SOLVENT ACTION O F VAPORS BY A.
T. LINCOLN
T h e existence of solid solutions has been thoroughly demonstrated and the study of liquid solutions, wherein one or both of the components may be a liquid, has been and still is a very considerable field of investigation. Since the solvent action is a function of the temperature and pressure and in general increases with the increase of the former, it seems hardly probable that this specific action which is manifested by the substance in the solid phase and more markedly in the liquid phase should become nil as soon as the vapor phase has been reached. Some attention has been devoted to the solvent action of substances in the vapor phase, and the object of this investigation is to see if some additional data could not be collected which would aid in the solution of this problem. We will first present the theoretical aspect of the subject and then give the experimental data which we hare collected. 4 s the solubility is a function of the temperature and pressure, it will be necessary that the diagram employed will have to represent the effects of these two factors. Neither a simple temperature-pressure nor a temperature-concentration diagram will do it ; but by means of a combination of these two, the resulting surface could be used to illustrate what we desire. Since, however, it introduces a solid figure, the coniplications which arise make the treatment rather difficult and confusing. T h e result may be reached in a simpler manner by a plain figure if we employ as our abscisse the temperature, and as the ordinates the volume concentration. For illustration, let us take the simple case for water as given in Fig. I. Since the density of water changes but slightly with the temperature, the portion
7x6
.4. T. Lincoln
of the curve, SPW, representing the solid phase of water, will be nearly horizontal as far as the melting-point M. Here there is a decided change in the density and the volume concentration increases, hence the break extends upward. T h e liquid phase portion of the curve L P W will gradually drop down until the critical point C is reached. For the volume concentration of VOL-
conc.
t Fig.
I
the vapor phase of water, we shall have the curve VPW, which is very much lower than those for the liquid and solid phases. Now let us consider the case of two pure components for temperatures below the critical point, say up to the melting-point of the higher melting substance. In Fig. 2 let SPA, LPA and VPA represent the curves for the volume concentration of the solid, liquid and vapor phases of A respectively. As a rule, the liquid phase is less dense than the solid ; we will indicate this by a decrease of the volume concentration at the melting-point of the substance instead of an increase as in the case of water. I n an analogous manner for the other component B, we will represent the relative volume concentration for the solid, liquid and vapor phases by the curves SPB, L P B and VPB respectively,
Solvent Action of Vapors
717
the melting-points at MI and MZ, while Cr and C2 represent the critical points. T h e volume concentration at some temperature, such as the eutectic temperature, we may designate by the location of E, then the volume concentration of pure A in the liquid phase for the temperatures between this and its own melting-point will be represented by the curve EMT,while between the eutectic temperature and the melting-point of the other component B, by the curve EF. In a like manner we have the values for the
conc. -'O"
I
volume concentration of pure B in the liquid phase between the eutectic temperature and the melting-point of the component A, represented by the curve ed, while the curve r?M2 represents the volume concentration between the eutectic point and the melting-point of B. For the components in the vapor phase we shall have the curve ab for the volume concentration between the melting-point of A and the eutectic temperature, while the curve bF represents it between the eutectic temperature and the
melting-point of B ; the curves dc and cf representing the corresponding values for B in the vapor phase. In the preceding, we have been dealing with the pure coinponents. Now let us consider the solvent action of the components and at temperatures ranging above the critical temperature. In Fig. 3 we will represent the solid, liquid and vapor phases of the two components A and B as was done in Fig. 2. This diagram differs from Fig. 2 in that the values for the lower melting substance in the solid and liquid phases are lower than those for the higher melting component, and this is the case generally, but the relative position of these two curves is immaterial. Let us consider first the liquid phase. For A we would have between the melting-point of A and the eutectic temperaVOL-
conc.
t Fig. 3
ture the volume concentration represented by EMI, while at the critical temperature of A there would be some of the component €3 present, and hence the volume concentration at the critical
Solveat Actioiz
of' Vapor
719
temperature CI would be represented by some point as D, then the curve ED will represent the volume concentration of B from the eutectic temperature to the critical temperature of A, while the curve CID represents the limiting surface. In like maimer for the component B in the liquid phase, the curve ce represents the volume concentration between the melting-point of A and the eutectic temperature, but for temperatures higher than this what direction will the curve take? This depends upon whether A above its critical temperature has any solvent action on B. I t either has or it has not. If it has not, then the volume concentration of B above the critical temperature of A would lie in the line VPBC,, and if it has a solvent action, it would lie. above. As early as 1880 Hannay and Hogarth' showed that KI, KBr, and CaClz were dissolved by alcohol vapor above its critical temperature. A solutioii of sulfur in CS2 did not separate sulfur at 50' above the critical point, and selenium also remained in solution. Resin in some light above the critical paraffins remained in solution at 360'-100' point. And in the case of CoC12 in alcohol, the same characteristic absorption bands were present at 300' as at 15'. Further, they showed that the decomposition-product of chlorophyll by acids gave the same characteristic bands in alcoholic vapor at 350" as in alcoholic solution. Villard' has shown that iodine is dissolved by COa above the critical point and that the absorption spectra of the vapor does not show the least characteristic of gaseous iodine. Even at 20'-25' there was an appreciable coloration of the COz, due to the iodine dissolved. That vapors manifest a solvent action has also been shown by Pictet.3 Wood4 demonstrated that iodine and bromine are soluble in CS,above its critical temperature and confirmed the work of Hannay and Hogarth concerning the solubility of K I in alcohol vapor. He further showed that Hg12 is soluble in ether vapor. I
Proc. Roy. SOC.30, 178 (1880). Comptes rendus, 120, 182 (1895).
Ibid., 120,64 (1895). Phil. Mag. 41, 423, (1896).
720
A. I: Lzncoln
From the experimental results just considered, we see that above the critical temperature many substances do manifest a solvent action. Now referring to Fig. 3 again, we will locate that point above the critical temperature at T, thus showing that the quantity of the component B present is more than would be represented by the curve VPBC, at that particular temperature, i. e., some has been dissolved. Now considering the concentration in the vapor phase in a manner analogous to that employed for Fig. 2, we see that the curve ab represents the volume concentration of A between the melting-point of A and the eutectic temperature, while bD represents it up to the temperature where A ceases to exist as a liquid. For the component B, the curve cd represents the volume concentration between the temperature of the melting-point of A and the eutectic temperature. If the vapor of A does not exert any solvent action, then the point will fall on the line VPB, but we do not know whether it does or whether it does not at this particular temperature, but at this low temperature, these values must approximate each other so closely that at some temperature they may be considered as identical. Then the curve which represents the volume concentration between the temperature represented at d and T must pass through these two points. Whether this boundary curve takes the directiou d G T or dHT, we do not know ; but it is presumable that the change in the concentration wi!l fall off much more rapidly at the higher temperatures than at the lower, and therefore d H T would be the more probable curve, and hence the volume concentration curve would drop rapidly until it came near the vapor pressure curve for pure B and would no doubt approach it asymptotically. Hence we see that in order to determine whether the vapor has a solvent action and to measure this, it would be necessary to work at the higher temperatures where the curves dHT and VPBC, are some distance apart. I n other words, we would have to work at such temperatures where the difference between the partial pressure and vapor pressure of one component was greater than the experimental error.
Solveitt Actioiz of Vapor
721
I t was hoped by choosing salicylic and benzoic acids that we had substances which would give us vapors, the composition of which could be readily determined and at the same time, that the difference between the partial pressure and the vapor-pressure would be much greater than the experimental errors. T h e presentation of the experimental part will now be given. As the vapor pressure of salicylic and benzoic acids could not be found in the literature, it was necessary to determine these values for the lower temperatures at least. T h e method employed was substantially the same as that used by Ramsay and Young.’ T h e acids were fused npon the bulb of a standardized thermometer and this placed into a IOO cc distilling flask and then heated by means of an oil-bath. T h e bath was kept at 3oO-40~above the temperature recorded by the thermometer in the distilling flask. When the flask was immersed directly into the bath, the vaporization of the acids was not sufficiently rapid to keep the temperature constant and coiisequently the difference between the readings of the two thermometers would be only a few degrees. By this method it was impossible to raise the temperature of the bath much above the temperature registered by the thermometer of the flask. In order to get the temperature of the bath sufficiently high above that of the thermometer within the flask it was found necessary to place the flask into a small beaker and pack cotton batting tightly around the flask, thus making an air-bath. T h e beaker with its contents was then immersed nearly to its top in the oil. By this modification, we were able to keep the bath at any desired temperature above that recorded by the thermometer in the flask. This flask was connected with a manometer which read to millimeters and this in turn with a Boltwood mercury pump. ,411 joints were made by fusing the glass tubes together, except two:- the corinection with the mercury pump, which was a mercury joint and the rubber stopper through which the thermometer, on which was the fused acid, passed into the distilling ~
Phil. Trans. 175, 37 (1884)
A. T. Liizcohz
7-22
flask. This latter joint was made tight by pressing the stopper below the top of the neck of the flask and sealing by means of a mixture of beeswax and rosin. With. the connections made in this manner, and all of the stop-cocks well greased with a lubricant consisting of pure India rubber and beeswax, any pressure desired could be obtained and retained for hours. With the apparatus in this form, the vapor pressures of salicylic and benzoic acids were obtained. T h e values are given in Tables I. and 11. respectively. In the first column is given the temperatures as read from the thermometers, while the corrected values are given in the second column. These corrections are the corrections which are due to the portion of the thermometer thread which is exposed to the air and which extends outside of the flask. T h e corrections were made from ihe table as given in Landolt and Bornstein's Tabellen. In the last column is given the pressure in millinieters of mercury. TABLE _-_ _ _ ~
I
Vapor Pressure of Salicyclic Acid ____
~
I
Temperature
I
Observed
Corrected
82.5' 86.6 103.2 103* 4
82.5' 86.6 103.2 I03 4 116.1
I I
0.3 0.5 I .o I .o
3
115.4 I 14.2
IIj.0
115.2
115.9
121.2
122. I
131.2 137.0 139.9 139.5 147.0 145.4 '49.2 153.2 155.0
132.2 138.2 140.3 140.8 148.6 146.9 150.9 155.0 156.9
Pressure mm .
~
2.0
2.5 2.5 3.0 6.5 11.0
11.5 12.0 21.0 22. j 27.0
37.0 44.0
'
723
Sol-denf Actioiz of Vapor
Temperature
I I
Pressure
Observed
Corrected
71.3O 70.8 92 .o 99.0 101.6
71.3O 92.0 99.0 101.6
107.0
107.0
109.8 I 18.0 131.3 145.4 147.3 153.3 153.9
I 10.3 118.8 132.3 146.9 148.9
23
155.1
30.5
70.8
155.8 I 60.8 162.3 167.4 167.5 171.4
I 58.8
160.2 16j.o 165.1 '169.0 169.8 172.8 172.9 176.0 176.0 '79.2 182.0 182.I 18j.3 187.6 187.7 I 90.0
,
mm. 0.2
..
0.4
I .o
1.8 2.0
3.0 3.2 5.5 11.0
21.5 0
31.2
40.0
41.0 50.0
51.0 60.2
172.2
62.2
'75.4 175.5 I 78.7 178.7 181.9 184.8 184.9
71.0
188.2
190.5 190.6 193.0
70.5 80.0
80.5 91.0 101.0
103.0 I 15.0 127. j
126.0 I 36 .o
I n Fig. 4' will be found a graphical representation of the vapor pressures of these two acids. T h e tetnperatures are plotted along the abscissze and the logarithm of the pressures along the ordinates. The value marked K is plotted from a value determined by Kahlbaum. Since there is a decided break in the vapor-pressure ciirve of benzoic acid and the vapor condenses in the neck of the flask, this would make a very good laboratory experiment.
724
A. T. Lincoln
I t will be noticed that the values for salicylic acid are very much higher than required by the curve prolonged. This is no doubt due to the experimental difficulty in reading the low pressures by means of a mercury manometer graduated to millimeters. The same is true for the first few values given for benzoic acid. SO
2.00
.so
I1.00
.50
10.00
.50
9.00 O0
T h e distillation of the saturated solutions was carried out by placing in a retort considerably more of the solute than would be dissolved by the solvent at the temperature of the experiment. Since solubility determinations are not given, this had to be de-
Solveiat Action of l7apor
725
termined experimentally. T h e mixture was then boiled by usually heating the retort in a glycerol bath, but it was found that it was immaterial whether the heat was snpplied in this manner or by direct flame since the distillate contained practically the same per cent of the solute. I n order to prevent condensation on the neck of the retort and to insure that all of the vapor-both of the solute and solvent -was carried over, the neck of the retort was heated electrically. When water was used as solvent, i t was found that considerable of the solute would separate in the solid phase when the distillate was cooled. So in order to be sure that all of the distillate was collected in the receiving flasks it was necessary that all of the condensation should take place in the flasks. Hence these flasks were siirrounded by ice and successive portions of the distillate collected in different flasks. Considerable difficulty was, however, experienced when it was necessary to distil under diminished pressure. I n order to collect different portions and at the same time have the condensation take place in the receiving flask, a Y-tube was placed on the end of the delivery tube, and on the two ends of this tube were placed two small flasks. When a sufficient quantity of the distillate had been collected in one of them, the ice-bath was removed and the tube rotated so that the two flasks would exchange places and then this second flask was surounded by the ice and the distillate collected in it. These particular substances were chosen for experimentation so that it would be a simple matter to analyze the distillates for the acids by titrating with a standard alkaline solution. For this purpose about a n / 5 0 solution of barium hydroxide was prepared ; but it was not an easy matter to find a suitable indicator. For the analysis of salicylic acid in water, acetone and benzol solutions, Walker and Wood' recommend the use of Congo-red as an efficient indicator. My experience, however, with the particular sample of Congo-red used, was very unsatisfactory. I n exceedingly dilute solutions the end reaction is apparently fairly delicate, but in moderately concentrated warm aqueous solutions the end reaction is not sharp, but there is a gradual transition in color and no distinct change on which to set. Jour, Chern. SOC.73, 619 (1898).
726 Several other indicators were tested and the results for phenolphthalein and paranitrophenol are given in Table 111.
TABLEI11 Salicylic Acid in Aqueous Solutions Indicator -Paranitrophenol
_____
Grams of acid
I
Taken
1
--I
0.229
I
[
2
0.202
I
i 1
1
Found
~
-
0.2284 0.1957
i-
1
Percent . -
99.72 96.9
Indicator - Phenolphthalein 0.1624
I 2
0.2161
I
630 0.2175 0.I
1
100.3
100.6
Walker and Wood state that they did not find phenolphthalein to give good results. For the determination of benzoic acid in aqueous solutions, rosolic acid was used as an indicator and the preliminary determinations gave the following results : ~~~
__
~~
-
__
~~~
Grams of acid ~
_
~
-~ _
j
Taken ~
Found
Perceii t
727 TABLEIV Benzoic Acid in Benzol Indicator - Rosolic Acid
___
~ ~.. -_ _ _
~ _ _ _ _ _
Grams of acid
'
I 2
' I
3
_ -
Found
Taken
0.0468 0.0394 0.0286
I
I
Percent
~
0.04689 0.03978 0.02843
100.2
I O 1 .o
99.4
Indicator -Paranitrophenol I
I
I
I
2
0.02j
j
0.02723
~
100.3 99.0 100.j
Paranitrophenol can be used with about as fair a degree of accuracy as rosolic acid provided one sets on the appearance of the canary yellow coloration, i. e., on the color of the aqueous solution of paranitrophenol. Rosolic acid was used as the indicator in the analyses of the distillate of both the aqueous and benzol solutions of benzoic acid. Acetone was first employed as solvent for salicylic acid, but owing to its low boiling-point, a mere trace of the acid was found in the distillate. Water was next employed, but upon distilling a saturated solution of salicplic acid, a distinct odor of phenol was detected and upon further examination, it was found that carbon dioxid wa.s given off. Although considerable salicylic acid was detected in the aqueous distillate as well as in the distillate from benzol solutions, it was advisable owing to the unfavorable action of the acid to devote our attention to benzoic acid. I n preparing a saturated solution of benzoic acid in water, it was found that two liquid layers were formed before the temperature of the boiling-point of the mixture was reached. T h e distillate was collected in the manner described Bbove, and two determinations were made under diminished pressure, with
the aqueous solutions. In Table V. is given the results of these determinations. I n the first coluinn is the pressure under which the vaporization took place, and in the second column is the temperature of the boiling mixture. T h e third column contains the percent of benzoic acid in the distillate, while the next to the last column contains the partial vapor pressure due to the benzoic acid in the distillate. T h e value of this partial pressure was calculated, from the following formula :
wheregl is the number of grams of the solute in one hundred grams of the distillate, and g, the number of grams of solvent. Whereas MTand M, represent the molecular weights of the solute and solvent respectively, P represents the total pressure of the distillate, and p the partial pressure due to the vapor pressure of the solute. In the last column appears the vapor pressures of the pure solvent at the respective temperatures, as interpolated from Fig. 4. TABLEV Benzoic Acid Solvent -Water I
1
Pressure mm.
Temp.
,
Vapor-pressure of solute Pct. acid
Found
I
587 028 73: 735 741.6 741.6 745.7 745.7 746.4 746.4 746.4 746.4
Theory
I
91.4O 94.6 99.6 99.6 99.8 99.8
'
'I
1
to *
IO0 120.0
100.4 100.6 100.8 101.1
0.832 1.275 0.965 1.040 1.023 I .032 I ,027
1.376 0.946 I .o 58 1.108 1.144
0.7 1.2 1.2 1.1
1.1 1.1 1.1 I.j 1.0
1.2 1.2 I .j
i 1 ~
I ~
1.1
1.3 1.3 1.8 1.85 1.85 1.87
-
1.9 2.0 2.0 2.0
A. T. LincoZn
729
Some difficulty was experienced in preparing a saturated solution of benzoic acid in benzol under atmospheric pressure owing to the great solubility of the acid in this solvent. By the addition of benzoic acid, the boiling-point of benzol was raised about thirty-six degrees and yet the soltition was apparently not saturated. Therefore satnrated solutions were prepared under diminished pressure, the distillates collected and analyzed and in Table VI. are given the results thus collected. T h e columns have the same respective significance as in the preceding table.
TABLEVI Benzoic Acid Solvent - Benzol Pressure mm.
Temp.
542 590 606 606 613 623 632 633
104-6' 102-5 86-88 89 101-4 94-96 98-100
I I
i ~
,
I
'
Vapor-pressure of solute ~-
Pct. acid
Found
Theory
0.537 0.389
2.0
2.5
1.5
2.4 0.85 0.96 1.3
0.150
0.6
0.177
0.7 I .o
0.264 0.405 0.290 0.320
I .6 1.2
1.3
2. I
1.4 1.8
From an examination of the results given in Tables V. and VI., it will be readily observed that the partial pressure due to the solute, benzoic acid, is approximately equal to the pressure which this substance would exert if it were in the free condition at the respective temperatures. In other words, the values of the partial pressure for benzoic acid and for its vapor pressure are within the limit of expermental error, and therefore as in Fig. 3, represent that portion of the curves for the volume concentration at that range of temperature where they lie very close together. Hence, in order to prove whether these solvents exert any solvent action when in the vapor phase, upon benzoic acid, it would be necessary to conduct the experiments at a much higher temperature.
730
Solvenf Actioiz of Vapoy
That the same is also true concerning the solvent action of ethyl alcohol vapor toward camphor and methyl alcohol becomes apparent from a comparison of the partial pressure of these substances in the distillate from saturated alcoholic solution and determined experimentally by Talmadge,' and the vapor pressures of these substances as recently published by ,411en." In Table VII. will be found in the first column the temperatures at which the distillate passed over, in the second column the partial pressures as determined experimentally by Talmadge and calculated in the manner described above, while in the last column is given the values of the vapor pressures of camphor and naphthalene as interpolated for these particular temperatures, from the determinations by Allen.
TABLEVI1 Ethyl Alcohol Solutions Camphor
I
Pressure in mm.
Temp.
Found
:Io
3.4
86
6.7
~
Theory
I I
,
4.6 9.2"
Naphthalene ~
~
51° 56 57 59 60 66
0.7
0.8
1.2
1.3
1.3 1.6
1.j
1.7
1.7 I .8
2.6
2.8
67
2.8
3.0
As in the case of benzoic acid, camphor and naphthalene give values for their partial pressures, which are slightly under the values of .the vapor-pressures of these substances at
' Jour. Phjs. Chem. a
I,
Jour. Chem. SOC.7 3 , At 80°.
547 (1897). 415 (igco).
41 I ,
A. T. Lz'izcohz
731
the respective temperatures, all of which are within the limit of experimental error. T h e results presented in the preceding pages may be siitiiniarized as follows : I. T h e coordinate system volume concentration and temperature has been employed to represent the concentration, pressure and temperature effects. 2. It has been pointed out that theoretically the vapor should have a solvent action ; but whether this is demonstrable experirnentally depends upon whether the difference between the partial pressure and the vapor pressure of the component called the solute is greater than the experimental error. And only at high ttmperatures in the vicinity or above the critical point of the solvent is this probably the case. 3. T h e distillate from saturated aqueous, benzol and acetone solutions of salicylic and benzoic acids contains some of these acids ; but the quantity present exerts a partial pressure about equal to the vapor pressure of the pure solute and the difference is within the limit of error. This is also true for camphor and naphthalene in vapors of ethyl alcohol. 4. T h e vapor pressure data for salicylic and benzoic acids over a range of temperature for about 70' to 190' have been presented. In conclusion, I desire to express m y gratitude to Professor Bancroft for his assistance during this investigation. Cornell Univeisi2y, June, r900.
