T H E MOLECULAR LOWERING O F THE FREEZING POINT .OF IJQUID AMMONIA’ BY LOUIS D. ELLIOTT
Introduction The development by Franklin and coworkers 0:’the ammonia system of compounds in which liquid ammonia occupies a position analogous to water in our familiar water system has opened up an extensive field of research. Step by step the parallel relationships between ammonia and mater have been brought out by numerous investigations of reactions in liquid ammonia as a solvent and through studies of the physical properties of this remarkable
It was for the purpose of contributing to our knowledge in the latter field that the work upon the molecular lowering of the freezing point of ammonia was undertaken. No direct determination of the freezing point constant has hitherto been made. From the fact that the boiling and freezing point constants for water, 5 . 2 and 18.6,respectively, are relatively very low, and that the boiling point constant for a m m ~ n i a 3.4, , ~ is the smallest known we should expect the cryoscopic constant for ammonia to be even smaller than that of water. Masso1’s4value of 1838 calories per gram molecule for the heat of fusion of ammonia and that of de Fourcrand and Massol5 of 1950 lead to the calculated values of 7.1 and 6.6, respectively, for the molecular lowering of the freezing point of ammonia. The work of Rupert6 and that of Postma’ on the freezing point curve of the system water-ammonia, though not involving precise differential temperature measurements do, however, indicate a value, in the cnse of water as solute at least, only slightly under I O for the more dilute solutions measured. Thermometry A consideration of the thermomet>ricmethods available in low temperature measurements led to the conclusion that platinum resistance thermometry would lend itself most satisfactorily t80the accurate measnrements of small temperature changes in the temperature region involved.
* From a thesis presented to the Department of Chemistry and the Committee on Graduate Study of Stanford University, August 1923, in partial fulfillment of the requirements for the degree of Ph.D. * Some of the important papers in connection with this development are: Franklin and Kraus: Am. Chem. J., 20, 820 (1898); 23, 277 (1900); Franklin and Stafford: 28, 83 (1902); Franklin and Cady: J. Am. Chem. SOC.,26, 499 (1904); Franklin: 27, 820 (1905); Franklin and Kraus: 27, 191 (1905); Franklin: Z. physik. Cheh. 69, 272 (1909); Franklin: Am. Chem. J., 47, 285 (1912); Franklin: J. Am. Chem. SOC.,44, 185 (1922). Franklin and Iiraus: Am. Chem. J., 20, 836 (1898). Compt. rend. 134, 653, (1902). Compt. rend. 134, 743 (1902). J. Am. Chem. SOC.,32, 748 (1910). Rcc. trav. chim., 39, 515 (1920).
612
LOUIS D. ELLIOTT
As no suitable resistance thermometer was available it was thought worth while to de.sign one suitable for accurate measurements under the conditions involved which could be constructed out of materials a t hand. Although nearly all successful forms of platinum resistance thermometers have been of the mica cross type' it was thought advisable to discard the mica cross feature and t o construct a glass core thermometer which could be made hy a glass blower with the fewest materials possible. Since the practice of imbedding the wire by fusion into the glass core as in the Heraus and de Leeow type instruments2 is undesirable from the standpoint of possible strainsS in the winding it was decided to wind the wire loosely upon a glass core and protect it with B glass sheath. Thermometer No. 4 which was used throughout this investigation was constructed as follows: For the core A, Fig. I , ordinary soft glass tubing eight millimeters outside diameter, coated with paraffin, was placed in a lathe and double threads fifty to the inch, cut t'hrough the wax. By etching with hydrofluoric acid very satisfactory threads 5, around the glass were obtained. The thermometer core was made from a section of this tube about four centimeters long which was fused shut a t one end. The platinum resistance wires, fused to t'he silver lead-wires L4 (shown without their protecting tubes), threaded through the two small holes F situated 2 . 5 centimeters from E and fixed in position by two minute drops of fused lead glass were non-inductively wound upon the threads etched in A. The ends of the resistance wires were then fused together and held in position a t E by means of droplets of fused lead glass. The silver lead-wires were insulated from each other by placing in narrow bore thinC walled glass tubes. To obviate air currents up and down FIG.I the stem of the thermometer these three tubes passed through a small cork (not shown in Fig. I) of such a diameter as to fit snugly in the glass stem tube B. A thin-walled glass sheath C of the same diameter as B ( 2 cm) was placed over the core and sealed to B at H leaving an air space around the core tube of about one half millimeter thickness. The platinum resistance element on the core consisted of approximately I O O centimeters of pure platinum wire 0.0075 millimeters in diameter the length chosen being such that the resistance of the thermometer at oo would be in the neighborhood of twenty five ohms. With such a resistance a reading of 0.000 I ohms on the bridge employed would correspond to approximately O . O O I ~ . The wire was flash annealed before and after winding. Within the Callendar: Phil. Mag., 32,104(1891);T. S. Sligh: Bur. Standards, Scientific Paper
NO.407 (1921);J. Am. Chem. SOC.,43,470 (1921). * D e Leeow: Z. physik. Chem., 77,304 (1911).
Sligh: loc. cit. W.Siemens: Proc. Roy. SOC.,19, 351 (1871);Waidner and Burgess: Bur. Standard8 Bull. 6,152 (1915). 4
FREEZING P O I N T LOWERING O F AMMONIA
613
glass head D, fitted over the stem tube B with sealing wax, the silver leads were soldered to insulated stranded lamp cord wires which served as the three external leads t o the bridge. Entrance of these external leads to the head was effected through three holes in the side of the latter made tight with sealing wax. The total length of the thermometer was about, 35 centimeters. Resistances were measured upon a five-dial Leeds and Northrup Mueller type precision temperature bridge1 using a highly sensitive d'hrsonva! mirror galvanometer with larrip and scale. The bridge having been adjusted to accuracy by the manufacturers was not further calibrated for this work. Resistance-Temperature
Relationships
+
Between - 40'and I 100' the relationship between temperature and the electrical resistance of pure annealed platinum is accurately expressed by Callendar's parabolic equation of the form; R = Ro( I + u t +/3tz).2 Below approximately - 40' the relationship begins to deviate from this law. From Henning's calibration3 of a platinum resistance thermometer against a hydrogen thermometer it was shown that the above equation gives temperatures 0.08' too low at - 78' and over 2' too low at - 193'. The curvature of the deviation curve at - 7 7 ' ) that is, the rate of change of deviation from the Callendar formula is too great to allow of the use of interpolated corrections in this temperature region. Rather than attempt to use any of the several equations of higher degree it was decided to make use of a relationship found by Henning connecting the resistance of any two thermometers. In order to use this relationship it was necessary to calibrate our thermometer at only one point, preferably at liquid air temperature, other than the ice and steam points. From the value for the resistance at oo (ro)we could derive the resistance ratio (RI)of our thermometer, that is, the ratio of its resistance (rJ at a specified temperature to its resistance (yo) a t 0 ' . Henning carefully calibrated a standard platinum resistance thermometer, P. T. R. No. 32, against a hydrogen thermometer in the interval between - 193' and +7' thus connecting the resistance ratio (R) of his thermometer to temperature. It now only remained to find a functional relntionship bet,ween the resistance ratio of our thermometer and that of No. 3 2 . This was accomplished by using the relationship found by Henning to exist between the resistance ratios of any two thermometers expressed by the formula: (I)
R ~ - R = M ( R - I ) + N ( R - I ) ~ , in which M = a1 -((~-~ooc)-~,and B
For a description of this type of bridge see Mueller: Bur. Standards Bull. 13, (1917); Waidner, Dickinson, Mueller and Harper: Bull. 11, 571 (1915). Callendar: Phil. Trans. 178 A, 161 (1887); Sligh: loc. cit. Henning: Ann. Physik, (4) 40, 635 (1913).
614
LOUIS D. ELLIOTT
a and a.l are t,he fundamental coefficients of No. 3 2 and our thermometer, respectively, obtained from ice and steam point calibrations. rloo -ro in which rloo= resistance at 100’. c, a con(2) a= J ooro
stant determined by calibration of our thermometer at liquid air temperature is found by means of the equation:
This may be put into a more convenient form by using the concept of “platinum temperature” (pt) which is related to resistance by the linear equation: q-ro , rt being the resistance a t the temperature in rloo -ro question. Equation (3) now becomes: (4)
pt=100-
e=-
ptl-pt in which ptl = platinum temperature of our pt(pt,- IOO),’ thermometer and pt = platinum temperature of standard thermometer. Having determined R1 experimentally R may be calculated and the corresponding temperature obtained from Henning’s table of values of R.1 (5)
Calibration of the Thermometer The steam point of our thermometer was det>erminedby the usual hypsometer method.* The value for the resistance at I O O O (rlo0),32.3433 ~ 0 . 0 0 0 4 ohms was arrived a t by averaging five values, corrected to standard conditions, obtained on five different days, and calculating the probable error. The ice point was determined as follows; an ice sheath of inside diameter slightly larger than the diameter of the thermometer was made from distilled’ water and the sheath surrounded with shaved commercial ice. The sheath was filled with distilled water cooled to oo and the thermometer inserted. From a series of thirteen readings agreeing to within 0.002 ohms the value for the resistance a t oo (To) was determined to be 23.2811 ohms with a calculated probable error of 0.0001ohms. Upon recalibration at the ice point after sis months the resistance was found to be unchanged. From these two calibrations the characteristic constants for thermometer No. 4 were derived. F. I. (fundamental interval), rloo-ro, was found to be 9.0622 ohms and a, the fundamental coefficient, F I/Iooro is 0.003892. The fundamental coefficient of a platinum thermometer is a criterion of the purity of the platinum and should not be less than 0.00388.~ 2
Ann. Physik, (4) 40, 635, Table 9. Winkelmann: Handbuch der Physik, 3, I, 22. Bur Standards, Scientific Paper No. 407, p. 52.
615
F R E E Z I N G POINT LOWERING O F AMMONIA
The calibration at liquid air temperature was carried out by the vapor pressure method of Henning' for the boiling point, of liquid oxygen, the oxygen being purified by the method of von Siemens2. As the temperature of the liquid air bath surrounding the bulb of liquid oxvgen rose slowly a series of va.por pressure readings for the oxygen were taken as shown in the table below. The pressure readings were made upon a wood scale calibrated with a cat*hetometer. Time in minutes
Corrected Dressure In mm.
Resistance in ohms
Calculated temperature
5.0936 - 190.22' -190.11 5.1030 5.1135 - 190.06 31 5.1148 - 190.06 337.8 37 -190.04 339.3 5.1158 41 The temperature was calculated from the vapor pressure equation of von Siemens : logp=-- 3 99 +1.7jlogT-o.o1292 T + j , o j 2 7 , T 7
22
331.8 335.9 337.8
in which p =pressure in millimeters and T = absolute temperature. Solving equation ( 5 ) c = 5.01 X IO-?. N therefore becomes negligible and R1-0.0056. drops out of equation (I), and M becomes -0.0056, hence R = 0.9944 T o expedite conversion of resistance to temperature a table of corresponding R1 values covering the range of temperatures involved was made, thus allowing the temperature corresponding to R1 to be read off from our table directly.
Description of Apparatus The apparatus as finally adopted and used throughout the work is shown diagramatically in Fig. 2 . The Dewar vessel A, the inside dimensions of which were 33X8.5 centimeters served as a container for a cooling bath of petrolenm ether. The receptacle for holding liquid air, the refrigerant employed, was a pyrex bulb B suspended in the ether bath by its entrance and exit tubes C and C', respectively, which passed through the cork stopper F as indicated. The lower half of bulb B was made double walled though not evacuated. A preliminary bulb, double walled nearly to its top did not permit of rapid enough heat transfer, while with the bulb adopted the temperature of the bath and apparatus could be quickly reduced to the desired region by keeping it full of liquid air. When maintaining a constant temperature a little liquid air in the lower part was sufficient. The freezing cell D (28x3 centimeters) is shown in place, separated from the bath by its air mantle tube E. This gave an air space around D of five millimeters thickness which was found to give a very satisfactory rate of cooling. The side tube of the cell was closed 1
Ann. Physik, (4),43, 282 (1913); Bergstrom: J. Phys. Chem., 26, 363 Ann. Physik, (4) 42, 871 (1913).
(1922).
616
LOUIS D. ELLIOTT
with a phosphorus pentoxide drying tube. The petroleum ether was agitated by means of a very efficient stirrer (omitted in Fig. 2 ) consisting of a brass cylinder ( 2 5 X z centimeters) within which a rapidly revolving brass rod equipped with small propellor blades a t three intervals drew the liquid in through apertures near the bottom and forced it out at the top of the cylinder near the surface of the ether bath. The temperature of the bath was observed
W FIG.2
on a pentane thermometer (not shown in the figure )passing through stopper F. Passing through the cork stopper Q of the freezing cell were the resistance thermometer, G, a platinum stirrer (omitted in the figure), and the small tube H of capillary dimensions for introducing the solvent. The platinum stirrer consisted of a stiff platinum rod which passed through the mercury seal as indicated a t I and extended to the bottom of the freezing cell. Near the lower end three platinum rings were fastened t o the rod in a horizontal position a t intervals in such a way as to thoroughly stir the ammonia solutions. The up and down motion of the stirrer was effected by means of a suitable pulley belted through gear reducing wheels to a motor. In order to melt the crystals of ammonia after each freezing point reading, a coil of platinum wire was fused into the bottom of the freezing cell, the copper leads to which it was soldered passing up through the air jacket and to the outside between R and D. A coil of No. 30 enameled nianganin wire about 60 centimeters long, was wound around the upper portion of the cell so that heat might be applied to loosen
FREEZING POINT LOWERING O F AMMONIA
617
slight crusts of ammonia crystals which tended to form at the surface of the ammonia. The three terminal wires from these two separate heating coils were attached through suitable switches, a variable rheostat and ammeter t,o a source of potential. The calibrated ammonia-filling pipette J was attached by means of the two-way stopcocks 1. and K to the tubes 0 and 0’ in such a manner that dry air could be by-passcd around J through stopcock S and allowed t o circulate through the freezing cell or could be made to force the ammonia sample from J into the freezing cell. The Dewar tube M containing commercial ammonia was used as n bath for J when distilling dry ammonia from a ten-pound steel bomb into J through 0”. Beiore attaching to the apparatus by fusing a t 0 and 0’ the filling pipette J was calibrated with water and its volume to the tip of the pointer P found to be 80.8 cubic centimeters. Assuming the density of liquid ammonia a t -33.3O to be 0 . 6 8 2 , ~the weight of the ammonia to the pointer while surrounded with a boiling ammonia bath was calculated to be 5 5 . I grams. The three lead wires from G were connected with the bridge and galvanometer and the readings made as described in connection with the calibration of the thermometer. After each run the introductory tube was severed at H and the apparatus disassembled for cleaning. After reassembly preparatory to a new run the introductory tube was resealed to the pipette and the two corks, Q and R, made tight with paraffin. Purity of the Ammonia The ammonia used as the solvent in this investigation was prepared as follows: liquid ammonia from a tank of ordinary commercial ammonia was distilled into the ten pound steel tank containing sodium amide. The tank was connected to J by a lead tube fastened to the glass at 0” by means of sealing wax. Midway of the lead tube a short piece of glass tube containing a little ignited asbestos was interposed to trap any material that might carry over mechanically. After distilling into J the doubly distilled product was considered to be pure and dry.2 During the course of the work several new lots from the same tank of commercial ammonia were taken and once an entirely new lot was drawn from another cylinder. I n no instance could’ any effect upon the freezing point be detected by the change of source of the ammonia. Mode of Procedure Each run consisted of freezing point measurements in increasing concentrations of one solute preceded in each case by a determination of the freezing point of the pure solvent. After filling the Dewar vessel A, Fig. 3, with the bath liquid through a siphon tube inserted through the stopper F (omitted in 1 Fitagerald: J. Phys. Chem., 16, 654 (1912); Cragoe and Harper: Bur. Standards, Scientific Paper No. 420 (1921). Franklin and Kraus: Am. Chem. J., 23, 285, (1900).
618
LOUIS D. ELLIOTT
the figure) and starting the bath stirrer, a large vacuum bottle serving as the liquid air reservoir was connected to the inlet tube C by means of a double walled tube evacuated between the walls. Stopcocks L and K were turned to communicate with 0” and N, respectively, and a slow stream of ammonia gas passed through J to sweep out moisture and air. The gas passed out through a rubber tube fastened to N and was caught in a water bottle. At the same time air from the phosphorus pentoxide drying train was passed by way of S and H into the bottom of the freezing cell and the current of air maintained while the temperature of the system was being lowered by pumping liquid air into B. In preliminary experiments in which this stream of dry air was omitted there was invariably a deposition of water and consequent ice formation upon the wall of the freezing cell during tlie cooling period. While cooling was in progress stopcock K was closed and the ammonia bath M brought up around J at the same time. Ammonia from the bomb was then passed into J more rapidly until the pipette was filled to the pointer with the condensed ammonia. Stopcocks L and K were then both closed, M withdrawn, and a similar tube substituted containing petroleum ether temporarily borrowed from A a t a temperature of about -1100’. After the ammonia was cooled well below its boiling point, the three stopcocks were so manipulated that liquid ammonia was forced into the freezing cell by air pressure. The ammonia was carried to the very bottom of the cell and the small amount of vapor momentarily escaping before the exit was covered was immediately frozen upon the wall of the cell. When transference was complete stopcocks L and K were closed and the ammonia was brought down to its freezing point. The platinum stirrer was started when the solvent was transferred to the cell and a uniform speed of one Ptroke per second maintained by means of a rheostat interposed between the actuating motor and the stirrer. The length of the stroke was so adjusted that the column of liquid was well stirred the whole of its length by the stirrer rings. It was found by experience that the best rate of heat transfer was obtained by holding the bath temperature in the neighborhood of -88’ to -93’. The supercooling throughout the work never amounted to more than 0.2’. No corrections for concentration due to separation of solid ammonia have been made for the reason that the small amount of observed supercooling taken in connection with the exceptionally high heat of fusion of ammonia gave a corrected value without significance. The lag of the thermometer was very slight, the galvanometer responding to the appearance of crystals within a few seconds. The temperature rise from the lowest reading to the maximum required usually from three to five minutes. It was noted throughout that in the case of the pure solvent the crystals formed had a tendency to adhere to the stirrer or to one another in the bottom of the cell. On the other hand even from the most dilute solutions employed ammonia crystals separated which did not show this tendency and were easily kept in motion throughout the liquid by the stirrer. In determining the freezing point of the pure solvent and of the subsequent solutions, several readings were taken, melting and recooling between
FREEZING POINT LOWERING OF AMMONIA
619
each reading. The application of about forty watts of energy to the lower coil for a few seconds usually sufficed to melt the crystals. Although the surface of the ether bath was held at different levels a t different times above that of liquid in the freezing cell no effect was noticeable upon the readings. It was usually kept, however, about two or three centimeters above that of the contents of the cell. Liquid solutes were introduced through the side tube by means of an Ostwald pipette taking precaution that each drop reached the surface of the liquid in the cell. Solid solutes when readily soluble or when small in quantity were added in pellet form. For the weighing tube A, Fig. 3, was utilized. B, over the lower end of which was fitted the rubber tube C, was weighed with A and the stopper D. When introducing the solute into the freezing cell the tube B was inserted into the side tube so that C reached nearly to the surface of the liquid and the material from A allowed to pass through B and C directly into the solution. After withdrawing B and C and allowing the moisture condensed upon the cold tube to evaporate they were again weighed with A and the weight of the sample thus obtained by difference. In weighing out the larger quantities of solute where it was only necessary to weigh to the first or second decimal this method proved very satisfactory and entirely obviated the possibility of solid solute adhering to the sides of the cell. To avoid radiation of heat through the Dewar vessel A containing t,he petroleum ether bath the former was surrounded by a black cloth FIG. 3 while readings were in progress. In order to ascertain what effect possible dissolved gases might have upon the freezing point air in one instance waq passed through the ammonia in the cell for five minutes. The subsequent freezing point reading agreed with the previous reading to within 0 . 0 0 2 ~ .
Accuracy of the Measurements In considering the accuracy of temperature measurements distinction must be drawn between the absolute accuracy of the temperatures measured and the accuracy of the temperature differences in any one set of measurements. Henning's functional relationship1 between resistance ratios gives temperatures which are accurate to within about 0.02'. However, the accuracy of the temperatures obtained in the calibration a t the boiling point of oxygen, the least accurate of the calibration points, is about f.0.03' and this figure LOC.cit. and Ann. Physik, (4)43,
282
(1914).
620
LOUIS D. ELLIOTT
may be considered to represent the absolute accuracy of our measurements. Since the measuring instruments employed were sensitive to differences of less than 0.001' the limiting factor in the accuracy of temperature diferences becomes the closeness with which conditions may be controlled during any one run. It was found that duplicate readings usually agreed to within 0.0004 ohms. Taking into account the number of duplicate readings made the accuracy of the temperature differences is approximately 0.002'. Freezing Point of Ammonia Since each run was preceded by a determination of the freezing point of the pure solvent a series of forty such determinations were obtained whose range of values is shown in the distribution table below. The first column gives the total temperature range of observed freezing points divided arbitrarily into ranges of 0.01'. The second column shows the number of times the freezing point measurements fell into that particular range. From the table it is seen that twenty-nine out of the forty observations agreed to within 0.04'. The average of the forty freezing point temperatures is -77.726'. Taking into account the absolute accuracy of our temperature measurements the value for the normal freezing point of liquid ammonia is found to be -77.73'0rt.03'. Employing a platinum resistance thermometer and the regular Callendnr formula with Henning's corrections the U. 5. Bureau of Standards1 determined the temperature a t the triple point to be -77.70'. Bergstrom2 calcnlated the temperature at the triple point from vapor pressure data to be - 77.9'. Table of Freezing Points Range
Frequency
- 7 7 .680', to - 77 :689'. . . . . . . . . . . . . . . . . I .690 " .699 . . . . . . . . . . . . . . . . . 2 .703 1, . 709 . . . . . . . . . . . . . . . . . .6 11 ,710 . 7 I9 . . . . . . . . . . . . . . . . . . 7 )l .720 . 7 29 . . . . . . . . . . . . . . . . . . 7 1) . .739 . . . . . . . . . . . . . . . . .9 .730 .740 " .749 . . . . . . . . . . . . . . . . . .2 .750 " .759 . . . . . . . . . . . . . . . . .. 3 .760 " . 7 69 . . . . . . . . . . . . . . . . . . I .770 " .779 . . . . . . . . . . . . . . . . . .2
Results of Measurements The order of accuracy of the measurements of temperature differences made the determinat,ion of the freezing point constant in concentrations of less than about 0.006 moles per IOO grams ammonia impracticable. The lower concentrations employed are of the same order of magnitude as those employed by Franklin and Kraus3 in their work on the boiling point rise of ammonia 1
3
McKelvy and Taylor: Bur. Standards, Scientific Paper No. 465 (1923). J. Phys. Chem., 26, 367 (1922). LOC.cit.
62I
FREEZING POINT LOWERING O F AMMONIA
In the case of nearly every solute two and sometimes three or more series of measurements were made, each series being one clay's run consisting, as previously indicated, of the determination of the freezing point of the solvent followed by the freezing point determinations of the solutions of the particular solute under study in increasing concentrations. Tables I to 9,inclusive, show the results upon such substances as might be expected to give fairly normal values for the constant at, least in the lower concentrations. Table I O to 16,inclusive, give the data for substances expected to show some dissociating effect of the solvent upon the solute. In the headings for the columns of the tables, g=grams of solute used, M = o n e hundredths moles per IOO grams
Series
A I EI B I D I D2
g
CI
0.196 0.209 0.216 0.219 0.361 0.407 0.440 0.680 0.691 0.697 0.734
A3
0.758
E2
A2 D3
B2 E3
E4
I .006
A 4 D 4
1.18 I .18
E5
I .51
B3 c2
1.57 I .88
D5
2 .oo
A5
2.13
B 4 E 6
2.32
B5
2.82 3.07 3 $40 3.56 3.99 4.05 4.62 4.94 6.08
D 6 c3
D7 E7 D 8
E8 E 9 c 4
2.50
TABLEI Urea M 0.593 0.634 0.651 0.663 I .og
dT 0.062~ 0 .os9 0.066 0.062 0 .IO1
K 10.5 9.3 10.1
9.4 9.3 9.2
1.34 2.06
0.113 0.135 0.190
2 .IO
0.201
2.11
0.201
2.23 2.30
0.231
10.1
0.275
9.1
0.339 0.328 0.409 0.449 0.524 0,542 0.591 0.677 0.655 0.821 0.815 0.902 0.919 I .014 I .03
9.5 9.2 8.9 9.5 9.2 9.0 9.2 9.7 8.7 9.6
I .23
3.03 3.56 3.56 4.57 4.75 5.71
6.06 6.43 7.03 7.58 8.53 9.29 10.31 IO .8 12.1
12.3
14.o I 5 .o
0.218
I.I5 I.23
Solubility exceeded.
10.1 9.2
9.6 9.5 9.8
8.8 8.8 8.5 8.4 8.4 8.2 8.2
LOUIS D. ELLIOTT
622
solvent, dT, the observed depression of the freezing point and K, the molecular lowering of the freezing point. The weight of solvent in all cases was 55.1 grams. The value for K was calculated by the equation: gm. solvent used X mol. wt. solute X depression K = gm. solute X IOO The results upon each solute are tabulated in the order of increasing concentration irrespective of the order of series PO as to bring out more clearly the agreement between results of different series. Series are indicated by letter and the individual measurement by number. Soon after adding C 4 it was observed that the solubility was exceeded and as the temperature continued to fall more urea precipitated until the cryohydric point was reached. From the observed reading at this point and assuming a value for K of 8 at this concentration the concentration of the saturated solution was calculated to be approximately 9.5 grams per IOO grams of ammonia at the cryohydric temperature, - 78.9’. In a similar manner the solubilities of some of the other solutes in ammonia at low temperatures were obtained. The urea employed in Series A was a sample of Kahlbaum’s second grade dried in a desiccator. For Series B the same sample was recrystallized from amyl alcohol, the last traces of the latter removed with ether, the sustance recrystallized from absolute alcohol and dried in a vacuum desiccator. In the remaining series a sample of Kahlbaum’s “Special” obtained before the war was employed. TABLE I1 Ethyl Alcohol I< M dT Series @; 11.1 0.280 1.11 0.123’ A I 10.4 1.56 0.163 B I 0.397
B2 A 2 A B A B A A B A B
3 3 4 4 5 6 5 7 6
0.602 0.663 I .41 2.47 2.74 4.51 5.03 7.97 8,75 I 2 .oo I 3 ’ 58 14.6 16.5 17.5
2.38 2.61 5.57 9.76 10.8 17.8 19.9 31.4 344 47.3 53.9
0.237 0.253 0.542 0.924
IO .o
I .82
9.7 9.7 9.5 9.5 9.2 9.2
2.75
8.8
2.96 3.91 4.35 4.68 5.13 5.48 6.59
8.6 8.3 8.1 8.1 7.9 7.9 7.3
I ,027
I
.63
A 8 57.7 65.1 B7 69.0 A 9 2 2 .Q A IO 90 - ..3A freshly prepared sample of 99.97% ethyl alcohol was used in the measurements upon alcohol.
.
FREEZING POINT LOWERING OF AMMONIA
TABLE I11 n-Propyl Alcohol Series
A I A:! A A A A
3 4 5 6
A 7 A 8
g
0.487
0.878 I .52 3.29 5.67 9.63 14.8 20.8
M I .48 2.65 4.60
9.96 17.1
29.2 44.8 63 . I
dT 0.140' 0.246 0.418 0.844 1.34 2 .OI
2.75 3.51
For the measurements upon n-propyl alcohol the sample employed had been purified by repeated distillation from a snmple of U. S. Industrial Alcohol n-propyl alcohol. After standing over calcium oxide for several wedm its boiling point was found to be between 97.0' and 97.3'.
TABLE IV Acetamide Series
A I B I A2 B2 A 3 B 3 A 4
-45 B 4 B 5
g
0.243 0.249 0.556
0.556 1.086 2.00
2.15 3.95 4.43 7.47 8.0;
A 6 A 7
10.57
B 6
11.27
B7 A 8
12.57 14.30
M 0.748 0.767
1.71 1.71 3.34 6.17 6.60 12.16 13.62 23.0 24.8 32.6 34.7 38.6 44.1
dT 0.069' 0.076 0.165 0.167 0.325O 0 * 596 0.638 1.153 1.293 2.23 2.34 3.03 3.29 3.6.5 4.05
IC 9.2 9.9 9.7 9.8 9.7 9.7 9.7 9.4 9.5 9.7 9.4 9.3 9.5 9.5 9.2
The acetamide was twice recrystallized in chloroform from a post-war Kahlbaum article. The purified product was odorless and melted sharply at 81'. The limit of solubility was not reached but from the sluggishness with which A 8 dissolved it should be not far fyom that concentration.
624
LOUIS D. ELLIOTT
TABLE V Water Series
g
A I
0.387
A 2
0.870
A 3 A 4
B I A 5 B2 A 6
CI c2
c 3
dT 0.383'
8.78
0.855
M
.65 3.35 5.76 6.41 8.98 11.45 12.9 13.6 14.6 I
3.91 16.7 33.9 58.1 64.7 90.8
116. 130. 136. 147 '
I .62 3.29
5.75 6.41 9.35 12.35 14.12
K 9.8 9.7 9.7 9.7 9.9 9.9 IO . 3
10.7 10.8
15.10
I 1 .o
16.60
11.3
The values in Table V carry the concentration to the neighborhood of the cryohydric concentration for the system NH3-Hz0. 2NH3.
TABLE VI o-Nitrophenol Series
A I B I A 2
B3 B A B B A
3 3 4 5 4
g
0.946 1.362 1.86 2.04 3.03 3.44 3.53 3.73
5 .OS
M 1.24
1.78 2.44 2.67 3.96 4.50 4.60 4.87
dT 0.113' 0 .I 7 5 0.226 0,259 0.377 0.404 0.434 0.458
K 9.1 9.8
9.3 9.7 9.5 9.0 9.4 9.4
Solubility exceeded.
The cryohydrie temperature was found to be - 78.2' and the cryohydric concentration approximately 6 . 7 grams per IOO grams of ammonia. The sample of o-nitrophenol was purified by recrystallization from absolute alcohol and dried in vacuo at room temperature. The product melted sharply a t 44.8".
625
F R E E Z I N G POINT L O W E R I N G O F A M M O N I A
TABLE VI1 Sucrose Series
A I EI B I FI DI A 2
F2 B 2
F3 D2
C I
F 4 E2
D3
F5 B3 D4 F 6 c2
E3
D5 E4
g
M
8.6
I .20 I .32
0.121
9.2
I .42
0.130
I .60
I .72
0.158 0.146 0.162
I .88
0.195
I .go
0.220
9.2 9.9 9.0 9.4 10.4 1 1 .6
I .91 2.12
0.182 0.219
.IO.3
2.36
0.239
IO.2
2.59 2.68 3.16
0.251
9.7 9.3 10.4
0,889
I .87
0.990
2 .OI
I .07
2.26 2.48 2.67 3 .OI 3.04 3.25 3.54 3.58 3.59 4 .oo
I .62
4.43 4.87 5.94 5.96 5.97 6.02 6.66 8.OI
K
0.076' 0.086 0.098 0.106
I .67
5 .os
dT
3.17
3.19 3.20 3.54 4.26
0.250
0.330 0.305 0.352 0.332
8.7 9.2
8.8
9.5
9.6 11.1
10.4
0.380
10.7
0.431
IO.I
For all the series upon sucrose a sample of J. T. Baker's c.p. sucrose was employed the purity of which had been established in another connection.
626
LOUIS D. ELLIOTT
TABLE VI11 Aniline g
M
dT
K
B I A I A 2
0.590 0.648 I .26
1.15
11.8 8.9
B2
I .28
A 3
2.48 2.49 3.36 3.37 4.41 5.50 5.51 7.44 8.91 944
0.136’ 0.113 0.248 0.256 0.426 0.490 0.534 0.631 0.753 0.938 0,976 I .236 I .48 1.54 1.94
Series
B3 A B A A
4 4 5 6
B 5 -47 B 6 A 8
IO. 72
10.74 14.6 17.4 18.6 23.8 24.9 31.9 35.0 35.6 41.4 48 .o
12.22
B 7 A 9
12.76 16.2 17.9 18.3
I38
A IO B 9 B IO B 1.1
I .27
2.46 2.49 4.83 4.86 6.55 6.58 8.59
21.2
24.6
2.21
2.46 2.68 2.73 3.11 3.51
IO.I
10.3 8.8 IO. I
8.2 9.6
8.8 8.8 9.1
8.5 8.5 8.3 8.2 8.9 7.8 7.7
7.7 7.5 7.3
Series A was conducted with a redistilled Bausch and Lomb product while in Series B a redistilled sample of Kahlbaum’q “zur Analyse” aniline was used. A curious phenomenon noted in connection with aniline was the complete invisibility of the crystals of ammonia while the concentration was passing through the neighborhood of 39 grams per IOO grams of ammonia.
TABLE IX Pyrocatechol Series
A I A 2
g
0.513 I .044
M 0.846
dT 0.088’ Solubility exceeded.
K 10.4
The cryohydric temperature was - 7 7 .8’ and the concentration approximately I . z grams per IOO grams ammonia assuming 10.0for the value of K. The solubility of this substance is surprisingly low in view of the fact that Franklin and Krausl report it to be very easily soluble in ammonia at higher temperatures. 1
Am. Chem. J.,20,
820
(1898).
FREEZING POINT LOWERING OF AMMONIA
627
Discussion of the Results on Non-electrolytes The outstanding feature of the results on these solutes is the variation of the const,snt with concentration. In only two cases, that of acetamide and thst of ortho nitrophenol are we justified in arriving a t the value for the freezing point constant by simply averaging the values obtained in the case of each. Even with acetamide a slight tendency toward association of the molecule with increasing concentration can be detected. With ortho nitrophenol the low solubility prevented an extensive series of measurements. With urea, ethyl alcohol, normal propyl alcohol and aniline the tendency to associate is clearly brought out by the decreasing value for the constant with increasing concentration. Except in the case of ethyl alcohol this is in agreement with the behavior of the boiling point constant for these solutcs as determined by Franklin and Kraus.' In the case of ethyl alcohol they obtained a constant value for the boiling point constant through a wide range of concentrations. With normal propyl alcohol and aniline a comparison of our results with theirs shows the tendency of these two substances to associate to a greater degree at lower temperatures. For urea the degree of association for equal concentrations is about the same at the freezing point as at the boiling point of ammonia. Water which gives a constant valuc for thc boiling point constant does likewise in the case of the molecular freezing point lowering up to relatively high concentrations after which a gradual increase occurs.
CHART1
The results on sucrose, while unsatisfactory from the point of view of agreement between series, do however, show clearly when considered as a whole a gradual increase beginning with values slightly less than the normal constant. The sluggishness with which the sucrose dissolved after the last few additions of this solute rendered measurements in higher concentration, impracticable. The one value obtained for pyrocatechol is of minor significance. LOC.oit.
62 8
LOUIS D. ELLIOTT
The best means of arriving a t the normal value of the molecular lowering of the freezing point is to examine the results on each solute with a view t o determining whether or not there is a range of concentrations throughout which constant values occur. In Chart I are plotted the values of the constant against concentration for seven of the nine solutes. It is clear from the graphs that from initial concentration up to that of about 0.06 moles per IOO grams ammonia the constant lies between 9 and IO. With some of the solutes a fairly constant value is maintained up to far greater concentrations. Taking up each solute individually we may proceed to obtain the normal value for the constant. Urea: Fairly constant values are found up to a concentration of about 0 . 0 4 moles per roo grams ammonia. The average of the first fifteen values in Table I is 9.6. Ethgl AZcoiioZ: The range of constant values lies bet ween concentratione of about 0 . 0 2 and 0.10moles. While there are not as many points on the curve as in the case of urea the range of constant values is much wider and the results within this range are in closer agreement. The average of the second to the seventh value, inclusive, of the constant in Table I1 is found to be 9.8. Normal Propyl Alcohol: Only three values were obtained in concentrations of less than 0.05 moles, the average of which is 9.3. Acetamide: The average of all the determinations is 9.5. Water: This solute gives constant values for a wide range of concentrations. The determined points within this range are few but conform closely to a straight line. The average of the first six values in Table V is 9.8. 9rtho Nitrophenol: The average of seven determinations is 9.4, Anilzne: Unfortunately the range of concentrations within which the constant is apparently normal is a range in which individual results were somewhat contradictory. Omitting the first value in Table VI11 which appears unreasonably high, thc average of the results up to B 4, inclusive, is found t o be 9.4. Leaving out of consideration the above value for normal propyl alcohol based upon too few observation and also omitting the average for aniline we arrive, by averaging the remaining six individual avcrages, at a value of 9.7 for the normal molecular lowering of the freezing point of ammonia. This is considerably higher than that calculated by Massoll from his experimental value for the heat of fusion of ammonia. Calculating back from our freezing 0 .02T2
point constant of 9.7 by van’t Hoff’s formula, W = --’
we obtain for the K value of W, the heat of fusion, 78.7 calories per gram, almost the same as the corresponding value for water, or 1338 calories per gram molecule as against 1838 determined by Massol. Raoult found that the lowering of the freezing point produced by one molecule of a solute in one hundred molecules of any solvent is very nearly 1
LOGcit.
FREEZING POINT LOWERINQ O F AMMONIA
629
a constant, 0.63'. By dividing our value of 9.7by the molecular weight of ammonia we obtain the value 0.57' for Raoult's constant while Massol's data give a value of 0.42'.
Measurements on Electrolytes Franklin and Kraus' noticed that dilute solutions of sodium nitrate, ammonium nitrate, and potassium iodide in liquid ammonia which are good conductors of electricity gave a molecular boiling point rise considerably lower than expected from a consideration of their relatively high conduct,ivities. It was later established by Franklin and CadyZfrom a study of the velocity of ions in liquid ammonia solutions that the dissociating power of this solvent is more in accord with the value for its dielectric constant of 22 than with conductivity data. The high conductivity of electrolytes in dilute ammonia solutions is explained by the relatively higher speed of the ions. Working with more concentrated solutions Franklin and Kraus3 later found that the molecuTABLE
x
Sodium Nitrate Series
g
C I B I
0.247 0.470 0.491
c2
A I B2 c3 n3 c 4
0.505
,
0.582 0.742 0.784 1.044
A2
I .045
B 4 CS A3
1.53 2.03
B5
3 .I5 3.53 3.53 6.49 6.53 8.04
C 6 A 4
2 .os
1 M 0.528 I .004 I .os
I .08
1.24 I .58 I .68 2.24 2.24 3.28 4.34 4.39 6.73 7.54 7.54 13.9 14.o
,dT
K
0.067' 0.109 0.116 0.138 0 . I33 0.166
12.7
0.173
10.3 IO .3
0.230
0.261 0.337 0.439 0.480 0.690
0.757
10.9 11.1
12.7
10.7 10.5
11.7
10.3 10.1
IO .9
10.3 10.0
0.806 10.7 IO.? 1.49 A 5 10.3 1.44 c7 A 6 17.2 I .90 I1 .o C 8 8.10 I .86 10.7 17.3 B 6 8.16 I .88 10.8 17.4 22.4 IO.46 B7 11.3 2,54 A sample of Kahlbaum's pre-war sodium nitrate was used after drying at I IO'.
LOC.cit. J. Am. Chem. SOC.,26,499 (1904). J. Am. Chem. SOC.,27, 191 (1905).
63 0
LUOlS D. ELLIOTT
lar conductivities of tfhese salts in solutions of ammonia were less than in aqueous solutions of the same concentration notwithstanding the greater mobility of the ions.
It was thought of interest to investigate the freezing point depression of these three salts and a few other substances which give good conducting solutions in liquid ammonia. In addition to the three salts mentioned silver iodide and strontium nitrate whose conductivities in ammonia are abnormal were investigated. Two organic compounds, phthalimide and trinitraniline, which give good conducting solutions in ammonia were also included.
TABLE XI Ammonium Nitrate Series
6
M
A I B I B 2
0.394 0.467 0.668
0.894 1.06
B 3
0.897
A 2
I .OI
B 4 A 3 A 4
I
.26
I
.52
2.83
dT
I
