902
J. Chem. Eng. Data 2007, 52, 902-905
Solubility Pattern of CaSO4·2H2O in the System NaCl + CaCl2 + H2O and Solution Densities at 35 °C: Non-ideality and Ion Pairing Arvind Kumar,* Rahul Sanghavi, and V. P. Mohandas Central Salt and Marine Chemicals Research Institute, Bhavnagar-364002, India
The solubility of CaSO4·2H2O was determined in aqueous NaCl solutions up to very high salinities and constant CaCl2 concentrations at 35 °C. Addition of CaCl2 into the aqueous NaCl system reduces the solubility of CaSO4· 2H2O quite dramatically while maintaining the basic pattern of the solubility curve. Mean ionic activity coefficients γ( of CaSO4·2H2O derived using the extended Debye-Hu¨ckel law with quadratic terms decrease with an increase in ionic strength. A comparison of the value of the thermodynamic solubility product constant, Ksp(th) of CaSO4· 2H2O to the solubility product Ksp obtained from the observed solubility data indicate sizable differences. This has been explained through a combination of the extended Debye-Hu¨ckel law and ion association theory. We also measured accurate densities for the quaternary system CaSO4·2H2O + NaCl + CaCl2 + H2O at 35 °C. The density of the solution increases linearly with an increase in concentration. Solutions containing higher amounts of CaCl2 were found to be less dense when compared at the same ionic strengths. Solubility and density data as a function of concentration have been correlated using polynomial and linear fits by the method of least squares. These studies are of relevance in the production of salt with low impurities of Ca2+ and SO42- ions.
Introduction Physicochemical properties of multicomponent electrolyte mixtures in water are important in understanding ionic equilibrium, ion-solvent and ion-ion interactions in natural waters. Accurate and reliable data on physicochemical properties of aqueous salt systems are necessary for many industrial processes where these systems are used as feed. We are continuing our research program on aqueous electrolyte solutions saturated with CaSO4·2H2O, which is a predominantly sparingly soluble electrolyte present in seawater and industrial water systems. It precipitates and can form scale once its saturation limit exceeds a certain threshold limit. Therefore, it is quite important to have accurate data on solubility and other physical properties of the systems where CaSO4·2H2O is an important constituent. In earlier reports,1-3 we studied a number of physicochemical properties for the ternary systems CaSO4·2H2O + NaCl + H2O and CaSO4·2H2O + CaCl2 + H2O. Addition of NaCl increases the solubility of CaSO4·2H2O initially and then decreases after reaching a maximum value, whereas addition of CaCl2 decreases the solubility of CaSO4·2H2O sharply at lower concentrations and decreases further with an increase in concentration. Therefore, we find it worth examining the counter effects of Ca2+ and Na+ ions on the solubility of CaSO4·2H2O in the quaternary system CaSO4·2H2O + NaCl + CaCl2 + H2O up to very high salinities. A research paper by Atkinson et al.4 provides a complete review of the solubilities of CaSO4·2H2O in water as well as in aqueous solutions of different electrolytes. Despite a number of investigations on the subject, a systematic study of the effect of added CaCl2 on the solubility and other physicochemical properties of CaSO4·2H2O in saline water is lacking. In this paper, we determined the solubility behavior of CaSO4·2H2O in the quaternary system CaSO4·2H2O + NaCl + * Corresponding author. Tel.: +91-278-2567039. E-mail: mailme_arvind@ yahoo.com or
[email protected].
CaCl2 + H2O using analytical methods and present accurate results of density measurements as a function of concentration at 35 °C. While dealing electrolyte with solutions at high concentrations, departure from ideality and ion association become quite significant due to Coulombic interactions between ions. In addition, the ion-solvent and solvent-solvent interactions that are present even in dilute solution also become increasingly modified as the concentration increases, contributing further to the nonideality. The nonideality could be evaluated by activity coefficients, which can be based upon some theoretical models.5-7 The ion association contribution can be studied both theoretically and experimentally.8-13 From experimental results of solubility measurements we have evaluated deviations from ideality using a combination of the extended Debye-Hu¨ckel law and ion association theory.
Experimental Section CaSO4‚2H2O and NaCl (g 99.8 % mass fraction) obtained from S. D. Fine Chemicals, Bombay, India, were used after drying in an oven at 70 °C without further purification. Fused CaCl2 obtained from E. Merck (India) Limited, Mumbai (g 98 % mass fraction), was recrystallized using Millipore grade water and was dried at 120 °C under vacuum. All solutions were prepared by weight, using an analytical balance with a precision of ( 0.0001 g (Denver Instrument APX-200) in Millipore grade water. Four aqueous CaCl2 solutions with different concentrations were prepared by dissolving known amounts of CaCl2 in Millipore grade water. Stock solutions were prepared by adding oven-dried NaCl to the solutions containing fixed amounts of CaCl2. A range of solutions with different concentrations of NaCl saturated with CaSO4‚2H2O were then made by diluting stock solutions with initially prepared aqueous CaCl2 solutions and adding excess CaSO4‚2H2O. The resulting solutions were stirred in a thermostatically controlled water bath. After the
10.1021/je0604941 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/08/2007
Journal of Chemical and Engineering Data, Vol. 52, No. 3, 2007 903 Table 1. Molal Solubilities and Mean Ionic Activity Coefficients of CaSO4·2H2O in Aqueous NaCl Solutions of Fixed CaCl2 Concentrations at 35 °C NaCl m1 0 0.1796 0.3184 0.4777 0.7754 1.1295 1.4890 2.0024 2.5153 3.0303 3.4906 3.8350 4.2002 0 0.7960 1.0378 1.4765 2.2545 2.8867 3.3263 3.8598 4.2968 4.4488 4.9943
CaSO4 m2
γ(
I
0 m CaCl2 0.0151 0.0604 0.0250 0.2796 0.0295 0.4364 0.0350 0.6177 0.0410 0.9394 0.0460 1.3135 0.0490 1.6850 0.0520 2.2104 0.0529 2.7269 0.0529 3.2419 0.0510 3.6946 0.0493 4.0322 0.0471 4.3886 0.0396 m CaCl2 0.0109 0.1624 0.0278 1.0260 0.0301 1.2770 0.0337 1.7301 0.0368 2.5205 0.0377 3.1563 0.0374 3.5947 0.0351 4.1190 0.0328 4.5468 0.0317 4.6944 0.0280 5.2251
0.4256 0.2477 0.209 0.1836 0.1584 0.1428 0.134 0.1277 0.1258 0.1269 0.1299 0.1333 0.1379 0.3038 0.1529 0.1440 0.1333 0.1262 0.1265 0.1290 0.1343 0.1404 0.1428 0.1536
NaCl m1 0 1.0078 1.4003 1.8413 2.3786 2.7680 3.1576 3.5513 3.9921 4.3834 0 0.8494 1.2374 1.5797 1.8811 2.3294 2.6792 3.0296 3.3195 3.7567 4.2999
CaSO4 m2
I
0.1072 m CaCl2 0.0102 0.3625 0.0195 1.4075 0.0227 1.8128 0.0248 2.2622 0.0252 2.8011 0.0250 3.1897 0.0245 3.5773 0.0231 3.9654 0.0217 4.4006 0.0200 4.7851 0.3676 m CaCl2 0.0080 1.1348 0.0097 1.9910 0.0100 2.3802 0.0101 2.7229 0.0102 3.0247 0.0103 3.4734 0.0100 3.8220 0.0094 4.1700 0.0087 4.4571 0.0079 4.8911 0.0073 5.4319
γ( 0.2239 0.1401 0.1319 0.1274 0.1258 0.1266 0.1289 0.1325 0.1381 0.1445 0.1491 0.1297 0.1267 0.1258 0.1261 0.1282 0.1310 0.1349 0.1390 0.1465 0.1585
solutions were stirred with an electrical paddle for about 24 h, liquid samples were withdrawn periodically and analyzed for different ions as described elsewhere.1,2 The densities of the solutions were measured with an Anton Paar (model DMA 4500) vibrating-tube densimeter with a resolution of 5 × 10-2 kg‚m-3. The densimeter was calibrated with doubly distilled and degassed water, with dry air at atmospheric pressure, and also against the densities of NaCl(aq),14 with an accuracy of 0.01 %. The temperature of the apparatus was controlled to within ( 0.03 K by a built-in peltier device. Reproducibility of the results was confirmed by performing at least three measurements for each sample.
Results and Discussion Solubility. Experimental results of the solubility of CaSO4‚ 2H2O in the aqueous NaCl solutions with fixed CaCl2 concentrations are given in Table 1 and are visualized in Figure 1. Uncertainty in the solubility measurements was estimated to be less than 0.02 %. The composition dependence of CaSO4‚ 2H2O solubility in aqueous NaCl solutions of fixed concentration was correlated by means of a polynomial type equation: F(Q) ) A0 + A1 (mNaCl) + A2 (mNaCl)2 + A3 (mNaCl)3
Figure 1. CaSO4‚2H2O solubility at constant CaCl2 concentration but varying NaCl concentration at 35 °C. Table 2. Parameters Ai and Standard Deviations σ of Equation 1 for the System CaSO4·2H2O + NaCl + CaCl2 + H2O at 35 °C CaCl2 m
A1
A2
A3
Solubility/mol‚kg-1 0.0365 -0.0109 0.0179 -0.0028 0.0125 -0.0020 0.0035 -0.0006
A4 0.0010
s
0 0.0396 0.1072 0.3676
0.0150 0.0104 0.0062 0.0049
0.0010 0.0020 0.0006 0.0003
0 0.0396 0.1072 0.3676
Mean Ionic Activity Coefficients γ( 0.3447 -0.3320 0.1428 -0.0182 0.2926 -0.1970 0.0665 -0.0066 0.2119 -0.0669 0.0122 0.1468 -0.0220 0.0058
0.0447 0.0163 0.0113 0.0020
0 0.0396 0.1072 0.3676
0.9945 1.0023 1.0063 1.0272
Density, F/10-3 kg‚m-3 0.0379 0.0369 0.0365 0.0357
0.0039 0.0017 0.0011 0.0014
in the solution. For the same ionic strength, the maximum in the solubility curves appears to shift toward higher concentration with an increase of the CaCl2 concentration in solution. The solubility equilibrium under study is CaSO4‚2H2O(s) S Ca2+(aq) + SO42-(aq) + 2H2O(l) (2) that can be expressed by the solubility product constant Ksp ) aCa2+‚aSO42-‚aH2O2 ) s2‚γ(2‚aH2O2
