the hydrogen and chlorine were recovered in the free state.

Another bulb was now heated for ten minutes to a higher tempera- ture, namely, 450O. Still no explosion occurred. Analysis showed that combination had taken place, but not completely; about 80 per cent. of the mixture had combined. On examining the bulb, it was evident that the phosphoric oxide had fused, and some had volatilised. It is unfortunately necessary to scatter the phosphoric oxide over the whole of the glass in order to remove moisture a s quickly as possible, chlorine and water-vapour being likely to react with one another. We were not able to heat the gases without a t the same time heating the phosphoric oxide.

(3) Action of Sunlight.-On exposure to bright sunshine a t Wye, in June, no explosion took place. The bright sunshine lasted for three days. At the end of that time the bulb was opened and the gases analysed. About 30 per cent. of the hydrogen and chlorine had combined.

Our experiments, therefore, lead t o the conclusion that an electric spark causes an explosion in the dried, just as readily as in the moist, gases. Combination is complete, showing that the action is propagated through the whole mass of the gas. Neither heat nor sunshine

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1280 MELLOR: THE UNION OF HYDROGEN AND CHLORINE.

causes explosion in the dried gases; combination, when it occurs at all, is very slow and incomplete, and not transmitted through the whole of the gas as in the previous case. Why there should be this difference, and whether the slow combination which we find is a direct action or due to a surface action, we are not at present in a position t o say.

Our best thanks are due to Professor Dixon for much kindly help and encouragement in this research, and to Mr. W. A. Shenstone, F.R.S., for supplying us with some of his electrodes and for giving us some valuable suggestions as to the preparation of the chlorine.

THE OWENS COLLEGE, MANCHESTER, AND

SOUTH EASTERN AGRICULTURAL COLLEUE, WYE.

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