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1168
PERMAN : VAPOUR PRESSURE OI?
CXV.- Vc~ipou7*
Pressure of Aqueous Ammonia S o h i o n .
Part 1..
By EDGAR
PHILIP
PERMAN.
IN a former paper (Trans,, 1901,79, 71S), I have published the results
of the determination of the vapour pressure of aqueous ammonia
solution for various temperatures and concentrations. F o r the sake
of reference and for comparison with the results obtained by another
method, these data have been further elaborated. The values of the
vapour pressure for percentage concentrations of 2.5, 5.0, &c., were
read off from the original isothermal curves, and the valueli for each
concentration were then plotted against temperature, giving series of
curves which I suggested (Trans., 1902, 81, 483) should be called
isosthens," or lines showing the relation between pressure and
temperature for solutions of equal concentration. These curves mere
continuous, and were drawn without difficulty with the help of a set
of curved rulers, or in some cases with a thin steel strip kept i n
position by a series of projecting brass rods, which could be screwed
to a brass framework.
These curves may, without appreciable error, be considered as being
straight for a variation i n temperature of not more than 2'. The
values of vapour pressure have therefore been read off at intervals of
2", and are arranged in the table on p. 1169.
The horizontal columns give the isothermals, and the vertical
columns the isosthens. The values for intermediate temperatures or
concentrations can be deduced without serious error by taking proportional p a r t s ; if greater accuracy is required, the curves can be
constructed from the tabulated numbers. The pressures are given
t o the nearest half millimetre, which is about the limit of accuracy
attained in the measurements.
The P a r t i d Qapour Pressures of Aqueous Ammonia Xolution.
I n order to measure the partial pressures of the ammonia and the
water vapour given off by a n aqueous ammonia solution, it was necessary to devise some method of estimating the quantity of each constituent in the vapour t h u s evolved ; the method which seemed to me
t h e most promising was t o aspirate a current of air through the
solution and then through suitable absorbents, Eefore adopting this
process, I made some preliminary experiments on pure water, which
showed t h a t the vapour pressure of water could in this way be found
with great accuracy. I have since made further experiments up t o
90' with a similar result (Proc. Roy. Soc., 1903, 72, 72).
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AQUEOUS AMMONIA SOLUTION.
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Perceyatnge of rlrmiorzicc,
T.mp.1
0.
____
I I 1 1
2'5
5.0
7'5
__
10.0
__
~
12.5
1 1
15'0
~-
1169
PART 11.
1 1 1 I
17'5
20'0
22'5
---__--
25'0
27'5
30 0
Vnpour pressure in inin. of niercury.
13
13'5
14
15
16
18
20
20'5
21
20
4'5
5'5
G
7-0
8
9
10'5
12
13-5
15.5
17.5
22
20
28.5
32'5
36.5
2L
22'5
41',5
il9.5
2ti
23
46.5
2s
2h.5
31'5
35.5
40
44'5
49'5
55
61.5
61'5
56.5
62.5
6 (I
66
74
S3
$12
102
iti
112
123.5
s3.5
91
100
110
120
1.32
146
161
177
104.5
136
149.5
163'5
234
255'5
122.5
134 5
147'5
161
175.5
192
210
230
252
275
300
327
335
0"
2
4
6
8
10
12
14
16
18
30
32
34
3G
38
40
42
44
4e
4s
50
62
54
56
58
GO
62
6S
75-5
63'5
92 5
102
112.5
20
22
25
213'5
22'5
24 '5
27
30
33'5
37
42
47'5
53
27'5
25'5
YO
32'5
3 ti
40
44
50
56
62.5
70
77'5
$5'6
95
104'5
115
i27
140
153.<-I
167.5
183
199
216.5
27G
257.5
2s1
306
332
360'5
391
425
460
35
45
57
:?ti.Z
3:I
4i.5
60.5
51
6ii
43
4s
54
GO
67
75
s4
56
72.5
93
103
114
1%
1YS
153'5
l(jS.5
1%
202
221
241
263
2b6
310
3 3 ti
363'5
395
427'5
4ti1.5
49s.5
539
5S2
so
li2
;:
11
85'5
!)5'5
lo(;
11s
131
145
160
lit;
1!12'5
"2'5
233 '5
255
S!)
99
110.5
1'23
1365
I >l
lt17.5
lh5.5
204
223
206
2g11
31s
346
377'5
1:1n
17ti
104.5
215'5
412
"7
627
2W'G
2se
313
342
5S6
(39
095.5
24 i
:;73
267.2
407'5
444.5
292
260
212
93
101
110
120
131.5
144
450
492
i64
hl7
8>4
964
4 b:? .-1
1027
.5
110.5
1189
355
3,%
420
31s
317.5
""
3 1 1 '5
410
411
451
531-5
454
Gtj4
491'5
530'5
609
ti56
70A
759
834.5
s97
1033
1109
816.5
11111
278'5
303'5
330
5i2
617-5
6Oti
717
I
---b'iS.5
<"I,-
5N.5
ti17
1276
1367
I460
tititi
71!1,5
-
Cr 1
-
I rb
-
9ti3
-
-
1279
As t h e method proved satisfactory for water, I employed it for the
ammonia solution; in order to estimate both the ammonia and the
water, the mixture of air, ammonia, and water vapour was made t o
pass through dilute eulphuric acid of measured volume and strength,
and then through strong sulphuric acid ; the total increase in weight
gave the weight of ammonia and water carried off by the air current ;
the weight of ammonia was found by titration, t h a t of the water being
obtained by difference.
I have already shown that the air passing through the solution
takes u p the ammonia and water vapour, and becomes saturated with
great rapidity, also that the ammonia is very rapidly absorbed by the
dilute acid solution. The air current was always slower than 0.1 litre
per minute, and considerably less than this with the more concentrated
solutions. The experiments were therefore of a tedious nature, but
the method adopted was, in my opinion, t h e only one practicable.
Apparatus.-The
apparatus employed was nearly the same as t h a t
used i n my experiments (Zoc. cit.) on the vapour pressure of water,
the chief difference being the introduction of a second absorption
VOL. LXXXIII.
4 K
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1170
PERMAN: VAPOUR PRESSURE OF
apparatus for t h e ammonia. This vessel was similar t o the one used for
t h e absorption of the water, but was larger, the capacity of the bulbs
being about 100 C.C. and 20 C.C. respectively; i t was placed between
the flasks containing the ammonia solution and the absorption-bulbs
for the water. There were only two other essential differences: (1)
t h e last flask, containing the ammonia solution, could be detached from
t h e rest, so t h a t the strength of the solution i n i t might be estimated ;
(2) the air was freed from carbon dioxide, before passing into the
ammonia solution, by means of a tube of soda-lime.
Method of Vork.-In general, t h e procedure was t h e same as in the
experiments on water, but in this case the operation included t h e
titration of the dilute acid or ammonia in the absorption flasks.
It was necessary to find what pressure was required to drive t h e
air through the whole apparatus: without neutralising a n y of the acid.
This was effected by ascertaining the pressures necessary to drive the
air (1) through the ammonia solution alone, and (2) through t h e acid
solution and the rest of the apparatus; the sum of the two pressures was the amount required, and the gauge (Proc. Boy. Xoc.,
ibid., Fig. 1) was adjusted to this pressure before beginning the
experiment.
The strength of t h e ammonia solution i n t h e last flask did not
usually change appreciably during t h e experiment, the maximum
alteration being only about 1 per cent., but i n every experiment the
strength was found before and after aspiration, and the mean taken of
t h e two determinations. From 2 t o 10 C.C. were withdrawn by carefully
standardised pipettes, run into a n excess of ,standard acid, and titrated
with standard caustic soda solution, t h e temperature being maintained at 15’. The percentage strength mas calculated from Lunge and
Wiernik’s data (Landolt and Bornstein’s Tabellem, 221). The neutralisation of the acid in t h e absorption flask was shown by some methyl
orange present, and the aspiration was not allowed to continue long
after this point was reached. Acid solutions of various strengths had to
be used to suit the varying concentration of the ammonia solution and
the different amounts of ammonia solution withdrawn ; these were
standardised by means of sodium carbonate, and also with ammonium
chloride and a caustic soda solution.
A series of experiments was always begun with a strong solution,
and the remaining solution was diluted down for the succeeding
experiment. The ammonia solution employed was obtained by distilling
t h e pure concentrated solution with barium hydroxide, and passing
the gas evolved through barium hydroxide solution and into distilled
water ;in this way, it was obtained so free from carbon dioxide t h a t it
gave, i n the worst cases, only a very slight turbidity with bariumchloride.
A trace of some of the amines was probably present, but this has
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AQUEOUS AMMONIA SOLWTION.
1171
PART 11.
been shown not to affect appreciably the vapour pressure of the solution
(Trans., 1901, 79, 721).
Experiments were made a t every 10' from 0' to 60'. Unfortunately,
the method is not applicable over a very wide range of temperature or
with very varying concentrations of the solution, for when the vapour
pressure becomes nearly equal t o t h e atmospheric pressure, a very
little air will draw off a verylarge quantity of vapour, and, moreover,
the evaporation of the ammonia becomes so rapid t h a t it is impossible
to keep the temperature constant.
measurements already described,
Calculation of h?esults.--The
together with t h a t of the barometric height, afford the following
data :
w, = weight of ammonia withdrawn.
ww=
,, water vapour ,,
(by difference).
P = pressure in last aspiration flask.
p=pressure of air in aspirator (corrected for the vapour pressure
of water).
5'"absolute temperature of aspirator.
V=volume of water drawn from aspirator.
Assuming t h e truth of Dalton's law of partial pressures, t h e
following relationship holds good :
pressure of ammonia
total pressure
- volume of ammonia
I
total volume
'
which works out t o the following expression (putting p , and p , =partial
pressures of ammonia and water vapour respectively) :
w, x 1.312 x 760 x P x T
p'= 5"x 760(w,x 1.312 + w, x 1 4 4 2 ) + 673 x Y x P '
and similarly for the water vapour :
w,x1.242x76OxPxT
p w = T x 760(w, x 1.242 + w, x 1.312) + 273 x Y x p '
At 0' and under 760 mm. pressure, the specific volume of ammonia
is assumed to be 1.312, whilst that of water vapour is taken as 1.242.
These values are calculated from the density of oxygen and the molecular
weights of the gases, I have shown t h a t this assumption of normal
density is justifiable in the case of water vapour (Yroc. Roy. A ~ O C . ,1903,
72,80), but there is some doubt as to whether this is the case with
ammonia.* L e Due found that the density of ammonia at O'and
* The value given is 0.5971 (air =1) (Comnpt. r m d . , 1897, 125, 573). This gives
the specific volume as 1'295 instead of 1.312, the number employed above.
4 K ' L
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1172
PERMAN : VAPOUR PRESSURE OF
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760 mm. was more than 1 per cent. above the normal value,
but when mixed with large quantities of air and water vapour, t h e
density will probably be more nearly normal, assuming that no
chemical action takes place, I n the absence of exact data on this
point, it was thought best to calculate the results on the assumption
t h a t the density was normal throughout, especially a s the deviation
FIG. 1.
mm.
0
5
10
15
20
25
per cent. NH,
from normal valne can hardly be more than the probable experimental
error of these results.
Eesults obtained-The actual experimental results are shown in the
table on p. 11’73.
From these results, a series of isothermal curves was plotted, Fig, 1
for ammonia, and Fig. 2 (p. 1176)for water vapour, and from them the
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1173
PART 11.
AQUEOUS AMMONIA SOLUTION.
Temperature.
Percentage of
animoiiia.
Partial 1mssnre of
nmmonia ( p a ) .
Partial presznre of
water vaponr ( p w ) .
0"
4.72
9.15
14'73
19'62
22.90
11-4 mm.
24'8 ,,
51-3 ,,
82.5 ,,
116.5 ,,
5.1 mm.
5.3 7 ,
4'1 9 ,
3.0 9 ,
2.8 2,
10"
4.16
8.26
12.32
15'88
20'54
21 -83
16,5
37.2
64.2
95.1
149'2
169'8
19-90
4.18
6.50
6 *55
7 -72
10.15
10.75
16.64
19-40
23'37
30 09"
40"
50"
60"
3 '93
7 -43
9.75
12.77
17.76
17'84
21.47
3.79
7.36
11.06
15.55
17-33
20-85
3 '29
5'00
8.91
11-57
14.15
14'91
33 6
5.77
7 -78
9 '37
11'31
,,
,,
,,
,,
,,
,,
27'4 ,,
45.8 ,,
46.0 ,,
56'2 ,,
80.6
86.3 ,,
166.1 ,,
215% ,,
302.4 ,,
41.2 ,,
86.3 ,,
120.0
175.0 ,,
290.2 ,,
291.1 ,,
),
y,
404.6
,,
,,
,,
,,
,,
,,
,,
79.1 ,,
151.3
246'6 ,,
341.7 ,,
451.4 ,,
487'1 ,,
136'9 ,,
215.9 ,,
300.4 ,,
375'7 ,,
475.8 ,,
61-1
133'0
218.5
353%
427'7
576.1
,)
9.1
8-8
7'6
7.0
7'2
5.5
,9
,,
?,
,Y
9,
9 ,
,,
,,
,,
1 5 % ,,
16'4
16'1
16.0
,,
,,
12.9 ,,
12.3 ,,
10.3 ,,
15.1
14'7
31.1
29.2
,,
,,
,,
,,
24'8 ,,
24'3 ,,
22.1 ,,
28'5
26'6
53-5
50.7
49'1
44'1
-
,,
,,
,,
,,
,,
8 9 % ,,
87'1 ,,
83'0 ,,
80'6 ,,
77-0 ,,
37'8
75.2
,,
144'1
,,
,,
),
,,
-
138.5
135.5
130'4
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1174
PERMAW : VAPOUR PRESSURE OF
pressures were read for concentrations of 2.5, 5.0, &c., per cent. of
ammonia, The (' isosthens " were then constructed from the numbers
obtained; but it has not been thought necessary to reproduce them.
These curves, which were drawn directly without smoothing through
the points from t h e isothermals, may be regarded as being straight
lines over interv:zls of 2O, and the values have been read off and
tabulated, both for ammonia and water vapour. The following table
furnishes the isothermals when read horizontally and the isosthens
when read vertically ; intermediate values may be found with fair
accuracy by taking proportional parts.
Percentage OJ Ammonia.
Temp.
-
1 1 1 I 1 I I 1 I
2.5
-
5.0
7.5
~
~
10.0
12.5
15.0
17.5
~
~
_
_
20.0
22'5
_
~
Partial pressure of ammonia in mm.
0"
2
6
4
7
6
7'5
8
9
10
11
12.5
14
15'5
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
6 -5
17
18'5
20 -5
22 -5
25
27'5
30
32.5
35
38
42
46
50
54.5
59
64
69.5
75
80
85.5
13
14.5
15
16.5
18
20
22
24 -5
27
30
33-5
37
40 *5
44'5
49
54
58'5
64
70
76 '5
83.5
91
99
10i-5
116'5
126
136
147
158.5
170.5
183
---
20
22
24
26.5
29-5
32.5
36
40
44 *5
49
54
59 -5
65 *5
72
78-5
86
94
103
112.5
122'5
134
145'5
158
172
186
200-5
217
233.5
251
270
291
28.5
31.5
34.5
38
42.5
47'5
52
58
64
71
78 *5
86 ' 5
95
104
114
124 -5
136
149
162
176
191
208
225-5
244
263
284.5
306
329
354
380
406.5
40
43.5
48
53
58 -5
65
71.5
79
87.5
97
107
117
128
141
154.E
169
184'5
20 1
219
238
258 -5
280
302'5
327
352
380
-
-
53.5
58.5
64'5
71
78
86 *5
95.5
105
115'5
127.5
141
155
170
186
202'5
220
241
263
286.5
311
336
363'5
392.5
4225
455.5
491'5
-
-
69
76
83.5
92
102
112
123.5
136
150
165
181
198
217
236-5
258
281
306
334
363
395
429
466
-
-
-
-
87-5
97
107
i18
130
143
1575
173
190
208.5
228'5
250
27 3
298
324'5
353
382.5
415
451
491
635
-
-
-
-
---- -
111
123
135-5
149
163.5
179.5
196.5
215-5
236
2585
283
309
337
368
403
441.5
-
-
-
-
-
I
-
-
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AQUEOUS AMMONIA SOLUTIOX.
1175
PART 11.
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Percentage @ Ammonia.
Partial pressure of water vaponr i n mm.
0"
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
-
4 -5
5.5
6
4.5
5
6
7
7
8
9
10.5
12
13'5
15.5
17.5
20
22'5
25
28 '5
31.5
35.5
40
44'5
49'5
55
61'5
68
75 -5
83 -5
92 5
102
112'5
123.5
136
149'5
8
9
105
12
13.5
15
17
19
21.5
21
27
30.5
34.5
38.5
43
48
54
60
66.5
73 -5
81'5
90
99
109
120
132.5
148.5
4
5
6
4
5
5.5
6 -5
7
8
7.5
9
8.5
10
9.5
11'5
11
12
13
14
14-5
16'5
15-5
18.5
17'5
20
21
23 '5
22
25
26 *5
28 *5
30
32 -5
34
36.5
38
41
42 -5
46
47.5
51.5
53
58-5
57
63.5
65
71.5
70
77'5
79
85'5
375
94
96'5
103'5
37
114'5
18
126'5
30
139
1-3
-
4
4'5
5
6
7
8
9
10'5
12
13'5
15
17
19.5
21'5
24 *5
28
31.5
35'5
39.5
44 -5
49.5
55
61'5
68
75.5
82.5
91
100.5
111
122
134
3 *5
4
4.5
5.5
6.5
7 *5
8.5
10
11.5
13
14.5
16'5
18'5
21
23.5
26.5
30
33'5
37 '5
42'5
47 *5
53
59
65
72
79'5
87.5
96
-06
-17
.28-5
3 :*
4
4'5
5 '5
3.5
4
4.5
5'5
6'5
6.5
7 '5
7'5
8 '5
8.5
9 '5
9.5
11
11
12-5 12.5
14
14
16
16
18
18
20 -5 20
23
22
25 -5 24'5
28'5 27.5
32
31
36
34.5
40.5 38
45
42.5
50.5 47
56
62
68-5 75.5 83
I
-
-
-
-
3
3.5
3
3.5
4
4.5
5
6
4
4.5
5.5
6.5
7'5
8.5
10
11.5
13
14.5
16.5
18'5
20'5
23
26
89
32
35.5
39.5
43.5
7
8
9
10
11.5
13
145
16.5
18 -5
20-5
23
26
29
32
36
-
-
Relation between the Partial Pressures and Concentration.-As
-
-
-
--
-
I
pointed out i n a former paper (Trans., 1901, 79, 724), a n aqueous
solution of ammonia may be regarded as a mixture of two liquids,
t h e boiling points of which are far removed from each other, and t h e
vapour pressures should therefore follow the laws of vapour pressure
of mixed liquids as worked ont by Duhem, Margules, and others.
Excellent accounts of t h e subject are given by Zawidski (Zeit.
physikaZ. Chem., 1900, 35,157) and by Ostwald in his Lehybuch der
allgeminen Chemie (2nd edition, vol. ii, p. 636). The relation between
t h e partial pressures and the concentration of t h e solution, deduced
from thermodynamical considerations, is
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PERMAN: VAPOUR PRESSURE OF
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or
where
p , = partial pressure of one constituent (say ammonia),
P2 =
x =
9,
,, the other ,, (water),
no. of molecules of ammonia in the liquid mixture.
Total no. of molecules,
This equation can only be integrated by making certain arbitrary
assumptions as to the relation between the partial pressure of one
FIG. 2.
mm.
150
100
50
0
5
10
15
20
25
per cent. NH,
constituent and the vapour pressure of the pure liquid. It has been
thought best, therefore, to test the differential equation a s it stands.
The values of dp, have been obtained from the table already given, the
average of two differences, each over a variation of concentration of
2.5 per cent., being taken.
Values of dp2 are very difficult to obtain with any accuracy owing
to their small magnitude.
The curves at Oo and 10' are straight, and therefore dp2 has a
uniform value, which has been calculated directly from the first and
last readings of p,.
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AQUEOUS AMMONIA SOLUTION.
PART 11.
117'7
At the higher temperatures, the values have been found by drawing
tangents by eye ; this method is capable of considerable accuracy, as
was shown in a previous paper (Trans., 1901, 79, 723). The results
are now tabulated :
Temp.
~
0"
1-x.
2.
_
PI.
_
7
110.5
36.8
52 .O
76.3
110.5
0.375
0.375
39.9
50.6
61'9
83-4
31.3
43.2
62.7
90'0
19.25
26.5
37
51
0 $0
0.60
0'825
0.825
36'5
44'6
54 '0
66.6
32.1
44'2
44.9
61.8
30
28
25 *5
23
30.5
415
56 0
80.2
0.975
1*1
1'2
1.3
32'4
37 '9
46.3
58.2
31 *3
37 -7
83.5
191
336
53
49'5
45
48.0
62-2
3 '34
1-85
2'44
28 *3
32.9
40'1
35.8
0.9473
0.8950
126
284.5
87.5
70.7
82.5
89.7
2 -32
2 '98
25.7
29'4
30.5
30.1
0'9473
153
143
102.7
3.76
23.0
27 '3
0.19
0.19
0'19
0'19
0'9473
0.8950
0'8429
0.7912
13
28-5
53 -5
87.5
4.0
4'0
3-5
3'0
10
14'5
21
0'0527
0'1050
0'1571
0'2088
0.9473
0.8950
0'8429
0'7912
20
47 *5
86.5
143
9
8
75
6.5
11-75
16.2
23-5
33.75
0'375
0.375
20"
0'0527
0'1050
0'15T1
0*2088
09.173
0'8950
0'8429
0.7912
33'5
78 -5
141
228.5
16.5
15
14
13
30"
0.0527
0*1050
0'1571
0.2083
0.9473
0.8950
0'8429
0'7912
54
124.5
220
353
0,0527
0'1050
0.1571
0.9473
0'8950
0.8429
0.0527
0.1050
0.0527
40"
5 0"
60"
Lip2
--
0.0527
0'1050
0.1571
0'2085
10"
$231.
dp2.
232.
85.2
58.4
60 *7
82'0
46'7
61.7
33%
34.9
I
The agreement between the values of '&! and - fi . '-2 is as close
dP2
Pz
as can be expected, considering the very large errors which may arise
in determining dp2,and the differences are evenly distributed.
The partial pressure of the ammonia does not follow Henry's law,
as can be seen a t once from the curvature of the isothermals. The
curves for water are, however, straight, or very nearly straight, over
a considerable range of concentration, t h a t is to say, the partial pressure of the water vapour is proportional t o 1 - x (lam of Raoult and
van't Hoff ), p2 = P2(1- x), where 2, = vapour pressure of pure water.
The agreement is found to be very good up to a 10 per cent. ammonia
solution :
View Article Online
11'78
PERMAN: VAPOUR PRESSURE OF
~~
~
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Temperature.
0"
10"
4.6
9.2
(1- x).
P2(1- x).
P,.
0'947 *
0.895
0.843
0.791
4 -3
4 '1
3 *9
4-0
4'0
0'947
0'895
0-843
20"
3b"
40"
17'5
3 1 -7
55.2
3.5
3-0
3%
8 *7
9.0
8-0
7.5
8'2
0.791
7 '8
7'3
6.5
0'947
0.895
0'843
16.6
15'7
14'8
165
15.0
14.0
0.791
13'8
13'0
0.947
0.895
0.843
0.791
30.0
28'4
26.7
25 -1
30.0.
28 *o
25.5
23.0
0.947
0.895
0,843
0.792
52.3
53.0
49'4
46 5
43.7
49.5
45.0
39.5
50"
92 *3
0.947
0.895
0.843
87.5
82 -7
77.8
87.5
82.5
75 -5
6 0"
149.3
0.947
1 4 1-4
133 *6
143.0
134 -0
0-895
* These values of 1--x correspond with 5 , 10, 15, and 20 per cent. of ainmonia
respectively.
This close agreement shows that the molecular weight of ammonia
in solution is normal, and that there cannot be a large amount of any
hydrate formed.
Relation between Partial Pressure and Temperature.
The reIation appears to be simplest a t the highest concentrations ;
with 22.5 per cent. ammonia solution, the expression logp = a + bt holds
good to within 1 per cent. With a concentration of 20 per cent.,
another term must be introduced, whilst at lower concentrations still
more terms would be necessary.
View Article Online
AQUEOUS AMMONIA SOLUTION.
logp = a + bt + ct2, a = 1.942, b = 0.02195,
c = - 0.0000575,
(20 per cent.)
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1179
PART 11.
Temperature.
p (found).
p (calculated).
O0
87.5
143
228.5
353
535
(87.5)
143
10
20
30
40
(22.5 per cent.)
228
354
535
logp=a+bt, a=2.051, b=0-0199.
Temperature.
p (found).
p (calculated).
O0
111
149
196.5
258.5
337
441.5
112
148
195
256 *5
33s
445
6
12
18
24
30
As the formula is purely empirical, it has not been extended to suit
the lower concentrations.
No simple formula has been found to represent the relationship
between temperature and the partial pressure of the water vapour.
Comparison of the Total Qapozcr Pressures obtccined by the Two
Methods.
I n t h e following table are given the vapour pressures measured by
the statical method (static), and the sums of the partial pressures of
the ammonia and the water vapour (air-current) :-
6 0".
Pcrcen tage of
ammonia Static. Aircurrent.
~
Static.
_______
~
10.0
234
327
425
539
12.5
15'0
17'5
20'0
22.5
-
2.5
5.0
7-5
50".
-
232
326
430
540
-
146
210
281
363'5
454
564
-
40".
Aircurrent.
Static.
-
Aircurrent.
AirStatic* current.
____
____.
149
214
286
367
459
567
30".
91
134.5
183
241
303'5
377'5
465-5
569.5
-
93
136
185
240
306
381
471.5
574
-
~
56-5
83
115
153'5
1935
245
305.5
373
455.5
56
84
115
152.5
196
246
305
376
4635
View Article Online
1180
PERMAN: VAPOUR PRESSURE OF
~
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20".
Percentage
of
ammonia.
Stat,ic.
12.5
15.0
151
17.5
20'0
22 *5
190
237
291
10.0
10".
0".
--___
32.5
47.5
70
93
118
2.5
5'0
7.5
1
--
Aircurrent.
32
50
70
94
122
155
Static.
194
18
27
40
54
69
89
115
241
294
144
180.5
Aircurrent.
18
29
41
56
73
94
119.5
150
186
Static.
13
20
27.5
35
45
57
75
93
117
Aircurrent.
10.4
17
24
32
43'5
57
72
50-5
114
A t the higher temperatures, the agreement is seen t o be fairly close,
and a s a rule within 1 per cent. A t the lower temperatures, the
experimental error is much larger in both methods ; in the statical
method, a trace of air would produce a considerable error, whilst in the
air-current method, the amounts of ammonia and water vapour aspirated
off are comparatively small. On the whole, there is a tendency for the
results by the air-current method t o be somewhat higher than the
others, and I believe this to be due to the abnormal density of ammonia
gas. F o r want of exact information on the matter, the density has
been taken as normal in calculating out the vapour pressures. If the
density had been taken as being somewhat smaller, and the specific
volume therefore as greater, the agreement would have been closer.
It follows from these results t h a t the sum of the partial pressures is
equal to the total pressure, and there is no large deviation from
Dalton's law in the case of mixtures of air, ammonia, and aqueous
vapour; consequently there cannot be any appreciable quantity of a
hydrate of ammonia present in the vapour drawn off from the
solution.
Rekction hetween Partial Pressure and
((
Eate of Escape."
I n a former paper (Trans., 1898,73,515),it was shown that if a current
of air is aspirated through a single flask of ammonia solution, the
quantity of ammonia in solution q, after the passage of Vlitres of air,
can be accurately represented by the equation log q = a b V, a and 6
being constants ( a is the logarithm of the amount in solution at the
beginning of the experiment). By differentiating the equation, we
-
obtain - = bp; 6 represents, therefore, the amount of ammonia that
dV
would be drawn off by one litre of air, supposing the solution to con-
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PART 11.
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AQUEOUS AMMONIA SOLUTION.
1181
tuin altvays one gram of ammonia, and is therefore a measure of the
“ rate of escape ” of the ammonia.
I n order t o deduce the partial pressure of the ammonia from the rate
of escape, i t is necessary to find a mean concentration of the solution
so that a solution of this strength would give up t o t h e air as much
ammonia as was given up by the actual solution of varying concentration,
Let c = the real mean concentration (grams of ammonia in 50 C.C. of
t h e solution).
V=vol. of air (litres) passed through the solution measured as
dry air at the temperature and pressure of the solution.
W a = amount of ammonia drawn off (grams).
bc V
correcting for the reduction from natural
Thus w a =0.4343’
0.4343wa
logarithms to logarithms t o base 10, and c =
bV
Let I-’ = the total pressure.
pa = partial pressure of the ammonia.
PW =
9,
water vapour.
wa = volume of the ammonia drawn off.
sa = specific “volume of the ammonia a t the temperature and
pressure of the solution.
Then from Dalton’s law :
---.
V
=I+
CVb -l+0 *434 3“
0,4343
CbS,
]’-%
.*.pa c
0.43431 f--cbs,
Several cases have been tested, i n which all the data are derived
from the paper on the “ r a t e of escape” except the values of pw,
which are taken from the present communication.
20’. P = 766, pw = 16, sn = 1,397, b = 0.00655, c = 2.880 (5.90 per
cent. NH,).
pa = 42.9. Read from isothermal 41.2.
30’. P=758, p w = 3 0 , sa=1.460, b=0.01034, c=2*869 (5.88 per
cent. NH,).
pa = 66.0, Read from isothermal 65.0.
40’. P = 7 6 5 , p w = 5 3 , ~ a = 1 * 4 9 4 b=0*0165,
,
~ ~ 2 . 4 4(5.00
9
per
cent. NH,).
p a = 86.9, Read from isothermal 84.0.
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1182
PERMAN: VAPOUR PRESSURE OF
The agreement is not good, and there appears to be some constant
error. The present experiments are far more accurate than the older ooes,
and the latter are probably not of much use for giving absolute values
of the partial pressure.
Latent Heat of Evaporation of Aqueous Antmonicc Xolution.
The latent heat of evaporation of solutions of various concentrations has
T - s)*
been calculated from the theriuodynamical equationL = -(s'
J
d T'
The
specific volume of the mixed vapour, s', has been taken as the sum of
the specific volumes of the ammonia and water vapour present ; s has
been neglected.
Values of
*
have been calculated from the table
dT
of total pressures, and also from the partial pressures (by taking the
sum of the two differences); the mean of the two results has been used
in the calculations.
-dp a t 13".
dT
Percentage of
ammonia,
From
total
pressure.
Mean.
A''.
I;.
3'625
4.375
5 -5
6.75
8.375
15-66
12.27
9 *53
7.47
5.97
4T92
3.847
3.132
2.548
516.3
488.3
27.5
30-0
3.5
4-25
5.75
6.75
8 -5
10.0
12.25
14.75
17-25
AL.
-
~~
10.0
12.5
15-0
17.5
20 *o
22.5
25.0
L'.
Prom
partial
pressures.
3.75
4 -5
5 '25
6.f5
8 '25
10.0
10'0
-
-
-
-
476.7
458.6
454.8
435'9
428'7
420.2
399'8
5 60
552
545
537
530
523
515
508
501
44
64
68
78
75
87
86
88
101
The column headed 1;' contains the values of the heat of evaporation
calculated on the assumption that it is simply the sum of the heats of
evaporation of the ammonia and the water present in the mixture.
The column A L contains values of L' -A, which should represent the
heat evolved on mixing the ammonia, in the liquid form, with the
water.
The heat of solution of ammonia is given by Thomsen (Thermochemische Untersuchungen, vol. 2, p. 68) as NH,,Aq = 8430 ; this gives the
heat of solution per gram of ammonia as 496, and subtracting from
this the heat of evaporation of ammonia, 295, we get 201 as the
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AQUEOUS AMMONIA SOLUTION.
PART 11.
1183
heat evolved on mixing 1 gram of liquid ammonia with a large quantity
of water. The value L = 2 9 5 is obtained from Regnault’s results,
taking the mean of the values of L and the mean of the temperatures
(Landolt and Eornstein’s Tabellen).
The above values of A L are vitiated by the large error inherent
in the ratio
9
from which L is calculated, but, nevertheless, they are
dT
of the order which would be expected from the numerical data just
given.
Xurnmayy and General Conclusions.
(1) The partial vapour pressures of aqueous ammonia solution have
been found and tabulated for concentrations up t o 22.5 per cent.
ammonia, and at temperatures from 0 to 60’.
(2) The relationship between the partial pressures and the concentration of the solution is found to be that deduced by Duhem and
others for binary mixtures of liquids.
(3) The lowering of the vapour pressure of water by the ammonia
follows Raoult’s law clcsely, showing t h a t the molecular weight of
ammonia in solution is normal, and that no large quantity of a hydrate
is formed.
(4) The relation between the partial pressure o€ the ammonia and
temperature is expressed by a formula of the form, logp = a + bt .,
(5) The sum of the partial pressures is equal t o the total pressure
found by the statical method; consequently there is no appreciable
quantity of hydrate present in the gaseous mixture given off by the
solution.
(6) The relationship between the partial pressure of the ammonia
rate of escape ” has been deduced.
and the ‘&
(7) The approximate latent heats of evaporation of aqueous ammonia
solution of various concentrations have been calculated.
+ .
The experimental part of this work was carried out at the Physikalisch-chemisches Institut, Leipzig, and I am much indebted to the
director, Prof. Ostwald, and the sub-director, Dr. Luther, for the
facilities they so readily afforded me.
The expense incurred for apparatus was covered by a grant from the
Research Fund of the Chemical Society.
View Article Online
1184
VAPOUR PRESSURE OF AQUEOUS AMMONIA SOLUTION.
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Tern
poratu
of
solation.
0"
10"
19.9"
Appendix.-Experimental
~~~
P
TII m
19'62
14.73
9.15
4-72
22-90
60*00"
absolute
P
Pa
II
-
0.827.
0'499,
0.2351
0'1051
1*257
2'010
15.38
0'9955
6.50
6.55
4'18
10.75
77 2
23.37
16.64
0'230i
0.459:
0'265:
0'9064
3.5594
2.918
1.977
1.8450
4,845
17-84
7.43
333
7
mm.
ni In.
8.26
4-16
2u-54
12.32
21 -83
0-358t
0'154:
1.355
0'6455
0'032
0.0411
0'052
0.0501
0 0321
734.7
737.9
745'2
742.5
739'4
10
10
10
10
10
296.C
293'7
292.2
292-C
292'6
703.7
212.1
i23.5
719.5
713'8
0 *089!
0.0891
734.7
726.8
741.5
731.5
735'4
739'0
10
10
292.5
292'4
291.8
291 ?3
293'4
292-7
709.8
702.2
0.071(
0,080:
0'069(
0'077i
0'085i
0'1692
0.168;
O'l63C
0'163E
0'102E
3 .I f25
3'1670
)*1710
12.77
9 .75
2 1 '47
!-328
)'4665
1'2090
!-304
.098
1.6843
;397
)'2050
)*l667
)*1663
).2080
1.1765
15-55
11'06
7 '36
3.79
17-23
20'85
I'445
'541
1.7995
1*3280
1.135
;*539
)'4535
1.3660
) .3220
b.3035
14.94
8'91
I1.57
5.90
3'29
.4'15
,258
-208
1'944
.1 .31
7 -78
-924
'336
9.37
57 7
3.16
-171
1.8470
1.6510
*827O
*3719
'4007
4455
17'76
50*00"
Y
litres.
inonin.
19'40
40*00"
.
T
~
Partial
pressures.
Per-
10'15
30'09'
Data.
'8115
*3766
-488
)*1717
1.2540
-
1'4530
1.6945
1'4294
1'7340
1'4937
1,4509
1.2610
-
UNIVERSITY
COLLEGE,
CARDIFF.
745.5
739'4
739.4
738'7
742.5
740'0
736'0
730.9
738.0
8.084
10
10
10
5
10
!O
10
LO
6
LO
LO
10
5
236.0
134.4
735'1
7415'
740.7
740'3
733.5
5
2882
290'6
291 -8
292'8
294.3
294.8
293.7
291-0
29 1*O
716.2
706.2
710.7
713'3
724'7
718-3
717.8
715.6
717.0
711.6
711-2
707.9
715.7
4 '1
5 '3
116.6
2'8
8.8
16.5
149'2
64-2
169'8
95 '1
9 '1
7-2
7.6
5.5
7.0
45 '8
46 '0
27'4
86 '3
56 *2
302'4
166'1
1 6 '1
16'0
16'4
14.7
10.3
12.9
80'6
15 -1
12.3
24'3
29.2
31'1
5
710.0
291'1
86'3
4 1 '2
290'2
175'0
120'0
404'6
743 9
747 '6
747.0
749'4
7bO-0
746-0
5
5
5
5
5
2.218
292-0
292-4
292.4
292.1
292.4
292.3
719.1
722.9
722 '8
726.3
723.7
721'6
953.6
218'5
133.0
6 1 -1
427.7
576.1
736.1
i35.9
730.2
735.9
i36.1
i36.9
2.305
3
4
4
4.030
1
294'0
293.2
292-2
292.0
291'6
292'0
707.9
710.7
711.6
713.5
713.6
487.1
246'6
341 *7
151'3
79 '1
451.4
137'9
140'6
742'0
'41'4
'40%
1.217
2
2
1
2
292.8
292.6
293%
292.8
292-0
711.6
717.0
716.2
717'3
717'3
475'8
300.4
375.7
215.9
136-9
5
5
707.5
1.5'6
215'6
713'6
711-1
713'8
718.1
717.9
716.3
5
5-1
37 '2
290.5
291'0
291'0
290-2
290.4
290.6
291.5
5
3.0
82.5
5 1 '3
24.8
11'4
24 '8
26 '6
25.5
22'1
44 '1
49'1
50.7
53.5
-
37'8
75.2
83.0
80'6
87.1
89'6
77.0
130.4
1385
138.5
-
144.1