Enthalpies of solution and solubilities of calcium chloride and its lower hydrates

O-l 19 .I. Chem.

Thermodynamics

1985, 17, 893-899

Enthalpies of solution and solubilities calcium chloride and its lower hydrates

of

G. C. SINKE, E. H. MOSSNER, Chemicals Research Department, Michigan Division, The Dow Chemical Company, Midland, Michigan 48667, U.S.A.

and J. L. CURNUTT Central Research Laboratories, The Dow Chemical Company, Midland, Michigan 48667, U.S.A. (Received 15 November 1984: in revised.form

26 March

1985)

New measurements on (CaCI,+rH,O) in the temperature range 329 to 559 K have revealed the existence of a hydrate having r = l/3. This hydrate was characterized as to composition, X-ray diffraction pattern, and enthalpy of solution in water. For comparison, enthalpies of solution of the monohydrate, dihydrate, and anhydrous CaCI, were determined.

1. Introduction Anhydrous calcium chloride is an industrial chemical manufactured on a large scale by dehydration of the aqueous salt obtained from various sources. Knowledge of the path of dehydration is important in efforts to minimize the energy required for this energy-intensive process. The only phase relations in (CaCl, + rH,O) above 383 K were reported by Roozeboom. (l) He determined 448.7 K to be the incongruent melting temperature of CaCl, .2H,O to give (CaCl, . H,O + saturated solution) but did not detect a melting temperature for CaCl, * H,O although his solubility measurements extended to 508 K. We have redetermined the (CaCl, + rH,O) phase diagram in the temperature range 329 to 559 K and discovered that the monohydrate melts incongruently at 460 K with the formation of a fractional hydrate.‘2’ We have measured the composition and enthalpy of solution of the fractional hydrate and for direct comparison have also measured the enthalpies of solution of anhydrous CaCl, and several compositions of varying degrees of hydration. These results should be of value to the current CODATA project on thermodynamic properties of compounds of calcium.‘3’

2. Experimental All samples were prepared from J. T. Baker Chemical Company “Baker Analyzed Reagent” calcium chloride dihydrate. The manufacturer’s analyses of the several 0021L9614/85/090893 +07 $02.00/O

c) 1985 Academic Press Inc. (London) Limited

894

G. C. SINKE.

E. H. MOSSNER.

AND

J. L. CLJRNITT

lots used varied only slightly from the typical values (all in mass per cent): sulfate. 0.005; ammonium, 0.003; barium, 0.003; magnesium, 0.003; potassium, 0.002; sodium, 0.01; strontium, 0.03; heavy metals (as Pb), 0.0002; and iron, 0.0003. The average purity of the dry salt was better than 99.9 mass per cent as confirmed by heating the dihydrate to 673 K and titrating the anhydrous salt for calcium with standardized ethylenediamine tetracetic acid solution. Solubility measurements were made by sealing samples in heavy-walled 25 cm3 glass ampoules, each of which contained a magnetic stirring bar. Each ampoule was heated in an oil bath and the temperature at which the last crystals melted in the stirred liquid was visually determined. The heating rate as the sample approached complete melting was 14 PK. s- i. Samples were made up by adding water to or removing water from portions of a carefully analyzed homogeneous batch of dihydrate. The compositions of individual samples were calculated from the masses of dihydrate and water charged into each ampoule. A correction was applied for the amount of water in the 12 cm3 vapor space above the liquid. Temperatures were measured with calibrated mercury-in-glass thermometers. As an aid in locating transition temperatures, differential thermal analyses were run on CaCI, .2H,O and CaCI, * H,O in sealed containers. Thermograms were of the type obtained by Kessis (4) for SrCl,. 2H,O, and transition temperatures were determined by the intersection of the forward extrapolated base line and the backward extrapolated initial sides of the endotherms.(5) The enthalpy-of-solution measurements were made in a glass solution calorimeter similar in design to that described by Vanderzee and Myersc6) The calorimetric vessel was a 700 cm3 silvered Dewar vessel fitted with a 1.3 cm brass flange, O-ring seal, and a brass lid. Sample ampoules were blown from precision-bore glass tubing and sealed with plastic-plug stoppers. Separate calorimetric experiments revealed no significant thermal effect from breaking empty ampoules in water at 298.15 K. Agitation was provided by a rotary propeller located inside a concentric stirring well extending to near the bottom of the Dewar vessel. The stirrer was driven at 3.3 Hz by a synchronous motor. In operation, the calorimeter was submerged in a water bath whose temperature was controlled at (298.250+0.005) K. Electrical calibrations were made immediately before and after each dissolution experiment, using an energy-equivalent circuit of conventional design.“’ The 45 R calibration heater was constructed of 0.25 mm diameter manganin wire wound noninductively on a borosilicate-glass spool. The heater was contained inside the calorimeter in a glass well filled with Dow Corning Sylgand 184 encapsulating resin used as a heat-transfer medium. The power supply was a constant-current source, John Fluke Mfg. Co., Model 382A, operated at 0.30000 A. The duration of the electrical energy input was measured with a computer clock. Xerox Data System Sigma 3, which had a resolution of 10 ms. The calibration experiment was run for approximately 140 s and produced a temperature rise of about 0.2 K. Five electrical calibrations performed with 587.19 g of water showed that the energy equivalent of the calorimeter was reproducible to within kO.17 per cent. Temperatures were measured to 1 x 1O-4 K with a quartz-crystal thermometer. Hewlett-Packard, Model 2801 A, connected by an interface unit to the Sigma 3

A,,,H,(CaCI,

895

rH,OJ

computer for producing the time-against-temperature results in the form of punched computer cards. The corrected temperature rise was determined by the integration method,@) using a Fortran program’9’ rewritten to analyze the quartz-crystal thermometer output on a Burroughs 5500 computer. As a check on the performance of the solution calorimeter, a series of measurements was made on the reaction of tris(hydroxymethyl)aminomethane with excess 0.1 mol. dm- 3 HCl. The TRIS sample was obtained from the National Bureau of Standards (SRM724) and used according to the recommendations specified in the provisional certificate issued with the material. For 11 experiments, we obtained a mean value at 298.15 K of AIH, = - (29748 + 27) J. mol- ’ in which the uncertainty interval is twice the standard deviation from the mean. This result is in good agreement with the weighted mean, - (29744 f 5) J . mol- ’ . of 29 values”” reported for this test reaction during the period from 1964 to 1975.

3. Results and discussion Results of the solubility measurements are listed in table 1 and plotted as a phase diagram in figure 1. The transition temperatures of 449, 460, and 503 K determined by thermal analysis correlate very well with breaks in the solubility curve. At 449 K the expected transition of dihydrate to (monohydrate + saturated solution) occurs. At 460 K a previously unreported transition of monohydrate to (a new solid phase + saturated solution) takes place. The new phase, in turn, is converted to (anhydrous CaCl, + saturated solution) at 503 K. Solubilities of Roozeboom”’ shown in figure 1 are uniformly higher for a given temperature in the dihydrate region. The agreement of his dihydrate transition temperature with ours must be regarded as fortuitous since it appears to be derived from the intersection of the dihydrate curve and values for metastable fractional hydrate. We have observed that slurries of fractional hydrate can be supercooled completely through the monohydrate region without spontaneous formation of the monohydrate phase. Recent solubility results by Assarsson and co-workers are discordant; a paper on (calcium chloride + potassium chloride + water)‘“’ gives results in good agreement with our values but a later publication on (calcium TABLE T/K

10Zm(CaCl,)/{m(CaC1,)+m(H,O)}

1. Solubility

of CaCI,

in water

T/K

10’m(CaClz),/(m(CaC12)+m(HzO)~

329.1 368.7 388.9 407.7 423.7

56.96 60.10 62.13 64.35 66.75

469.6 486.2 495.7 499.2 516.7

75.17 75.94 76.44 76.59 77.00

446.3 447.7 449.0 451.8 459.6

71.75 72.30 72.90 73.34 74.69

525.2 527.1 542.7 558.7

77.10 77.20 77.58 77.88

896

G. C. SINKE, E. H. MOSSNER. AND J. L. CURNITT

6ol

60

65

70

75

NJ

102m(CaC12)/(m(CaC12) + m(H?O)} FIGURE 1. Phase diagram for (CaCl,+rH,O). s(r), solid with mole ratio r of H,O to CaCI,.

0. This work; +, Roozeboom:“’

1, liquid solution;

chloride + strontium chloride + water)‘12’ quotes solubilities even higher than those on Roozeboom’s curve. A single point by Lightfoot and Prutton(13’ is intermediate between Roozeboom’s results and ours. The composition of the new phase was determined by heating two platinum crucibles, one containing anhydrous CaCl, and the other containing a much larger amount of CaCl, . H,O, in a sealed container for 3 d at 423 K. Upon cooling and analyzing, the anhydrous CaCl, was found to have been converted to CaCl, .0.340H,O. The experiment was repeated with a small amount of CaCl,. H,O and a much larger amount of anhydrous CaCl, in the respective crucibles. The CaCl,. H,O in this case was found to have been converted to CaCl, -0.332H,O. We conclude that the new phase is an exact hydrate with I = l/3. This new phase had a unique X-ray diffraction pattern as collected on a Guinier camera with Cu Ka, radiation (I = 0.1540598 nm) and aluminum foil as an internal standard. The strongest lines as obtained by the procedure of Frevelo4’ are given in table 2. Equilibrium vapor pressures to be reported in a later paper show the new phase to be stable over a range of conditions which includes 443 K and 47 kPa partial pressure of water vapor. A sample prepared by holding CaCl, at this

&,H,(CaCI, TABLE

2. X-Ray

d/rim

diffraction

measurements

897

rH,O)

of lattice spacing CaCI, .$H,O

IJP

d/rim

l/l”

dlnm

16 16 15 20 22 26

0.3671 0.3148 0.28285 0.27981 0.26917 0.26246

11 17 100 27 53 14

0.25995 0.25674 0.24823 0.24545 0.24361 0.23859

0.6042 0.4968 0.4325 0.4134 0.407 1 0.4018

d and

relative

line

l/P 30 16 26 21 35 10

intensity

f/I”

for

d/rim

1jP

0.23023 0.22862 0.21927 0.21480 0.21353 0.21188

15 30 27 12 18 15

temperature and water vapor pressure for 24 h was found to have the composition CaCl, .0.334H,O. Several samples of CaCl, of varying degrees of hydration were prepared for enthalpy-of-solution studies by heating in a closed container for 3 d at 450 K. This heat treatment insured that only the equilibrium phases were present. The sample compositions as determined by mass loss on heating to 673 K and the phases observed by X-ray diffraction are listed in
Sample no.

3. Characterization

of samples

used for enthalpy-of-solution fraction of H,O

Phases by X-ray diffraction

10zw

Sample no.

I

24.23

2

13.50

CaCI, CaCI,

.2H,O H,O

3

7.45

CaCI,

fH,O,

CaCI,

H,O

4

5.54

CaCl,

$H,O.

CaCI,

H,O

measurements;

IO5V

w denotes

the mass

Phases by X-ray diffraction .--________- ..~~~

5

5.22

6 7

3.71

CaCl, CaCI,

0

CaCl,

fH,O .;H,O.

CaCI,

898

G. C. SINKE. TABLE

4. Calorimetric

results

m

r

E. H. MOSSNER. at 298.15

AND

J. L. CURNITT

K for the enthalpies

of solution

(.s(calor))

AlI,

J.K-’

K

J

A,i,H J

i

AH(corr.)

of CaCI,

.rH,O Asot KiM J,g-’

1.970

1.6949 1.3799 1.2513

2733.7 2734.1 2732.0

0.18683 0.15261 0.13692

0.3 0.2 0.2

-12.6 -9.5 -8.2

0.961

0.8437 0.8550 0.8134

2731.2 2729.3 2737.7

0.12532 0.12830 0.12168

0.0 0.0 0.0

-5.8 -5.9 -5.6

-412.6 -416.5 -416.4

0.496

1.0269 1.1867 1.0945

2735.2 2724.8 2731.7

0.20629 0.23877 0.21967

0.1 0.1 0.1

-8.2 -9.9 -9.2

- 557.4 - 556.5 - 556.6

0.361

0.9200 0.9876 0.7276

2741.7 2731.4 2744.4

0.19911 0.21396 0.15695

0.0 0.1 0.0

-7.2 -8.1 -5.2

-601.2 - 599.9 - 599.1

0.340

0.8930 0.9254 0.9413 0.6978 0.7527 0.6874 0.8064 0.8964 0.8880

2719.4 2717.8 2740.7 2721.9 2724.8 2714.0 2716.8 2706.8 2727.5

0. I9702 0.20396 0.20586 0.16391 0.17659 0.16164 0.21669 0.24055 0.23782

0.0

-7.0 -7.3 -7.4 -5.1 -5.5 -5.0 -6.7 -1.4 -7.4

-607.8 - 606.9 -607.3 -646.1 -646.6 -645.5 -738.4 -734.6 -738.8

0.237

0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0

- 308.6 -309.1 - 305.3

r = 1.970 from table 4 were used to derive by extrapolation

the molar enthalpy of solution for r = 2 given in table 5. The results are in good agreement with the selected values of Parker, Wagman, and Evans(‘6’ listed in table 5. Much of the data base for the selected values was created nearly a century ago. Samoilov and Tsvetkov(“’ reported the molar enthalpy of solution of anhydrous CaCl, in 500H20 as - 8 1.7 kJ mol- ‘. When adjusted to infinite dilution, their molar enthalpy of solution is - 83.7 kJ. mol- ‘. reported the molar enthalpy of solution at infinite Perachon and Thourey”‘) dilution to be - 79.03 kJ. mof -1. Their sample was vacuum dried at 723 K while our material was dried in air at 673 K. detected fractional Thermal analyses by Buzagh-Gere, Gal and Simon(“) hydrates in (calcium chloride + water) and (sirontium chloride + water) but not in TABLE

5. Molar

CaCI,

enthalpies

.rH,O

of solution at 298.15 its lower hydrates

- A,,,H;/(kJ This work

CaCI, CaCl, CaCI, CaCI,

.fH,O H,O 2H,O

K of calcium

81.85 71.37 52.16 44.85

molParker

chloride

‘) rt ~1.“” 81.34 53.7 45.8

and

A,,H,(CaCI,

rH,O)

899

(barium chloride + water). However, Kessis,(‘“) Williams and Wendlandt,‘21 J and Wendlandt’22’ by thermoanalytical techniques found evidence for fractional hydrates in both (strontium chloride + water) and (barium chloride + water). The fractional hydrates were assumed by these workers to be hemihydrates. It would be of interest to measure carefully the stoichiometry of these hydrates to determine if Y = f is unique to (CaCl, + rH,O). The authors thank Laverne Ruhberg and Jim Edmonds of the Michigan Division Analytical Laboratory for obtaining the X-ray diffraction pattern of CaCl, .fH,O and Alan Syverud of Central Research Laboratories for the computerized curve fitting. REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. IO.

II. 12. 13. 14. IS. 16. 17. 18. 19. 20. II. ‘2.

Roozeboom, H. W. B. 2. Phys. Chem. 1889,4, 31. U.S. Patent 3.878.295: IS April 1975. CODATA Thermodynamic Tables. Tentative Selections for Some Compounds of Calcium and Related Mixtures. Garvin, D.; Parker, V. B.; White, H. J., Jr.: editors. To be published by CODATA Kessis, J. J. C. R. Paris 1%7, Ser. C264,2141. Schwenker. R. F., Jr.; Cam, P. D. Thermal Analysis. Vol. 2. Academic Press: New York. 1969. Appendix 3. Vanderzee, C. E.; Myers, R. A. J. Phys. Chem. 1961, 65, 153. Skinner, H. A.; Sturtevant, J. M.; Sunner, S. Experimental Thermochemistry. Vol. II, Chap. 6. Skinner. H. A.: editor. Interscience: New York. 1%2. Wadsii, 1. Science Tools 1%6, 13, 33. Curnutt. J. L. Ph.D. Thesis. Universitv of Nebraska. Lincoln. Nebraska. U.S.A. 1968. Montgomery. R. L.; Melaugh. R. A.;‘Lau. C.; Meier, G. H.: Chen. H. N.: Rossini, F. D. J. Chem Thermodvnamics 1977, 9, 915. Assarsson,G. 0. J. Am. Chem. Sot. 1950, 72. 1433. Assarsson, G. 0.: Balder. A. J. Phys. Chem. 1953, 57, 717. Lightfoot, W. J.; Prutton, C. F. J. Am. Chem. Sot. 1947, 69. 2098. Frevel, L. Anal. Chem. 1%6,38, 1914. Lange, E.; Strecck, H. 2. Phys. Chem. 1931. A152, I. Parker, V. B.; Wagman. D. D.; Evans, W. H. J. Phys. Chem. Rej: Datu 1982, 1I. Supplement No. 2. Samoilov, 0. Y.; Tsvetkov, V. G. Russ. J. Struct. Chem. 1%8, 9, 142. Perachon, G.; Thourey, J. Thermochim. Acta 1978, 27. I I I. Buzagh-Gere, E.: Gal, S.; Simon. J. Z. Anorg. ANg. Chem. 1973, 400. 37. Kessis, J. J. C. R. Paris 1%7, Ser. C264, 973. Williams, J. R.; Wendlandt, W. W. Thermochim. Acra 1973, 7. 275. Wendlandt. W. W. Thermochim. Acta 1975, 12, 359.