T H E HEATS OF VAPORIZATION O F ISO-PROPYL ALCOHOL AND ETHYL ALCOHOL BY GEORGE
S. PARKS
AND WILLIAM K . NELSON
A few years ago, in connection with another research, eome information waa needed concerning the heat of vaporization of iso-propyl alcohol. At that time the only two values existing in the literature were those of Luginin' and Brown.2 The former reported a result of 157.8 cal. per gram; the latter obtained a mean value of 161.I in a very careful and noteworthy investigation of the heat of vaporization of a number of substances. It seemed to us probable that these earlier investigators may have been seriously handicapped in obtaining accurate values by the difficulty attached to securing any large amount of pure iso-propyl alcohol. At any rate there is a 2y0 discrepancy between their results; and, in view of this situation, the present determination was undertaken. As will be shown subsequently in this paper, our mean result is 161.7 cal. per gram. In making the determinations to be described, ethyl alcohol was used for preliminary measurements in order to test the apparatus and to aid in remedying any sources of error that might become apparent. This was done because the supply of ethyl alcohol was more plentiful than that of iso-propyl alcohol. However, it was found that the values previously obtained for the heat of vaporization of ethyl alcohol showed great divergence, so after the development of our method two complete series of runs were made upon this substance. Our mean result in this case was 2 0 8 . 7 cal. per gram. Following the completion of our work, Mathews3 has recently published the data of a very interesting investigation on the heat of vaporization of a large number of liquids. For iso-propyl alcohol he obtained 159.23 and for ethyl alcohol 201.88 cal. per gram at the respective temperatures, 81.25' and 77.42' c. Experimental Materials. Commercially refined iso-propyl alcohol and ethyl alcohol served as the starting point in the preparation of the materials for the present investigation. Water being the chief impurity, these were first dehydrated by three successive treatments and distillations with lime in the ordinary manner. The resulting liquids were then carefully fractionated. In each instance the middle fraction, about 607, of the total, was selected for use in the measurements. I t showed a density of 0.78108 2S0/4O in the case of isopropyl a!cohol and of 0.78549 25'/4O for ethyl alcohol. On the basis of the criteria' previously employed, these values correspond to a purity of 9 9 . 9 0 7 ~ and 99.86Tc, respectively. l Luginin: Ann. Chim. Phys., (7) 13, 340 (1898). *Brown: J. Chem. SOC., 83,991 (1903). Mathews: J. Am. Chem. SOC.,48,572 (1926). Parks and Kelley: J. Phys. Chem., 29, 728 (1925).
62
GEORGE S. PARKS AND WILLIAM K. SELSON
Apparatus and Procedure. I n principle, the method was simple. It consisted essentially of supplying measured quantities of heat electrically to the alcohol, which previously had been brought to its boiling point, and of condensing and weighing the amount thereby vaporized. Thus, a careful weighing of the amounts vaporized per 15-minute period at two known rates of energy input gave, by difference, the necessary data for the calculation of the heat of vaporization. The apparatus, as finally developed after much preliminary experimentation, conformed closely to the second type described by Awbery and Griffiths.1 A z.j-liter, silvered, Dewar vessel served as the container for the alcohol which was to be vaporized. This vessel was of a carafe or bottle shape with a neck about 1 2 cm. long. Thus, with the carafe half-filled with liquid, the vapor produced had to pass vertically upward at least 20 cm. before escaping into the outlet tube. This distance should greatly reduce any error due to the production of spray and the consequent loss of liquid as such. The heating coil in this evaporator consisted of 1.5 meters of So. 30 (B. and S.) enameled Therlo wire, wound upon a small mica frame;it had a resistance of about 14 ohms. The carafe was fitted with a cork stopper (tin-foiled) which carried the mercury-filled glass tubes leading to the heating coil, a glass tube closed at the lower end and containing a thermocouple junction for measuring the temperature of the liquid, and a 1 2 mm. Pyrex outlet tube for the vapor. The last ran through the stopper vertically, then turned sharply downward a t an acute angle and passed out through an opening in the outer jacket to connect with an inclined condenser. Surrounding this Dewar evaporator was the outer container or thermostat, which was a vapor-tight, cylindrical vessel of copper about 60 cm. high and 20 cm. in diameter. It was jacketed with a layer of asbestos sheeting. The upper half of this container was then wound with a few turns of high resistance wire to serve as a heating coil and covered with another layer of sheeting. A form, wound with more resistance wire, covered the top of the cylinder. These coils, placed in parallel with one another but in series with a variable resistance, were supplied with a suitable current from the I Io-volt line. The outer container was ordinarily filled to a depth of about 3 cm. with some of the same alcohol employed in the evaporator. During the vaporization measurements this liquid was heated and kept a t its boiling point by means of a Bunsen burner placed directly beneath the cylinder, which rested upon a small iron tripod. To conserve the resulting vapor, the outer container was equipped with a reflux condenser, the connecting tube being large enough to insure smooth boiling and the easy return of condensed alcohol. The Dewar evaporator, being a commercial vacuum bottle, came supplied with a small metal jacket and this was left on in the present case to prevent any of the cooled condensate from coming into direct contact with the glass vessel. I n addition to the openings for the outlets to the two condensers, there were two more openings in the copper cylinder for the lead wires to the heatAwbery and Griffiths: Proc. Phys. SOC.London, 36,309 (1924).
HEATS O F VAPORIZATION
63
ing coil within the evaporator. Two wires were led in through each of these two openings. One pair of wires carried the current from the lead storage battery and the other pair connected with the voltmeter which was used to mertsure the fall of potential across the heating coil. All openings in the outer container were reinforced by soldering in short pieces of copper tubing of such diameten that standard rubber stoppers gave very tight seals. The lead wires were introduced through these stoppers as were also the glass outlet tubes for the liquid. The container w&s fitted with a leak-proof cover. A flat wooden block was used as a stand for the Dewar evaporator inside the copper cylinder. This block was drilled in several places to permit free circulation of the bath liquid. As noted before, the outer container was mounted upon an iron tripod stand. The condensers were supported by large iron ringstands. A large piece of asbestos was formed into a cylindrical screen about 90 cm. high and 40 cm. in diameter. Holes were made in this screen for the lead wires and the outlets to the condensers, and it was then placed around the entire apparatus. A circular piece of asbestos sheeting was used to cover the top. The copper-constantan thermocouple wires were led into the outer container through the reflux condenser and from there to the tube in the stopper of the Dewar carafe. The other junction was placed in a mixture of ice and water. The e. m. f. readings were made with a Leeds and Korthrup “thermocouple” potentiometer. The voltmeter employed for determining the fall of potential across the heating coil in the evaporator was a Weston instrument, reading t 3 0.01volt. The ammeter, in series with the coil, was of German manufacturc and could be read to 0.001ampere. The time elapsing during the collection of a sample of the condensed vapor was measured by a stopwatch. The voltmeter ammeter and watch were carefully calibrated at the time of the determinations and probably gave values for the energy input accurate to within 0.37~. The actual procedure in making the measurements of the heats of vaporization was extremely simple. The evaporator was about half filled with the particular alcohol under investigation. Enough of the same material was kept in the outer container to prevent it from becoming dry through evaporation losses, although the level was always maintained below the bottom of the Dewar carafe. The quantities of liquid in the two vessels having been satisfactorily arranged, the apparatus was heated up to the boiling point of the alcohol. To accomplish this the outer container was heated by means of the Bunsen burner and the external heating coils, while heat was supplied to the alcohol in the carafe by an electric current from a 20-volt storage battery discharging through the Therlo coil. After the condensate from the evaporator began to flow at a uniform rate and the thermocouple gave a reading corresponding to the boiling point of the alcohol, the applied voltage was reduced to approximately two volts. il half-hour was allowed to elapse, in order that equilibrium might be attained, and then the condensate was collected in small Erlenmeyer flasks and weighed. In each set of determina-
64
GEORGE S. PARKS AND WILLIAM K. NELSON
tions five portions were collected at this voltage, fifteen minutes being the duration of each collection period. The voltage was next increased t o the figure at which the measurement was to be made, and another half-hour was devoted to securing equilibrium. At the end of this interval the condensate was again collected over fifteen minute periods and weighed. Eight or nine portions were usually taken during a series of the determinations, great care being exercised to change the flasks at exactly the end of the fifteen minute periods without loss of liquid. The voltmeter and ammeter were read at one minute intervals during the collection of each sample of condensate and the average of these readings was taken for the calculations. The energy input at two volts and the amount of liquid collected at this voltage were used as reference quantities; the values thereby obtained were subtracted from those found for the higher voltages. Under these experimental conditions there could be but little energy exchange between the evaporator and its surroundings and the residual effect was allowed for by thus determining the energy required to produce a very slow rate of evaporation. Results. The individual results obtained in one of the series of determinations on iso-propyl alcohol are recorded in Table I. In order to show the calculation involved, it may be well to indicate the steps taken to find the first of these: Average lower voltage Average lower current Average condensate at lower voltage Average higher voltage Average higher current Condensate at higher voltage Time of collecting sample
I . 940 Volts 0.141amps.
0.700 gms.
13.585volts 0,987amps. 18.158gms. 900.o seconds
= 161.9cal./gm. (preliminary value).
To correct for the current passing through the voltmeter circuit and for the heat lost in the leads to the Therlo coil, we must now reduce this value by 1.570. Thus, the final result becomes 159.5calories per gram.
TABLE I Heat of Vaporization of Iso-Propyl Alcohol The individual results obtained in Series C. Determination number
Heat of Vaporization
159.5cal. 161.9 ” 3 161.2 ” 4 160.8 ” Mean result of Series C I
2
Determination number
Heat of Vaporization
5 6 7 8
161.5cal. 161.2 ” 161.7 ” 161.9 ” 161.21”
65
HEATS O F VAPORIZATION
In Table I1 we have summarized our experimental results for five series of determinations on iso-propyl alcohol and two series on ethyl alcohol. The data given serve to indicate the type of concordance obtained in successive series of determinations. TABLE I1 Summarized Data for the Heats of Vaporization of Iso-Propyl .&oh01 and of Ethyl Alcohol Determinations
Mean voltage Higher Lower
Iso-Propyl Alcohol Series A 12.914 B 12.846 C 13.586
D E
11.596 11.488
Final mean value
.. . .
,
Ethyl Alcohol Series A 1 3 . I47 B 13.117 Final mean value. . . . . .
Mean wt. of condensate Higher Lower
I . 892
0.675 0.869
Mean heat of vaporization
1 6 1 . 0 4 cal. 162.30 ” 161.21 ” 162.00 ”
16.282 1.882 16.194 1 940 17.990 1.942 ‘3.299 1.924 ‘2.993 ..................
1 6 2 .IO
”
......
,161.73
”
I ,912 1.912
1,015
0.789
208.82 cal. 208.52 ” , 2 0 8 . 67 ”
1 3 . I90 12.903
0.700
0.872 0.809
, . . , .
Discussion As shown in the preceding table, our mean result for the heat of vaporization of iso-propyl alcohol is 161.7 calories per gram at its boiling point (82.2’). Luginin and Brown in earlier investigations obtained I 5 7.8 and 161.I calories, respectively. Of these two values, that of Brown is undoubtedly the more trustworthy because of the relatively great care and precjsion of his experimental method. The recent result of Mathews, 159.23 cal. at 81.25’~ appears to be the product of an extremely refined apparatus and a painstaking procedure, although from his paper it is not clear that he was able to entirely avoid the production of fog or spray in the vaporization process. Of course, such spray tends to produce results that are too low and has constituted one of the major sources of error in past calorimetric determinations of heats of vaporization. However, in the present case this may not be the cause of the discrepancy between our result and that of hlathews, for it appears that he used material that differed appreciably from ours. He reports a density of 0.7830 25’/4’ for his iso-propyl alcohol and this, on the basis of our own standards of purity (essentially the density determination of Brunel’), corresponds to a liquid containing 0 . 9 4 7 ~water. Taking all these points ) into consideration, we therefore would like to suggest 161.0( ~ 1 . 5 calories per gram as the most likely value for the heat of vaporization of iso-propyl alcohol. In the case of ethyl alcohol, our mean result is 208.7 calories at the boiling point, 78.4“. The more important earlier values2 are 201.5 (Luginin), 2 0 5 . 1 ‘Brunel: J. Am. Chem. SOC.,45, 1336 (1923). “Tabellen,” I 480 (1923).
* Landolt-Bornstein-Roth-Scheele:
66
GEORGE S. PARKS AND WILLIAM K. NELSON
(Wirtz), 206.4 (Schall), 207.2 (Young), 216.5 (Marshall and Ramsay) and 216.4 (Brown). Mathews' recent determination gave 201.88 at 77.4'. The average of all these results, 207.9 calories, agrees very well with our own value and is about midway between those BO carefully obtained by Brown and by Mathews. Hence, aa a provisional value for use at the present time, 208 calories per gram is probably as satisfactory and reasonable aa any other that might be suggested. However, in the case of a very common and important substance like ethyl alcohol, such a marked variation in the experimental results is extremely unfortunate ; certainly the situation requires further research for its clarification. Departmat o j Chemis
Stanjord University, C%)ornia. July 1, f987.
