ELECTRIC DISCHARGES IN LIQUIDS. P A R T I- THE ARC DISCHARGE IN WATER.

BY HERBERT DYSON CARTER AND ALAN NEWTON CAMPBELL.

Received 8th February, 1932.

Davy was the first to observe that the electric arc would continue to “burn ” when immersed in water, and Masson2 fifty years later made observations on the spectra of such arcs. The decomposition of water vapour by electric spark discharges was noted and briefly studied by P e r r ~ t , ~ whose work was expanded by Buff and Hofmann.4 At the same time the spectra of such discharges received attention from Liveing and D e ~ a r , ~ Konenj6 and others. Lepsius had employed carbon electrodes in an arc burning under water, but his work was re- peated and extended by Lob,s with considerably different results : both workers studied the gaseous reaction products to some extent qualitatively, and Lob concluded that the reaction was a purely “ pyro- genetic ” one, analogous to that taking place when hot wires are plunged into liquids. Organic liquids as well as water were employed, but the solid products received little attention.

At this time Bredig9 commenced his work on the production of colloids by arcing metals in liquids, and SvedberglO noted a difference in the products of direct and alternating current arcs, and modified the Poulsen rectifier by burning the arc in organic 1iquids.ll Chapman and Lidbury,12 Holt and Hopkinson,13 and J. J. Thomson,l* working mostly with spark discharges in water vapour, observed a wide divergence from Faraday’s law, but failed to provide a suitable hypothesis for the reaction. The work of Kernbaum lS and Makowetzky l 6 has shown that the products are more complex than a t first believed, and that they vary greatly with the type of discharge and electrodes employed : theories of the reactions were also advanced. Muller l 7 studied the spark discharge in liquid air, showing how completely the reactions differed from those taking place in the ordinary air spark ; he also observed a solid product not colloidal in character. Recently Shipley

IDavy, J . Roy. Inst., I, 165, 1802. ZMasson, Ann. Chim. Physique, 31, 295, 1851. SPerrot, referred to by J. W. Mellor, Comprehensive Treatise on Inorganic

‘Buff and Hofmann, A n t . Chirn., 113, 129, 1860. 5 Liveing and Dewar, Collected Papers on Spectroscopy,” Cambridge

6 Konen, Ann. Physik, 4, 742, 1902. ‘Lepsius, Ber., 23, 1418, 1637 and 1642, 1890. SLob, Ber., 34, 915, 1901. 10 Svedberg, summarised in the text Formation of Colloids, 1921. IlSvedberg, Physik. Z., 15, 361, 1906. 12Chapman and Lidbury, J . Chem. Soc., 81, 1301, 1902. 1SHolt and Hopkinson, Phil. Mag., 16, 92. 1908. 14Thomson, J. J., Recent Advances in Electricity and Magnetism, p. 181,

15Kernbaum, Cornpt. Rend., 151, 319, 1910 (and other papers in this journal). IGMakowetzky, 2. Elektrochem., 17, 217, 19x1. “Muller, 2. anorg. Chem., 76, 324, 1912.

Chemistry, Vol. I, p. 493, 1922.

Universitv Press, 1915 ; papers in Proc. Roy. Soc., 1880 et seq.

Bredig, 2. Elektrochem.. 4 , 514, 1898.

1892.

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THE ARC DISCHARGE IN WATER

and Goodeve l8 investigated high voltage arcs in alkaline electrolytes with various electrode materials, but all of the products were not studied. Many papers, notably those by Tarczynski,ls and Fowler and Mardles,20 have been published dealing with discharges in organic liquids, but it may be said that the existing knowledge of these reactions is still incomplete. In no cases have the relationships between the electrical energy input and the heat energy developed been studied.

The Nature of the Problem.

The work to be outlined below was done with two general objects in view :

I . To determine qualitatively and quantitatively the complete pro- ducts of the various discharges, and

2 . To determine, if possible, the mechanism of the reactions taking place.

In all cases i t was recognised tha t not only do the electrodes and liquids employed affect the reactions, but that the nature of the discharge must be carefully controlled if the results obtained are to be comparative, a fact not always realised by earlier workers. In this part only the low tension arc discharge in liquid water is covered, carbon electrodes being employed in most of the experimental work and metal electrodes only occasionally.

I . Characteristics of the Arc Discharge. The essential feature of the arc discharge in gases is the production

of heat, light, and chemical change, and in this respect the arc burning in liquids differs but little. To the eye, however, the two types of dis- charge are quite dissimilar; the characteristics of low voltage arcs in water and organic liquids have been incompletely described by Konen,6 and those of spark discharges by Tian,21 Henri,22 H ~ w e , ~ ~ and others. Shipley and Goodeve l8 studied fully the appearance and certain electrical effects of high voltage A.C. arcs. in electrolytes.

The arcs discussed below have several very characteristic features. They are produced by striking, a method mentioned in only a few of the papers referred to above; that is, the electrodes are brought together while connected to the source of current, and then separated slightly. With low voltages (from 6 to 50 volts A.C. or D.C.) the arc is extremely short, and may best be maintained with the electrodes in actual contact, a distinguishing feature. It appears as a brilliant spot emitting a very intense light, the region of arcing occurring not a t the point of contact, and shifting continually as the electrode material is worn away. A sound of variable frequency is produced and the emitted light is rich in ultra-violet radiations, as many workers have noted, e.g. HOW.^^ At all times during arcing a stream of gas bubbles, giving off a distinct odour, rises from the incandescent region, violently agitating the liquid, and when the water is kept near the boiling-point large amounts of steam also are evolved. The heat developed makes cooling necessary,

Isshipley and Goodeve, Eng. Jour. Can., 10, 3, 1927.

19 Tarczynslu, 2. Elektrochem., 22, 252, 1926. 20Fowler and Mardles, Trans. Far. Soc., 23, 301, 1927. 21Tian, Compt. Rend., 1483, 1911. 23Howe, Physic. Rev., 8, 674, 1916.

Also Shipley, Can. Jour. Res., I , 305, 1929.

Z2Henri, Physik. Z., 14, 516, 1913.

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H. D. CARTER AND A. N. CAMPBELL 48 I

either by circulation or by supplying fresh water to the arc. The liquid rapidly becomes cloudy with particles from the electrodes, most of which are not colloidal.

A most important feature is that these arcs can be maintained only in water of low conductivity (distilled or good tap water). The addition of small amounts of electrolyte prevents the production of, and im- mediately inhibits any existing, arcing ; the arcing being replaced by extensive electrolysis. When arcing occurs a strong electric field, which may be detected by the fingers, exists in the surrounding liquid, similar to that observed by Pfanhauser 24 during electro-plating under special conditions.

These characteristics make i t impossible to secure an arc which is automatic in adjustment, since the electrodes must be continuously moved in a manner only developed by experience. If a mechanical vibrator be substituted for hand adjustment then the arc is frequently broken and when this occurs electrolysis takes place, giving products not as- sociated with the true arc reactions. For low voltage work hand manip- ulation of a t least one electrode is therefore necessary, and the lower the voltage employed the more frequent is the adjustment required. With metal electrodes " welding " is almost continuous unless over 50 volts be applied, when the ends may be separated slightly.

2. Apparatus. Direct current from a 120-volt main was the source of power in all

experiments, this being passed through two heavy line resistances. This precaution was necessary since the resistance of the arc is almost zero at the time of striking, but rises to about 2 ohms during arcing. By varying the line resistance the voltage across the arc may be altered, but this factor also depends upon the back E.M.F. of the arc, which is considerable. Generally the voltage across the arc fluctuates widely and with great rapidity while the current is relatively steady. Neither of these factors can be read precisely upon meters of ordinary construction, and only a rough eye approximation can be obtained. An apparatus for measuring accurately the mean amperage and voltage supplied to the arc has been constructed, and the results obtained with i t will be reported later.

The arc was operated in a small copper chamber fitted with a window of thin sheet mica for observation. The electrodes were horizontally opposed and entered the chamber through lubricated rubber sleeves. Outlets were provided for the removal of the gaseous products and the liquid remaining after arcing; the former was shielded by a baffle which prevented the escape of the liquid, but allowed free circulation of gases. The whole chamber was immersed in a metal calorimeter vessel of one litre capacity, the electrodes being admitted through seals similar to the above. This outer container was filled with water cir- culated by a stirrer, and served to keep the liquid in the inner arc chamber below 30" C. during a run; it was also employed as a calori- meter in measuring the thermal energy output of the arc.

To prevent water vapour or spray entering the gas absorption train a glass wool tube immersed in a freezing mixture was connected to the chamber outlet, and followed by a sulphuric acid tube. Carbon dioxide was removed by a train of KOH bulbs and soda-lime tubes, and the

24 Pfanhauser, 2. Elektrochem., 7, 895, 1901. 32

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482 THE ARC DISCHARGE IN WATER

9'9 9.8 11-5 8.1

residual gas was collected over water containing baryta. The volume of the gas was determined by displacement of water, and was measured at atmospheric pressure.

Certain experiments were facilitated by operating the arc outside of this apparatus in a small Pyrex beaker ; any such modifications em- ployed are given in their proper sections.

16.6 1'7 53'4 i 6.1 12-4 I 7-0 3'1 47'0 Traces. 16.3

5'3 I 6.9 ::': 1 1'4 12.7 18.1 3'8 20-9 4'6

3. The Gaseous Products of the Discharge. Several types of carbon electrodes were employed, but after analysis

a hard uncored '' projector " carbon was selected for this work ; it con- tained 96 per cent. carbon, the impurities being silica ( 2 per cent.), iron (1.4 per cent), alumina (0.6 per cent.) and traces of lime. As oxides these substances have little effect upon the reaction products, as will be seen later.

The water used in the runs was distilled and air- and carbon dioxide- free. After each run the liquid remaining was titrated for carbon dioxide, using excess baryta and succinic acid, but the correction was very small; the train was found to absorb all but traces of this gas. The air in the apparatus was either estimated and a correction applied, or the train was filled with the gas before weighing.

About one litre of gas, from which the carbon dioxide has been absorbed, collects during a five to ten-minute run, the amount depending upon many factors. The analyses were done in a Burrell apparatus, fully described in a U.S.A. Bureau of Mines Bulletin 25 ; carbon monoxide and hydrogen were estimated by the copper oxide method with electrical heating, and saturated hydrocarbons in a combustion pipette. The cuprous chloride method for carbon monoxide was found to be quite unreliable, in accordance with the results of many workers.

While the analyses of any one gas sample gave results checking to 0.2 per cent., successive gas samples showed a wide variation, as will be seen in Table I. The copper oxide was held a t 290' C. to 310' C. which was apparently high enough to burn all carbon monoxide and hydrogen; the "residue" is to a great extent nitrogen from the con- tained air, and the gas percentages were not corrected for this, for reasons given later.

TABLE I.

The carbon monoxide and hydrogen contents differ widely from those obtained by Lob,s whose results differed from those of Lepsius,' although similar methods of analysis were employed by these workers. Modern work would certainly point to the copper oxide method being the more accurate, and the use of cuprous chloride solutions for CO would perhaps

*Burrell, Siebert and Jones, U.S. Dept. of Commerce, Bureau of Mines, Bulletin No. 197, p. 40, 1926.

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H. D. CARTER AND A. N. CAMPBELL 483

explain these differences. Lobs gives percentages of CO,, CO and H, which add up to 99.8 per cent., and this throws suspicion upon his results. With water and carbon electrodes considerable quantities of nitrogen and oxygen are almost certain to be in the collected gas, unless special precautions are taken, and a non-combustible gaseous residue has always been observed by us.

While the gas percentages have not yet been correlated with the factors affecting the arc operation, i t seems apparent that the percentage composition depends upon these factors as well as upon the nature of the electrodes. In no case were any unsaturated hydrocarbons observed with fuming sulphuric acid or saturated bromine water as the absorbent ; the amount of saturated hydrocarbons varied from mere traces to 6 per cent. The smell would indicate the presence of traces of unsaturated hydrocarbons, though not observed in the analyses.

The question of free oxygen being produced in the arc is of importance theoretically, and special tests were made to clear this up. Estimation of the air content of the apparatus is subject to error, and therefore runs were made in which the gas was collected by displacement of water from a cylinder held directly over the arc. During the run the water, previously distilled and boiled, was kept boiling vigorously in a pyrex vessel to prevent any dissolving of air. The electrodes were heated white hot in air, and then boiled in distilled water before each run ; this treatment was found by Konen to be sufficient to remove air from such carbon rods. The gas produced was led directly through water-filled tubing into the burette of the gas analysis apparatus. Absorption with fresh alkaline pyrogallol solution showed only traces of oxygen, less than the limit of accuracy of the apparatus (0.2 per cent.), when care was taken to keep the arc in operation during the entire run. Since it is virtually impossible to prevent the arc breaking for a second or so during a long run, the electrolysis which occurs during these periods probably explains the presence of oxygen in the gas mixture, and i t may be said that no free oxygen is produced by the arcingitself. The oxygen percentages in Table I. are corrected for a large part of the air present in the apparatus, and are no doubt the result of electrolysis.

Gaseous Residue.-In every case there remained in the analysis burette a 5 to 6 per cent. non-combustible gas residue, not accounted for by the nitrogen in the contained air. When a run was made for free oxygen, as above, and the CO and H, and hydrocarbons removed, this residue was reduced to 4 per cent. ; since no air had been introduced into the apparatus i t could not be assumed that the gas was nitrogen. A special apparatus was constructed to remove all gases except nitrogen, and the residue was transferred to a pyrex tube containing calcium chips; on heating to yellow heat, the gas was entirely absorbed. The ammonia test for calcium nitride could not be applied because of the small amount of gas (5 C.C. from a 1000 C.C. sample), and a t the tem- perature employed the calcium would have absorbed many gases, in- cluding hydrogen.

A sample of the gaseous residue was dried over P,O, and passed into a Geissler tube. Examination in the spectrometer revealed an intense hydrogen spectrum, several lines of the secondary spectrum being observed; the tube was tested with dried air, and shown to contain no water vapour or hydrogen. It is evident, therefore, that there was at least a trace of hydrogen in the residual gas, this having resisted oxidation by the copper oxide. The major portion of the gas was

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484 THE ARC DISCHARGE IN WATER

undoubtedly nitrogen, although Konen found his treatment, as em- ployed here, sufficient to remove even spectroscopic traces of nitrogen from carbon rods; considering the absorbing powers of carbon this hardly seems likely.

A similar nitrogen residue was obtained with hard carbon and with soft graphite rods.

Kernbaumls found only hydrogen produced when water was decomposed by the action of light, radium and electric discharges, assuming that hydrogen peroxide accounted for the oxygen. In some papers dealing with the decomposition of water by several methods there has been mentioned an absence of stoichiometrical relations in the quantity of hydrogen and oxygen produced. Linder 26 observed this with the glow discharge in water vapour, and could not trace the oxygen to hydrogen peroxide or ozone. Urey and Lavin 27 failed to find any error in similar results with a discharge tube, and Bates and Taylor 28

reported the same thing in the photochemical decomposition of water, In many cases a gas remaining after analysis was assumed to be nitrogen.

Attempts to find a " mass balance" in the amounts of hydrogen and oxygen produced in the arc gave no conclusive results, the dis- crepancies being far beyond the limit of error of the methods employed. In most cases there was about 25 per cent. (by weight) excess of hydrogen over the theoretical amount corresponding to the total oxygen, free and in combination; this pointed to the existence of products other than the gases already discussed.

4. Solid Products of the Discharge. (a) Carbon Electrodes.-Practically every paper on arcs under

liquids mentions the formation of " cloudy dispersions " or " muddy '' liquids during the operation of the arc, whether carbon or metallic electrodes were used. Before the work of Bredig9 such effects were not further studied, but in later papers the dispersions were dismissed as being purely colloidal suspensions of the electrode material.

When carbon electrode arcs with about 15 amperes at 30 volts D.C. have been operated in pure water for a considerable time (IS minutes in 300 C.C. of water, externally cooled) the water becomes dense black, and if allowed to stand for half-an-hour a very characteristic substance settles out. It is very flocculent and light, closely resembling metallic hydroxides in character except that i t usually has only the faintest tinge of green or yellow colour. Under the microscope this substance is quite distinctive, although granules of solid carbon may be seen mixed with it. It takes the form of almost transparent plates and " strings," joined together in long chains; the plates are tinged green by trans- mitted light.

By carefully shaking a freshly produced dispersion of this sort the flocculent substance was obtained almost free of carbon particles, the latter sinking to the bottom relatively easily. The light substance could then be filtered off and dried.

Combustions of this substance were made in the same apparatus used for analysing the electrodes, in which the sample could be heated t o a white heat while visually observed. Observation was required,

26 Linder, Physic. Rev., 38, 679, 1931. 27 Urey and Lavin, J . A m . Chem. SOC., 51, 3290, 1929. "Bates and Taylor, J . A m . Chem. Soc., 49, 2438, 1927.

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485 H. D. CARTER AND A. N. CAMPBELL

since the heat source had to be removed once the combustion started, otherwise it proceeded rapidly with a small explosion in some cases, and thus collection of all the carbon dioxide produced was impossible. Bright white heat was necessary completely to burn the substance in oxygen.

The samples were air-dried and then desiccated over sulphuric acid at atmospheric pressure for forty-eight hours. On heating to red heat a partial combustion took place, the loss in weight being 24 to 25 per cent. ; very little carbon dioxide and some water was produced. Com- plete combustion gave the substance a percentage composition as follows : 2 to 3 per cent. hydrogen ; 65 to 70 per cent. carbon, and 25 t o 30 per cent. oxygen. The variations in the results were no doubt due to the varying amount of solid carbon contained in the substance, i t being impossible to eliminate this entirely. The composition was prac- tically the same whether the hard or soft carbon electrodes were used in the preparation of the sample, and extended drying had no effect.

It appears that this substance is similar in nature to the so-ca!led " graphitic acid " or oxide. While this has been shown to be not a true compound, combustions give formulae such as C,,H,O, and C,,H,O, ; M e l l ~ r , ~ ~ and Hulett and Nelson 290 give a summary of the work on this substance. At red heat graphitic acid changes t o a soft black substance (" pyrographitic " acid : C,,H2O4), the change representing a loss in weight of 23 per cent. approximately. This agrees well with the results given above, and since graphitic acid is roughly 2 per cent. hydrogen, 63 per cent. carbon, and 35 per cent. oxygen, the similarity in the two substances is evident.

Graphitic acid is usually explosive, bu t the material produced in the arc burns quietly in the bunsen flame. Dilute acids immediately floc- culate the suspended substance, and the gel formed is peptised by repeated washing with water ; this property is possessed by graphitic acid, and in addition the appearances of the two in the microscope are similar.

A determination of the heat of combustion of the dried material was made in a bomb calorimeter, giving a result of 8-4 cals. per gram. If i t is assumed to be a pure substance of the formula C11H405 then the heat of formation is about 1800 cals. per gram mol (positive). No data could be found on the heat of formation of graphitic acid.

(b) Metal Electrodes.-Electrodes of aluminium, copper, zinc, and iron yield solid substances colloidal in character but not sols, and closely resembling that from the carbon electrodes except in colour.

The copper compound is probably a hydrated hydroxide, mixed with hydrated oxides. It is a blue-green substance and is completely soluble in ammonium hydroxide and HCI, thus resembling cupric hydroxide. Prolonged boiling, however, has little effect, and this treat- ment turns cupric hydroxide black. The solution is distinctly alkaline with pH 8-5 when saturated, and gives tests for dissolved copper. Colori- metric determination gave 16 mg. of copper per litre as the solubility, but no data could be found as to the solubility of cupric hydroxide.

The zinc compound is partially soluble in ammonium hydroxide and acids, but is flocculated by these reagents ; i t is soluble in ammonium chloride, and is probably a hydrated hydroxide or oxide similar t o

2B Mellor : Comprehensive Treatise on Inorg. Chem., Vol. IT. , p. 825 (Graphitic acid) ; Vol. IV., p. 521 (Zinc hydroxide) ; Vol. II., p. go4 (Carbon suboxides).

29a Hulett and Nelson, trans. Am. Electrochem. SOC., 37, 103, 1920.

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486 THE ARC DISCHARGE IN WATER

'' zinc hydroxide " which Mellor 29 shows is not definitely a compound. A saturated solution of the substance is alkaline with p~ 7-6 approxi- mately.

The hydroxide nature of the compounds from the metal electrodes seems established, and possibly the carbon compound is similarly con- stituted. A t any rate all of the oxygen in the latter cannot be due merely to water of hydration, because of the proportions of hydrogen and oxygen occurring here; the existence of a compound resembling graphitic acid, regardless of its exact composition, indicated tha t the hydrogen-oxygen balance could not be obtained by consideration of the gaseous products alone. This will be discussed later.

5. Soluble Substances Produced in the Discharge. Fresh solutions from arcing carbon and metal electrodes in boiled

distilled water gave no tests for hydrogen peroxide, ozone, aldehyde, or ketone, and no definite iodoform reaction.

After several minutes arcing in pure distilled water vigorous electrolysis always occurred when the electrodes were separated, i .e. the conductivity of the water was greatly increased. The hydrogen- ion concentration of solutions in which various electrodes had been arcing was therefore taken, and the results are given below ; the " core " relates to soft carbon electrodes employed in ultra-violet therapy work, and the metals are present as oxides. All solutions were originally neutral with PH 7.0.

p , Value when Saturated. Core Substance.

Uncored hard carbon . . 6-0 Uncored soft carbon . . 5.8

Fe, Al, Ni, Si . . 6.0

The solutions reached these values after three minutes arcing in 200 C.C. water, the voltage being about 30 volts and the current 15 amps. ; the water was kept boiling to prevent dissolving of carbon dioxide.

As stated previously, arcing will not take place in strongly acidic or basic solutions, but weak acids such as boric do not inhibit arcing. The arc with zinc electrodes changes the p H of 200 C.C. of a boric acid solution from 5.0 to 7-5 after five minutes arcing, but in such solution the flocculent zinc compound discussed above is apparently not formed, only a heavy black amorphous substance settling out.

Evaporation of the filtered solution from metal electrode arcs revealed traces of the metal and the " hydroxide " compounds, while the same treatment of the liquid from the carbon arcs gave an almost white powder, evidently not the carbon compound previously described. This substance was insoluble in ether, somewhat soluble in water, and readily soluble in small amounts of ethyl alcohol. It melts above 350° C. but below a red heat.

The water solution of the substance was quite acidic, giving a p~ value of 4.0. It is therefore probably an acid such as mellitic or pyro- mellitic produced by the solution of higher oxides of carbon, some of which are anhydrides of these acids ; such oxides are produced in many types of electrical discharges, as mentioned by Mel10r.~~ The fact that the substance gives a p~ of 4, compared to a p H of 6, for the mixture of this, and the solid carbon compound, may indicate that the latter is basic in character, as suggested previously.

Cerium core . 3'0

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487 H. D. CARTER AND A. N. CAMPBELL

Water Temperature.

Distillation of the clear filtered liquid from carbon arcs failed to show the presence of any volatile substances ; similar results were obtained by Fowler and Mardles 2o with sparks in organic liquids, and these workers also obtained a solid substance of high melting-point.

Since the carbon arc under pure water gives the gaseous, solid, and soluble substances outlined above, in order to determine the mass relationships in the reactions taking place, all these products must be considered. An apparatus is now being constructed with which this problem may be fully investigated. I t is clear, however, that there exists no " water gas " equilibrium as was formerly thought to be the case, and that the reaction is not a simple thermal one between the carbon electrodes and the steam produced in the arc.

The remainder of this paper deals with experiments done with a view to determining the mechanism of the reactions taking place in the carbon electrode arc under water.

6. Effect of Temperature of the Water upon the Reaction. A rise in the temperature of the water was always observed to result

in an apparent increase in the gas produced by the arc in a given time, regardless of the electrode material. Since the gas flow is a measure of the velocity of the reaction or reactions taking place, the above effect was measured accurately. As the temperature of the water approached the boiling-point the vapour pressure rose rapidly and the evolved gas mixture became richer in water vapour. Since the heat supply was constant, that 'is, there was a constant rate of input energy, the evolution of water vapour depended directly upon the vapour pressure. To determine whether the apparent increase in gas flow was merely due to this effect a series of tests were made in which the gas-water vapour mixture was collected and measured a t the temperature of the water itself. The arc with hard carbon electrodes was operated in a large vessel, the temperature of the water being regulated to within I * C. by means of an electric heating coil, and the gas was collected by dis- placement of water in a measuring cylinder held above the arc. The measuring error was less than 2 per cent. and to average the errors due to variation in the arc operation ten or more tests were run at each tem- perature. The arc was hand operated, as this gives the most consistent results, and the runs were for one to five minutes, depending upon the volume of gas evolved.

Fig. I gives the results of these tests in graphic form, the data being in Table 11. With the curve is given the vapour pressure curve of water {same abscissze). It is seen that the increase in gas flow is not entirely accounted for by the increase in the partial pressure of the water in the gas mixtures collected.

TABLE 11.

Gas Flow. c.cjmin.

Water 1 I Tempprature.

~ Watpr Temperature.

4; c- 7

I 6" 20" 26" 29O 34"

13"

Gas Flow. c.c./min.

Gas Flow. c.c./min.

225 225

250 300 260 325 275

210

411 C. 43

61"

52" 55"

63" 70"

3 75 300 375 400 410 390 400

75" c. 82'

410 450 600 650

1250 2900

I I00

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488 THE ARC DISCHARGE IN WATER

An apparatus similar to that discussed in Section 2, but much larger, and constructed of iron pipe instead of sheet copper, was used to verify the above conclusion. The electrodes were mounted horizontally through the ends of a 2-ft. length of pipe (4 ins. in diameter). At the centre of this a second length of pipe was fixed vertically, so that the whole took the form of an inverted T. The water was electrically

‘-d

heated and the gas passed from the apparatus through a condenser. to a cooling vessel, being finally collected over water a t a fixed temper- ature. In this way the gas was measured under constant temperature and pressure conditions, regardless of the temperature of the water in which the arc operated. The results are shown in Fig. 2 ; there is a considerable increase in the rate of the reaction as the water tem- perature approaches the boiling-point, the curve indicating that past

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H. D.CARTER AND A. N. CAMPBELL 489

, 90'c. IOOO I 10° 120"

100' C. or so there is little further rise. this point because of the pressure required. are given in Table 111.

Tests were not continued past The data used in the curve

TABLE 111.

Water Temperature.

I- -____

10° c. I 8' 27O 36" 42O

Gas Flow. C.C. /min.

200 210 200 225 230

Water Temperature.

-

4 9 O c. 70° 60 "

80"

Gas Flow. cc./min.

2 2 0

235 260 270

Water ' Temperature.

Gas Flow. cc./miri.

350 .

325.

300.

-n 275- - n

P L f O - C

rn

c

2 t 2 5 '

Z O O I / W A T E R

T L M P L R ATURL

7. Measurement of the Arc Temperature. Many measurements have been made of the temperature of arcs

in air and various gases, but no similar data have been found relating to arcs in liquids. However, Wilson 30 has determined the temperature of under-water sparks with an applied potential of 20,000 volts ; spectra measurements involving Birge's formula and based on the quantum theory gave the temperature in this case as 51 15" C., which was claimed to be accurate. It was desirable to measure the temperature of the arcs used in this work with a view to determining whether or not the reactions were purely thermal in nature.

There are available several methods of measuring high temperatures, discussed by Mason 31 and Tanberg and B e r k e ~ . ~ ~ In the latter paper i t is shown that a very accurate method gave results close to those

30Wilson, J . Opt. SOC. Am., 17, 37, 1928. 31 Mason, Physic. Rev., 38, 427, 1931. 32Tanberg and Berkey, Physic. Rev., 38, 296, 1931.

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490 THE ARC DISCHARGE IN WATER

secured with an optical pyrometer, and the accuracy of the latter instru- ment is discussed.

In the following a " Holborn and Kurlbaum " type of optical pyro- meter was used, manufactured by Leeds and Northrup. Since the Cali- bration supplied did not extend into the temperature regions measured, it was necessary to increase the range. With this type of instrument a neutral absorption " filter " may be employed, and extrapolation from the known calibration curve carried out, employing Wien's law in modified form.

Calibration of such a neutral white glass filter was carried up to 2000' C., a smooth curve being obtained. Then the carbon arc in air was measured, and its temperature taken as 3500" C., this being the accepted value for the crater. The curve could then be continued to this point, and to verify the soundness of this method the formula of Pirani and Meyer, as cited by Burgess and Le Chatelier,33 was applied. It was found to agree to within 100' (maximum error), and this accuracy was sufficient since comparative temperatures rather than absolute values were desired.

The arcs were operated in a litre of water contained in a copper vessel fitted with a small window of extremely thin sheet mica. The arc was hand-operated with horizontally opposed electrodes held close to the window, and all measurements were made with the pyrometer the same distance from the arc as from the calibration sources. Because of the small size of the arc region i t was not possible to discriminate between cathode and anode temperatures. The arc was viewed at right angles to the electrodes and no difference in temperature through- out the brilliant source spot could be detected by the instrument. It is therefore evident tha t whatever temperature gradient may exist in these arcs, the mean temperature throughout was the value measured.

Three groups of tests were carried ou t : first, the effect of the temperature of the water upon the arc temperature, with constant voltage applied to the arc. Second, the effect of increasing the applied voltage on the temperature of the arc, the temperature of the water being 25-30' C., and third, the same effect with the water temperature at 100' C., i.e. boiling vigorously. The voltages given on the curves of Fig. 3 were the average figures read on the instruments and, of course, did not take into account wide fluctuations which occur momentarily, i.e. they are not " mean " voltages. The latter figures would have had no significance since the temperatures were measured during steady arc opera ti on.

Fig. 3 shows five curves plotted from the data in Table IV., each for a different type of carbon electrode, many of them with meta! oxide cores. It is seen that a 1500' rise in the arc temperature is the general effect when the temperature of the water is raised from 5" to 100' C. ; the significance of this will be discussed later.

In the following table and curves the clarendon letters used indicate the type of carbon employed, as follows :-

Lettering. Type of Carbon. A . . Soft, cerium core. B . . Hard, no core. c . . Soft, All Ni, Fe, Si core. D . . . . Soft, no core.

a3 Burgess and Le Chatelier : High Temperature Measurement, p. 328 (Wiley & Sons), 1912.

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H. D. CARTER AND A. N. CAMPBELL

Water Temperature.

B.

8" C. 60"

I 00"

D.

1.41 C. 25

I 00"

TABLE IV.

Arc Temperature.

1325" c. 2025' 2500'

2300' C. 2500" 3925O

491

Water Temperature.

A.

11" c. 25'

I 00"

C.

8" c. 25' IOOO

Arc Temperature.

2300' C. 2525' 2725"

2700' C. 2750" 3625"

FIG. 3.

The relationships between the arc temperature and the voltage were also determined, and will be reported when the corresponding effects on the reactions have been found.

The lowest temperature measured was 1300" C. with a hard uncored carbon arc at 25 volts, the water temperature being 8" C. Soft

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492 THE ARC DISCHARGE IN WATER

" graphite " electrodes with a strontium core gave the highest temperature, viz. 3600" C., arcing at 40 volts with the water boiling. Some thirty measurements were made between these two extremes, in steps of 75". The core substances have little effect upon the arc temperature, but the latter is greatly affected by the quality of carbon. Under the same conditions of voltage and water temperature the hard or " projector " carbons gave the lowest, and the soft or "graphite" carbons the highest temperature readings.

Since the sensitivity of the instrument used was lessened by the addition of the neutral filter, the temperatures read have probably a maximum error of 100" C. It is not unlikely that the high temperatures measured were actually somewhat lower than the true arc temperatures, since the water diffused the light considerably, and no correction was made for this. The temperature of the air arc used in calibrating the instrument, viz. 3500" C., was obtained with pure uncored carbon rods and a low D.C. voltage; hence the fact that a temperature of 3600" C. was obtained with cored graphite rods under water, with a higher voltage than the above, does not show that the under-water measure- ments are in error, for cored rods of this sort are known to give a much higher temperature arc than uncored types. Attempts are now being made to determine the temperatures of the various under-water arcs by measurements of their continuous spectra.

8. Effect of Pressure on Arcing.

Shipley and Goodeve,l* working with arcs in boiling alkaline electro- lytes, found that 3 lb. per sq. in. pressure above atmospheric inhibited

t 25

t 00

I15

IS0

125

1 0 0

1 5

5 0

Z!

P R 1 S t U e . l

0 1 0 0 PO0 3 0 0 400 5 0 0 6 0 0 700 8 0 0 900 1.000

FIG. 4.

all arcing with an applied potential of 700 volts A.C., while 31 lb. suppressed arcing with 2200 volts applied. It was thought that the

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H. D. CARTER AND A. N. CAMPBELL 493

arcs dealt with in this paper, i.e. “ struck ” arcs in water of low con- ductivity, were entirely different in mechanism and in products to those discussed in the above reference. To verify this the effect of pressure on the arc with carbon electrodes was determined.

The apparatus was practically the same as that used in determining the effect of water temperature (Section 6), being strengthened to stand high pressures. The adjustable electrode was fed in by means of a screw thread, the sealing arrangement being similar to that used in high pressure valves. The arcs were operated a t 30 volts, the current being about 15 amps., and the connections to the electrodes were made from the outside by means of heavy automobile “ spark plugs,” slightly altered.

In a closed system such as this a constant rise in pressure per unit time indicates a constant mass of gas produced in that time, assuming the Gas Laws to hold. Fig. 4 shows the rise in pressure against time, readings being taken about four times per minute, and the data is in Table V. The curve is quite linear, showing that the amount of gas produced was independent of the pressure within the limits used.

TABLE V.

Time. Seconds.

0

50 60 75

I45 I95 230 275

I I 0

Pressure. (lb./sq. in.)

I 0 I5 I7

30 35 42 50 57

20

Time. Seconds.

340 360 375 395 450 495 525 575 650

Pressure. (Ib./sq. in.)

68 72 77 85 92 98 I 06 118 125

Time. Seconds.

Pressure- (lb. /sq. in.)

720 730 770 810

835 880 940

I 040

To verify the above conclusion, tests were run a t pressures through- out the range of Table V., in which the gas evolved was expanded to atmospheric pressure and its volume measured a t constant temperature. The highest pressure employed was 350 lb./sq. in. or about 23 atmo- spheres. Regardless of the pressure the volume of gas evolved per unit time did not vary more than 5 per cent. from the mean, and there was no regularity in the fluctuation.

At pressures over five atmospheres the characteristics of the arc underwent some changes; the sound emitted, and the mechanical vibration of the heavy apparatus (weight about 75 lb.) increased greatly. This effect lessened and finally disappeared when the pressure was in- creased to eight atmospheres. Visual observation of the arc indicated little change, but this was not conclusive because of the heavy protective glass lenses used, the latter being set into the side of the apparatus.

It is evident, however, that pressure has little if any effect upon the production of gaseous products, and as this latter factor may be taken as a measure of the reactions taking place, it may have some bearing upon the mechanism.

Theoretical Considerations. The earliest workers in the field of electric discharges in liquids and

gases found the theory of “ electrolysis’’ to be untenable, because of

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494 THE ARC DISCHARGE IN WATER

the disagreement with Faraday’s law, but Chapman and Lidbury,12 with others, made use of the term “ electrolysis of steam.” In 1900 Lob 8 realised that the electrodes had a great influence upon the arcing itself, and upon the gaseous products; and we have shown that the formation of soluble and insoluble solid carbon compounds in addition to the gaseous products, complicates the study of the carbon arc under water.

The question arises as to whether the reaction is not a simple one between the steam produced in the arc and the carbon a t the high temperatures which have been shown to exist. Pring and H u t t o n s investigated the reaction between hydrogen and carbon heated to a high temperature by an electric current, and found acetylene produced in increasing amounts as the temperature was raised ; but the irregular results observed and the methods of analysis employed leave doubts as to whether the acetylene was a direct product of the reaction or due to impurities in the carbon. At any rate, with steam and carbon the reaction is quite different, and Thiele and Haslam 35 have shown the “ water gas ” reaction to be somewhat complicated, involving surface reactions. No electrical influences were employed in this work, and no mention is made of saturated or unsaturated hydrocarbons being produced.

Influence of the Water.-With the arc in liquid water a high temperature is produced when the electrodes are brought together, due to the great resistance of the irregular ends, and the formation of steam follows immediately. The arcing no doubt takes place in the steam, but separation of the electrodes is not necessary to secure arcing, as discussed previously. With a given power input to the arc the extent of the steam “ mantle” would depend upon the temperature of the surrounding water, as this affects the rapidity with which the steam condenses as it leaves the arc due to the buoyancy effect. As long as the arc is surrounded by steam a region of very high temperature can be produced and maintained because of the low heat conductivity of the steam; the sweeping effect of the evolved gases would tend to remove the steam from the upper part of the arc.

It is thus seen how a rise of 75” in the temperature of the water can result in a 1500~ rise in the arc temperature : as the boiling-point is approached the steam is removed much less rapidly from the arc region. In addition to this it must be considered that less energy is required to heat the water up to boiling, and this factor must be of considerable importance when the large amount of steam produced in the arc is taken into account, because this steam is rapidly condensed by the surrounding liquid. More energy is available for chemical reactions when the water is near the boiling-point, whether these be purely thermal due to the high temperature or of some other nature; this may explain the rise exhibited in Fig. 2.

Nature of the Reactions.-The dissociation of water vapour into its elements a t high temperatures has been studied by L ~ w e n s t e i n , ~ ~ Langm~i r ,~ ’ and others, and the appreciable extent of the dissociation over 2 0 0 0 ~ C. has been established. Bjerrum3* has carried out these

34Pring and Hutton, J . Chem. SOC., 89, 1591, 1906. 35Thiele and Haslam, J . Ind. Eng. Chem., 19, 882, 1927. 36 Lowenstein, 2. physzk. Chem., 54, 797, 1905 ; 56, 513, 1906. 37Langmuir, J . Am. Chem. Soc., 28, 1351, 1906. 38Bjerrum, 2. ph-ysik. Chem., 79, 513, 1912.

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H. D. CARTER AND A. N. CAMPBELL 495

investigations up to nearly 3000’ C. and observed at the latter tem- perature over 1 1 per cent. dissociation ; these results, obtained by the bomb method, agree well with those for lower temperatures. I t is possible therefore tha t the preliminary reaction in the arc is the thermal decomposition of the steam into hydrogen and oxygen at the high temperatures obtaining here.

The possibility that the primary reaction is of the “water gas ” type is evident, but the existing data on this equilibrium would tend to disprove this. Haber 39 gives a review of this work, and the percentage composition of the gases do not correspond a t all t o those from the arc, for similar temperatures. The existence of solid products is a further indication tha t there is no such simple equilibrium in the arc.

It is theoretically possible that the thermal decomposition of the steam, with subsequent union of the oxygen with the electrodes to produce gaseous and solid carbon oxide, constitutes the entire mechanism of the reactions. If this were true an increase in the temperature of the arc should result in a corresponding increase in the reaction velocity, the latter rising rapidly past 2000’ C. A consideration of Figs. 2 and 3 will show that the increase in gas flow as the water temperature rises is not accounted for by the accompanying increase in the arc tem- perature. Certain qualitative experiments, now being repeated care- fully, indicate that the reaction is to a large extent governed by the applied voltage, and this points to an ionisation effect.

There is a space potential gradient in these arcs amounting to a t least 300 volts per cm. when 30 volts are applied to the electrodes, the length of the arc being less than a millimetre. This potential is sufficient to produce a large variety of ions from substances available in the arc, as shown by Senftleben and Rehre11,~0 and Bleakney.*l

The electric discharge through water vapour has been known to produce the so-called “ water ” bands and lines, and Konen has ob- served these with certain under-water arcs. Watson 42 has recently shown these bands to be due to the OH ion, and Barton and Bartlett 43

have shown how extensive the production of these ions is. Lozier4* considers tha t the formation of the OH particle is a step in the dissoci- ation of water vapour, and considering these facts i t is possible that the most important preliminary reaction taking place in the under-water arc is the dissociation of water into H and OH particles. These may be charged during formation, or subsequently by the electron stream in the arc, and thus be given a high kinetic energy by the potential gradient existing between the electrodes. In this condition they would be capable of bringing about similar dissociation of other water molecules.

We consider tha t it is probable that the OH particles, which may be charged and highly excited, react directly with the electrode materials, forming metal hydroxides with copper, zinc, and other electrodes, and a “ graphitic ac id” type of compound with carbon electrodes. It is significant that no more than traces of free oxygen escape from the arc, as this would hardly be the case if the dissociation of water vapour were

39 Haber, Thermodynamics of TecJanical Gas Reactions (trans. by Lamb), p. 122, 1908 ; Oxygen-hydrogen reaction, p. 177.

40 Senftleben and Rehren, 2. Physik , 37, 529, 1926 ; Also Senftleben, 2. Physik , 37, 539, 1926.

41 Bleakney, Physic. Rev., 35, 1180, 1930. 4 2 Watson, Astrophys. J . , 60, 145, 1924. 43 Barton and Bartlett, Physic. Rev., 31, 823, 1928. 44 Lozier, Physic. Rev., 36;-1417. 1930.

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496 THE ARC DISCHARGE IN WATER

purely thermal, the equilibrium being " frozen " by the surrounding cold liquid. The fact that the oxygen is all in chemical union with the electrode material may indicate that the predominant reaction is electrical in nature, taking place on the electrode surface, or in the regions of great potential fall (amounting to millions of volts per cm.) which have been shown by Mason However, the purely thermal dissociation of the water must play some part, whether accompanied by electrical effects or not.

There is also the possibility that photochemical reactions play a part in these arcs, since under-water discharges have long been known to be sources of intense, low wave-length radiations. This point will be investigated later, when a search is made for further products, and some spectral evidence secured.

Energy relationship between the electrical input and thermal energy output are being studied, and in this connection there has been found much conflicting data as to the heat of formation of water from its elements, a t high temperatures. Haber and Bruner 46 found the heat of formation to decrease with rise in temperature, while Haber 39 reviews evidence to the contrary. The disagreement with thermodynamical theory would seem to lie in lack of accurate data on the variation of the specific heats of oxygen, hydrogen and water vapour a t high tempera- tures ; Bjerrum 38 has shown this variation in the case of hydrogen to b e considerable.

Summary.

and others to exist close to arc electrodes.

A special type of low voltage contact arc, operating under water, is described. The chemical nature of the gaseous, liquid, and solid products is examined in detail. With regard to gaseous products, the results of previous workers are qualitatively but not quantitatively confirmed, and considerable attention is given to a gaseous inactive residue. The solid products are shown to be hydroxidic in character ; the solid product from carbon arcs is shown to be a body of the nature of graphitic acid ; in the latter case, traces of a sparingly soluble, high-melting acid are simultaneously produced.

The apparent or mean temperatures of the under-water arcs are in- vestigated by means of an optical pyrometer, and their variation with temperature of the surrounding water. The variation of total gaseous yield with temperature and pressure is investigated.

The mechanism of the process is discussed in the light of the results communicated, and the following probabilities are arrived a t : (I) The pro- cess is not a mere thermal reaction ; (2) A certain proportion of the reaction is no doubt due to the fairly considerable dissociation of water vapour at the high temperatures in question, but (3) i t is shown that very probably an electrical dissociation of water vapour into H and OH ions is responsible for a t least the solid products; (4) Short wave radiation effects are not excluded.

Work is still proceeding on : (I) The spectrum of the arc, and ( 2 ) the energy relations.

Thanks are due to Mr. W. R. Carter for invaluable assistance rendered in the work outlined in Sections 6 and 8, and to Professor M. A. Parker for information and advice.

45 Mason, Physic. Rev., 38, 427, 1931- 46 Haber and Bruner, 2. Elektrochewz., 10, 697, 1904.

Department of Chemistry, University of Manitoba.

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