"benzine," "benzoline," "petrol," and "petroleum spirit" all refer to more or less volatile (the most volatile being mentioned first) and more or less thoroughly rectified products obtained from petroleum. They are mixtures of different hydrocarbons, the greater part of them having the general chemical formula C_nH_2n+2 where n = 5 or more. None of them is a definite chemical compound as is benzene; when n = 5 only the product is pentane. These hydrocarbons are known to chemists as "paraffins,"
"naphthenes" being occasionally met with; while a certain proportion of unsaturated hydrocarbons is also present in most petroleum spirits. The hydrocarbons of coal-tar are "aromatic hydrocarbons," their generic formula being C_nH_2^n-6, where n is never less than 6.] are allowed to vaporise in a room in which a light may be introduced. Less of the vapour of these hydrocarbons than of acetylene in the air of a room brings the mixture to the lower explosive limit, and therewith subjects it to the risk of explosion. This tact militates strongly against the use of such hydrocarbons within a house, or against the use of air-gas, which, as explained in Chapter I., is air more or less saturated with the vapour of volatile hydrocarbons. Conversely, a combustible gas, such as acetylene, may be safely "carburetted" by these hydrocarbons in a properly constructed apparatus set up outside the dwelling-house, as explained in Chapter X., because there would be no air (as in air-gas) in the pipes, &c., and a relatively large escape of carburetted acetylene would be required to produce an explosive atmosphere in a room. Moreover, the odour of the acetylene itself would render the detection of a leak far easier with carburetted acetylene than with air-gas.
N. Teclu has investigated the explosive limits of mixtures of air with certain combustible gases somewhat in the same manner as Eitner, viz.: by firing the mixture in an eudiometer tube by means of an electric spark.
He worked, however, with the mixture dry instead of saturated with aqueous vapour, which doubtless helps to account for the difference between his and Eitner's results.
_Table giving the Percentages by volume of Combustible Gas in a Dehydrated Mixture of that Gas and Air between which the Explosive Limits of such a Mixture lie._--(Teclu).
____________________________________________________________________ Lower Explosive Limit. Upper Explosive Limit. Description of ________________________ ________________________ Combustible Gas. Per Cent. of Gas. Per Cent. of Gas. __________________ ________________________ ________________________ ACETYLENE 1.53-1.77 57.95-58.65 Hydrogen 9.73-9.96 62.75-63.58 Coal-gas 4.36-4.82 23.35-23.63 Methane 3.20-3.67 7.46- 7.88 __________________ ________________________ ________________________
Experiments have been made at Lechbruch in Bavaria to ascertain directly the smallest proportion of acetylene which renders the air of a room explosive. Ignition was effected by the flame resulting when a pad of cotton-wool impregnated with benzoline or potassium chlorate was fired by an electrically heated wire. The room in which most of the tests were made was 8 ft. 10 in. long, 6 ft. 7 in. wide, and 6 ft. 8 in. high, and had two windows. When acetylene was generated in this room in normal conditions of natural ventilation through the walls, the volume generated could amount to 3 per cent. of the air-space of the room without explosion ensuing on ignition of the wool, provided time elapsed for equable diffusion, which, moreover, was rapidly attained. Further, it was found that when the whole of the acetylene which 2 kilogrammes or 4.4 lb.
of carbide (the maximum permissible charge in many countries for a portable lamp for indoor use) will yield was liberated in a room, a destructive explosion could not ensue on ignition provided the air-space exceeded 40 cubic metres or 1410 cubic feet, or, if the evolved gas were uniformly diffused, 24 cubic metres or 850 cubic feet. When the walls of the room were rendered impervious to air and gas, and acetylene was liberated, and allowed time for diffusion, in the air of the room, an explosion was observed with a proportion of only 2-1/2 per cent. of acetylene in the air.
_Solubility of Acetylene in Various Liquids._
_____________________________________________________________________ Volumes of Tem- Acetylene Solvent. perature. dissolved by Authority. 100 Vols. of Solvent. ___________________________ _________ ____________ __________________ Degs. C Acetone . . . . 15 2500 Claude and Hess " . . . . 50 1250 " Acetic acid; alcohol . 18 600 Berthelot Benzoline; chloroform . 18 400 " Paraffin oil . . . 0 103.3 E. Muller " . . . 18 150 Berthelot Olive oil . . . . -- 48 Fuchs and Schiff Carbon bisulphide . . 18 100 Berthelot " tetrachloride . 0 25 Nieuwland Water (at 4 65 atmospheres pressure) . . 0 160 Villard " (at 755 mm. pressure) 12 118 Berthelot " (760 mm. pressure) . 12 106.6 E. Muller " " . 15 110 Lewes " " . 18 100 Berthelot " " . -- 100 E. Davy (in 1836) " " . 19.5 97.5 E. Muller Milk of lime: about 10 grammes of calcium hy- 5 112 Hammerschmidt droxide per 100 c.c. . and Sandmann " " " 10 95 " " " " 20 75 " " " " 50 38 " " " " 70 20 " " " " 90 6 " Solution of common salt,5% 19 67.9 " (sodium chloride) " 25 47.7 " " 20% 19 29.6 " " " 25 12.6 " "(nearly saturated, 26%) . . 15 20.6 " "(saturated, sp. gr. 1-21) . . 0 22.0 E. Muller " " " 12 21.0 " " " " 18 20.4 " Solution of calcium Hammerschmidt chloride (saturated) . 15 6.0 and Sandmann Berge and Reychler's re- agent . . . . -- 95 Nieuwland ___________________________ _________ ____________ __________________
SOLUBILITY.--Acetylene is readily soluble in many liquids. It is desirable, on the one hand, as indicated in Chapter III., that the liquid in the seals of gasholders, &c., should be one in which acetylene is soluble to the smallest degree practically attainable; while, on the other hand, liquids in which acetylene is soluble in a very high degree are valuable agents for its storage in the liquid state. Hence it is important to know the extent of the solubility of acetylene in a number of liquids. The tabular statement (p. 179) gives the most trustworthy information in regard to the solubilities under the normal atmospheric pressure of 760 mm. or thereabouts.
The strength of milk of lime quoted in the above table was obtained by carefully allowing 50 grammes of carbide to interact with 550 c.c. of water at 5 C. A higher degree of concentration of the milk of lime was found by Hammerschmidt and Sandmann to cause a slight decrease in the amount of acetylene held in solution by it. Hammerschmidt and Sandmann's figures, however, do not agree well with others obtained by Caro, who has also determined the solubility of acetylene in lime-water, using first, a clear saturated lime-water prepared at 20 C. and secondly, a milk of lime obtained by slaking 10 grammes of quicklime in 100 c.c. of water. As before, the figures relate to the volumes of acetylene dissolved at atmospheric pressure by 100 volumes of the stated liquid.
_________________________________________________ Temperature. Lime-water. Milk of Lime. _______________ _______________ _________________ Degs C. 0 146.2 152.6 5 138.5 -- 15 122.8 134.8 50 43.9 62.6 90 6.2 9.2 _______________ _______________ _________________
Figures showing the solubility of acetylene in plain water at different temperatures have been published in Landolt-Bornstein's Physico- Chemical Tables. These are reproduced below. The "Coefficient of Absorption" is the volume of the gas, measured at 0 C. and a barometric height of 760 mm. taken up by one volume of water, at the stated temperature, when the gas pressure on the surface, apart from the vapour pressure of the water itself, is 760 mm. The "Solubility" is the weight of acetylene in grammes taken up by 100 grammes of water at the stated temperature, when the total pressure on the surface, including that of the vapour pressure of the water, is 760 mm.
_____________________________________________ Temperature. Coefficient of Solubility. Absorption. ______________ ________________ _____________ Degs. C. 0 1.73 0.20 1 1.68 0.19 2 1.63 0.19 3 1.58 0.18 4 1.53 0.18 5 1.49 0.17 6 1.45 0.17 7 1.41 0.16 8 1.37 0.16 9 1.34 0.15 10 1.31 0.15 11 1.27 0.15 12 1.24 0.14 13 1.21 0.14 14 1.18 0.14 15 1.15 0.13 16 1.13 0.13 17 1.10 0.13 18 1.08 0.12 19 1.05 0.12 20 1.03 0.12 21 1.01 0.12 22 0.99 0.11 23 0.97 0.11 24 0.95 0.11 25 0.93 0.11 26 0.91 0.10 27 0.89 0.10 28 0.87 0.10 29 0.85 0.10 30 0.84 0.09 ______________ ________________ _____________
Advantage is taken, as explained in Chapter XI., of the high degree of solubility of acetylene in acetone, to employ a solution of the gas in that liquid when acetylene is wanted in a portable condition. The solubility increases very rapidly with the pressure, so that under a pressure of twelve atmospheres acetone dissolves about 300 times its original volume of the gas, while the solubility also increases greatly with a reduction in the temperature, until at -80 C. acetone takes up 2000 times its volume of acetylene under the ordinary atmospheric pressure. Further details of the valuable qualities of acetone as a solvent of acetylene are given in Chapter XI., but it may here be remarked that the successful utilisation of the solvent power of acetone depends to a very large extent on the absolute freedom from moisture of both the acetylene and the acetone, so that acetone of 99 per cent.
strength is now used as the solvent.
Turning to the other end of the scale of solubility, the most valuable liquids for serving as seals of gasholders, &c., are readily discernible.
Far superior to all others is a saturated solution of calcium chloride, and this should be selected as the confining liquid whenever it is important to avoid dissolution of acetylene in the liquid as far as may be. Brine comes next in order of merit for this purpose, but it is objectionable on account of its corrosive action on metals. Olive oil should, according to Fuchs and Schiff, be of service where a saline liquid is undesirable; mineral oil seems useless. Were they concordant, the figures for milk of lime would be particularly useful, because this material is naturally the confining liquid in the generating chambers of carbide-to-water apparatus, and because the temperature of the liquid rises through the heat evolved during the generation of the gas (_vide_ Chapters II. and III.). It will be seen that these figures would afford a means of calculating the maximum possible loss of gas by dissolution when a known volume of sludge is run off from a carbide-to- water generator at about any possible temperature.
According to Garelli and Falciola, the depression in the freezing-point of water caused by the saturation of that liquid with acetylene is 0.08 C., the corresponding figure for benzene in place of water being 1.40 C.
These figures indicate that 100 parts by weight of water should dissolve 0.1118 part by weight of acetylene at 0 C., and that 100 parts of benzene should dissolve about 0.687 part of acetylene at 5 C. In other words, 100 volumes of water at the freezing-point should dissolve 95 volumes of acetylene, and 100 volumes of benzene dissolve some 653 volumes of the gas. The figure calculated for water in this way is lower than that which might be expected from the direct determinations at other temperatures already referred to; that for benzene may be compared with Berthelot's value of 400 volumes at 18 C. Other measurements of the solubility of acetylene in water at 0 C. have given the figure 0.1162 per cent. by weight.
TOXICITY.--Many experiments have been made to determine to what extent acetylene exercises a toxic action on animals breathing air containing a large proportion of it; but they have given somewhat inconclusive results, owing probably to varying proportions of impurities in the samples of acetylene used. The sulphuretted hydrogen and phosphine which are found in acetylene as ordinarily prepared are such powerful toxic agents that they would always, in cases of "acetylene" poisoning, be largely instrumental in bringing about the effects observed. Acetylene _per se_ would appear to have but a small toxic action; for the principal toxic ingredient in coal-gas is carbon monoxide, which does not occur in sensible quantity in acetylene as obtained from calcium carbide.
The colour of blood is changed by inhalation of acetylene to a bright cherry-red, just as in cases of poisoning by carbon monoxide; but this is due to a more dissolution of the gas in the haemoglobin of the blood, so that there is much more hope of recovery for a subject of acetylene poisoning than for one of coal-gas poisoning. Practically the risk of poisoning by acetylene, after it has been purified by one of the ordinary means, is _nil_. The toxic action of the impurities of crude acetylene is discussed in Chapter V.
Acetylene is an "endothermic" compound, as has been mentioned in Chapter II., where the meaning of the expression endothermic is explained. It has there been indicated that by reason of its endothermic nature it is unsafe to have acetylene at either a temperature of 780 C. and upwards, or at a pressure of two atmospheres absolute, or higher. If that temperature or that pressure is exceeded, dissociation (_i.e._, decomposition into its elements), if initiated at any spot, will extend through the whole mass of acetylene. In this sense, acetylene at or above 780 C., or at two or more atmospheres pressure, is explosive in the absence of air or oxygen, and it is thereby distinguished from the majority of other combustible gases, such as the components of coal-gas.
But if, by dilution with another gas, the partial pressure of the acetylene is reduced, then the mixture may be subjected to a higher pressure than that of two atmospheres without acquiring explosiveness, as is fully shown in Chapter XI. Thus it becomes possible safely to compress mixtures of acetylene and oil-gas or coal-gas, whereas unadmixed acetylene cannot be safely kept under a pressure of two atmospheres absolute or more. In a series of experiments carried out by Dupre on behalf of the British Home Office, and described in the Report on Explosives for 1897, samples of moist acetylene, free from air, but apparently not purified by any chemical process, were exposed to the influence of a bright red-hot wire. When the gas was held in the containing vessel at the atmospheric pressure then obtaining, viz., 30.34 inches (771 mm.) of mercury, no explosion occurred. When the pressure was raised to 45.34 inches (1150 mm.), no explosion occurred; but when the pressure was further raised to 59.34 inches (1505 mm., or very nearly two atmospheres absolute) the acetylene exploded, or dissociated into its elements.
Acetylene readily polymerises when heated, as has been stated in Chapter II., where the meaning of the term "polymerisation" has been explained.
The effects of the products of the polymerisation of acetylene on the flame produced when the gas is burnt at the ordinary acetylene burners have been stated in Chapter VIII., where the reasons therefor have been indicated. The chief primary product of the polymerisation of acetylene by heat appears to be benzene. But there are also produced, in some cases by secondary changes, ethylene, methane, naphthalene, styrolene, anthracene, and homologues of several of these hydrocarbons, while carbon and hydrogen are separated. The production of these bodies by the action of heat on acetylene is attended by a reduction of the illuminative value of the gas, while owing to the change in the proportion of air required for combustion (_see_ Chapter VIII.), the burners devised for the consumption of acetylene fail to consume properly the mixture of gases formed by polymerisation from the acetylene. It is difficult to compare the illuminative value of the several bodies, as they cannot all be consumed economically without admixture, but the following table indicates approximately the _maximum_ illuminative value obtainable from them either by combustion alone or in admixture with some non- illuminating or feebly-illuminating gas:
________________________________________________ Candles per Cubic Foot ______________ ___________________ _____________ (say) Acetylene C_2H_2 50 Hydrogen H_2 0 Methane CH_4 1 Ethane C_2H_6 7 Propane C_3H_8 11 Pentane C_5H_12 (vapour) 35 Hexane C_6H_14 " 45 Ethylene C_2H_4 20 Propylene C_3H_6 25 Benzene C_6H_6 (vapour) 200 Toluene C_7H_8 " 250 Naphthalene C_10H_8 " 400 ______________ ___________________ _____________
It appears from this table that, with the exception of the three hydrocarbons last named, no substance likely to be formed by the action of heat on acetylene has nearly so high an illuminative value--volume for volume--as acetylene itself. The richly illuminating vapours of benzene and naphthalene (and homologues) cannot practically add to the illuminative value of acetylene, because of the difficulty of consuming them without smoke, unless they are diluted with a large proportion of feebly- or non-illuminating gas, such as methane or hydrogen. The practical effect of carburetting acetylene with hydrocarbon vapours will be shown in Chapter X. to be disastrous so far as the illuminating efficiency of the gas is concerned. Hence it appears that no conceivable products of the polymerisation of acetylene by heat can result in its illuminative value being improved--even presupposing that the burners could consume the polymers properly--while practically a considerable deterioration of its value must ensue.
The heat of combustion of acetylene was found by J. Thomson to be 310.57 large calories per gramme-molecule, and by Berthelot to be 321.00 calories. The latest determination, however, made by Berthelot and Matignon shows it to be 315.7 calories at constant pressure. Taking the heat of formation of carbon dioxide from diamond carbon at constant pressure as 94.3 calories (Berthelot and Matignon), which is equal to 97.3 calories from amorphous carbon, and the heat of formation of liquid water as 69 calories; this value for the heat of combustion of acetylene makes its heat of formation to be 94.3 x 2 + 69 - 315.7 = -58.1 large calories per gramme-molecule (26 grammes) from diamond carbon, or -52.1 from amorphous carbon. It will be noticed that the heat of combustion of acetylene is greater than the combined heats of combustion of its constituents; which proves that heat has been absorbed in the union of the hydrogen and carbon in the molecule, or that acetylene is endothermic, as elsewhere explained. These calculations, and others given in Chapter IX., will perhaps be rendered more intelligible by the following table of thermochemical phenomena:
_______________________________________________________________ Reaction. Diamond Amorphous Carbon. Carbon. ________________________________ _________ ___________ ________ (1) C (solid) + O . . . 26.1 29.1 ... (2) C (solid) + O_2 . . . 94.3 97.3 ... (3) CO + O (2 - 1) . . . ... ... 68.2 (4) Conversion of solid carbon into gas (3 - 1) . . . 42.1 39.1 ... (5) C (gas) + O (1 + 4) . . ... ... 68.2 (6) Conversion of amorphous carbon to diamond . . ... ... 3.0 (7) C_2 + H_2 . . . . -58.1 -52.1 ... (8) C_2H_2 + 2-1/2O_2 . . ... ... 315.7 ________________________________ _________ ___________ ________
W. G. Mixter has determined the heat of combustion of acetylene to be 312.9 calories at constant volume, and 313.8 at constant pressure. Using Berthelot and Matignon's data given above for amorphous carbon, this represents the heat of formation to be -50.2 (Mixter himself calculates it as -51.4) calories. By causing compressed acetylene to dissociate under the influence of an electric spark, Mixter measured its heat of formation as -53.3 calories. His corresponding heats of combustion of ethylene are 344.6 calories (constant volume) and 345.8 (constant pressure); for its heat of formation he deduces a value -7.8, and experimentally found one of about -10.6 (constant pressure).
THE ACETYLENE FLAME.--It has been stated in Chapter I. that acetylene burnt in self-luminous burners gives a whiter light than that afforded by any other artificial illuminant, because the proportion of the various spectrum colours in the light most nearly resembles the corresponding proportion found in the direct rays of the sun. Calling the amount of monochromatic light belonging to each of the five main spectrum colours present in the sun's rays unity in succession, and comparing the amount with that present in the light obtained from electricity, coal-gas, and acetylene, Munsterberg has given the following table for the composition of the several lights mentioned:
______________________________________________________________________ Electricity Coal-Gas Acetylene ________________ __________________ _______________ _______ Colour in With Spectrum. Arc. Incan- Lumin- Incan- Alone. 3 per Sun- descent. ous. descent. Cent. light. Air. __________ ______ _________ ________ _________ _______ _______ _______ Red 2.09 1.48 4.07 0.37 1.83 1.03 1 Yellow 1.00 1.00 1.00 0.90 1.02 1.02 1 Green 0.99 0.62 0.47 4.30 0.76 0.71 1 Blue 0.87 0.91 1.27 0.74 1.94 1.46 1 Violet 1.08 0.17 0.15 0.83 1.07 1.07 1 Ultra- Violet 1.21 ... ... ... ... ... 1 __________ ______ _________ ________ _________ _______ _______ _______
These figures lack something in explicitness; but they indicate the greater uniformity of the acetylene light in its proportion of rays of different wave-lengths. It does not possess the high proportion of green of the Welsbach flame, or the high proportion of red of the luminous gas- flame. It is interesting to note the large amount of blue and violet light in the acetylene flame, for these are the colours which are chiefly concerned in photography; and it is to their prominence that acetylene has been found to be so very actinic. It is also interesting to note that an addition of air to acetylene tends to make the light even more like that of the sun by reducing the proportion of red and blue rays to nearer the normal figure.
H. Erdmann has made somewhat similar calculation, comparing the light of acetylene with that of the Hefner (amyl acetate) lamp, and with coal-gas consumed in an Argand and an incandescent burner. Consecutively taking the radiation of the acetylene flame as unity for each of the spectrum colours, his results are:
__________________________________________________________________ Coal-Gas Colour in Wave-Lengths, _______________________ Spectrum uu Hefner Light Argand Incandescent ___________ _______________ ______________ ________ ______________ Red 650 1.45 1.34 1.03 Orange 610 1.22 1.13 1.00 Yellow 590 1.00 1.00 1.00 Green 550 0.87 0.93 0.86 Blue 490 0.72 1.27 0.92 Violet 470 0.77 1.35 1.73 ___________ _______________ ______________ ________ ______________
B. Heise has investigated the light of different flames, including acetylene, by a heterochromatic photometric method; but his results varied greatly according to the pressure at which the acetylene was supplied to the burner and the type of burner used. Petroleum affords light closely resembling in colour the Argand coal-gas flame; and electric glow-lamps, unless overrun and thereby quickly worn out, give very similar light, though with a somewhat greater preponderance of radiation in the red and yellow.
____________________________________________________________________ Percent of Total Light. Energy manifested Observer. as Light. ____________________________ ___________________ ___________________ Candle, spermaceti . . 2.1 Thomsen " paraffin . . . 1.53 Rogers Moderator lamp . . . 2.6 Thomsen Coal-gas . . . . . 1.97 Thomsen " . . . . . 2.40 Langley " batswing . . . 1.28 Rogers " Argand . . . 1.61 Rogers " incandesce . . 2 to 7 Stebbins Electric glow-lamp . . about 6 Merritt " " . . 5.5 Abney and Festing Lime light (new) . . . 14 Orehore " (old) . . . 8.4 Orehore Electric arc . . . . 10.4 Tyndall; Nakano " . . . . 8 to 13 Marks Magnesium light . . . 12.5 Rogers Acetylene . . . . 10.5 Stewart and Hoxie " (No. 0 slit burner 11.35 Neuberg " (No. 00000 . . Bray fishtail) 13.8 Neuberg " (No. 3 duplex) . 14.7 Neuberg Geissler tube . . . 32.0 Staub ____________________________ ___________________ ___________________
Violle and Fery, also Erdmann, have proposed the use of acetylene as a standard of light. As a standard burner Fery employed a piece of thermometer tube, cut off smoothly at the end and having a diameter of 0.5 millimetre, a variation in the diameter up to 10 per cent. being of no consequence. When the height of the flame ranged from 10 to 25 millimetres the burner passed from 2.02 to 4.28 litres per hour, and the illuminating power of the light remained sensibly proportional to the height of the jet, with maximum variations from the calculated value of 0.008. It is clear that for such a purpose as this the acetylene must be prepared from very pure carbide and at the lowest possible temperature in the generator. Further investigations in this direction should be welcome, because it is now fairly easy to obtain a carbide of standard quality and to purify the gas until it is essentially pure acetylene from a chemical point of view.
L. W. Hartmann has studied the flame of a mixture of acetylene with hydrogen. He finds that the flame of the mixture is richer in light of short wave-lengths than that of pure acetylene, but that the colour of the light does not appear to vary with the proportion of hydrogen present.
Numerous investigators have studied the optical or radiant efficiency of artificial lights, _i.e._, the proportion of the total heat plus light energy emitted by the flame which is produced in the form of visible light. Some results are shown in the table on the previous page.
Figures showing the ratio of the visible light emitted by various illuminants to the amount of energy expended in producing the light and also the energy equivalent of each spherical Hefner unit evolved have been published by H. Lux, whose results follow:
_______________________________________________________________________ Ratio of Ratio of Mean Energy Light Light Spherical Equiva- Light. emitted to emitted to Illuminat- lent to 1 Total Energy ing Power. Spherical Radiation. Impressed. Hefners. Hefner in Watts. ____________________ ____________ ____________ ____________ ___________ Per Cent. Per Cent. Hefner lamp 0.89 0.103 0.825 0.108 Paraffin lamp, 14" 1.23 0.25 12.0 0.105 ACETYLENE, 7.2 litre burner 6.36 0.65 6.04 0.103 Coal-gas incandes- cent, upturned 2.26-2.92 0.46 89.6 0.037 " incandes- cent, inverted 2.03-2.97 0.51 82.3 0.035 Carbon filament glow-lamp 3.2-2.7 2.07 24.5 0.085 Nernst lamp 5.7 4.21-3.85 91.9 0.073 Tantalum lamp 8.5 4.87 26.7 0.080 Osram lamp 9.1 5.36 27.4 0.075 Direct-current arc 8.1 5.60 524 0.047 " " enclosed 2.0 1.16 295 0.021 Flame arc, yellow 15.7 13.20 1145 0.041 " " white 7.6 6.66 760 0.031 Alternating- current arc 3.7 1.90 89 0.038 Uviol mercury vapour lamp 5.8 2.24 344 0.015 Quartz lamp 17.6 6.00 2960 0.014 ____________________ ____________ ____________ ____________ ___________
CHEMICAL PROPERTIES.--It is unnecessary for the purpose of this work to give an exhaustive account of the general chemical reactions of acetylene with other bodies, but a few of the more important must be referred to.
Since the gases are liable to unite spontaneously when brought into contact, the reactions between, acetylene and chlorine require attention, first, because of the accidents that have occurred when using bleaching- powder (_see_ Chapter V.) as a purifying material for the crude gas; secondly, because it has been proposed to manufacture one of the products of the combination, viz., acetylene tetrachloride, on a large scale, and to employ it as a detergent in place of carbon tetrachloride or carbon disulphide. Acetylene forms two addition products with chlorine, C_2H_2Cl_2, and C_2H_2Cl_4. These are known as acetylene dichloride and tetrachloride respectively, or more systematically as dichlorethylene and tetrachlorethane. One or both of the chlorides is apt to be produced when acetylene comes into contact with free chlorine, and the reaction sometimes proceeds with explosive violence. The earliest writers, such as E. Davy, Wohler, and Berthelot, stated that an addition of chlorine to acetylene was invariably followed by an explosion, unless the mixture was protected from light; whilst later investigators thought the two gases could be safely mixed if they were both pure, or if air was absent. Owing to the conflicting nature of the statements made, Nieuwland determined in 1905 to study the problem afresh; and the annexed account is chiefly based on his experiments, which, however, still fail satisfactorily to elucidate all the phenomena observed. According to Nieuwland's results, the behaviour of mixtures of acetylene and chlorine appears capricious, for sometimes the gases unite quietly, although sometimes they explode.
Acetylene and chlorine react quite quietly in the dark and at low temperatures; and neither a moderate increase in temperature, nor the admission of diffused daylight, nor the introduction of small volumes of air, is necessarily followed by an explosion. Doubtless the presence of either light, air, or warmth increases the probability of an explosive reaction, while it becomes more probable still in their joint presence; but in given conditions the reaction may suddenly change from a gentle formation of addition products to a violent formation of substitution products without any warning or manifest cause. When the gases merely unite quietly, tetrachlorethane, or acetylene tetrachloride, is produced thus:
C_2H_2 + 2Cl_2 = C_2H_2Cl_4;
but when the reaction is violent some hexachlorethane is formed, presumably thus:
2C_2H_2 + 5Cl_2 = 4HCl + C_2 + C_2Cl_6.
The heat evolved by the decomposition of the acetylene by the formation of the hydrochloric acid in the last equation is then propagated amongst the rest of the gaseous mixture, accelerating the action, and causing the acetylene to react with the chlorine to form more hydrochloric acid and free carbon thus;
C_2H_2 + Cl_2 = 2HCl + C_2.
It is evident that these results do not altogether explain the mechanism of the reactions involved. Possibly the formation of substitution products and the consequent occurrence of an explosion is brought about by some foreign substance which acts as a catalytic agent. Such substance may conceivably be one of the impurities in crude acetylene, or the solid matter of a bleaching-powder purifying material. The experiments at least indicate the direction in which safety may be sought when bleaching- powder is employed to purify the crude gas, viz., dilution of the powder with an inert material, absence of air from the gas, and avoidance of bright sunlight in the place where a spent purifier is being emptied.
Unfortunately Nieuwland did not investigate the action on acetylene of hypochlorites, which are presumably the active ingredients in bleaching- powder. As will appear in due course, processes have been devised and patented to eliminate all danger from the reaction between acetylene and chlorine for the purpose of making tetrachlorethane in quantity.
Acetylene combines with hydrogen in the presence of platinum black, and ethylene and then ethane result. It was hoped at one time that this reaction would lead to the manufacture of alcohol from acetylene being achieved on a commercial basis; but it was found that it did not proceed with sufficient smoothness for the process to succeed, and a number of higher or condensation products were formed at the same time. It has been shown by Erdmann that the cost of production of alcohol from acetylene through this reaction must prove prohibitive, and he has indicated another reaction which he considered more promising. This is the conversion of acetylene by means of dilute sulphuric acid (3 volumes of concentrated acid to 7 volumes of water), preferably in the presence of mercuric oxide, to acetaldehyde. The yield, however, was not satisfactory, and the process does not appear to have passed beyond the laboratory stage.
It has also been proposed to utilise the readiness with which acetylene polymerises on heating to form benzene, for the production of benzene commercially; but the relative prices of acetylene and benzene would have to be greatly changed from those now obtaining to make such a scheme successful. Acetylene also lends itself to the synthesis of phenol or carbolic acid. If the dry gas is passed slowly into fuming sulphuric acid, a sulpho-derivative results, of which the potash salt may be thrown down by means of alcohol. This salt has the formula C_2H_4O_2,S_2O_6K_2, and on heating it with caustic potash in an atmosphere of hydrogen, decomposing with excess of sulphuric acid, and distilling, phenol results and may be isolated. The product is, however, generally much contaminated with carbon, and the process, which was devised by Berthelot, does not appear to have been pursued commercially. Berthelot has also investigated the action of ordinary concentrated sulphuric acid on acetylene, and obtained various sulphonic derivatives. Schroter has made similar investigations on the action of strongly fuming sulphuric acid on acetylene. These investigations have not yet acquired any commercial significance.