VAPOR PRESSURES OF SOLUTIONS

The millimeter scale for this manometer was calibrated ... pressures are expressed in centimeters of mercury at 0°C. and a gravita- tional accelerati...
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LIQUID Ai'vIMONIA AS A SOLVENT. I1

VAPORPRESSURES OF SOLUTIONS AT 25OC.l HERSCHEL HUNT

AND

W. E. LARSEN

Department of Chemistry, Purdue University, Lafuyette, Indiana Received September 26, 1933

These investigations were made in order to establish a significant scale of relative acidities, basicities, and salt-like properties in liquid ammonia and water. The present part deals with the vapor pressures of the more concentrated solutions of ammonium salts in liquid ammonia. These data may help to interpret the complex ammonia system of electrolytes. EXPERIMENTAL

Manometer The vapor pressures were measured directly on a manometer capable of reading fifteen atmospheres pressure (figure 1). The millimeter scale for this manometer was calibrated with a U. S. Bureau of Standards steel tape. The readings were corrected for temperature and gravity so that the vapor pressures are expressed in centimeters of mercury a t 0°C. and a gravitational acceleration of 980.665 cm. per sec.2 It was found that the correction for the compressibility of water only amounted to 0.001 per cent at 750 cm. pressure; it was therefore neglected. The manometer was filled with boiled water, all air bubbles being carefully removed. Columns A and F were carefully dried after the manometer was filled. The pressure of the water columns, calculated to standard conditions, was subtracted from the mercury reading and the barometric pressure added. As a final check on this method of ineasuremsnt the vapor pressure of pure ammonia was measured. We obtained a value of 750.8 cm. as compared to 752.05 reported by Cragoe (1). When the initial pressure was not, more than three atmospheres (concentrated solutions), st simple manometer was used. Temperature A double-walled copper container of 1-gallon capacity was immersed in a 75-gallon water thermostat. The Dewar containing the cell was placed This paper is a part of a thesis presented by W. E. Larsen in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Purdue University. 80 1

802

HERSCHEL HUNT AND W. E. LARSEN

inside this adjustable container. This triplex system made it convenient to surround the cell with a carbon dioxide-ether bath for chilling and also to minimize temperature changes. The room was kept at a temperature

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FIQ. 1. MANOMETER

such that the vapor pressure of ammonia was several centimeters above the vapor pressure of the solution, so that ammonia would not condense in the exposed tubes. The thermostat was kept at 25°C. f. O.O25"C., so the Beckmann (U. S. Bureau of Standards calibration) thermometer in the Dewar was constant to =tO.OOl"C. over a long period of time.

803

LIQUID AMMONIA AS A SOLVENT. TI

Cell Figure 2 shows the electrically stirred cell used for the solutions. An intermittent current passed through S. V is an iron needle valve used in valving the ammonia when it was desired to change the concentration: The cell had a volume of 100 ml. The vapor space over the solution and in the tube Ieading to the manometer was measured gravimetrically. This space is reduced to a minimum.

Measurements Reagent grade salts were recrystallized from water three times and dried for several hours a t 110°C. They were placed in the cell and an TABLE 1 Vapor pressures of aininonium nitrate in liquid ammonia at 26°C. SERIES

M O L E S "8

MOLE

NH4NOa

P

I

11

SERIES

NH? N&NOa

MOLES MOLE

cm. Hg

crn. H g

.4 B h C C B A D D C D C C C D D D C

'

0.744 0.808 1 22 1.30 1.56 1.76 1.85 1.93 2.20 2.58 2.84 3.09 3.45 4.01 4.55 5.23 6.30 7.05

86.3 86.3 88.0 97.0 123.0 143.4 152.9 163,5 192,6 235.8 266,7 291.4 331. I 385.9 438,6 489.1 551.1 580,7

P

8.01 9.10 11.4 12.2 15.6 21.7 32.6 47.3 51.8 60.0 69.7 80.6 91.4 111 126 135 140

613.0 638.1 672.6 682.7 699.2 716.6 727.9 735.5 736.6 738.7 740.4 741.7 742.6 743.7 744.4 744.8 745.0

excess of ammonia condensed into it. The cell was then sealed in place and the system evacuated while chilled. The cell was then warmed with the valve closed until the vapor pressure was several atmospheres. By repeated warming, chilling, and pumping out, the cell and manometer were freed from air. I n order to keep the ratio of ammonia in the solution to ammonia in the vapor phase high, it was necessary to use larger amounts of ammonia for the dilute solutions. The series varied from 13 g. of salt and 3 g. of ammonia to 0.4 g. of salt and 20 g. of ammonia. From three t o

804

HERSCHEL HUNT AND W. E. LARSEN

six hours were allowed for equilibrium to be established for the first reading and at least two hours for each subsequent reading. The manometer readings did not vary more than 1 mm. on standing an hour after equilibrium was reached. The concentration was changed by allowing ammonia to escape through V and absorbing it in concentrated sulfuric acid. The concentration of the TABLE 2 Vapor pressures of ammonium iodide i n liquid ammonia at 85°C. SERIES

MOLES "a MOLE

"a1

MOLE

NfIaI

P

'I

1

cni. IIg

B A

C B

A A A

C A E C E E D E D F D D D

E F F D

1.98 1.99 2.40 2.59 3.00 3.26 3.57 3.68 3.79 3.84 3.94 4.03 4.25 4.35 4.55 4.87 5.03 5.37 5.88 6.29 6.51 6.97 7.74 8.48

73.6 73.8 84.2 96.0 132.8 158.7 195.7 210.3 221 .o 232.4 241.1 255.4 282.3 301.3 316.7 358.2 375.5 406.0 448,6 477.3 487.5 517.4 553.0 580.7

______

I

cm. Hg

D E E F G G G F F

,

I ~

G H H H H H I I I J J J J J J

9.64 10.2 11.1 12.3 15,5 16.9 18.4 20.1 21.2 23.8 30.2 36.7 44.6 48.0 60.0 66.5 83.1 99.9 115 135 147 161 177 200

612.4 621.8 638.3 654.7 683.3 691.1 697.5 704,1

708.0 712.4 722.0 727.8 732 1 733.4 737.2 738.8 740.9 742.6 743.6 744.9 745.1 745.7 746.0 747.1

solution was calculated from the solvent and solute present in the cell a t each reading, allowance being made for the ammonia in the vapor phase. The weight of salt was determined at the end of a run by weighing the cell and salt and then weighing the cell after the salt had been dissolved out. RESULTS AND CALCULATIONS

The values given in tables I, 2, 3, and 4 are the values obtained from several sets of independent measurements. These values of the vapor pressures are plotted in figures 3 and 4, where the concentration is ex-

805

LIQUID AMMONIA AS A SOLVENT. 11

TABLE 3 Vapor pressures c zmmonium bromide i n liquid ammonia at 85°C. SERIES

NH3 NfIakh

MOLES

MOLE

P

/

SERIES

NHs NH&r

MOLES MOLE

cm. H g

cm. Hg

A A B A

C C R D

c C C C

D D C

D D C

1 56 1 83 2 29 2 50 2 56 2 77 3 00 3 29 3 53 3 93 4 30 4 69 5 17 5 57 5 74 6 80 7 39 9 03

P

162 3 162 3 162 3 184 2 196 5 226 0 257 9 305 5 340 6 396 2 440 1 480 8 522 6 550 7 559 7 609 0 628 1 662 5

9.65 10.7 12.3 14.2 17.5 21.6 24.6 35.1 39.9 44.0 56.3 71.6 94.0 129 146 159 172

671.1 683.4 695,1

704.3 713.5 722.2 725.4 732.5 734.9 735.9 739.2 741.5 743.2 745.1 745.5 745.9 746.3

TABLE 4 Vapor pressures of ammonium chloride in liquid ammonia at 25°C. SERIES

NHa NHaCI

MOLES

MOLE

P

SERIES

NH3 NHaCl

MOLES MOLE

cm. Hg

A 13

13 B R B A

R B A A C

C C A B

C

2.06 2.38 2.62 2.90 3.10 3.37 3.67 3.90 4.32 4.89 6.21 6,7l 7.48 8.55 9.41 9,96 12.2

314 0 314 9

530 3 558 2 595 8

P em. Hg

I

‘

1

D C D D D E F

13 8 16 0 17 9 20 0 24 9 32 0 34 7 51 3 60 7 69 0 86 4 105 111 122 133 143

725.9 729,0

730.7 732,4 735 2 737.6 738.6 741,7 742.5 743.4 744.2 745.2 745.5 745.9 746.8 747.1

I

0

I 5

I

I

I

10

I 15

I

I

I

20

Moles NHe/Mole Solute 3. VAPOR P R E S S U R E S O F SOLUTIONS I N THE CONCENTRATED REGIOX I, ammonium chloride; 11, ammonium bromide; 111, ammonium nitrate; TV, ammonium iodide. FIG.

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Moles NI&/Mole Solute F I Q . 4. VAPORPRESSURES O R SOLUTIONS IN THE DILUTE REGION I, ammonium chloride; 11, ammonium bromide; 111, ammonium nitrate; IV, ammonium iodide. Curves I, 11, and I11 have been displaced upward on the

ordinate 60, 40, and 20 cm., respectively. 806

LIQUID AMMONIA A S A SOLVENT. I1

807

pressed in moles per mole of salt. The tables include only a fraction of the measured pressures. DISCUSSION O F RESULTS

By this method it is impossible to carry the solutions to the desired dilutions so that we can calculate the activity of the solute. However, we are measuring the freezing points of the dilute solutions and will be able to furnish additional data in a subsequent article. Since the dielectric constant of ammonia is low, one expects a low activity of the electrolytes in this solvent. However, because of the great proton accepting power of ammonia one expects these ammonia salts to have a high activity. Our vapor pressures indicate that the solute ions are associated in our most dilute measurements. The order of association is what is to be predicted from the magnitude of the ion diameters. The extremely low concentration of H' or protons in (1) NH3 leads the authors to avoid the term, acids in (1) NH,, in the usual sense that the term acid is used. SUiMMARY

The vapor pressures of solutions of ammonium nitrate, ammonium chloride, ammonium bromide, and ammonium iodide in liquid ammonia have been determined over as wide a concentration range as possible at 25OC. REFERENCE (1) CRAGOE, C. S.,MEYER,C. H., AND TAYLOR, C. S.: J. Am. Chem. SOC. 43, 206 (1920).