T H E SOLUBILITY OF CERTAIN SALTS PRESENT IN ALKALI SOILS’ BY F. K. CAMERON, J. M. BELL, AND W. 0. ROBINSON
Statement of the Problem.-In previous papers from this laboratory results have been reported for a number of the possible systems which may exist containing the salts commonly found in the soils of arid regions. In the present investigation one of the simplest systems was taken as a starting point and by the successive addition of other components more complex systems were realized, since in this way the bounding isotherms and fields could be most readily determined. As the primary object of the investigation was to obtain information regarding the salts occurring in nature as “alkali,” certain conditions were imposed on the several systems, thus approximating field conditions and at the same time materially simplifying the problem. The data here given are for equilibrium conditions at 25’ C. The first results given are for the mutual solubilities of the chlorides of calcium and sodium, and they show that solid calcium chloride does not exist in contact with a solution containing sodium chloride except at very low concentrations of the latter, and at very high concentrations of calcium chloride. It is, therefore, highly improbable that solid calcium chloride exists under normal alkali conditions. The second system contains sodium chloride, sodium sulphate and water. The solubility curves and the solid phases in contact with the several sets of solutions were determined. This system shows three solubility curves, one representing solutions in contact with the solid decahydrate of sodium sulphate, a second curve for solutions in contact with the solid anhydrous sodium sulphate, and the third curve Published by permission of t h e Secretary of Agriculture. For illustrations of the occurrence of calcium chloride under unusual conditions, see Bull. No. IS, Division of Soils, U. S. Dept. Agriculture (~gox), PP. 44-45. 1 2
SoZzrhiZitj1 of Ceviaia Salts Present
iit
A l k a l i Soils
397
in contact with sodium chloride. This system was the starting point for the more complex systems considered in this investigation. The third system was realized by adding calcium sulphate to the second system. In addition to the fields for the decahydrate of sodium sulphate, anhydrous sodium sulphate, and sodium chloride, this system was also found to contain fields for gypsum and for a double sulphate of sodium and calcium whose composition has been determined. The position of the boundary curves and the composition of all constant solutions was also determined. Several systems containing calcium carbonate have also been studied, zriz., the solubility of calcium carbonate in various sodium chloride and sodium sulphate solutions, from which free carbon dioxide has been excluded; also the solubility of calcium carbonate in sodium chloride solutions saturated with carbon dioxide. System VI1 was realized by adding calcium carbonate to System 11, taking precaution to exclude any free carbon dioxide. The boundary curves in System I1 were but slightly displaced by the addition of the new component to the system, and but one new field was thereby introduced, namely, the field for solid calcium carbonate. System VI11 was formed by adding both calcium sulphate and calcium carbonate to System 11, taking precautions as in the case of System VI1 to exclude free carbon dioxide. I n this case it was found that the bounclary curves as they exist in System I11 are but slightly displaced. No new fields were found, though now, of course, each field represents solutions in contact with two solid phases, one of which was calcium carbonate in every case. The composition of all constant solutions and the position of the boundary curves were determined. Finally the ninth system was obtained by saturating all solutions of System VI11 with carbon dioxide at a pressure equal t o the atmospheric pressure; that is, the total pressure of the vapor phase was equal to the atmospheric pressure.
398
F . K . Cameron, J . M . Bell and W . 0. Robinson
In this system none of the solid phases in System VI11 disappeared, nor were any new phases recognized. There was, however, a well-marked displacement of the position of some of the boundary curves as they exist in that system, and consequently in the composition of the constant solutions. Each field in this system represents but two solid phases, although it might be supposed that, by the introduction of the new component, carbon dioxide, a field should represent three solid phases; but coincident with the introduction of the new component the condition was imposed that the pressure of the vapor phase should be the atmospheric pressure, thus arbitrarily reducing the variance of the system by one, or in other words, to the same variance as that of System VIII. Analytical Methods.-In this work it has been necessary to determine sodium chloride, sodium sulphate, calcium sulphate, calcium carbonate, carbonic acid otherwise com bined, and, in a few cases, water. The sodium chloride was determined volumetrically by titrating with standard silver nitrate solution, using potassium chromate as indicator. The sulphates were determined by precipitation with barium chloride in the usual manner. The sodium was not directly determined but was calculated by difference. The calcium was precipitated as the oxalate, with due precautions, and, in general, weighed after blasting to the oxide. When small in quantity, however, the calcium oxalate was titrated in a warm dilute sulphuric acid solution with a dilute solution of potassium permanganate. The carbonates were determined by adding a slight excess of standard sulphuric acid, boiling to expel the freed carbon dioxide, and titrating the excess of acid with a standard solution of sodium hydroxide, using phenolphthalein as indicator, the results being calculated as calcium carbonate. System I . Calcizcm Chloride, Sodium Chloride, Water.The mutual solubility of calcium chloride and of sodium chloride in aqueous solution was determined at 2.5'. A preliminary experiment showed that there could be but two curves; i. e., but two stable solid phases under these con-
Solubility
of
Certain Salts Present in Alkali Soils
399
ditions. It was found from data for the cooling curve that the temperature at which calcium chloride hexahydrate changes to the tetrahydrate in the presence of solid sodium chloride As the stable hydrate of calcium chloride at 2 5 O , was 29.0'. both when no other salt is present and also in the presence of sodium chloride is the hexahydrate, this is the only stable form a t that temperature. TABLE I ~~
Mutual solubility of calcium chloride and sodium chloride in water at ~ 25' C -~__-__ ~ -
_
~
~~~
Density
250
250
I
CaCI,
1 per too grams H,O Grams
-
84.0~
1.444 1 1.3651
1.3463 1.2831 1.2653
1
58.48 53.47 36.80 30.08
I
~
_
Solid phase
per Ioogranis H,O NaC1
1
_
~
-_~_-
Grams
CaC1,.6H20 CaCl,.6H,O,NaCl NaCl
0.0
1.846 1.637 1.799 7.77 10.70
~
~
( L
j
I ( 16
40
30 20
IO 20
40
Fig.
60 I
sulphate and water, has been investigated by Meyerhoffer and S a u n d e d while working on the effect of foreign salts, in solu1
From Landolt-Bornstein, Physikalisch-Chemische Tabellen ( rgog), p .
526.
Zeit. phys. Cheni., 31, 373 (1899).
400
F . K . Cameyon, J . M . Bell an,d W . 0. Robinson
tion, upon the inversion points of hydrated salts. It was found that the change from Glauber's salt to anhydrous sodium sulphate, which takes place in aqueous solution at 32.8', occurs at 17.9' when the solution is saturated with respect to sodium chloride. It is apparent that at intermediate temperatures either form of sodium sulphate may be stable in solutions containing sodium chloride, depending upon the concentration of the latter salt. Of course for a definite temperature, the solutions in contact with the decahydrate are less concentrated with respect to sodium chloride than the solutions in contact with the anhydrous sulphate. Brownel has also investigated this system, in part, when testing the " indirect method of analysis " proposed by Schreinemakers2 and Bancroft3 for the determination of the composition of solid phases which would undergo decomposition if washed with water to remove the mother-liquor. I n this work by Browne solutions of sodium chloride containing from o to I I .7 percent sodium chloride were saturated with sodium sulphate at 33' and cooled down to 23'. The solid crystallizing out was found to be the decahydrate of sodium sulphate. Seidell,4 in this laboratory, studied the problem through the entire range of concentrations and at various temperatures between IO' and 35'. His results confirm the observations of Meyerhoffer and Saunders. Below 18' he found that the complete solubility curve consisted of two branches along which the stable solids were the decahydrate of sodium sulphate, and sodium chloride. Between 18' and 33' there were three branches representing solutions in equilibrium with the decahydrate of sodium sulphate, anhydrous sodium sulphate, and sodium chloride. Above 33' the decahydrate no longer exists and there are, therefore, but two branches. The curves at 25' have been redetermined and the stable Jour. Phys. Chem., 5, 287 (1902). Zeit. phys. Chetn., 11, 81 (1893). Jour. Phys. Chem., 6, 178 ( 1 9 0 2 ) . 4,Am. Chem. Jour., 28, 52 (1902).
Solubility of Certain. Salts Present in Alkali Soils
401
solid phases present have been established by the indirect method. The following Table (11) gives the data, and Fig. 2 their graphical presentation. TABLE I1 Data for the solubility curves in the system, sodium chloride, sodium sulphate and water, a t 25O C
__-___ ~
1 5O Density 2-
25O
Na,SO, in
NaCl in -
io0 gms. IOO cc solution solution H,O IOO cc
I ~
IOO gms.
H,O
Grams , Grams
1.2180 1.2173 1.2162 1.2150 1.2275 1.2385 1.2571 1.2476 1.2429 1.2438 1.2451 1.2453 1.2309 1.2 162 1.2002
2.77 5.37 7.92 12.20
14.38 17.10
20.67 23.81 27.26 28.22 28-59 29-44 30.30 31.52
2.96 5.79 9-90 '3.43 15.82 19.13 23.22 26-54 3 1.06 32.41 33.00 33-81 34.60 35-80
26.93 24.88 22.62 19-73 I 7.86 17.86 18-55
14.50 I 1.32 8.76 8,65 8.54
I
5.81 2.97 0.00
1
~
I1
Solid phase
1
28.55 , Na,SO,.roH,O & I 26.60 1 ' I 24.32 li 21.41 I 1 19.62 19.64 20.73 Na,SO,. roH,O ; Na,SO, 16.28 Na,SO, ' I 12.62 9.98 " 9.93 9.84 Na,SO,; NaCl 6.66 NaCl I I 3.38 Id 61
1'
0.00
F . K . Cameron, J . M . Bell and W . 0 . Robinson
402
Density
F K
L
,
1
NaCl
Per cent
8.26 14.96
1.2201
1.2512 1.2431 1.2431
~
22. I O 22-10
1
Na,S04
19.01 15.93 13.40 7.12
j.12
cen;t1;~
NaCl
Na,S04 Per cent
G H
~
M N ' ' N' R S ~
Sitzungsber. Akad. Wiss. Berlin, 1899,810,
1.68 2.57 8.80 7.88
.9.21 58.46 62.30
33.74 35.62 49.49 66.80 59.30 2.39 0.p
Solubility
of
Certain Salts Present in Alkali Soils
403
Fig. 3
System I I I . Sodium Chloride, Sodium Sulphate, Calcium Sulphate and Water.-This system may be considered as made up of the following components, namely, Na,O, CaO, HC1, SO, and H,O. It is therefore a five-component system: But since in this investigation the condition was imposed that the sum of the bases should be exactly equivalent to the acids one degree of freedom is thereby lost and the system, t o all intents and purposes, becomes one of four components. It will be simpler for the present purpose to consider as components, NaC1, Na,SO,, CaSO, and H,O, and to regard the problem presented by this system as the solubility of calcium sulphate, or of a double salt containing it, in mixed solutions of sodium chloride and sodium sulphate. It will be desirable therefore to consider first the solubility of calcium sulphate in solutions of each of the sodium salts. The solubility of calcium sulphate in aqueous solutions of sodium chloride has been studied by Droeze,l Tilden cnd
' Ber. chern. Ges. Berlin,
IO, 330
(1877).
F . K . Cameron, J . M . Bell and PI/. 0. Robimon
404
Shenstone,' Lunge,' C a m e r ~ n Orloff , ~ ,4 Cloez' and d'Anselme.6 The details of these several investigations have been brought together recently in another publication.' It appears that the solubility curve at ordinary temperatures for solutions in which calcium sulphate or gypsum is the solid phase, passgs through a maximum point. A t 26' the concentration is approximately 140 grams sodium chloride and 7.2 grams calcium sulphate per liter of solution, and these figures must be nearly correct for 2 5 O , since it has been repeatedly shown that small differences in temperature in this neighborhood produce very small differences in the solubility curve. The concentration of the "constant solution'' with both calcium sulphate and sodium chloride present as solid phases, at 25' has been found by Cameron and Brown' to be 318.3 grams sodium chloride and 5.52 grams calcium sulphate per liter. In this system no new solid phases appeared. The solubility of calcium sulphate in aqueous solutions of sodium sulphate has been studied by Droeze,@Cameron and Seidell," Cameron and Breazeale," and details of the hvestigations have been brought together recently in the bulletin cited The solubility curve for 25' with calcium sulphate as the solid in contact with the solutions, 'passes through a minimum point at a concentration, approximately, of I 5 grams sodium sulphate and 1.4 grams calcium sulphate per liter of solution. The concentration of the solution in contact with both solid sulphates at 2 5 O , has been found by Cameron Proc. Roy. Soc., 38, 331 (1885). Jour. SOC. Chem. Ind., 4, 3 1 (1885). Jour. Phys. Chem., 5, 556 (1901); Bull. No. 18, Divisioii of s o i l s U. Dept. Agriculture (1901). Jour, Russ. Phys. Chem. SOC.,34, 949 (1902). BulL SOC. Chim. Paris [a], 29, 167 (1903). e, Ibid., ag, 372 (1903). 7 Bull. No. 33, Bureau of Soils, U. S. Dept. Agriculture ( ! 9 0 6 ) . a Jour. Phys. Chem., 9, 1 x 0 (1905). Ber. chern. Ges. Berlin, IO, 330 (1877). lo Jour. Phys. Chem., 5, 643 (1901). Ibid., 8, 335 (1904). l2 Bull. No. 33, Bureau of Soils, U. S. Dept. Agriculture (1906). a
Q
s.
.
Solzcbility of Certain Snlls Present in Alkali Soils
405
and Brown to be 254.6 grams sodium sulphate and 2.58 grams calcium sulphate per liter. At this temperature no new solid phase appeared in this system. The positions of some of the boundary curves for this four-component system were found by adding an excess of gypsum to each of the solutions described in Table 11. When equilibrium had been established, each solid was found by microscopic examination to be composed of at least two and sometimes three kinds of crystals. In cases where two solids were found the solutions were on boundary curves and where three solids were found, the solution was a “constant solution,” as at a fixed temperature the greatest number of solids which can exist in such a system is three, unless the temperature happens to be that of an invariant point, when four solid phases may coexist. The examination of the solids under the microscope showed the presence of one new crystalline form, which existed over a rather wide range of concentration. This was found to be a double sulphate of sodium and calcium and for the present it will be designated as “ double sulphate.” Analyses of these solutions for the various components are given in Table IV, and also the solid phases with which each solution is in contact have been given. By comparison of Table I1 and IV it will be seen that the presence of gypsum in solution depresses the solubility of sodium sulphate considerably and also depresses the solubility of sodium chloride but to a lesser extent. The position of the points B and C of Fig. 2 have been slightly displaced also by the addition of calcium sulphate. The new positions of these points have been shown in Fig. 4 A, B, C and D. This figure has been distorted somewhat t o magnify some of the differences which if drawn to scale would not be recognizable. The quantity of calcium sulphate in solution at these points is also widely different and this has been indicated by the width of the strip ABCDD’C’B’A’, the points B and B’ and the points C and C’ respectively being quite close together, The table also indicates that for a certain distance along A’B’ and for a certain distance along D’C’ gypsum is the solid
406'
F . K . Camerolz, J . M . Bell and W . 0. Robinson
a
3
W +
rd
n a
a x
z
4!z
N
N
I.. 9
m
P 9
.d
R
1
Solubility of Certain Salts Present in Alkali Soils
407
Fig. 4
phase. While the remainder of the curve A’B’C’D’ represents solutions in contact with the ‘‘ double sulphate” and another solid phase or phases. The part of the diagram A’B’C’D’G must therefore be divided into at least two fields, one for gypsum and one for the double sulphate. The boundary between the gypsum and “double sulphate” fields was found t o be a continuous curve between the point E on the curve A’B’ to the point F on the curve C’D’. It is therefore apparent that there are’no fields, other than these two. The data for the boundary curve between these two fields are given in Table V. It was thought at first that ,an anhydrite field would be found in this system. Actually, no anhydrite crystals could, be found in any of the solutions. Van’t Hoff and Armstrongl have shown that in any system where the solution has a vapor pressure above 1 7 . 2 mm a t 2.5’ gypsum is more stable than anhydrite and below this vapor pressure anhydrite is the more stable form of calcium sulphate. Thus the boundary curve at 2 5 O between a gypsum field and an anhydrite field becomes a curve of constant vapor pressure. In the present system no Sitzungsber. Akad. Wiss. Berlin, 1900,p. 559.
F . K . Cameron, J . M . Bell and W . 0 . Robinson
408
TABLE V Data for boundary curve between the field for gypsum and the double sulphate of calcium and sodium iu the system, sodium chloride, sodium sulphate, calcium sulphate and water, at 25' C ~_
_
_
~
~
~
~
_
~
~~
~~
~
NaCl in 100 c c .
solution
1.2119 1.2250 1.2223 1.2249 1.2312 I . 2208 I . 2234 1.2293
1
14.20 15.73 i 15.80 , 18.29
21.38 23.82 27.25 29.23
I 1
Na,SO, in
gms. H,O
solution
15.96 17.j o 17-47 20.69 23.70 26.79 30.80 33.07
17.39 16.44 16.46 13.42 11.06 8.92 6.96 4.78.
io0
~
~
I 0 3 cc
1
gn1s. H,O
IO0
19.54 18.29 18.20 14.9' 12.24 10.09 7.85 5.4'
solution could be found which had a vapor pressure of 17.2 mm or less and consequently if any form of calcium sulphate is stable, it is gypsum and not anhydrite. The solutions which would have the lowest vapor pressures in this system would be those along the boundary curves ABCD and A'B'C'D'. The vapor pressure of a saturated solution of sodium chloride a t 25' is 17.7 mm; a solution saturated with respect to anhydrous sodium sulphate, and sodium chloride has a vapor pressure of 17.5 mm. As sodium sulphate decahydrate and sodium sulphate anhydrous coexist at 25' at a vapor pressure of 19 mni, the vapor pressure at the point B of Fig. 4 is 19 mm, and no point on the curve AB will have a vapor pressure below 19 mm. The two solutions E and F of Fig. 4 lost over one percent in weight when standing over a solution of sulphuric acid having a vapor pressure of 17.5 mm. From these results it follows that there is no solution having a vapor pressure as low as I 7.2 mm and consequently there can be no anhydrite field. The composition of the constant solutions have been tabulated below, together with the solid phases with which the solutions are in contact.
Solubility of Certain Salts Present in Alkali Salts
409
TABLEV I Constant solutions and corresponding solid phases at ~~~~~
Point
Concentration of solution n grams per roo grams H,O
A
0.00
B C D A'
19-13 33.0 35.80 0.00
27.12
E
15-96
19.54
B' C'
19-23 32.18 33.07 35-09
21.79
F D'
~
~
28.55 20.73 9.84
0.000 0.000
11
~. ~
~ ~~
25'
~~ ~
~
-
~~
-
Solid phase
Na,SO,. 1oH,0 Na,SO,. IOH,O and Na,SO,
0.00
sulphate 0.0354 Na,SO,. 1oH,0, double sulphate and Na,SO,
9.50
5.4' 0.00
The Double Sulphate of Calcium and Sodium in Sodium Chloride Solutions at 25' C.-The artificial preparation of a double sulphate of calcium and sodium in the wet way was first accomplished by Fritzsche' while preparing hydrochloric acid by mixing sulphuric acid and sodium chloride containing gypsum. He obtained long, acicular needles which somewhat resembled gypsum crystals in appearance but from the action of water upon it, it was apparent that it was a double salt. Fritzsche also prepared the compound by treating gypsum with a saturated solution of sodium sulphate at 80 C. By analysis he decided the composition of this feathery-like compound t o be represented by the formula 2Na,SO,.CaSO,. zH,O. By continued heating of the pasty mass of featherlike crystals and' mother-liquor, it was found that the crystals disappeared, the mother-liquor appeared to clear up, and rhomboidal crystals precipitated. These last crystals could be readily filtered from the mother-liquor, and the substance was found t o have the composition Nq,SO,.CaSO, and to be in fact an artificial glauberite. Subsequently Hannay' found this same compound in the flues of a chemical factory and Jour. prakt. Chem., 72, 291 (1857). Chem. News, 34, 256 (1876).
F. K.
410
Carneyon,
J . hf. Bell and W . 0. Robinsort
Volhardl obtained it as a by-product in the manufacture of sodium acetate from calcium acetate and sodium sulphate. Van’t Hoff and Chiaraviglio2 prepared glauberite by the reaction of gypsum with a solution containing 33 percent sulphuric acid saturated with Glauber’s salt a t 80’. They prepared the acicular compound of Fritzsche by bringing together 53 grams of dry calcium chloride and 34 grams of Glauber’s salt in a half liter of water and evaporating. Gypsum first formed but subsequently disappeared, the acicular double salt being formed. On continued boiling, this latter was entirely transformed to glauberite. The Fritzsche formula for these acicular crystals has generally been misquoted as Na,S0,.CaS0,.2H20; and van’t Hoff , 3 from the analogies this substance shows to syngenite, concluded that it contained but one molecule of water and designated it as sodium syngenite. In a paper just cited van’t Hoff has demonstrated that at 29’ a solution saturated with both Glauber’s salt and gypsum undergoes a change and a new solid phase, glauberite, crystallizes out. In other words, there is an invariant point at 29’, three solid phases being in equilibrium. It is also shown that another invariant point exists at 30.2’, a t which point the three phases, Glauber’s salt, gypsum and “sodium syngenite” are present. It is obvious that in the neighborhood of 29’ the stable double salt is glauberite, and that the second of these invariant points is metastable. In the work which has been described in the preceding chapter, the double salt, was, however, composed of acicular crystals and no evidence of any rhomboidal crystals at any time appeared. The following dilatometric measurements were made in order to determine which of these double salts was the stable form under the conditions of the experiments. Four dilatometers were charged with these acicular crystals which had been freed, as far as possible, from the mother4
Chem. News, 4 3 , 6 (18Sr). Sitzungsber. Akad. Wiss. Berlin, ~ S g g p , , 810. Ibid., 1905,478.
Solubility of Certain Salts Present i n Alkali Soils
41 I
liquor in which they were prepared, and the solutions used were those at the points E,F,B’ and C’, respectively, of Fig. 4. Four similar dilatometers were charged with glauberite prepared by Fritzsche’s method and the solutions were the same as for the other four dilatometers. If the field EFB’C’ of Fig. 4 were in reality the sum of two fields, one for each double salt, such would have been indicated, for the stable double salt could not have been the same at all four points. As a matter of fact, these measurements showed conclusively that in all cases where the solid was composed of the acicular crystals, there was no appreciable change of volume in the system, but. where the rhomboidal glauberite crystals were used, there was in all four cases a decided increase in the volume of the system, showing that the acicular crystals are stable under these conditions. The composition of the double compound has been determined by three methods, all of which indicate that the acicular crystals are not sodium syngenite, but correspond t o the formula 2CaS0,.3Na2SO,. The first method was by the use of two triangular diagrams, which method has been described by Bell.’ The solutions and residues have been analyzed and plotted on two such diagrams, and in spite of the fact that the points are too close together t o prove absolutely the formula which has been given above, yet the lines joining the corresponding points pass very close to the point representing the above conipound, and the lines do not pass near the point which would represent a compound of formula CaSO,.NaSO,.H,O. The quantity of calcium in the solutions was so small that it was not necessary to determine it at all. It will be observed that in the first diagram the lines cross on one of the sides of the triangle. The solid compound therefore contains no sodium chloride, and from the position of the point of intersection it contains 61 percent sodium sulphate which is the quantity present in 3Na2S0,.2CaSO,. Neither diagram, however, proves conclusively that the ___-____ Preceding article.
F . K . Cameron, J . M , Rdl and W . 0. Robinson
412
TABLE VI1 Composition of solutions and residues
_ _ ~ -
~-
~
I
I 11
!
-
Solution
1
__
-
~
~~
--
NaCl
14.00
~
.-I
Na,SO,
1-
p
-
~
~
I
~
~
~
_
_
-
-
~
~~
Residue ~
1
NaCl
Na,SO,
~~-
1
1
12.36
~
Percent 12.82 ‘
17.17
H,O
CaSO,
- -
Percent Percent 6.66 ~ ~ I 8 71~
11.75
-
I-
~~
~
1
-
Percent
i
4.16 449
1
Percent
64.38
1
65.82
crystals contain no water. Owing to the feather-like nature of the crystals the quantity of adhering mother-liquor was very large and the points in the diagrams lie close together. A small experimental error in any determination would cause a great difference in the crossing point of two lines which meet a t such a small angle. To find the quantity of water contained in the crystals a “zero method” was employed. To 180 cc of a solution in the double salt field, which had reached equilibrium with the double salt, were added 4.26 grams anhydrous sodium sulphate and 2 . 7 2 grams calcium sulphate, which are in the ratio corresponding t o 3Na,SO,.zCaSO,. The density of the solution was determined before and I O days after the addition of the salts, as was also the sulphuric acid content of the solution. The densities were 1.2188 and 1.2190, respectively, and the barium sulphate from I O cc of the solution weighed 2.561 and 2.563 grams, respectively, indicating that the solution had undergone no change. As the solution in the field contains but very little calcium sulphate, the union of the sulphates in any other proportion than 3 t o 2, would have augmented or decreased the sodium sulphate content of the solution, which would have changed the density and also the sulphuric acid content as determined by precipitation with barium chloride. From this experiment it follows that either no water was removed t o form the double salt, or else there were two bhanges, one in sodium sulphate and the other in water m-hich counterbalanced and gave a solution of the same density and the same sulphate content as the original solution.
Solubility of Certain Salts Present in Alkali Soils
413
This last possibility was excluded by the following experiment. Duplicate mixtures were prepared synthetically which would give a solution in the double salt field. As no sodium chloride exists in the double salt, the same ratio of sodium chloride to water, before and after the double salt is formed would indicate that no water has been removed from solution. This was actually found t o be the case, as the following table shows. TABLE VI11 Composition of the original mixture and of resulting solution ~~
Original mixture
20
grams Na,SO,. . . . . . . . . H,O .. . . . . . . . . .
~~~
~-
~
~
~~
Resulting solution
16.89 grams Na,SO, per
IO
grams H,O
IOO grams
Jour. Phys. Chem., 6, 50 (1902);Bull. No. 18,Division of Soils, U. S. Dept. Agriculture ( I 90I ) .
F, K. Canzevou,]. M. Bell and h? 0. Robz'izson
414
TABLEI X Solubility of calcium carbonate in sodium chloride solutions free from carbon dioxide a t 25' -
-
~
-
~
__
~~~
NaCl in
1 CaCO,
H,O
IOO grams
Grams
in
IOO grams
H,O
Gram
1.601 5.177 9-25 I I .48 16.66 t
~
1
22.04
30.50
0.0079 0.0086 0.0094 0.0I 04 0.0106 0.0115 0.0119
consisted entirely of carbon dioxide and water vapor was one atmosphere. This was attained by saturating the solutions with carbon dioxide at a lower temperature than 25' and removing the stopper of the bottles at intervals to allow the excess of the gas to escape after the bottles had been brought to the temperature of the experiment. TABLE X Solubility of calcium carbouate in sodiiim chloride solutions saturated with carbon dioxide at 25' and one atmosphere pressure ____
~
-___
~
Density
25O o2.5
NaCl
-~
ill 100 grams
~
~
~~~~~
_
_ ~
CaCO, in roo grams H,O
H,O
.
Gram
Grams
1.0129 1.0499
0. I 50
1.45 . 5.69
1.0501
6.48
1.0759 I.XOI5 1.1246 1.1789 1.1957
I I .06 15.83 19.62 29.89
35.S5
0.160 0.173 0. I74 0.172 0.159 0.123 ~
0.103
System V I . Sodium Sulphate, Calcium Caibonate and Water.---These experiments were, similar to those in System IV, except that sodium sulphate was used instead of sodium chloride.
TABLE XI Solubility of calcium Carbonate in sodium sulphate solutions free from carbon dioxide at 2 5 O ~
Density
y5-: -
1.1200
1.1539 1.1615 1.1837
~
Na,SO, in ioogms. water CaCO, in
25
1.0081 1.0161 I .0363 I. 1084
:-
~-
I
~~
..__
~~
100grams
___
-
H,O -
Gram
Grams
0.97
0.0151
1.65 4.90 12.69 14.55 19-38
0.01 80
0.0262 0.03 I 3 0.0322
0.0346 0.0343 0.0360
2 I .02
23-90
Svstern V I I S o d i u m .Sulfiha.te, S o d i u m ChJoride, C a l c i u m Carbonate und V’utey.- In the work on the solubility of calcium carbonate in sodium sulphate solutions and in sodium chloride solutions by Cameron and’ Seidell, the solutions were in equilibrium with the air and consequently contaified a small quantity of free carbon dioxide. It was found that with increasing concentration of sodium sulphate, the solubility of calcium carbonate continually increases ; on the contrary in sodium chloride solutions, a maximum solubility of calcium carbonate is reached a t about 60 grams per liter. It was brought out that at all concentrations thk calcium carbonate was much more soluble in sodium sulphate solutions than in sodium chloride solutions. Cantoni and Goguellal have recently shown that the solubility of calcium carbonate increases with increasing concentrations of sodium chloride up t o a concentration of about 2 0 grains of the chloride per liter, which was the most concentrated solution used by them. An excess of solid calcium carbonate was added to solutions containing varying quantities of sodium chloride and sodium sulphate, and after three months at 25*, with frequent shaking, the solutions were analyzed. Table XI1 gives the data obtained in this work. Bull. S O ~C. h i n ~ .Paris [3], 33,
24
(1905).
416
-
~~
F. K. Cameron,]. M. RcZl and U.: 0. Robzmo~z
TABLEXI1 Solubility of calcium carbonate in mixed solutions of sodium chloride and sodium sulphate at 25' C . _-__~-__~_______
Density
-!Lo
1
'
1.2570
1.2435 I .2442 1.2434 1.2470
1
1.2122
1
1.2020
1
~
Na,SO, in
I
~
25
1.2115 1.2380 1.2378 1.2427
I
NaCl in
^-o
IOOCC
~
r o o g H,O
6.43
6.93
10.00
10.78
10.07 14.62 17.16 23.90 27.30 27 4 3 28.32 30.38 31.52
10.89 I 6.07 19.18 26.66 31.r.5 31.52 32.17 3437 35.70
100
cc
2 6.90 24.83 21.67 19.82 19.39 I 8.24 18.43
11.30
roo g H,O
28.48 26.47 23.36 21-37 20.98 20.07
20.74 12.58
8.79 8.88 6.74
10.00 10.20
2.08 0.00
2.35
7.65 0.00
'
CaCO, i n 00 g H,O
0.0239 0.0192 0.0137 0.0 I34 0.0137 0.0119 0.0116 0.0044 0.0046 0.004 I 0;0043 0.0037 0.0036
These figures are- in accord with the previous results in showing that calcium carbonate is more soluble in a saturated sodium sulphate solution than in one saturated with sodium chloride. It will b e observed that as the concentration of sodium chloride in solution increases, the quantity of calcium carbonate decreases. The solutions described in this table all lie on the curves of Fig. 2 , and consequently the presence of calcium carbonate in these small amounts has not appreciably affected the solubility of the more soluble chlorides and sulphates of sodium. In all these solutions calcium carbonate was the solid phase, and not a double compound. If a double compound of two calcium salts had been formed, there would have been sodium carbonate in solution which would have caused the solutions to show a decided alkaline reaction. As the solutions were but very faintly alkaline, no such change could have taken place. Further, if a double carbonate had resulted, the solution would have been rich in calcium salts and as the solution carried very little lime, it is again apparent that such a change did not occur.
Sodz.diZity of Certnz'n Salts Present zn Adkalz' Soids
417
System V I I I . Sodium Sulphate, Sodium Chloride, Calcium Sulphate, Calcium Carbonate and Water.-Owing to the very slight solubility of calcium carbonate in solutions of sodium chloride and sodium sulphate, it was expected that the presence of a salt with a common ion (CaSO,) would depress this solubility even more. By the same procedure as was described in the preceding chapter, it was demonstrated that calcium carbonate underwent no change in these solutions. Therefore, this system cannot differ essentially from that described by Table IV, as the solubility of calcium carbonate is very slight, and as no new solid phase can appear. An extensive investigation of this system was not necessary as the solutions would have been practically identical in c0.mposition with those of Table IV. These conclusions are confirmed by the data in the following table. TABLE XI11 Data for the solubility of calcium sulphate and calcium carbonate in mixed solutions of sodium chloride and sodium sulphate at 25O C ~~~~~
~-
Density
~~
'5" 11 250
Io0 cc
_ _
~
1.2109 1.2113 1.2442
1 1
~
I
NaCl i n
~_
1 '
Ioog
11,o
2.76 2.94 5.52 I 28.12 1 32.18
1
Na,SO, in Ioocc
1 23.78
i___
I
H,O contains
r o o g ~ , CaSO, ~
1 ~
8.52
1 IOO g
I
25.35 23.39 9.50.
i ~
CaCO,
-I___
0.2290'0.0163 0.1992 I 0.0155 0.0301 0.0065
System I X . Sodium Sulfihate, Sodium Chloride, Calcium Sulphate, Calciunz Carbonate and Water Saturated with Carbon Dioxide.-- It has been shown above that the variance of this system is not increased over that of the precedbg system by the addition of the new component, carbon dioxide, for at the same time the further condition was imposed, that the pressure of the vapor phase equals atmospheric pressure. In this case a field will represent solutions in contact with two solid phases, a boundary line, solutions in contact with three sclid phases. " Constant " solutions will be in equilibrium with four solid phases.
F. K. Cameyou,J. M. Bell a m ? W. 0. Robinson
418
For the experimental work on this system solutions saturated with either sodium sulphate or sodium chloride were
Fig. sa
put in
several solutions were saturated with carbon dioxide by passing the gas under pressure into the solutions at a low temperature. The bottles were then shaken at constant temperature, 2 5 O C., and frequently unstoppered so that the pressure of the vapor phase was finally the atmospheric pressure. In all cases solid calcium sulphate, either as gypsum or a double sulphate, was found. Table XIV gives the analytical data obtained for this system.
TABLE XIV Solubility of mixtures of calcium sulphate and calcium carbonate in mixed solutions of sodium chloride and sodium sulphate saturated with carbon dioxide a t atmospheric pressure, a t 25’ C NaCl in 5O Density 2-
25O .
___
~~
1.2158 1.2142 I .2 109 1.2143 1.2248 I .2224 1.2281 1.2388 1.2590 1.2554 1.2332 1.2474 1.2429 1.2135 1.1957
IOO
cc
____ 0.00 2.02
4.00 6.00 9.98 9.95 I 1.31 13-78 I 8.67 I 8.80 2 I .46 27.78 28.47 30.50 31.14
I
IOO g ~
I
i
, 1
H,O
.-
0.00
2.13 4.28 7.03 10.77 10.75 12.31 15.50
20.40 2 1.04 23.81 31.70 32.73 34.48 35.46
~
100
cc
loo g H,O
____
25.86 24.23 22.81 21.61 19. 53 19.73 19-15 I 8.72 19.80 16.81 I 1.61 8.70 5.56 2.20 0.00
27.12
25.52
24.22 23.10 21.09 2 1.32 20.84 20.59 21.61 18.81 12.88
9.92 6.26 2.49 0.00
Calcium as calcium oxide in IOO g H,O
_
_
_
0. I430 0.1212 0. I 148
0.1069 0.0812 0.0826 0.0726 0.0673 .-
0.0358
-
0.0183
0.0291 0.0484 0.0490
The diagram representing the system would be almost identical with that shown in Fig. 4, except that each field would represent solutions in contact with two solid phases, one of which would be that shown in Fig. 4 and the other in every case calcium carbonate. The data for the “constant” solutions are given in Table XV.
~
420
F. K. Canzeron,J. M. BeZl and W. 0. Robzrzsorz
TABLE XV Data for cotistant solutions in the system sodium chloride, sodium sulphate, calcium sulphate, calcium carbonate, carbon dioxide and water, at 25' C ~~
~-
~
Grams in io0 grams H,O
~
~-
1 Solid phases
-
' Na,SO,.roH,O, CaS0,.2H20 and CaCO, 0.0673 Na,SO,.IoH,O, CaS0,.2H,O, CaCO, 1 and 2CaS0,.3Na2S0, 0:0360 Na,SO,. roH,O, CaC0,,2CaS0,.3Na,S04, ~~
0.1430
!
and NaSO.
1
-
I
7
Summary.-In the foregoing pages data have been given for typical systems encountered in studying the chemistry of alkali, or the accumulation of readily soluble salts found in the soils of arid regions. Incidentally it has been shown that solid calcium chloride is not to be expected normally as a component of alkali soils, nor is anhydrite to be expected normally where the salts of sodium and calcium predominate. Further, it has been shown that under certain conditions of concentration a double sulphate of calcium and sodium can exist. At higher temperatures this double salt is the wellknown mineral glauberite ; at lower temperatures the double salt has been shown to have the composition 2CaSO4.3NaSO,. Bureau of Soils, U. S. Department of Agvicultuve, Washington, D.C.
