T H E CONDENSATIQN O F WA4TER BY ELECTROLYTES' BY F. K . CAMERON A N D 117. 0 . ROBINSON
In aqueous solutions the sum of the volunies of the components is seldom the same as the volume of the resulting solution. The case of cane sugar and water is a well-known exception t o this generalization, following closely the law of mixtures. But in dealing with dilute solutions it frequently happens t h a t the weight of water, obtained by subtracting the weight of salt from the weight of the solution, is greater than t h a t which could have occupied the same volume as the solution under the same pressure and temperature. Since the assumption of a negative volume for the salt is absurd, it must be concluded t h a t the water has been condensed. That the salt has also been condensed is reasonably certain,. as shown b y experiments with other substances. Consider, for instance, two mixtures of alcohol and water, one containing a proportionally small amount of alcohol and the other a large amount of alcohol. There is a marked condensation in each case. I t is plain that both components of the solution have been contracted. It is difficult t o apportion the total condensation between the solvent and the solute. However, if the solutions be very dilute, the magnitudes of the condensation of the same solvent by different solutes niay be determined with sufficient accuracy for purposes of comparison by assuming t h a t the volume of the salt in the solution differs from the volume in the pure anhydrous state by a quantity negligible in comparison with the total condensation of the solution. This comparative condexisation of the solvent by different solutes is believed t o have an iniportance in the study of the properties of water and its relations to solids in contact with it, as for instance, in the extreme cases where on the one hand, finely divided solids, such as clays or silts, are susPublished by permission of the Secretary of Agriculture.
2
F . K . Came"r'n and W . 0. Robinson
pended in the water, and on the other hand, all the water is distributed over the surface of the solids, as in the case of soil below the optimum water c0ntent.l Michel and Icraft,' in an attempt t o use the determination of specific gravities of simple salt solutions as a method of analysis, stated t h a t the specific gravity of a solution was proportional t o the concentration. Kremers3 showed t h a t the conclusion of Michel and Kraft was not strictly true, for salt solutions showed a marked contraction when mixed with water. That is, the volume of the resulting solution was always less than the sum of the volumes of the salt solution and the water with which it was mixed. Gerlacli4 showed t h a t there were changes in volume departing from the law of mixtures, where aqueous solutions, containing reacting weights, were mixed. For instance, in the neutralization of solutions of potassium hydroxide with hydrochloric acid there was a large expansioii in volume. Tissier' developed this idea further and added more experimental data confirming the work of Gerlach. He found t h a t there was a contraction in volume when ammonia solutions were neutralized by an acid, whereas there occurred an expansion if a fixed alkaline hydroxide were used. &'hen solutions of aluminum sulphate and potassium sulphate were mixed there was no change in volume. J. Regnauld6 attempted t o explain why ammonia solutions and solutions of the fixed alkalis showed opposite volume changes when mixed with acids. He considered that the hydrates of the fixed alkalis and acids were true compounds and that when these compounds neutralized one another the water was set free, occupying a greater volume as water molecules than when in cotnbination; ammonia, however, did not form Cameron and Gallaglier: Bull. Xo. j o , Bureau of Soils, U. Agriculture (1907). Ann. Cliini. Phys. ( 3 ) , 41,471 ( I S j 4 ) . Pogg. Ann., 95, 1 1 0 ; 96, 39 (ISjj). Jahresliericlite, 44 (rS59). Ibid., 45 (Isj9). I M . , G9 (15Gj).
S. Dept.
Condensation of lYater by Electrolytes
3
true hydrates with water. ,Ostwald' made many measurements on the changes in volume observed when various bases and acids, were neutralized, for this purpose calculating the relative affinities of acids for the. bases, sodium, potassium, and ammonium. Valson2 calculated the specific gravity of salt solutions of certain concentrations b y the use of stoichiometrical relations. The difference between the specific gravity of solutions of different metals in combination with the same acid was found t o be constant for the same concentration. Thus, knowing the specific gravity of a .normal solution of ammonium chloride it was only necessary to add a characteristic number for potassium to obtain the specific gravity of a normal solution of potassium chloride. Valson called these numbers moduli. From the amount of condensation observed with calciuni chloride solutions, Charpy3 concluded that the dissolved calcium chloride was combined with six molecules of water. Schneider4 studied the densities of crystalline hydrates and their aqueous solutions. He calculated that the average density of a water of crystallization was 1 . 2 . Nico15 measured the molecular volumes in solution of a number of salts a t concentrations of I , 2 and 3 tnols of salt to IOO mols water. By the molecular volume in solution is meant the .i-oluiiie of water displaced by a gram molecular weight of the solute per liter of solution. For the concentrations abm-c mentioned Nicol found tliat the molecular voluiiies of the radicals C1, SO,, NO,, C10, and OH \\'ere the same, regardless of the base with ~vhichthey were cotnbined. The molecular volume of a salt increases with the concentration. %-ater of crj-stallization exerts no appreciable effect upon tlie molecular \.olunie. That is, tlie molecular volume of the '
~.
~
~
~~~
Jour. pralit. Cliem. (z), IS,353 ( 1 ~ 7 s ) . Coinptes rendus, 73, 441 (1874). '' Ihitl , 109,299 (1889). Sitz~ingslier.Aliad. Wiss. ll'ien, gg, May, rb'yo. ~Ic~natslieft, 11, 1890. Phil. Nag. [j], 16, 131 (1883): [j], 18, 179 (rssj).
F . K . Cameron and W . 0. Robinson
4
SO, radical is the same in solutions of Na,SO,.IoM,O as in K,SO,, or the water of crystallization has the same density as water of solution. As a result of experiments on phosphoric acid and disodium phosphate, the author concludes t h a t the water of constitution is more dense than the water of solution. According t o Mendelejeff,’ there is a combination between the solute and the solvent when solution of a solid in a liquid takes place. Were solubility a purely physical effect it is probable t h a t a n expansion would take place, for when a solid goes into a liquid state or a liquid goes into agaseous state, there is an expansion of volume. Since a contraction and not an expansion takes place usually during solubility, it follows t h a t the phenomenon is probably a chemical one. It is true t h a t the volume change is small, but it must be borne in mind t h a t solids and liquids have small coefficients of compressibility. MendelejeK considered that hydrates were formed in solution. Traube’ has collected and examined critically a large amount of data on specific gravities. From the specific gravities he has calculated the molecular volumes in solution. These values decrease with dilution, and for a few salt solutions there are obtained negative values. Traube found t h a t , in general, salts which crystallized with water of crystallization showed a greater contraction than did salts crystallizing without water. He considered t h a t pure water forms a complex molecule and t h a t this is broken down by other molecules or ions t o form water of crystallization or hydration which occupies a smaller space. Thomsen3 has shown t h a t the heat capacity of electrolytes in water solution diminishes with the dilution, and in dilute solutions it has a negative value. It is obvious that a part of the water, at least, has been so changed that it has not retained its pristine heat capacity.
3
Principles of Chemistry, I , 56 (1891). Longmans, Green & Co. Zeit. anorg. Chern., 3, 11 (1893). Thermochem. Unters., I , 53 (1S82).
Condensation of Water by Electrolytes
5
Tammann’ has shown t h a t a salt solution has many analogies t o the pure solvent under external pressure. Riintgen and Schneider’ found salt solutions t o be less compressible than pure water. They found t h a t certain elements and radicals had a constant effect on the coefficient of compressibility of the salt solution. The recent work of Jones and his associates3 and the transference experiments of Washburn,‘ and the investigations of Kahlenberg‘ on osmotic pressures, indicate t h a t salts in aqueous solution are combined with water,6 or that the character of the water has been materially altered by the presence of the solutes. The data contained in the researches cited above, while sufficing t o show t h a t a contraction or condensation of water takes place when an electrolyte is dissolved, are very seldom, however, of such a character as to permit comparisons in any satisfactory way. It has been deemed desirable, therefore, t o obtain further experimental data, with this purpose directly in view. While it is customary t o think of condensations in terms of volume changes, this point of view has serious disadvantages for the present purposes, in that the volume changes are relatively small, and small volume changes can not be measured with any high degree of accuracy. On the other hand, condensatior, in terms of mass changes can be computed from specific gravity determinations, which can be made comparatively readily and with great precision to at least seven significant figures. To this end, condensation of water produced by the introduction and solution of an electrolyte is here defined as the difference in weight of the water in a solution over the weight of pure water occupying the same Uber die Beziehung zMischen den inneren Kraften und Eigenschaften der Losungen. Hamburg and L e i p i g (1907). * ii’ied. Ann., 29, 165 (18S6). RIonograpli Carnegie Inst. Pub. No. 60 (1907). Tech. Q m r t . , 21, No. 2 , June, 1908. Jour. Phys. Chern., IO, 141 (1906). E See E. \i‘LVashburn, , “Hydrates in Solution, Tech.” Quart., 21, No, 4, December (190s).
6
F . K . Cameron and kV. 0. Robinson
volume. Let us consider a volume V of solution, with a density D, containing s grams of solute. Then, VD --s is the weight of water in the solution. The volume actually occupied by this water is, V-v, where ZJ is the volume of the solute. If D, be the density of pure water, then condensation, C, is given by the formula
C = VD-S-D~~(V-ZJ). If the condensation be determined for a standard volume, V becomes unity; and taking the density of pure water a t the temperature of the experiments as iinity, tlie formula is C = D-s--(I-v).
If d be the density of the solute, then, substituting for get
C = D-s-
(I
--)sd
ZJ,we
*
Assuming that no material error is introduced by using the value of d obtained from measurements with the dry solid, this formula contains only quantities which can be determined readily with a high degree of precision. In the present investigation, the deterniinations were made for a liter at 2 jo C with a number of acids, bases, and certain of their salts, which have a special interest in affecting aqueous suspensions, or the '' crumb structure of agricultural soils. The picnometers used in this research were of the Sprengel type, of IOO cc capacity. The exterllal opening of the capillaries were very srnall. IYeighings were made against a tare at frequent intervals until the weight had become constant. As the weights of the solutions ivere all compared t o water of equal volume and tlie differences i n density were hut slight, no vacuum corrections were necessary. I n tlie calibration of the picnometers it was not difficult to obtain duplicates varying not more tllall two or three units in the seventh decimal place. The temperature was carefully regulated a t 2 j o by a thermometer standardized by the Bureau of Standards, and I '
independently by another thermonieter standardized by determining the reading for the inversion point of Glauber’s salt to anhydrous sodium sulphate (32.38’ hydrogen scale), the z j o mark being estimated b). breaking off a short thread of mercury and calibrating the bore of the capillary. No difference between the two temperatures could be observed with a Beckinann thermometer reading to 0.01 O C. The bath was regulated t o within o.oojo C. The method of procedure was as follows. The picnometers were filled with the solution to be measured a t a temperature soniewhat lower than 2 j 0 . Fine rubber tubes, filled with the solution, were then drawn over the capillaries and the whole immersed in the bath with the ends of the rubber tubes coming out of the water. It was found t h a t the temperature of the solution became constant within half an hour. The rubber tubes were then removed, allowing only the capillaries to come above the surface of the water. The drops were carefully flicked off, the stoppers put on, the whole apparatus was carefully wiped, and the picnometer hung in the balance case. The water used in this research was distilled from a solution of alkaline permanganate through a block-tin condenser and then redistilled immediately before use. Standing in glass a few days increased sensibly the specific gravity. The air was sucked out of the solutions by connecting the picnometer with the vacuuni pump. Kahlbaum‘s dry chemicals and Baker & Adaimson’s acids were used without further purification. A solution of the material to be investigated \vas made up stronger than required and analyzed carefully. From these stock solutions there were prepared b j ~dilution the solutions actually emplojred in the nieasurenients. The tables gi1.e the obserirecl and calculated data, and are self-esplanator!.. It must be borne in m i n d that no clajm is made of the rigid \-aliditjr of the calculations. The formula used, page 6, in]-olves certain assumptions that cannot be prolren. For instance, it presupposes that the volume of the salt in solution has not changed from that which it occupied in the solid state; and also that tlie “cornbinetl” water to-
F . K . Cameron and W . 0 . Robinsom
8
gether with the material, should such exist, fills the same space as the material itself. The examination of the table shows but few generalities.
TABLEI-ACIDS
',
Density of Zoticentration solution
Acid
~
P
I
I
'
Ratio of nols H,O ondensed to mols acid
I H,O H,O lensity'Volurne~cond. cond. If acid of acid per per g m . I 1 liter acid _ ~
25°/250
I
~
~
_
N/IO N/25 N/50 N/roo
_
' 1 1
_
I .003420
1.001410 1.000719 1.000402
,
1.94
N/200 1 I . 0 0 0 2 I O
N/IO 1 N/25 N/jo I N/IOO N/zoo I
1.001813 1.000709 1.000338 1.000151 1.000065
1.46
N/IO
1.003366 1.001330 I a00654
I .52
~
N/25 N/5o N/roo N/zoo N/IO
1
1.000145
~
'
1.002701
1.000860 1.000350
1 N/5o
1.000184
; N / I O O 1.000075 1 N/zoo 1.000050 ~
0.54 0.50
~
0.43
~
0.29
0.99 0.99 0.94 0.88 0.63
~
~
I .88
I
1
3.6 3.9
4.199 1.183 0.282 1.658 0.468 0.281 I 0.829 0 . 2 2 3 0.268 ' 0.414 0.104 0.251 1 0.207 0.037 0.18 1.741 0,349 0.175 0.087
~
2.9
0.13
N / j o I 1.000572 N / I O O 1.000300 N/zoo 1 1.000151
I N/25 1
2.4 2.6
0.008 0.064 I
0.700
CH,COOH N/ro
0.443 0.485 0.546 0.671 0.73
2.498 , 0.665 0.266
, 0.125
1.000322
1
1.181 0.514 0.291 0. I79 0.ogs
i 1.000 I 0 . 2 5 1 10.250 l 0.497 I 0.107 0.215 I 0.250 0.036 0.144
1.001100
~ / 2 5
I
~
2.665 I .006 0.533 0.267 0.133
I -04
1.070 0.490 0.256 0.148 0.074
,
' 0.615
I ~
10.700 i 10.731 ' i 0.846 1 0.851
I 1
5.772 0.633 0.109 2.309 0 . 2 58 0.112 , 1.154 0.137 0.119 1 0.577 0.052 0.090 i 0.288 0.038 0.131
3.3 3.8 3.9 4.6 4.6 0.36 0.37 0.40 0.30 0.44
There is little parallelism between the chemical nature of the solute (i. e . , acids, bases, or salt) and the relative power t o condense water. It may be seen t h a t those substances which
-
Interpolated.
Condensation of W a t e r by Electrolytes
9
show large heats of solution or have a tendency t o crystallize with water of crystallization show relatively a higher condensiiig power. TABLBI I - h K A L I S _
~ _~
_
~
~ ~
~ __ _ _ ~_ ~ ~ ___ _ _ __ -
bask I
NaOH
I
N/IO ,I.O04j47 ' N/25 ~ 1 . 0 0 1 8 j1 N / 5 0 I .000900 N / IOO I .000448 N/zoo r.ooo2o7 1
___ 2 .os
!----
1.924 2.467 0.796 0.770 0.985 0.794 10.384 0.484 0.780 0.788 0.700
2 .oo
0.499 0.507 0.516 0.521 0.527
0.9j.4. l 0.j61 0.487 10.281 ! 0.245 0.140 0.124 1.122
N / I O O I.oooj2j 1.00026j
N/200
4.003 1.602 0.800 0.400 1 0.200
0.870 0.349 0.165 , 0.062 I 0.036
10.356 0.178 0.089 ' 0.045
1.182 1.006 0.602 ' 1.073 0.293 1.046 , 0.151 1.08
0.600
1 1
'
2.7 2.7
2.6 2.6 2.4 2.6 2.7
2.7 1
2.7 2.7
0.362 0.363 0.348
1.0
0.28
0.8 0.8
0.30
1.0
1.0
1
3.1 3.3 3.3 3.4
I I
0.716 1.178 0.356 0.598 , 0.178 0.290 0.089 0.1j o
0 2.08
Ca(OH),
I
I
1
It will be noticed that this specific condensing power, in most cases, varies with the concentration, generally in-
creasing with the dilution. Interpolated.
Hydrochloric and nitric acids
F . K . Cainei.oi2 a?id 1.V. 0. Robiiisoii
IO
are exceptions, and with these solutes there is an increase of condensation with increase of concentration. For more concentrated solutions the relative amount of water condensed decreases with the concentration, the condensation passing through a maximum value. This maximum occurs, commonly, with solutions of about I percent. It is evident, t h a t this contraction is the resultant of two forces a t least.
Salt
CaSO,
,
Concetitratioii
1
~
per liter
Hio
colltl. per qiu.
sait
Ratio of mols H2°
to salt
, 1
0.533 0.288 , 0.184 0.69 0.72 0.26j o . I j o 0.194 0.133 0.070 0.183 0.69
1.004609 I 1.001879 I N/ 5 0 1.000944 I N / I O O 1.ooojoI N / ~ o o 1.000277 I
2.22
2.477 1.584 0.996 0.675 0.498 0.342 0.249 0 . 2 0 0 0.125 0 . 1 2 7
N/IO N/2 j N/jo
2.22
2.13j 2.012 0.425 0.853 0 . 8 2 0 0.433 0.426 0.417 0.440 0.213 0.204 0.132 0.107 0.09j 0.401
34 3.5 3.5 3.5 3.2
2.48
2.13S 2.361 0.443 0.856 0.959 0.154 0.428 0.495 0.466 0.214 0.234 0.441 0 . 1 0 7 0.1 1 7 0.441
2.6 2.7
~
I
N/IO 1 N/Zj ~
COIltl.
2-95
N/IO N/2j N/so
I%/
I
.
N/roo N/?oo Na,CO,
~
1.001317 N,/IOO 1.000666 1 N/zoo 1.000326
I Na,HPO,
,
Hzo
Vol. of , of salt ~ ~ salt ~ 1
N/50
I
CaCl,
Deiisity o f
100
N/ZOO
r.oo461j 1.001861 I .00093 7 1.000464 1.000225
1.00jj28 I.oo2-?2Lj
1.001124 I .000jj I 1.0002 7 j
0.286 0.307 0.311 0.363 0.462
1.7
1.9 1.9 2.2
2.8
2.8
2.6 2.6
It might be thought a t first sight that the niaxitnuni point was due t o the concler~satio~~ of the solute beconiing appreciable in comparison with the condensation of the solvelit, b u t this is impossible, considering their relative masses, and it must' be adniittecl t h a t a satisfactory explanation of the observed phenomena is not yet at hand.
Condensation
OJ
IVater by Electrolytes
I1
Investigation of the properties of aqueous solutions have in the past been niacle mainly from the point of view t h a t the variation in properties of the solutions were due directly to the properties of the solute. This point of view has appeared t o be wanting in certain investigations, as in the study of flocculation of suspended solid:;, crumbing of soils, movements of film water, surface tensions, etc., and it has been thought possible to get further light on these phenoniena by considering them as clue t o changes in the water itself, which changes were induced by the presence of electrolytes. Although the data given above do not encourage greatly the idea t h a t this point of view will prove profitable, the paucity of experimental data suitable for comparison does not justify a positive statement, and further work in this direction will be presented as opportunity offers. E / i r c n i i of Soils,
li. S.Dcfinr f l i 1 C i i t ui Agl z c z l l i ~ ~ e , TI aslizrigtuii, D . C .
