T H E VAPOR-PRESSURE
METHOD OF DETERMINING
MOI.,ECUI.,AR W E I G H T S
W. R. ORNDORFF AND H. G. CARRELL
Preliminary Paper An accurate and simple method for the determination of the vapor pressures of solutions at low temperatures would be of great value to the physical chemist not only on account of the great theoretical importance of these figures but also because the method could be used to determine the molecular weight of any dissolved substance. T h e simplest method of determining the vapor pressure of a solution in the laboratory is the one suggested by Ostwald and worked out to some extent by Walker' and later by Will and Bredig . Walker passed a current of air through three Liebig potash bulbs, two containing the solution and the third the solvent (water), and then through a U-tube containing pumice stone saturated with concentrated sulfuric acid. The loss of weight of the solvent bulb and the gain in weight of the U-tube, after the experiment had continued for twenty-four hours, furnished the necessary data for calculating the niolecular lowering of the vapor pressure. Will and Bredig substituted alcohol for water as it is a much better solvent for organic substances. They also determined the amount of the solvent evaporated by weighing the solution and solvent bulbs both before and after the experiment. Instead of using an aspirator atid placing the bulbs in an air bath they forced the air through the bulbs, which were placed in a large water bath kept at a constant temperature by means of a thermoregulator and a mech'Zeit. phys. Chem. 2, 602 (1888). 'Ber. 22. 1084(1889).
754
W. R. Orizdorf and H. G. Carrel1
anical stirring device. T h e loss of weight (s’), of the solution bulbs corresponds to the vapor pressure of that solution ( p ’ ) ,while the loss of weight, (s), of the bulbs containing the solvent corresponds in like manner to the difference in vapor pressure between the solvent and the solution, ( p -9’). Substituting these values s and s’ for p -p’ and p‘ it is very easy to calculate the molecular weight of the dissolved substance. In order to saturate the air completely with the alcohol vapor Will and Bredig used a modified Liebig potash apparatus with nine, instead of three, absorption bulbs. They passed the current of air through the apparatus generally for twenty-four hours at the rate of one liter per hour, T h e work of Will and Bredig demonstrates clearly that the method is practicable and in the hands of careful workers can be made to yield good results. T h e present investigation was undertaken in the hope that it might be possible to simplify the method still further and reduce the time required for each determination and thus to make it a laboratory method for the determination of molecular weights of dissolved substances. W e used the method and apparatus described by Will and Bredig forcing the air through the bulbs instead of aspirating it as we found that by this method the air current was easier to regulate and gave a much more regular stream of air through the bulbs. As this regularity of the air current was a very important factor in the success of the ‘method we also arraiiged the two carboys, used to give air under pressure, in such a manner that the air in the lower carboy was always under the same water pressure. In order to do this the siphon tube leading from the upper carboy was made to terminate in a U-tube just below the rubber cork in the lower carboy, while a tube open at both ends passed through the cork stopper below the surface of the water in t h e upper carboy. T h e pressure was thus kept constant being always equal to the pressure exerted by a column of water equal in height to the distance from the bottom of the open tube in the upper carboy to the top of the U-tube in the lower. This pressure could be varied by raising or lowering this open tube in the upper carboy and thus changing the distance between the ends of the two tubes. T h e U-tube in the lower carboy was so arranged that it touched the neck of the carboy
Method of Determiniitg Molecular Weights
755
and the water ran down the side of this receptacle instead of dropping to the bottom. Thi? arrangement we fonnd to work very satisfactorily and to give a very constant and regular current of air through the bulbs. After the water in the upper carboy had flowed into the lower it was drawn back again by exhausting the air in the upper carboy by means of a Chapman water pump and at the same time connecting the lower end of the siphon tube with a rubber tube leading to ,the bottom of the lower carboy. T h e same water was thus used over and over again so that the- air passed through the bulbs was always at the room temperature. After much experimenting with various forms of absorption apparatus including Geissler potash bulbs, U-tubes filled with glass beads, Liebig potash bulbs, Winkler’s spiral, Mitscherlich’s tube improved by de Koninck and the modified form of the Liebig apparatus of Will and Bredig having nine instead of three absorption bulbs, we found that this apparatus of Will and Bredig gave the best results. We therefore used this apparatus in our work. I n having the apparatus made, however, we had the tubes connecting the absorption bulbs, (the nine small bulbs at the bottom of the apparatus), as well as those leading to the bnlbs made of very small diameter, so that the bubbles of air passing through these bulbs should be as small as possible and thus facilitate the saturation of the air with the vapor of the liquid. Of course in using this apparatus it was slanted in such a manner that the current of air had always to force its way through a current of the liquid flowing in the opposite direction. At first we used a large water bath kept a t a constant temperature by means of a thermoregulator and a mechanical stirrer kept in motion by means of a Raabe’s turbine. T h e bulbs were fastened to a wire frame immersed in this bath and were connected in the bath with a lead coil of pipe ten feet in length, through which the air was passed in order that it might have the temperature of the bath before passing through the bulbs. T h e air was dried by passing it through two calcium chloride towers filled with dry, porous calcium chloride. By means of this water bath it was possible to keep the temperature constant within half of a degree for twenty-
756
W. R. Orndorf and H. G. CarreZZ
four hours and after the turbine was once set in motion the apparatus required no further attention. I t was soon found, however, that the water in the bath became very dirty and that it was difficult to wash the absorption apparatus clean. Washing the apparatus with alcohol and then with ether and allowing to dry gave unsatisfactory results. For these reasons and in order to get rid, if possible, of this cluinsy piece of water bath apparatus, which required constant attention to keep it in order, charging the water and regulating the stirrer, etc., we made several experiments without this bath, simply passing the air current through the bulbs left in the air. T h e temperature variation amounted to as much as z to 6 degrees at times but was generally not over I degree. T h e results obtained by this modification were fully as good as those obtained by Will and Bredig using their complicated water bath so that we at once discarded this part of their apparatus. In beginning the work we thought that the method might be improved by passing the air current through the solvent first and then through the solution under investigation. We thus thought to get rid of changes in concentration which are sometimes quite large when the air is first passed through the solution. We expected that the loss in weight of the solvent bulbs would be proportional to the vapor pressure of the solvent while the solution bulbs would gain in weight owing to the distillation of the solyent from the point of higher vapor pressure to that of the lower. This modification of the method, however, was found not to give satisfactory results, notwithstanding its promises and hence was abandoned. I n order to get rid as far as possible of the influence of these changes in concentration in the solution bulbs we slanted the apparatus as much as possible thus preventing the passage of the solution from one set of the three bulbs to the other in the same piece of apparatus. I n this way the evaporation of the solvent took place almost entirely in the first set of three bulbs in the modified form of the Liebig apparatus of Will and Bredig, while in the other two sets of bulbs only slight changes in concentration took place. T h e air passed through a solution of lower vapor pressure in the first set of bulbs to one of higher in the second and third and was thus gradually saturated up to the vapor pressure of the solution when it left the third set of bulbs. After one experi-
Method of Deteerminivg Molecular Weights
757
ment was completed the solution in the three sets of bulbs was thoroughly mixed by drawing the liquid over into the large bulb and the apparatus was then ready for another experiment without recharging ; for the loss in weight represented only loss of solvent and the new concentration of the solution was easily calculated from this loss. I t was thus possible to make a series of four or five determinations with increasing concentrations exactly as is done in determining molecular weights by the boiling-point method. This makes the method more accurate and less troublesome than where single determinatiobs are made as was done by Will and Bredig. In the preliminary work it was soon found that the solvent bulbs invariably lost more in weight than they should theoretically. I t was thought that this might be due to the fact that the solvent evaporated directly into the air. We therefore tried connecting the solvent bulbs with a U-tube filled with glass beads and containing a sniall quantity of the solvent. This device we found to be of great value and made use of it in all of our experiments. This U-tube was not weighed but was merely used as a precautionary measure. Notwithstanding that the bulbs were kept tightly closed with small rubber tubes plugged in the usual manner with pieces of glass rod we found that in twenty-four hours a loss of over I O niilligrams took place, so that it is advisable in working with this method to reduce the time of the experiment as much as possible. The rate at which the air is run through the bulbs is a very important factor in this method. With some substances like urethane good results may be obtained using almost any rate while generally there is one rate which gives the best results. I n the table we give some results in which we used alcohol as a solvent. T h e alcohol was carefully purified by distillation over lime until it was free from water. I n calculating the molecular weight we used the formula
in which M = the molecular weight of the dissolved substance g = the number of grams of the substance in IOO of the solvent s = the loss of weight of the solvent bulbs
W. R. O m d o r f and H. G. CarreZZ
758
s'= the loss of weight of the solution bulbs 46 = the molecular weight of the alcohol used as a solvent. Urethane,
I
I____
Concentration in
IOO grams
of solvent
'
Mol. wt. found
of solution
i
Duration hours
--
---__ ___
No. of iters of air
Temp.
-__
variation
g
23.9452 15.2312 15.2312 16.6506 17.3083 18.401 7
88
30 23
5
I
20
0.0796 0.1306
1
1.3752
I ' IO3 89
5.75
1
13 13 13
1.5: 2.3 3.4' 0.7' 0.8' 1.10
Diphenylamine, (C,H,),N
7.9902 8.3200 1
O.OIj0
0.0494
' ~
0.6016 2.0680
147 1 I60
Nitrobenzene, C,H,NO,,
7.5199 1 0.0589 8.3176 I 0.0949
I 23
131
6.1'
2.0848
I22
2.7357
I IO
2.9; 0.7
a-Nitronaphthalene, C,,H,NO,,
I 73
Phenol, C,H,OH, 94
33.4394 41.9522 7.5355 8.2204 8.7473
0.9420 0.6270 0.1018 0.0677
4.0622
0.1117
1.8484
2.2726 2.1422 1.4204
,
66 70 73 79 67
23 57 55 I5 I7
40 34 23 16 I7
1.1'
1.8'
1.7; 1.9 1.6'
,
Method of Determining Molecular Weights
759
Urea, OC(NH,),, 60 _ _ _ _ _ _- _ ~ ~~~
I
Concentration ~ o s of s wt. in roograms
-1
of solvent
of solution
found
S'
m 55 46
~
Temp. ~
variation
I
2.0171 1.9260 3.1641 3.4406
Dura- No. of tion liters of hours air
'I
I9 25 2.5
0.0648 0.0670
44 22
1
25
1.10
I8 .26 26
o.go
29
3.a0
2.O
I.IO
I t will be seen from this table that the rate at which the air current passes through the apparatus influences markedly the results. I n some cases, however, results agreeing with the theoretical are obtained whatever be the rate as is seen in the case of urethane while with phenol the results do not agree very well with the theoretical whatever the rate. It is quite likely that some other factor comes in here, possibly that the surface tension of the solution plays an important part. Further work will be undertaken in this laboratory with this method, using other solvents like acetone and chloroform.
cornell University,July, 1897
.
