1. Introduction

The solubility of solutes in mixed solvents is of great practical importance since many industrial process as well as laboratory procedures call for the use of solvent mixtures. The solubility of solutes in mixed solvents depends primarily on the solvation of solutes or their constituent ions by the components of solvent mixtures[1]. Studying the thermodynamics of different salts, is important for evaluating the single ion thermodynamic parameters which help in explain the preferential solvation of the ions[2].

Removal of heavy elements and sulphate ions from an alkaline medium using solvent extraction was very important to get rid of these hard ions[3].

2. Experimental

The used calcium carbonate (CC) and ethanol (EtOH) were supplied from Merck Co. The saturated solution of calcium carbonate (CC) was prepared by dissolving little solid amount in closed test tubes containing different EtOH-H₂O mixtures. The mixtures were then saturated with nitrogen gas as inert atmosphere. The tubes were placed in a shaking thermostat (Model GEL) for a period of one week till equilibrium reached.

The solubility of CC in each mixture were measured conductmetrically exactly (three times minimum) by using conductmeter of the type YSI model-35 and it was connected with an ultra-thermostat of the type Kottermann-4130. All conductance were measured at 301.15 K. The accuracy of the solubility data is in average of third number aftercoma, as in previous work[4].

3. Results and Discussion

The molar solubility for calcium carbonate (CC) at 301.15 K were measured conductmetrically and the –log S values are cited in Table water, ethanol (EtOH) and their mixtures. The solubility of (CC) in water agreed well with that in literature[5].

The activity coefficients were calculated by the use of Debye-Hückel equation[6].

(1)

Where S is the molar solubility. Their data were tabulated also in Table 1. The solubility product was calculated by the use of equation 2[2].

(2)

Table 1. Molar solubilities (S), activity coefficient (      ), solubility products (pksp) and salvation free energies (      ) for molar solubilities, and free energies CaCO3 (CC) in different ethanol-water mixtures at 301.15 K Xs (mole fraction) ethanol - log S log pKsp k J/mole k J/mole 0 4.2097 - 0.00347 8.4116 48.5026 0 0.031 4.3519 - 0.00337 8.69706 50.1486 1.6460 0.072 4.4087 - 0.00316 8.81073 50.8044 2.3018 0.110 4.5648 - 0.00236 9.1243 52.6121 4.1095 0.171 4.6594 - 0.00196 9.3141 53.7066 5.2040 0.231 4.8249 - 0.00156 9.6459 55.6198 7.1172 0.310 5.0236 - 0.00113 10.0441 57.9159 9.41330 0.553 5.2980 - 0.00075 10.5937 61.0849 12.5823 0.735 5.6528 - 0.00055 11.3042 65.1818 16.6792 1.0 6.1495 - 0.00042 12.29908 70.9185 22.4158

Where S is the molar solubility of (CC) in mixed EtOH-H₂O solvents. pK_sp data are given in Table 1. From these solubility products the Gibbs free energies of solvation and the transfer Gibbs free energies from water to mixed solvents were calculated by using the following equations[4] and[5]. Their values are tabulated also in Table (1).

It was concluded that the Gibbs free energies of transfer increase in positivity by increasing the mole fraction of ethanol in the mixtures. This is due to more difficult solvation in the mixed solvents than that of water. Polar solvents like H₂O or EtOH or their mixtures cannot penetrate (CC) lattice due to the very high heats of formation, free energy, entropy and heat capacity for (CC) salts[7], calcite and aragonite.

Table 2. Molar thermodynamic properties of pure (CC) solid H°f G°f S° Cp° k.J/mol k J/mol J/K.mol J/K.mol CaCO3(calcite) -1206.92 -1128.79 92.9 81.88 CaCO3(aragonite) -1207.13 -1127.75 88.7 81.25

Ethanol is much less polar than water. Since (CC) is insoluble in water. Therefore (CC) would be even less soluble in ethanol.

According to this small solubility, applying some thermodynamic model was done to explain the precipitation of some insoluble salts[8].

Table 3. Molar (VM), van der (VW) and electrostriction volumes (Ve) for (CC) in mixed EtOH-H2O solvents at 301.15 K (in cm3/mol) Xs (EtOH) VM VW Ve 0 100.089 66.158 -33.931 0.03 107.864 71.298 -36.566 0.07 111.0174 73.383 -37.634 0.11 111.452 73.669 -37.783 0.17 111.561 73.742 -37.818 0.231 111.800 73.899 -37.901 0.310 112.107 74.1027 -38.004 0.553 120.825 79.865 -40.960 0.735 121.999 80.641 -41.358 1.0 140.571 92.917 -47.674

The molar volumes (V_M) for (CC) in mixed EtOH-H₂O were calculated by dividing the molecular weight by the exact solution densities and their values are tabulated in Table (3). The packing density (P) as explained by Kim (in ref. 6), i.e., the relation between Van der Waals volume (V_w) and the molar volume (V_M) for relatively large molecules was found to be constant and equal 0.661.

The electrostriction volumes (V_e) which is the volume compressed by the solvent can be calculated by using equation (6) as follows:

All the different volumes for (CC) in mixed EtOH-H₂O solvents were represented in Table (3). The data in Table (3) indicate that the volumes increase by more adding alcohol favoring more energy required for salvation, i.e., less salvation process.