1. Introduction

The solubility of any electrolyte in solvents depends on the properties for both solte and solvent.Debye-Huckel recognizes solvents by their bulk properties, namely relative permittivity, viscosity and density of pure solvent.Also the bulk properties of the solvents decrease as the electrolyte concentration increase. On the molecular scale solvents may be classified according to hard and soft donor and acceptor properties of both solvent and solute[1]. Interactions of solvent and solute depend on the electron distribution between donor and acceptor atoms in these substances. Theoretical approaches on Born-Oppenheimer (BO) level take into consideration internal charge distributions of solvent molecules with the help of molecular properties such as partial charges, dipole, quadrupole, higher n-pole moments, polarizabilities, etc. McMillan –Mayer (MM) approches by takingaccount of them intheir potential mean force. Chemical models (CM) on these levels yield importent progress as compared to the classical Debye-Huckel theory with its exclusive columbic, ion-ion interactions in a dielectric medium[2]. Applied solution chemistry successfully uses empirical parameters to describe a solvent’s ability to interact with acceptors such as protons, cations or Lewis acids. Examples of these are Guttmann’s donor number DN and Kemlet and Taft’s B parameter. The ability of a solvent to interact with donor’s is commonly characterizes by Dimroth and Richard’s ET parameter. Mayer Gutmann and Gorger’s acceptor number AN or Taft and Kamlets α parameter. The main parmeter used for the assessment of the polarity and polarizability of a solvent is solvatochromic parameter π*. A commonly used rough classification distinguishes between protic, aporotic and inert solvents. We adopt this classification taking into account the acid- base properties, polarity and polarizability.

On the other hand the solutes can be classified to ionophores or ionogens according to their ions in solutions. Ionophores are substances which in their pure state exist as ionic crystal, where their ions already exist as the component particles within their structure. Dissociation of the ionic crystal yield a first step the solvated cations and anions followed by the formation of solvatedion pairs. Symmetrical electrolytes, associate together to form neutral ion pairs, whereas the ion pairs from unsymmetrical electrolytes are charged. Ionogenes are substances which form ions through chemical reactions, either with solvent molecules or with suitable added species in the solution. In their pure states at the temperature and pressure of investgation, they exist as neutral molecules[3].

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[4]. The solubility of solutes in mixed solvents depends primarily on the solvation of solutes or their constituent ions by the components of solvent mixtures[4]. 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[5].

Removal of sulphate ions from an alkaline medium using solvent extraction was very important to get rid of these hand ions[3]. The aim of the work is to estimate the solubilities of (BS) in the mixed EtOH-H₂O solvents and its solvation parameters. Several treatment methods have been developed to reduce high sulphate concentration from different waste water, i.e, the limestone process, the barium salt process, the cost-effective sulphate removal and Saving process. The barium process is the best as described by precipitation of sulphate as barium sulphate.H₂S and BaS as by product for this last process can be removed by stripping with CO₂ and the barium carbonate is obtained which is the stating material. Therefore barium carbonate is recycled[6]. Giving date about the solubilities and solvation of (BS) can help us in future studies and applications for water purification.

2. Experimental

2.1. Materials

The used barium sulphate (BS) and ethanol (EtOH) were supplied from Merck CO. The saturated solution of barium sulphate (BS) was prepared by dissolving little solid amount in closed test tubes containing different EtOH-H₂O mixtures. The mixtures were then saturated with N₂ gas as inert atmosphere. The tubes were placed in a shaking thermostat (Model GEL) for a period of one week till equilibrium reached.

2.2. Solubility Measurements

The solubility of BS in each mixture were measured conductometrically exact (three minimum times) by using conductometer of the type YSI model-35 and it was connected with an ultra-thermostat of the type Kottermann-4130. All conductances were measured at 301.15 K. The conductivity was easily measured, reproducible and also cheap method. The conductance values were measured directly without waiting, i.e consuming time. The deviation in the experimental conductance’s are very small .Since the solubility of (BS) is sparingly ones, therefore conductivity is very good method for the analytical evaluations. The accuracy of the solubility data is in average of third number after coma, plus or minus as cited in previous work[7].

3. Results and Discussion

3.1. Solubility Results

The molar solubilities for barium sulphate (BS) at 301.15 K were measured conductometrically and the 10 g values are cited in Table 1 in water, ethanol (EtOH) and their mixtures. The solubility of (BS) in water agreed well with that in Literature[8]. The activity coefficients were calculated by the use of Debye-Hückel equation [9].

(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)

Where S is the molar solubility of (BS) 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 equations[4] and[5].

Their values are tabulated also in Table (1).

(3)

(4)

It was concluded that the Gibbs free energies of transfer increase in positivity by increasing the mole fraction of EtOH 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 can not penetrate (BS) lattice due to the very high heats of formation, free energy, entropy and heat capacity for the salt[10].

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

According to this small solubility, applying some thermodynamic model was done to explain the precipitation of (BS) [11].

Table 1. Molar solubilities (S), activity coefficient (), solubility products (pKsp) and solvation free energies () for (BS) in different ethanol-water mixtures at 301.15 k Xs (mole fraction) ethanol - log S log pKsp k J/mole k J/mole 0 4.9788 - 0.00164 9.9543 57.3981 0 0.031 5.0911 - 0.00144 10.1793 58.6954 1.2974 0.072 5.1471 - 0.00135 10.2916 59.3430 1.9449 0.110 5.2143 - 0.00125 10.4261 60.1185 2.7204 0.171 5.3989 - 0.00101 10.7958 62.2503 4.8522 0.231 5.5108 - 0.00088 11.0199 63.5425 6.1444 0.310 5.7065 - 0.00070 11.4118 65.8022 8.4042 0.412 5.9415 - 0.00054 11.88206 68.5138 11.1158 0.553 6.2661 - 0.00037 12.5328 72.2748 14.8767 0.735 6.6857 - 0.00022 13.3718 77.1039 19.7058 1.0 7.2732 - 0.00011 14.5512 83.9015 26.5064

Table 2. Molar thermodynamic properties of pure (BS) solid H°f G°f S° Cp° K J/mol K J/mol J/K.mol J/K.mol BaSO4 -1473.2 -1362.2 132.2 101.95

Table 3. Molar (VM), Van der Walls and electrostriction volumes (Ve) for (BS) in mixed EtOH-H2O solvents at 301.15 K (in cm3/mol) Xs- (EtOH) VM VW Ve 0 23.0 153.352 - 78.648 0.031 2441 159.367 - 81.733 0.072 255.2 168.687 - 86.513 0.110 276.0 182.436 - 93.564 0.171 298.1 197.044 - 101.056 0.231 301.3 199.159 - 102.140 0.310 302.2 199.754 - 102.445 0.412 302.5 199.952 - 102.548 0.553 303.1 200.349 - 102751 0.735 304.0 200.944 - 103.056 1.0 315.2 208.347 - 106.853

3.2. Volumes Results

The molar volumes (VM or V_M) for (BS) 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 represented by Kim (in ref. 9), i.e., the relation between Van der Waals volume (V_w) and the molar volume for relatively large molecules was found to be constant and equal 0.661.

(5)

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

(6)

All the different volumes for (BS) in mixed EtOH-H₂O solvents were represented in Table (3). The data in Table (3) indicate that the volumes increase by more adding alcohol (organic solvent) [12-18] favouring more energy required for solvation.

4. Conclusions

This work provide an analyst a lot of data which help hem in the determination of (BS) in solutions, Since the solubility of barium sulphate is very low, therefore volume measurement can help for the analytical evaluation.