T H E TRANSITION POINTS O F SALT HYDRATES I N VARIOUS NON-AQUEOUS SOLVENTS WALTER W. LUCASSE
AND
HAROLD J. ABRAHAMS
T h e J o h n Harrison Laboratory of Chemistry, T h e University of Pennsylvania, Philadelphia, Pa. Received August 16, 1988
Many methods have been used to determine the transition points of polymorphous solids and of salt hydrates. For both types of systems conductance methods have been used in various forms, taking advantage, particularly, of the difference in conductance due to the different solubilities of the two forms. In an earlier paper (1) it has been shown that with a constant amount of solvent and solute a break occurs in the resistancetemperature curve a t the transition point when the solution of a salt hydrate in a non-aqueous solvent is heated over a short range of temperature in this region. The break in the curve is presumably due to the change both in the salt a t this point and the simultaneous change in the solvent medium. The earlier study was limited t o solutions of various concentrations in ethyl alcohol and alcohol containing small amounts of water. In the present paper are given results obtained with a number of additional salts in methyl, ethyl, propyl, and isoamyl alcohols, pyridine, acetone, and ethylene glycol. APPARATUS, MATERIALS, AND METHODS
The cell used was in the form of a U-tube with the electrodes sealed in one arm and the thermometer placed in the other. The design permitted the solution to be transferred from one arm to the other to insure uniformity and obviated any change in the cell constant due to slight differences in proximity of the thermometer to the electrodes. The similar placement of the bulb of the thermometer and the electrodes, together with the very slow rate at which the temperature of the bath was increased, made it possible to assume that the temperature in both parts of the cell was the same. The thermometer was graduated in tenths of a degree and had been checked against a standard thermometer. Connection was made through the stopper bearing the thermometer with a mercury trap which prevented any loss of the vaporized solvent from the system and minimized concentration changes due to the high vapor pressure of the solvents. The ethyl alcohol was prepared by allowing ordinary commercial alcohol 51 1 THE JOURNAL OF PKYBICAL CHEMISTRY, VOL. XXXVII. NO. 4
512
WALTER W. LUCASSE AND HAROLD J. ABRAHAMS
to stand over freshly broken lime for several days, after which it was distilled over a slow water bath, only the middle portion being retained. The acetone was purified with sodium bisulfite and dehydrated in the usual manner. The other solvents and the salts used were all high grade products and no attempt was made to purify them further. I n most of the determinations saturated solutions were used. A quantity of the solvent was shaken for some time in a dried, air-tight container with an excess of solute and after being allowed to settle the clear supernatant liquid was transferred to the cell. I n some determinations the saturated solution was diluted with pure solvent before being introduced into the cell, and in others less solute than that required to make a saturated TABLE 1 LiNOaa3HzO = LiNO3.$H2O 2iH20 in pyridine (series I), in acetone (series in amyl alcohol (series 3)
+
+ 2Hz0 in propyl alcohol
CaC12.6HzO = CaClz.4Hz0@ MERIES
Temperature
-22 24 26 28 30 32 34 36 38 40
1
SERIES
.
2
..
SERIES
3
a),
(series 4 ) MERIES
4
Resistance
Temperature
Resistance
Temperature
Resistance
Tempersr ture
Resistanoo
1193 1181 1169 1157 1146 1137 1125 1116 1107 1099
24 25 26 27 28 29 30 31 32 33.2 34 35
46.96 46.77 46.21 45.84 45.47 45.11 44.93 44.57 44.21 43.68 43.51 43.16
24.2 25.3 26 27 28 29 30 31 32 33 34 35
505.0 492.9 486.9 473.4 463.9 454.7 448.4 438.6 428.2 419.7 411.4 403.2
25 26 27 28 29 30.1 31.1 32.1 33.0 34 35
319.5 314.3 309.9 305.5 301.8 296.9 293.4 288.7 285,2 281.8 277.8
solution was used. I n each case the bath was brought up to a temperature a few degrees below the transition point of the salt before introducing the cell, after which the temperature of the bath was raised a t the rate of about eight degrees an hour. Frequent readings of the resistance were then taken simultaneously with those of the temperature within the cell. RESULTS
I n table 1are given the results obtained with lithium nitrate in pyridine, acetone, and amyl alcohol. I n the first column of each series are given the temperatures and in the second the corresponding resistances. Plots were made of the various determinations, using both the resistances and the logarithm of the resistances as ordinates against the temperatures as
TRANSITION POINTS OF SALT HYDRATES
85
30
Tem eraturr
P
FIG.1
513
W
514
WALTER W. LUCASSE AND HAROLD J. ABRAHAMS
abscissae. In both plots curves were obtained which approached straight lines in the neighborhood of the transition points. With the logarithmic plot the curves seemed to remain straight over a longer temperature range and so in the final plots and in the calculations the relationship between the temperature and the logarithm of the resistance was used. Such a plot of the data given in table 1 appears as figure 1, where the ordinates of the individual curves were adjusted in order to show several curves on a single plot. In each case smooth curves are drawn through the experimental points and the curve obtained below the transition point extended beyond this temperature as a broken line for emphasis. The value for the transition temperature calculated from the data obtained from solutions of TABLE 2
+ 3 H ~ O i nmethylalcohol (series I), in ethyl alcohol (series 8 )
= CdBrz.HzO CdBrz. Zn(NOs)z.6Hz0 = Z
+
~ I ( N O ~ ) ~ . ~ 3Hz0 H ~ Oin methyl alcohol (series 3), i n ethyl alcohol (series 4 )
~
SERIES
Temperature
26 28 30 32 34 36 38 40 42 44 46
I
1
I
8ERIEB 2
Resistance
Resistance
62.87 61.61 60.50 59.41 58.22 57.29* 56.15 55.03* 54.16 53.19 52.24
28 30.5 32 34 36 38 40 42 44
556.2 546.2 538.6 530.0 522.6 515.3 508.1 501.1 495.1
- I1
SERIES
28 30 32.2 34.2 36 38 40 42
8
Resistance
Temperature
Resistance
113.2 112.3 111.4 110.5 110.1 109.2 108.3 107.5
28 30 32 34 36 38.2 40 42 44
196.5 192.6 188.8 185.5 182.1 179,3 177.1 174.7 172.2
lithium nitrate in pyridine agrees well with that found by Donnan and Butt (2) from solubility measurements, i.e., 29.6"C. The method of calculation as shown below depends upon the abrupt change in the slope of the resistance-temperature curve a t the transition point, and the identity of the resistance at this temperature. The curves for lithium nitrate in acetone nnd in amyl alcohol (Curves I1 and 111) are of interest even though they do not permit of the calculation of the transition point by the method here used. I n both cases there is an abrupt change in the curve between 29°C. and 30"C., indicating the transition point between these temperatures, but t'he solvent medium and the number and nature of the conducting particles change in such a way as to make the temperature coefficient above and below the transition point almost identical. Thus in these two solvents parallel lines were obtained in each
515
TRANSITION POINTS O F SALT HYDRATES
case in place of intersecting curves, making calculation of the exact transition point impossible. The last series in table 1 and Curve IV are based on the data obtained for solutions of calcium chloride in propyl alcohol. The value calculated for the transition point in this solvent is in fair agreement with that previously found for the salt in ethyl alcohol and with the TABLE 3 ZnBrl.2Hz0 = ZnBrz 2H20 in pyridine (series I) CoC12.6Hz0 = CoClz.Hz0 5H20 in glycol (series .8), in pyridine (series S), in methyl alcohol at different concentrations (series 4,5, and 6 )
+
+
SERIES
Temperature
1
SERIES
Resiatanoe
Temperature
8982 8766 8660 8487 8334 8184 8021 7925 7768 7675 7553 7493
25.1 26 27 28 29 30 31.1 32.1 33 34.1 35
Temperature
Resistance
Temperature
24 25 26 27 28 29 30 31 32 33 34 35
336.3 334.3 330.9 329.6 327.6 325.6 323.6 321.7 319.1 317.8 315.9 314.0
24 25 26.1 27.1 28 29 30.1 31.1 32.1 33 34
25 27.5 29 31 33 35 37.5 39 41 43 45 46 SERIES
4
2
-
SERIES
Resistance
788.2 762.6 738.1 714.4 691.5 666.8 648.3 627.7 610.3 591.1 574.7
BERIES
Temperature
SERIES
78.19 77.54 77.06 76.58 75.95 75.48 74.86 74.40 74.10 73.79 73.49
Resistance
22670 22240 21880 21530 21040 20470 20010 19700 19170 18710 18190 17680
20 22 24 26 28 30.5 32.5 34 36 38 40 42
5
Resiitance
3
6
Temperature
Resistance
24 25 26 27 28 29 30 31 32 33 34 35 36
32.54 32.22 31.96 31,70 31.51 31.32 31.07 30.82 30.58 30.44 30.27 30.08 29.85
value given by Bancroft (3) for the equilibrium temperature for the hexahydrate and the P-tetrahydrate. In table 2 are given the results obtained with cadmium bromide and with zinc nitrate, each in methyl and in ethyl alcohol. Solubility or other determinations leading to values of the transition point of an accuracy
516
WALTER W. LUCASSE AND HAROLD J. ABRAHAMS
comparable to those used in the earlier paper for these and the remaining salts considered here do not seem to be available. I n the previous work the average deviation from the established value of the transition found by this method using the above cell was about two-tenths of a degree, The emphasis in the present study, however, was placed upon extending the method to a larger number of solvents rather than establishing the exact values. The transition point of cadmium bromide according to Mellor (4) from consideration of the available data is 36°C. The value for zinc nitrate is given as about 35°C. Wasilieff (5) claims to have made a tetrahydrated salt and that it forms a n eutectic with the hexahydrate a t 35.4"C. Calculation of the data for this salt in ethyl alcohol gave 35.4"C. as the transition point; in methyl alcohol a curve was obtained similar to those for lithium nitrate in acetone and amyl alcohol. I n table 3 are given the data obtained from solutions of zinc bromide in pyridine and of cobalt chloride in ethylene glycol, in pyridine, and in methyl alcohol of various concentrations. The transition point of zinc bromide is given by Mellor (6) from consideration of solubility data as 35°C. According to a plot of the data compiled in Seidell (7), the transition point of cobalt chloride between the hexahydrate and monohydrate is a t about 31°C. Landolt and Bornstein (8), on the other hand, indicate the transition point of the hexahydrate and dihydrate a t about 50°C. and the possibility of a polymorphous transition of the hexahydrate between 30°C. and 35°C. Compilations of the solubility data for cobalt chloride in water show a curve which rises gradually as an almost straight line to about 30"C., a t which point there is a n abrupt curvature to about 50"C., where the line is cut by an almost straight line giving the solubility a t higher temperatures. Corresponding with these points, the color of the solution changes from rose to violet and finally to blue. Numerous theories have been advanced to explain the changes in color and solubility, including equilibria of various salt hydrates, double salts, complex ions, and hydrated ions. The several viewpoints are summarized by Friend (9), who also mentions the change in color in aqueous salt solutions and in non-aqueous solvents. In table 3 we have indicated the transformation as being that between the hexahydrate and the monohydrate, and calculation led to a transition temperature a t about 31°C. The real change taking place a t this temperature may be of quite a different nature and much more complicated. Whatever the cause, however, we found quite pronounced breaks in the resistance-temperature curves in all cases. The different solvents and concentrations' gave different colored solutions, but in each case the break in the curve seemed to come a t about the same point. The single exception was for the solution in glycol, where the calculated point was found to be 1.6"C. below that of the average in the four other solutions-in pyridine and in methyl alcohol, The deviation being so much greater than
517
TRANSITION POINTS OF SALT HYDRATES
the average deviation would indicate an influence of the solvent rather than experimental error. A single determination was made with this salt in the vicinity of 50°C., using et'hylene glycol as solvent. A pronounced break was found in the curve a t about 47°C. which again, owing to the solvent, 'may be lower than the true transition point for the change taking place at this higher temperature. In view of the findings given here it would appear that there is some abrupt change which takes place a t about 31"C., and a further transformation a t the higher temperature where the sharp change in the solubility curve is found.
a
BOLVENT
SALT
lO3b
a'
1086'
t
t'
Lithium nitrate
Pyridine Acetone Amyl alcohol
3.1255 -2.2203.1145 -1.847
29.5 29-30 29-30
29.6 29.6 29.6
Calcium chloride
Propyl alcohol 2.6587 - 6.190 2.6513 -5.932
28.7
29.2
Cadmium bromide
Methyl alcohol 1.9054 -4.120 1.8986 -3.927 Ethyl alcohol 2.8415 -3.4332.8246 -2.960
35.2 35.7
ca. 36 ca. 36
Zinc nitrate
Methyl alcohol 34.2-3t ca. 35 Ethyl alcohol 2.4108 -4.2002.3692 -3.026 35.4 ca. 35
Zinc bromide
Pyridine
4.0542 -4.0384.0346 -3.483
35.3
Cobalt chloride
Glycol Pyridine Methyl alcohol Methyl alcohol Methyl alcohol
3.2632 -14.623.2162 -13.04 4.4394 -4.1624.4873 -5.690 2.5916 -2.721 2.5849 -2.510 1.9666 -3.063 1.9295 -1.860 1.5896 -3.252 1.5712 -2.663
29.7 31.3 31.8 30.8 31.2
35.0
ca. ca. ca. ca.
31 31 31 31 ca. 31
CALCULATION O F THE TRANSlTlON POINT
As indicated above, the curves giving the relationship between the temperature and the resistance of solutions of salt hydrates in non-aqueous solvents approach straight lines at the transition points which in most cases intersect at this temperature. Equations giving the resistance as a function of the temperature may be developed, and since the resistance becomes identical a t this point, the temperature may be calculated. I n the present study it was found more advantageous to use the logarithm of the resistance, and equations for the two parts of the curve over the range where they appeared to be straight lines were calculated by the method of least squares in the form log R = a
+ bt
518
WALTER W. LUCASSE AND HAROLD J. ABRAHAMS
The results of such calculations are given in table 4. In the first and second columns are given the various salts and solvents. The constants a and b, given in the third and fourth columns, refer to the equation below the transition point and the values a’ and b’, in the fifth and sixth columns, are for the corresponding curve above the transition point. Solution of the equations for 1 yields the values of the transition points given in the next t o the last column. In the final column, under t’, appear such comparative data as can be found in the literature. In the tables above, all points obtained in the temperature interval are given. In the determination of the equations of the curves by this method, however, a few points may legitimately be omitted if from the plot they seem to be in error. This should be done particularly when such a point lies a t or near the end of the curve and would thus, owing to the small number of points, have a considerable effect upon the slope of the curve and wipe out the selfconsistency of the earlier points. This was found necessary in the present study only in the case of cadmium bromide, and the points omitted in the calculations are indicated in the table by asterisks; the remaining points in this curve and all of the points in the other curves were given equal weight. If from the graph a point appeared to be a t the intersection of the lines it was included in the calculation of both curves. Obviously in order t o achieve a high degree of accuracy with this method of determining the transition point, a larger number of points should be obtained for each branch of the curve so as to make the method of least squares truly significant. SUMMARY
In the above study the transition points of a number of salt hydrates have been determined by observing the abrupt change of the resistancetemperature curves of solutions of these substances in several non-aqueous solvents. I n most cases the numerical value of the point can be calculated, since merely a change in slope occurs a t the point. Examples are also given, however, where the slope remains nearly constant, but the magnitude of the resistance changes abrqptly. In all cases except for solutions of cobalt chloride in ethylene glycol the transition point was found t o be independent of the solvent and, within the accuracy of the experiment, comparable with that found by other methods. REFERENCES (1) LUCASSEAND HARRIS:J. Phys. Chem. 30, 930 (1926). (2) DONNAN AND BUTT:J. Chem. Soc. 83, 335 (1903). (3) BANCROFT: The Phase Rule, p. 71. The Journal of Physical Chemistry, Ithaca, New York (1897). (4) MELLOR:Comprehensive Treatise on Inorganic and Theoretical Chemistry, Volume IV, p. 567. Longmans, Green and Company, New York (1923).
TRANSITION POINTS O F SALT HYDRATES
519
(5) Landolt-Bornstein Tabellen, Volume I, p. 687. J. Springer, Berlin (1923). (6) Reference 4, p. 566.
(7) SEIDBLL: Solubilities of Inorganic and Organic Substances, Volume I, p. 256. D. Van Nostrand Company, New York (1919). (8) Reference 5, p. 648. (9) FRIEND: A Textbook of Inorganic Chemistry, Volume IX, Part I, p. 39. J. B. Lippincott Company, Philadelphia (1920).
