April. 1924
AQUEOUS PRESSURE O F I-IYDRATGD CRYSTALS.
I1
923
the packing of molecules, in some directions at least, is of the type in which separate molecules cannot be distinguished, that is, in which there is continuous bonding throughout the crystal.
Summary Measurements of the planar spacings in crystalline benzene are described, and the results compared with those of Broom& Comment is made upon the interesting coincidence of some of the diffraction maxima found for liquid benzene with those of the crystals. BERKELEY, CALIFORNIA [COXTRIBUTION FROM THE
T. JEFFERSON COOLIDGE, JR., CHEMICALLABORATORY, 1 HARVARD UNIVERSITY
THE AQUEOUS PRESSURE OF HYDRATED CRYSTALS. 11. OXALIC ACID, SODIUM SULFATE, SODIUM ACETATE, SODIUM CARBONATE, DISODIUM PHOSPHATE, BARIUM CHLORIDE €317
GREGORY P
RAXTER AND w:ILLlAM
c. COOPER, Jli.
RBCSIVBDJANUARY 2Y, 1924
In a recent paper by Wilson1 the critical discussion of methods for determining vapor pressures of salt hydrates contains the implication that in the gas-current method of determining vapor pressures, equilibrium may be established only from the side of under-saturation. That such an implication is unwarranted is shown by experiments described in this paper, in which equilibrium in the gas phase is approached from the side of over-saturation. The results of these experiments agree within the limit of error with those obtained in the usual way. The apparatus used for our work corresponded very closely with that used by Baxter and Lansing2 and for details their paper should be consulted.s Furthermore, two of the substances examined by them, oxalic acid and sodium sulfate, were re-investigated, as well as four others. Two sets of experiments were made with each substance a t each temperature. In one set, dry air was passed over the carefully prepared mixture of hydrated crystals containing a t least one-tenth of the first product of efflorescence. In the other set the air was first drawn over sulfuric acid solution of considerably higher aqueous pressure than that of the crystals, and then over a mixture of dehydrated crystals with a liberal proportion of hydrated material. The aqueous pressure of the sulfuric acid could in no case have fallen below that of the crystals 1 R. E. Wilson, THIS JOURNAL, 43,709 (1921). Baxter and Lansing, ibid., 42,419 (1920). has recently suggested improvements on this method, ibid., 45, 342 (1923).
* Schumb
924
GREGORY P. BAXTER AND WILLIAM C. COOPER, JR.
Vol. 46
because the air drawn in, possessed initially the aqueous pressure existing in the room, which in many cases was higher than that of the sulfuric acid. Calculation of the aqueous pressure of the crystals was made .from the weight of water collected in the weighed phosphorus pentoxide tube and the volume of air drawn over the salt after correction to standard conditions. The volume of a mole of water vapor is assumed to be 22.41 liters under standard conditions. While no particular difficulties were encountered with five of the substances examined, barium chloride was found to be extremely troublesome because of the slowness with which equilibrium is established from either direction, a fact which has already been noted by many other investig a t o r s . ’ ~ ~ By * ~ suitable choice of conditions false equilibria may be set up almost a t will. Using very slow currents of air with salt columns of extreme length (1.2 X 280 cm.) we finally succeeded in eliminating a large part of the uncertainty. Schumb reached the same end by using a salt column of large diameter and considerable length (5 X 50 cm. a t a maximum). In our experiments, and also in Schumb’s, the results with barium chloride lack the consistency obtained with the other salts. Furthermore, our results with barium chloride are perceptibly higher than Schumb’s a t 25 O , and since ours agree better with those of Wilson a t this temperature obtained by another method, the possibility exists that equilibrium was not quite reached with this substance in Schumb’s experiments. It is to be noted that in Wilson’s experiments as well as in ours equilibrium was approached from both sides. TABLE I AQUEOUSPRBSSURES HzCzO4.2HzO-HzC204 Temp.
oc. 0
Initial state No. of of air tubes current
1
dry
Weight
of water G.
Volume of dry air at O’and 700 mm. Liters
RRte
per
hour
Liters
Average interior pressure Mm
Bqueous pressure Mm.
0,0029 ,0057 .0055 .0056
7.5 16.5 17.7 15.7
2.1
3.1 2 8 2.8
759 75s 77 1 764
0 37
,0028
7.4 16.3 17.6 16.8
2.1 3.1 2.7 3.0
755 752 765 759
1.5 1..5 1.5
Av. Av. of all 765 764 754
.33 .30 .33 Av.
2
wet
.0058 .0060 .0061
25
4
2
dry
.0221 ,0214 ,0220
7.57 7.58 7.43
Partington, J . Chem. Soc., 99, 471 (1911)
.33 .35
.34 .32 .34 .34 .34 2.77 2.67 2.77
April, 1924
AQUEOUS PR6SSURE OF HYDRATED CRYSTALS.
925
11
TABLE I (Colztinued) Weight
Initial state Temp. No. of of air OC. tubes current
of
water
G.
.0213
Volume of dr air a t OJand 760 mm. Liters
7.45
Rate per hour Liters
Average interior pressure Mm.
1.5
753
Aqueous pressure Mm.
Av. 2
50
.0211 .0104 .0213 ,0191
wet
7.59 3.73 7.55 6.71
1.6 1.o
1.6 1.5
758 763 764 764 Av. Av. of all 747.2 746.2 752.3 750.6 Av. 745.4 744.8 751.4 748.6 Av. Av. of all
2
dry
.1249 ,1242 .1235 ,0883
7.130 7.260 7.255 5.198
1.6 1.4 1.6 1.4
2
wet
,1232 ,1253 .1251 .lo12
7.131 7.291 7.359 5.865
1.6 1.4 1.6 1.6
1
dry
0.0226 .0226 .0218 .0229 .0222
7.43 7.58 7.68 7.50 7.50
2.0 1.5 2.7 2.5 2.1
757 762 764 746 751
1
wet
.0230 ,0224 .0215 .0222 .0226
7.43 7.56 7.65 7.47 7.46
2.0 1.4 2.7 2.5 2.1
757 762 763 745 750
2.67 2.72 2.61 2.64 2.67 2.70 2.66 2.69 (15.94) 15.55 15.60 15.53 15.56 15.69 15.59 15.56 15.73 15.64 15.60
Na&304.10HzO--Na&04 0
Av.
20
3 3 I 1 1
dry
1 I
I
1
3 3 1
wet
,1055 ,1055 ,1051 ,1073 .lo69 .lo75 .lo67 .lo66
7.564 7.330 7.348 7.444 7.488 7.630 7.422 7.423
1.5 1.6 1.6 1.4 1.3 1.4 1.6 1.4
.lo71 .lo62 .lo68
7.411 7.456 7.622
1.4 1.3 1.5
Av. Av. of all 764.1 752.6 753.4 755.9 761.4 769.0 754.1 756.2 Av. 754.9 760.5 770.6
2.85 2.82 2.69 2.82 2.75 2.79 2.90 2.80 2.78 2.74 2.81 2.81 2.80 (13.03) 13.22 13.17 13.31 13.28 13.25 13.25 13.27 13.25 13.33 13.24 13.20
926
GREGORY P. BAXTfiR AND WILLIAM C . COOPER, JK.
1'01. 4G
TABLF, I (Conlinued) Temp. No of OC.
tubes
Initial state of uir riirrrnt
1 I
Volume
Weight of water G.
of dry air at OOand
.lo69 ,1063
7.418 7 411
760 mm. Liters
Kate
per hour Liters
1.6 1 5
Average interior pressure Xm.
Aqueouq pressure Mm
.
1:3.30
753 2 757 2
13 27
Av. 13.27 h v . nf all 13 20 0
I
dry
0066 007 1 ,0071 .0066
7.18 7.19 7.13 7.15
1.7 1.6 1 .8 I .6
0,0068 ,0070 .om3 , iKXii-;
7.37 7.37 7.30 7.94
J .6 1 .ti
,
,
r
.,,
i 63
756
750 762 Av.
0
2
w t
1. R
1.5
763 -rI a:) 748 r ih2
.SG
1,
A\,. A\,. of all 13
1
dry
,0227 ,0226 .0229
7.14 7.20 7.16
1.1 1.3 1.4
764 767 762
2
wet
,0234 ,0234 ,0239
7.30 7.37 7.33
1.2 1.4 1.6
764 787 762
Av.
25
2 1 2 1 1
dry
1
,0604.0488 ,0409 ,0249 ,0484 ,0182
7.35 7.16 6.01 3.67 7.10 6.97
1.o 1. 6 1.7 1. D 1.6 2.0
,0504 ,0498
7.37 7.16
1.7 2.1
Av. A4v.of all 754
--I
110
76 1 '762 m - r
--a0 I
&)
i
Av. 2
40
wet
1
dry
,1436 ,1447 ,1440 ,1440
7.050 7.072 7.075 7.141
1.6 2.0 1.5 1.8
2
wet
.1489 ,1490 ,1492
7.230 7.265 7.324
1.6 1.5 1.8
0.87 ,93 .93 .87 .90 0.87 .89
c--
loo
749
Av. Av. of all 752.7 754.1 753. A 761.6 Av. 751.2 752.9 760,8 Av. .4v. of all
,85 ,89
.90 3.01 2.98 3.02 3.00 3.03 3.02 3.08 3.04 3.02 6.38 6.35 (5.38 6.37 6.35 6.40 6.37 6.37 6.43 6.40 6.39 18.60 18.72 18.61 18.64 18.64 18.76 18.73 18.80 18.77 18.70
April, 1924
AQUEOUS PRESSURE OF HYDRATED CRYSTALS.
92 7
I,I
TABLB I (Continued) NazCOa. IOHzO-Na&Oo.7HnO Temp. "C.
0
Weight of water G.
Volume of dry air at Oo and 760 mm. Liters
dry
0.0203 ,0198 ,0212
7.26 6.89 7.39
1.9 2 2 2 1
757 768 76 1
wet
,0199 ,0197 ,0212
7.08 6.79 7.38
1 8 2 1 2 1
755 766 758
Initial state No. of of air tubes current
1
1 1
Rste per hour Liters
Average interior pressure Mm.
Aqueous pressure Mm.
Av. 2 2 2
Av. Av. of all
13
7.16 7.42 7.23
2 0 1 8 1 8
,0716 ,0703
7.38 7.64 7.56
2 1 1 8 1 8
dry
,1404 ,1400 .1370 ,1218
7.277 7.225 6.979 6.167
1 2 1 1
8 1 6 9
wet
,1412 ,1410 , 1376 ,1356
7,274 7.222 6.997 6,794
1 2 1 2
8 1 6 1
1 1 1
d1y
2 2 2
wet
1
,0665 ,069 1
0672
-
-c
iai
763 762 Av.
25
1 1
1 2 2 )
i
2
0693
2.62 2.74 2.71 2.69 2.63 2.75 2.70 2.69 2.69 8.65 8.72 8.71 8.69
756 76 1 760
8.72 8.77 8 70 Av. 8.73 Av. of all 8.71 764.2 17.91 763.7 17.97 755.0 18.00 755.9 18.12 Av. 18.00 762.6 762.3 753.7 754.3
17.98 18.07 18.00 (18.27) Av. 18.02 Av. of all 18.01
NaZHPO4.12He0--Na~HP0~.7H20 0
2
dry
0.0209 ,0214 ,0209 ,0211 ,0212
7.58 7.63 7.48 7.35 7.32
1.5 1.3 1.2 1.5 2.0
760 768 762 755 751
2
wet
,0212 ,0204 .0211 .0209 .0206
7.53 7.34 7.46 7.32 7.31
1.5
759 768 762 755 752
Av.
1.8 1.2 1.5 2.0
Av. Av. of all
2.60 2.67 2.63 2.69 2.70 2.66 2.65 2.65 2.67 2.67 2.68 2.65 2.66
928
GREGORY P. BAXTBR AND WILLIAM C. COOPER, JR.
Vol. 46
TABLE: I (Continued) Weight of water
G.
Volume of dry air at 0' and 760 mm. Liters
Temp, OC,
No. of tubes
Initial state of air curreni
Rate per hour Liters
15
2
dry
.0717 .0657 .0709 ,0716
7.48 6.84 7.60 7.44
1 .s 1.8 1.8 1.8
757 759 768 760
2
wet
,0717 , 0643 .on0 .0716
7.47 6.71 7.61 7.43
1.8 1.8 1.8 1.8
757 759 768 76 1
Average interior pressure Mm.
Aqueous pressure Mrn.
Av.
8.92 8.96 8.80 8.99 8.92 8.93 8.95 8.82 9.02 8.93 8.93 18.99 19.05 19.06 19.03 18.98 19.08 19.08 19.05 19.04
Av. AV. or ail .1510 7.320 1.8 759.0 25 2 dry ,1522 7.370 1.8 760.7 ,1538 7.367 2.1 733.1 Av. .I508 7,323 1.8 759.8 2 wet 761.1 1.8 .1524 7.372 . '1538 753.5 7.365 2.1 Av. Av. of all B~C~Z.~H~O-B~C~~,I-I~O In the experiments at 15' six of the saturating tubes contained a mixture averaging BaCl2.1.3Hz0 while the seventh contained BaC12.1.9HzO. In all the other experinients four of the tubes contaiqed BaClz.I.QH2O. 756 2.98 2.55 0.5 15 7 dry 0.0081 2.57 767 2.63 .5 .0071 . 2.90 760 2.40 .6 .0074 .4 753 2.77 2.39 .0071 Av. 2.82 3.67 .6 760 2 80 7 wet .0109 7.56 2.90 .6 3.59 .0111 758 2.85 4.29 .6 .0130 Av. 2.85 Av. of all 2.83 754 5.84 3.60 .6 25 7 dry .0226 754 5.66 3.60 .7 .0219 748 5.79 .5 3.60 .0226 760 5.69 .6 3.61 ,0219 Av. 5.75 742 5.73 .6 2.46 7 wet , ,0154 754 5.63 3.58 .6 .0216 756 5.78 .6 3.44 .0213 Av. 5.71 Av. of all 5.73 752 15.59 2.361 0.4 40 7 dry 0.0402
April, 1924
AQUEOUS PRESSURE OF HYDRATED CRYSTALS.
029
I1
TABLE I (Concluded) Initial state Temp. No. of of air OC. tubes current
7
wet
Weight Of
water
G.
Volume of dry air a t 0' and 760 mm. Liters
Average interior pressure Mm.
Rate per hour Liters
.0430 .0388 .0425 .0437 .0631 .0428 .0449 .0319 ,0434 .0430 .0271 .0418
2.558 2.328 2,527 2.442 3.534 2.477 2.590 I.828 2.509 2.500 1.566 2.411
.4 .3 .3 .5 .6 .6 .4 .4 .4 .4 .6 .4
758 766 759 748 737 748 756 755 751 751 755 757
.0438 ,0429 .0628 .0627 ,0610 .0455 ,0425 ,0423
2.478 2.525 3.733 3.649 3.587 2.475 2.345 2.476 2.447 2.422 2.421 2.389
.4 .4 .6 .6 .6 .4 .4 .5 .4 .5 .4
746 752 762 749 749 753 746 755 751 748 751 746
,0421
.0423 .0427 .0429
Aqueous pressure Mm.
15.52 15.56 15.57 16.29 16.00 1.5.74 15.06 16.03 15.83 15.74 15.90 15.98 Av. 15.75
16.04 15.57 15.62 15.67 15.52 ( 16.82) (16.45) 15.70 15.74 15.90 16.13 16.30 Av. 15.82 Av. of all 15.78
.5
Although in the greater proportion of cases the average result with dry air is a trifle lower than that with wet air, it is by no means universally the case and in no instance is the difference so pronounced as to be beyond the experimental error. The results with oxalic acid and sodium sulfate when compared with those obtained by Baxter and Lansing show satisfactory agreement. HsCnOc2HzO 00
Baxter and Lansing.. . . . . . . . . . . 0.38 .34 Baxter and Cooper. . . . . . . . . . . . Na&304.10He0 00
Baxter and Lansing.. .......... 2.77 Baxter and Cooper.. .......... 2 -80
25 '
2.65 2.69
50 '
15.55 15.60
200
13.33 13.26
(interpolated)
The curves represent aqueous pressure plotted against the temperature, and the logarithm of the aqueous pressure plotted against the reciprocal
930
GREGORY P. BAXTER AND WILLIAM C. COOPER, JR.
VOl. 46
of the absolute temperature. In the latter case the points lie very nearly upon straight lines, which may be represented by the following equation^.^ 11042.9 T 280.1 3614 51 . Na2COs.lOHzO log p = 11 8071 - ( T 46.45 5172 NazHP04.12H~Olog p = 14.1572 T + 103.63 9839.58 BaC12.2HzO log 9 = 17.9678 I' 273.75 NaAc.3HtO log p = 19.5821 -
++
+
In Table I1 the observed values are compared with those calculated from the equations] and the calculated values are extrapolated beyond the range Mm. 0
1 _. T
1 2
3 4
E
6 7
8
9 10 11 12 13 14 15 16 17 18 19 20
0.00380
40 '
0.00370
35"
0.00360
30
0.00350
25
0.00340
20"
0.00330
15"
0.00320
10 5"
0.00310
0"
0.00300 5
6
7
8
9
1 0
2
3 4 5 Log P.
6
7
8
8
1 2 3 4 5 1
of the experiments. Similar equations and tables have already been prepared for oxalic acid, strontium chloride and sodium sulfate.2 Using the Clausius-Clapeyron equation, [Heat of efflorescence = T dP (V vapor - V solid)]l and values for the vapor pressure found from Antoine, Compt. rend., 110, 632 (1890); log p = A
B + .-Tic'
April, 1924
AQUEOUS PRESSURE OF HYDRATED CRYSTALS.
Temp., "C.. Obs.. . . . . . . Calc.. . . . . .
AQUEOUS PRESSURES, MM. NaCtH802.3H~O~ -25 -15 0 5 10 15 ... . . . 0.90 . . . . . . 3.02 0.11 0 27 0.94 1.39 2.06 3.02
931
11
TABLE I1
Temp., OC.. Obs. . . . . . . . Calc. . . . . . . Temp., "C.. Obs.. . . . . . . Calc. . . . . . .
...
... ... ...
..
... ...
.. .,.
...
... ... 25 30 35 40 50 ... ... ... 6.39 . . . . . . 18.70 . . . ... ... ... 4.40 6.39 9.19 1.3.15 18.70 37.20 iVa2COs.lOH20 20 25 32.02' -25 -15 0 5 10 15 . . . 18.01 . . . ... . . . 2.69 . . . ... 8.71 0.29 0.74 2.69 4.03 5.96 8 . 7 1 12.59 18.01 29.26 20
...
NazHP04.12HzO8 20 25 35 Temp., "C.. -25 -15 0 5 10 15 ... ... . . . 2.66 . . . . . . 8.93 . . . 19.04 Obs.. . . . . . 0.28 0.72 2.66 4.03 6.03 8.93 13.10 19.04 39.13 Calc. . . . . . . Temp., "C.. 0 10 Obs.. . . . . . . . . . . . . Calc.. , . , , , 0.94 1.97
15 2.83 2.83
BaC12.2H209 20 25 ... 5.73 4.04 5.73
30
40 50 15.78 . . . 11.32 16.78 30.15
35
. . . . . . 8.08
the logarithmic vapor-pressure equations'O for 2 intervals, the heats of efflorescence per gram of water have been calculated. Table I11 contains also the heat of evaporation of water a t the same temperatures.ll Since the heat absorbed in efflorescence is the heat evolved in the combination of water vapor with the less hydrated phase, it may be considered12 0 Lescoeur gives the following values [Compt. r e d , 104, 60 (1887); Z . physik. Chem., 2, 761 (1888)l.
15" 20" 30" 40 " 50 2.4 4.4 10.5 20.6 35.3 Transition temperature, Richards and Fiske, THISJOURNAL, 36,485 (1914). 6 The following values were found by Frowein, Z. physik. Chem., 1, 362 (1887). O0 15" 25" 2.63 (interpolated) 8.84 18.71 (interpolated) Dickenson found 19.13 mm. a t 25", THISJOURNAL, 43, 723 (1921). Frowein, 2. physik. Chem., 1, 5, 362 (1887), interpolated 15O 2.19 mm.
25 5 . 13 nim.
40 16.60 mm.
Partington, J . Chem. Soc., 99,466 (1911); 25.01", 5.26 mm. Footeand Scholes, THIS JOURNAL, 33, 1309 (1911); 25', 4.8 mm. Wilson, Ref. 1, p. 704; 25", 5.8 mm. Schumb, Ref. 3, p. 351; 25", 5.50 mm. Io Baxter and Lansing's equations are used for oxalic acid, strontium chloride and sodium sulfate. Ref. 2. l1 Smithsonian Physical Tables, 1920, 7th ed., p. 234. l2 van't Hoff, "Lectures,'' 1898, I, 59.
932
GREGORY P. BAXTER AND WILLIAM C. COOPER,
JR.
Vol. 46
TA~LE I11 HEATSOF EFFLORESCENCE PER GRAMOF WATERIN 15' CALORIES 'C.
1x20
0 15 25 40 50
595 588 582 574 568
HzCz0a.- NaCzH80a.- SrC1z.2Hz0 3Hz0 6HzO
672 709 725 762 786
660 698 720 757
NazS04.- NazC0a.- NazHPO4.- B a c k lOHzO lOHzO 12Ha0 2HzO
670 695 712
705 697 698
... ...
...
676 686 693
...
. .
...
...
693 713 732
...
... 661 681 713
...
to be made up of (1) the heat evolved in the condensation of the water to the liquid state and ( 2 ) that evolved in the combination of liquid water with the less hydrated phase. The latter quantity, therefore, is obtained as the difference between the heat of efflorescence and the heat of evaporation of water at the same temperature. I n Table IV these values are given in kilogram calories per mole of salt, and in Table V per mole of water. TABLEIV HEAT^ Temp. "C.
HzCz04
0 15 25 40 50
2.76 4.37 5.14 6.77 7.84
+2Hz0
NaCIHaOz f3HzO
OF REACTION I SrC1z.2Hz0 NazSOa $4Hz0 +10H20
3.49 5.97 7.44 9.88
5.38 7.74 9.35
19.8 19.7 20.8
..
..
..
..
..
KG. CAI, NaaC0s.7Hz0 i-3WaO
NazHPO4.7Hi0 BaCIzHzO +5Hz0 +HzO
4.36 5.32 5.98
8.79 11.30 13.49
..
...
..
. .
..
1.32 1.78 2.61
..
TABLE V HEATSOF RFACTION PER MOLE OF WATER KG. CAL. Temp.
'C.
0 15 25 40 50
HzCzOa +2Hz0
1.38 2.19 2.57 3.39 3.92
SrCl2.2Hz0 $4Hz0
NazS04 +loHz0
1.16 1.99 2.48 3.29
1.35 1.94 2.34
1.98 1.97 2.08
..
..
..
NaCzHa08 3-3Hz0
NazC0a.- NazHPO4.7Hz0 BaCIz.Ha0 7Hz0 +5Hz0 +HzO $-3HzO
..
1.45 1.77 1.99
..
..
1.76 2.26 2.70
..
1.32 1.78 2.51
,.
..
..
..
If the heat of reaction is calculated according to the method suggested Ti Tz In -, Fz where F is the quotient of the by van't Hoff,la heat = R -
Tz - Ti F1 aqueous pressure of the salt divided by that of water a t the same temperature, values not very different from those in Table IV are obtained. It is to be noted that this method uses the experimental values for the vapor pressure of water instead of the experimental heats of evaporation of water. Aqueous pressures for the salts are found from the logarithmic equations in computing Table VI. 13
Frowein, Ref. 8, Ref. 12.
April, 1924 AQUEOUS PRESSURE OP HYDRATED CRYSTALS.
933
I1
TABLEVI I3EATS O F REACTION11 KG. CAL. Tpp. C.
HaCzOa +2H20
0-5 10-15 20-25 35-40 45-50
2.82 3.86 4.85 6.31 7.33
SrC1z.NnCzHaOa 2H20 $4Hz0 +3Hz0
3.73 5.37 6.70 9.28
.,
5.44 7.09 8.69
.. ..
NazSOa flOHnO
NnaC0a.7HzO +3Hz0
17.69 18.85 19.88
4.27 5.28 5.65
...
...
.. ..
NaaHPO4.7HzO BaCIP.Hz0 +bHzO +Ha0
8.74 10.51 12.18
...
...
..
1.10 1.59 2.32
..
Direct determination of the heat of reaction has been made by Thomsen by finding the heats of solution of the more and less hydrated phases. His experiments were carried out a t 18” and are expressed in 18” calories. Table VI1 gives a comparison of results calculated a t 18” in the above fashion and the values found directly. TABLE VI1 HEATSOF REACTION KG. CAL. Calc; I 18
H2Cz04 3. 2Hz0 . . . . . . . . . . . . . . . . . . . . . . . . . . 4.50 hiaCzHaOz 3Hz0.. . . . . . . . . . . . . . . . . . . . . . 6.43 8.14 SrC12.2Hz0 4 H 2 0 . .. . . . . . . . . . . . . . . . . . . . 20.18 Na~S04 10HzO... . . . . . . . . . . . . . . . . . . . . . . NazC03.7H~O 3HzO.. . . . . . . . . . . . . . . . . . . 5.51 5 H z 0 . . . . . . . . . . . . . . . . . . 11.71 NazHP04.7H20 HzO....................... 1.41 BaC12.H20
+
+ + +
+ +
;I
17.5
Found;‘ 18
4.30 6.11 7.81 19.14 5.23 11.19 1.32
6.33 8.68 9.60 19.2 5.49 11.22 3.83
Calc.
The agreement between the two sets of calculated results is on the whole better than that between the calculated and experimental quantities. Purthermore, although both sets of calculated results show a marked increase with rise in temperature, Pickering4 found experimentally the heat of hydration of anhydrous sodium acetate, sodium carbonate and strontium chloride to decrease somewhat with rising temperature.
Summary 1. The air-current method for determining the aqueous pressures of salt hydrates has been further tested by establishing equilibrium from the side of over-saturation of the air current as well as from the side of undersaturation and identical results have been obtained by both methods. 2 . Aqueous pressures of several salts a t different temperatures have been determined. 3. Heats of reaction of the wholly or partially dehydrated salts with water have been calculated a t different temperatures and compared with experimentally determined values of these quantities as found by Thomsen. CAMBRIDGE 38, MASSACHUSETTS l4
Thornsen, “Thermochem. Untersuch.,” 11, p. 293; 111, pp. 119, 123, 125, 157:
161, 191, Leipzig, 1882, 1883.
