THERMAL ANSLYSIS OF T H E SYSTEM BORON TRIFLUORIDEHYDROGEN SULFIDE BY ALBERT F. 0. GERMANTU’ AND HAROLD SIMMONS BOOTH
Since the discovery by Friedell in 1875 of the compound (CH3)20 . HCI much interest has attached to the study of molecular combinations between gases. Practically, this discovery was less amazing than the similar discovery by Priestley a century before that gaseous ammonia combines with gaseous hydrogen chloride to form a solid, S H 3 .HC1. But the experimental difficulties of assigning a formula to Friedel’s compound were tremendously greater than those surrouiiding the other, which was a well-known compound, whose composition was made evident by the new method of preparation. Since the discovery of Friedel’s cornpound, and his classic research into the nature of the compound, other methods have been developed which permit the detection and formulation of even unstable addition compounds be tween gases, by methods that are much simpler than those carried out by Friedel. Reference may be made, for example, to the methods used by McIntosh2and his co-workers, by Bagster3 and by Baume and his co-workers4for determining the existence and composition of addition compounds by thermal analysis. In the study of the properties of certain halide gases, the problem of preparing these gases in a high degree of purity confronted us. The best methods of accomplishing this involve the preparation of the desired gas (a) in vacuo, (b) from the purest materials, (c) by a method involving no other volatile products, (d) a preliminary purification of the gas from possible impurities by chemical means involving a gas absorption train, (e) condensation of the gas to a liquid, followed by a series of careful fractional distillations, and (f) if practicable, fractional crystallization. In spite of all these precautions, however, some impurities are very difficult to eliminate; for example, different investigators have found it diffiult to eliminate from hydrogen chloride some impurity which imparts a rose color to the frozen gas5. On the other hand, Ingliso found, in a study of chamber gases, that on certain occasions some impurity imparted a red color to the Bull. ( 2 ) 24, 160, 241 (1875). J. Chem. SOC.,85, 919 (1904); Rfaass: J. Am. Chem. SOC.,33, 71 (1911); 34, 1273 (1912); etc. J. Chem. SOC.,99, 1218 (1911). J. Chim. phys., 9, 245 (1911); 12, 2c6 (1914). SGray and Burt: J. Chem. SOC.,95, 1640 (1909); Cardoso and Germann: J. Chim. phys., 10, 517 (1912). 6 J. Soc. Chem. Ind., 25, 152 (1906).
3 70
ALBERT F. 0. GERMANN AND HAROLD SIMMONS BOOTH
frozen gas, usually white; on these occasions he mentions that the flue gases contained a chloride. Briner and llTroczynskilobserved that when a mixture of hydrogen chloride and nitric oxide is cooled in liquid air, the resulting solid has a wine-red color. Germann and Phillips2 have observed the production of a similar color with nitric oxide and boron trifluoride; and Germann has found that other halogen-containing compounds, for example phosgene, behave in the same way with nitric oxide at low temperatures, and he has used this property as the basis of a method for detecting nitric oxide in laughing gas. This red color is undoubtedly to be explained by the formation of an addition compound which halogen gases in general form with nitric oxide, Evidently the tendency of a gas toward compound formation with a given impurity, as well as the formation of constant-boiling solutions, would enhance the difficulty of making a separation of the impurity by fractional distillation, It seemed highly desirable, therefore, to have more information, particularly at low temperatures about the affinities of the gases being studied for other gases, especially those liable to be present as a result of the method of preparation. Boron trifluoride and phosgene were under investigation a t the time these studies were carried out (1919-ZI), the intention being to determine the weight of the normal liter of these gases; but owing to the removal of one of us from the Morley Laboratory of Western Reserve University, where the investigations were carried out, the program was not completed, except that some preliminary measurements were made in the case of phosgene3. In the preliminary study of these gases, it became evident that the elimination of some of the impurities was very tedious. It seemed, therefore, worth while to determine by the methods of thermal analysis as developed by Baume the possible addition compounds formed by these gases with the impurities likely t o be present. In the case of boron trifluoride, the method of preparation involved the action of sulfuric acid on calcium fluoride and boric anhydride, Gray and Burt have shown that in the similar reaction of sulfuric acid with sodium chloride or with ammonium chloride to produce hydrogen chloride a little hydrogen sulfide was always produced; they attributed its formation to reduction of the sulfuric acid during the reaction. The possibility of hydrogen sulfide being produced in the preparation of boron trifluoride due to the presence of small amounts of reducing substances, suggested the investigation of this system. Preparation of Boron Trifluoride4 7 0 grams of boric anhydride were dissolved with the aid of heat in 2 5 0 cc. of concentrated sulfuric acid. Analysis showed that the boric anhydride J. Chim. phys. 9, 116 (1911). Science, 53, 145 (1921). 3 Germann and Jersey: Science, 43, 582 (1921); Germann and Smith: Science, 57, 564 (1923). Gasselin: Ann. Chim. Phys., ( 7 ) 3, 9 (1894).
BORON TRIFLUORIDE AND HYDROGEN SULFIDE
371
contained 10% of water, I n one experiment, the effort was made to get rid of this water, by adding the calculated amount of pyro-sulfuric acid; but the gas obtained, when condensed to a solid, had a bright red color, obviously due to the same impurity, KO, which imparts a red color to solid hydrogen chloride. As the purity of the pyro-sulfuric acid used was questionable, it seemed probable that it might be the source of this impurity; in subsequent preparations, therefore, the fuming sulfuric acid was omitted, and the mixture made up of 300 cc. pure concentrated sulfuric acid, 7 j grams boric anhydride, and 2 0 0 grams calcium fluoride. Prepared in this way, the gas condensed as a white solid, without a trace of pink color, and melted to a clear limpid liquid. The use of fuming sulfuric acid thus seems inadvisable.
FIG.I
After cooling the solution of boric anhydride in sulfuric acid, it was mixed with 2 0 0 grams of calcium fluoride in a large porcelain mortar, and the mixture, which warms up and froths considerably, was stirred until the gas evolution ceased. The gases given off probably consisted principally of silicon tetrafluoride. The viscous mixture was then transferred to the reaction flask labelled A a t the left end of Fig. I , which shows the arrangement of the apparatus, and the mouth of the flask was then sealed off, as shown. Flask A communicated directly with a reflux condenser, C, and a safety manometer tube, MI, with a safety wash-bottle U containing a solution of boric anhydride in sulfuric acid, made up as described above; this was followed by a tube packed with alternate layers of glass wool and phosphorus pentoxide, and by a pair of condensing and distilling tubes, TI and Tz,each provided with a safety manometer tube Mz and MS. Thereaction mixture in flaskA was heated on asteam bath for an hour, while the air was pumped out of the apparatus; during this time no poticeable gas
ALBERT F. 0. G E R M A N 3 AND HAROLD SIMMONS BOOTH
372
evolution took place. Heated with a free flame, however, the reaction temperature was soon reached, and the positive heat of reaction was sufficient to maintain it. With the intervening stopcock open, and a bath of liquid air surrounding TI, the gas condensed to a white solid, and melted to a clear limpid liquid. On some occasions, the liquid boron fluoride contained particles of a white solid floating around, which may have been hydrogen sulfide, as reported by Gray and Burt, or carbon dioxide, or a product of the interaction of boron fluoride with phosphorus pentoxide, a reaction that seems to take place with some readiness. After collecting as much of the gas as was required, the reaction was interrupted by cooling flask A, and the condensed gas was subjected to a series of fractional distillations between TI and T2, rejecting the first and last fractions in each distillation, and the purified product was finally stored in F, a I j liter globe, previously evacuated and dried. When required for use, the gas was recondensed in T2with liquid air. During storage, some change occurred, evidenced by the fact that, on evaporation, a white deposit was left, which sublimed at a higher temperature; it is not clear what this was, though it may have been one of the fluorides of silicon, produced by the action of hydrogen fluoride on the glass1: the presence of hydrogen fluoride may be accounted for on the assumption of incomplete drying of the storage flask. A few additional fractional distillations sufficed to eliminate all traces of this impurity. Preparation of Hydrogen Sulfide Hydrogen sulfide was prepared by the action of constant-boiling hydrochloric acid (minimum vapor tension) on carefully washed zinc sulfide, freshly precipitated from a solution of zinc acetate: the reaction took placein a one liter flask, B, which contained the zinc sulfide; the acid was added as needed from the dropping funnel sealed into the neck of B. The evolved gas was freed from hydrogen chloride by bubbling it through a suspension of freshly precipitated aluminium hydroxide in water contained in the safety wash bottle R, and through the gas bubbler V, containing water, dried by contact with anhydrous calcium chloride and phosphorus pentoxide, and finally condensed in tube T h in a bath of carbon dioxide snow and acetone. The safety manometer, M?, interposed between the reaction flask B and the wash bottle W, is essential, as any clogging of tubes in W or in the drying tubes might give rise to serious consequences, Cardoso2 has very ably discussed the impurities accompanying hydrogen sulfide, which is difficult to obtain perfectly pure, and has devised elaborate precautions for their elimination. For our purposes the product obtained after three fractional distillations was of sufficient purity, and was stored in the globe marked R until required, when it was condensed in T3. See Germann and Booth: J. Phys. Chem. 21, 83 (1917). I, 153 (1921).
ZGazz., 51
BORON TRIFLUORIDE AND HYDROGEN SULFIDE
3 73
The Freezing Point Measurements The central portion of the apparatus was devoted to the measurement of the gases, the preparation of the solutions, and to the measurement of the freezing points. The details are similar to those described by Baume1. The gases were measured volumetrically in the calibrated flasks H and G, packed in cracked ice to maintain constant temperature. The pressure of the gas was measured on the baro-manometer P, one side of which could be evacuated by opening the stopcocks z and I leading to the vacuum apparatus, thus making the readings independent of atmospheric pressure. To measure a quantity of boron fluoride, for example, stopcock 4 of flask H, and stopcock 3 leading to the manometer were opened, and boron trifluoride was allowed to evaporate through stopcock 5 into the evacuated space thus made available, until the pressure reached a little more than one atmosphere. 5 was closed, and the pressure read and recorded; 4 was then closed, and the dead space between it and the manometer evacuated by liquid air condensation of the gas it contained into Tz, by opening j for a few moments. H then contained a known volume of gas under a known pressure, P’, a t a known temperature, 0’. The dead space between the mercury of the pressure manometer and the stopcock of the measuring flask contained the same gas at a pressure P”, equal to its vapor tension at the temperature of the cooling bath used (liquid air or liquid nitrogen, sometimes liquid oxygen). The gas in H was then transferred to the freezing point tube L by liquid air condensation, opening 4 and 6 for the purpose; when no further condensation occurred, 6 was closed, and the pressure P” was read on the manometer P ; the weight IT of the gas condensed in L was then (neglecting the small correction for the deviation of the gas from Boyle’s law) L v (P’-P”) L v Po = 1000 X 7 6 0 1000 X 7 6 0
w
where L is the weight of the normal liter of the gas, V is the volume of the measuring flask in cc., and Po is the pressure. Since, however, the molecular composition of the solution was required, rather than the composition by weight, the mass of the gas used was not actually needed. Assuming the perfect gas law, we have
-M, - - C pi Y Mh
Ph
where M, and Mh are the number of moles of the two gases measured in G and in H, Pi and Pi the pressures of the two gases, and c the ratio of the volumes of the two flasks, V h = cv,. Correcting for the deviation of the two gases from the perfect gas laws, we have 1
J. Chim. phys. 9, 245, (1911);12, 206, 216, 225, 242, 250, 256, 270, 276 (1914).
3 74
ALBERT F. 0. GERMANN AND HAROLD SIMMONS BOOTH
where A$]: and Ai]: are the compressibilities of the gases in G and H between zero and one atmosphere. The mole fraction Xg of the gas in G is given by
x,
=
M,
=
c Pi (760
+ Ail:)
The corrective factors are small and may be neglected in most cases, especiallyif the gases are in approximately corresponding states at oo and one atmosphere. I n this case the calculation takes the simple form
The volumes of the two globes were 8 9 3 . 5 3 ~for ~ . G, and 8 9 6 . 5 0 ~for ~ . H. Enough gas was condensed in L to cover the bulb of the thermometer; then the freezing point was determined, using as cooling agent a bath of liquid air, separated from the freezing tube by an air jacket J, containing a few pieces of anhydrous calcium chloride to prevent the formation of hoar frost in the annular space between the jacket and the freezing tube. The liquefied gas was stirred by the vertical motion of a spiral glass stirrer S, actuated by the attraction of an electromagnet E for a soft iron collar K attached to the top of the stirrer; the FIG.2 electromagnet worked intermittently, having an electrically connected metronome in the circuit.
A small amount of the second gas was then measured into the freezing point tube, and the freezing point observed again, and so on until the solution was approximately equimolar with respect to the two components. A similar routine, beginning with the second gas, and adding small amounts of the first, gave the data for the other half of the freezing point curve. The thermometer employed was a liquid propane thermometer, constructed by Mr. Pierre E. Haynes, Director of the Linde Air Products Research Laboratory, of Buffalo, N. Y . ,and presented by him. The correction
BORON TRIFLUORIDE AND HYDROGEN SULFIDE
375
curve for the emergent column was prepared by observing the freezing points of a number of pure liquids, chosen so as to give points at intervals along the scale. The corrected temperatures are probably accurate to one or two degrees. However, for our purposes, relative rather than absolute temperatures were required, so that an error from this source would not impair the validity of conclusions bearing on the chemical affinities of the substances studied. In Table I are the data with which the curve in Fig.
2
was constructed.
TABLE I ilverage
Pi HzS P i BFs Mol. Frac. Temp. F. P. mm.
mm .
3170.0 11
0.0
44.3
11
85.7
11
2j0.5
11
329.3 415.4 518.1 639.3 780. I 976.4
11 11
11 11 11 11
1170.7
1)
1361.8 1542. I 1761.4
11 11
11
2025.2
11 11 11 11
0.0
39.1
75 .o
2291.9 2540.3 2808.4 3059.7 2350.3
IIZS I
.ooo
,987 .974 ,927 .907 ,884 .860 '833 ,803 .765 .73I .700
'674 ' 644 .6r1 .581 . j56 431 .SI0 ,000
11
.or6
11
.03 I
PiHzS PE BF8 Mol.
Average Frac. Temp. F.P.
mm.
mm.
HZS
- 82 - 84%
108.6 156.2
2350.3
- 86% - 90% - 934 - 97 -IO0 -I02 - 104 - 1069
200.8
.044 .063 ,079 .095
O C*
-111
- 113 - 116 -119 - 125
- 1289 - I33 - I379 - 141 - 1284 - 131 - 130%
247.3 290.9 332.6 370.3 421.0
.3 594.9 689.2 781.6 885.0 1008. j 762.3 879.0 1013. I 1173.8 1347.0 1524.9 1947.6
11 11 1) 11 11
. I10 . I24
11
.136
11
.Ij 2
11
501
11 11
11 1) 11
1558.5 11 11
11 11 11 11
,176 ,202
.227 .250
.274 .301 ,329 .361 .395 ,430 ' 464 .495 .5j6
O C
-135 - 132 -137 -137 - I39 -139 - 141 - I43 - I44 - 146 - I474 - 146 - I44 - 142 - 141 - I394 --'I38 - I373 - 136 -137 - 130
For purposes of comparison, the data relative to other systems containing hydrogen sulfide as one component have been plotted along with the curve for the system here studied. These include systems of hydrogen sulfide with hydrogen iodide and with hydrogen bromide, by Bagster' ; hydrogen sulfide with hydrogen chloride, by Baume and Georgitses2; and hydrogen sulfide with methyl alcohol and with methyl ether by Baume and Perrot3. J. Chem. Soc., 99, 1218 (1911). J. Chim. phys., 12, 2 j 0 (1914). J. Chim. phys., 12, 2 2 5 (1914).
3 76
ALBERT F. 0. GERMANN AND HAROLD SIMMONS BOOTH
Discussion The system boron fluoride-hydrogen sulfide (see curve I ) presents a maximum at - 137’~with a solution containing equimolecular quantities of the two gases, showing the existence of the compound B F 3 . HzS. This compound is considerably dissociated a t the melting point, as shown by the fact that addition of either of the products of dissociation to the compound has little effect on the freezing point1, yielding a well rounded maximum. The curve shows two eutectics, one on either side of the maximum, one a t - 148’ with 2 2 7 0 of hydrogen sulfide, the other at - 140’ approximately, with 5376 of hydrogen sulfide. The form of the curve a t -99’ with 87$7G of hydrogen sulfide indicates that this is the transition point of a crystalline addition compound, whose formula seems to be BF3. 7HsS, and which dissociates into boron fluoride and hydrogen sulfide a t this temperature. Comparing with the diagrams for the other systems that have been recorded in the literature, it appears that boron fluoride has more affinity for hydrogen sulfide than have any of the other substances. Methyl ether, which is noted for the readiness with which it yields oxonium compounds, yields a diagram that is almost identical with that for boron fluoride, excep?. that there is no indication of a higher hydrosulfide. If we assume with Lewis2 that boron fluoride possesses three electron pairs in the valence shell of boron, we can easily represent graphically the structure of the first compound as one in which a lone electron pair of hydrogen sulfide is attracted to the boron atom in such a way as to complete its octet :-
..
:F:
.... .... +
:F:B
:F:
..
H .. . .:F: . . .H. :S: + : F : B :S: .. ...... :$’: H H
..
.I
.
Boron fluoride is known to form compounds with the ethers3, in which the union may be represented in the same way, oxygen occupying the place of sulfur, and methyl groups the places of the hydrogen atoms; but in the addjtion compound between methyl ether and hydrogen sulfide, the hydrogen bond4 evidently serves as the linkage between oxygen and sulfur, since the octet of each is complete: I€ .. H : C. : .H . .
:O:H:S:H
. . . .
H : C..: H H
Kremann: Monatshefte, 2 5 , 1215 (1904). “Valence and the Structure of Atoms and Molecules”, p. 98 (1923). 8 Gasselin: Ann. Chim. Phys., (7) 3, 28, 48 (1894). 4 Latimer and Rodebush: J. A4m.Chem. Soc., 42, 1431 (1920); Huggins: J. Phys. Chem., 26, 614 (1922). 1
BORON TRIFLUORIDE AND HYDROGEN SULFIDE
377
Judging from the form of the maxima, the sulfhydride of boron fluoride and that of methyl ether are dissociated at their respective melting points to about the same extent, indicating that the two types of linkages, that between sulfur and boron on the one hand, and that between sulfur and oxygen via hydrogen on the other, are about equally strong, neglecting the difference in the temperature. Any attempt to account for the non-formation of molecular compounds between hydrogen sulfide and the hydrohalogens and methyl alcohol would require a comprehensive analysis of all the known addition compounds of these substances, as well as of other similar ones. For example, why should the mere replacement of a methyl group in methyl ether by a hydrogen atom render the attraction of the oxygen atom for hydrogen sulfide inoperative? Is the temperature at which the alcohol-hydrosulfide eutectic separates so high that the addition compound is completely dissociated? That halogen atoms may unite with sulfur by means of the hydrogen bond is evidenced by the compound B F 3 . 7HzS; why do the halogen atoms in hydrogen chloride, hydrogen bromide and hydrogen iodide not yield addition compounds? It is true that Pollitzerl found the solubility of hydrogen sulfide in aqueous hydrogen iodide greater than in pure water (100% greater in 7N - HI, and 15% greater in I O K - HCl), which has been interpreted as evidence of compound formation2, but other evidence given by Pollitzer would seem to indicate that some other explanation would fit the facts better. The experimental work described here was performed in 1920 at the Morley Chemical Laboratory of Western Reserve University. The liquid air used was supplied by the Linde Air Products Co., to whom we desire to express our appreciation.
Summary The freezing point curve of the system boron trifluoride -hydrogen sulfide, has been investigated, and has been found to contain two eutectics, a maximum corresponding to BF3 . HZS, with freezing point - 1 3 7 ’ ~and a transition point of a compound B F 3 , 7HzS at -99’. Judging from the flatness of the curve a t the maximum, the compound, BF3, HZS, is considerably dissociated, so that there should be little difficulty separating H2Sas an impurity from BF, by fractional distillation. Laboratory Products Company, Cleveland, Ohio, and Western Reserve Unzversity, Cleveland, Ohio.
Z. anorg. Chem., 64, 121 (1909). Lewis and Randall: “Thermodynamics”, p. 106 (1923).
