Molar solubility, dissociation, association and solvation parameters of saturated o-chlorobenzoic acid solution in various solvents at 298.15 K
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https://www.eduzhai.net Food and Public Health 2012, 2(3): 65-68 DOI: 10.5923/j.fph.20120203.01 Molal Solubility, Dissociation, Association and Solvation Parameters for Saturated O -Chlorobenzoic Acid Solutions in Various Solvents at 298.15 K Esam A. Gomaa Chemistry Department, Faculty of Science, Mansoura University, Mansoura, 35516, Egypt Abstract The molal solubility for saturated solutions of O -chlorobenzoic acid at 298.15 K in various solvents was de- termined. The solvents used are, water (W), ethanol (Et), dimethylsulphoxide (DMSO), acetonitrile (AN), methanol (Me), 1-4, dioxane (Di) and N , N-dimethylformamide (DMF). From the experimental data for solubility, pH , densities, the different volumes, free energies, dissociation constants, association constants and solvation parameters were estimated. Also the free energies of dissociation and association were also evaluated. Other solvation parameters like the solvation numbers were cited here to help explaining the solubility trend.This work give a lot of data for the solubility of orthochlorobenzoic acid which help the biologist for using it as food preserver .The results were also discussed. Keywords Solubility, Dissociation Constants, Association Constants, Different Volumes, Solvation Parameters, Free Energies ,Orthochlorobenzoic Acid 1. Introduction The solubility of an electrolyte is influenced by a wide range of factors , including ion association ,variation in ionic activity coefficients , complexation and temperature. Solubility is an equilibrium property enable to thermodynamic parameters through the standard state free energy. Ion pairing can occur in dilute solutions for many electrolytes, particularly these with multivalent ions and for all electrolytes in concentrated solutions. Ion pairing is generally more pronounced in non-aqueous solvents which have lower dielectric constants than water. In effect, the ion pairs represent a reservoir of electrolyte in the solution and increase the solubility.The complexityof the system increases for unsymmetrical electrolytes or in mixed electrolyte systems. Bjerrum proposed that the motion of ions would be coupled when the energy of attraction between them exceeded the thermal energy. For solely columbic interactions his theory predicts a distance within which the electrostatic attraction between ions is greater than 2kT. Which will be sufficient to couple the motions of the ions. The treatment takes account of only electrostatic interactions and neglects the molecularity of solvent, Nevertheless in low concentration interactions between ions and solvent molecules resulting in ion pair formation. The three commonly * Corresponding author: firstname.lastname@example.org (Esam A. Gomaa) Published online at https://www.eduzhai.net Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved assumed structures are the first in which the ion retains their Individual solvation shells, and so is separated by two solvent molecules. The second in which the ions share some part of their solvation shells so are separated by one molecule and the third where the ions are in contact and share a common solvation shell. The presence of species such creates an experimental difficulty; the different techniques will have different sensitivities to the species present. Thus the conductance will see on the dissociated ions and the presence of ion pairs is determined by difference from experimental molar conductance and that expected for strong electrolyte. The formation of complexes (complexation) provides a route to increased solubility. Several equivalent representations of the speciation in these systems have been used. Many publications have appeared on the behaviour of weak acids in anhydrous solvents. Interesting work has been done by Kolthoff et al.[5,6]. Aleksandrov et al.[7,8] studied the dissociation of salicylic acid in butane-2-one. Kreshkov et al. studied the dissociation of amino acids (as weak) acids) in mixtures of formic and ethylmethylketone and in mixtures of acetic acid-ethylmethylketone. Gomaa et al. studied association, dissociation and hydrogen bonding of salicylic acid in water-N,N- dimethylformamide mixtures from solubility measurements. The aim of this work is to evaluate the solubilities of O-chlorobenzoic acid in different solvents and discuss in detail the solvation parameters for the solubility process for O-chloro benzoic acid which is one of the most widely used preservation materials .O-chloro benzoic acid has the ad- 66 Esam A. Gomaa: Molal Solubility, Dissociation, Association and Solvation Parameters for Saturated O -Chlorobenzoic Acid Solutions in Various Solvents at 298.15 K vantage of low cost, ease of in- corporation into products. Lack of colour and relatively low toxicity. Also knowing the other factors affecting the solubility is very important here. Is the electrostatic energy play important role in the solubility or not. Many authors like Bjerrum and other reported that the electrostatic energy plays important role in the solvation energy. In this work more effort was done to explain the main factors affecting the solubility of orthochlorobenzoic acid in different solvents 2. Experimental The O-chlorobenzoic acid 98 % m.p type Cambarian Chemicals was used. The solvents, ethanol (ET), dimethylsulphoxide (DMSO), acetonitrile (AN), methanol (Me), 1-4 dioxane (Di) and dimethylformamide (DMF) were supplied from Merck. The saturated solutions of O-chlorobenzoic acid were prepared by dissolving it in the solvents used. The solutions were saturated with N2 gas in closed test tubes. The tubes were placed in a shaking water bath of the type assistant for a period of four days, followed by another two days without shaking to reach the necessary equilibrium. The solubility of O-chlorobenzoic acid in each solution was determined gravimetrically by taking 1 ml of the saturated solution and subjecting it to complete evaporation using small aluminium disks heated by an infrared lamp. The pH readings of the saturated solutions were measured using a pH-meter of the type Tacussel/Minis 5000. The densities were measured by using weighing bottle 1 ml and analytical balance (4 digits) of the type Mettler Toledo DA. 3. Results and Discussion Determination of the dissociation constants of organic acids in aqueous and non aqueous solutions by different methods like condutometric method during a period of 100 years was discussed. From time of pioneering works of Arrhenius and Ostwald the conductometric method provide simple method for the determination of the dissociation constants of organic solvents in water. This method is based on the assumption that strong electrolytes are completely dissociated while weak electrolytes attain this state only at infinite dilution. At fixed temperature, an equilibrium exists in solutions of weak electrolytes between ions and undissocited molecules to which the law of mass action and the Ostwald dilution law can be applied. Therefore complete study need more work for this field which is the target of this work. The calculated molal solubilities (m) for O-chlorobenzoic acid saturated solutions were given in Table (1) from at least three average measurements. Also the measured densities and pH values of the acid used are also listed in Table (1). The molar volumes (VM) of benzoic acid were obtained by dividing the molar mass by the densities and their values are listed in Table (2). The packing density as reported by Kim et al. and Gomaa et al., i.e. the relation between Van der Waals volume (VW) and the molar volume (VM) of relatively large molecules (above 40) was found to be a constant value and equal to 0.661. P = VW = 0.661 ± 0.017 (1) VM The electrostriction volume (Ve) which is the volume compressed by the solvent, was calculated by using equation (2) after Gomaa. Ve = VW - VM (2) The molar, Van der Waals and electrostriction volumes of O-chloro- benzoic acid in various solvents at 298.15 K were tabulated in Tables (2). The apparent molar volumes Vφ[14,15] were calculated using equation (3). Vφ = M/do – (d-do/ddo) 1000/m (3) Where M is the molar mass of benzoic acid, m is the concentration, d and do are the densities of saturated solution and pure solvents, respectively. The values of Vφ for O-chlorobenzoic acid in various solvents at 298.15 K are presented in Table (2). Table 1. Molal solubilities (m), densities (d) and pH values for saturated O-chlorobenzoic acid solution in different solvents at 298.15 K Solvent m/(g.mol/kg solvent) d pH H2O 1.3319x10-2 1.00527 6.24 Ethanol (Et) 1.9168 0.95137 2.12 DMSO (Dimethyl sulphoxide) 1.1721 1.2527 3.25 AN (Acetonitrile) 0.6035 0.82307 3.14 Me (Methanol) 1.7169 0.91787 1.89 Di (1,4 dioxane) 3.1219 1.12743 9.01 DMF (dimethylformamide) 3.1431 1.16610 9.01 Table 2. Molar volumes (VM), Van der Waals volumes (VW), electrostriction volumes (Ve) and apparent molar volumes (Vφ) for saturated solutions of O-chlorobenzoic acid in different solvents at 298.15 K (in cm3/mole) Solvent H2O Et DMSO AN Me Di DMF VM 155.780 164.625 152.075 190.287 170.534 138.917 134.310 VW 102.972 108.817 82.674 125.779 112.789 91.824 88.778 -Ve 52.808 55.8087 42.401 64.508 57.845 47.093 45.5329 Vφ -454.940 83.445 46.077 29.793 93.156 123.715 101.718 The activity coefficient was calculated by using the rela- tion log γ± = -0.5062 m . Kass values were calculated from the ratios of asso- ciation constant to dissociation constant (i.e., K1/K2) for the dimers of O-chlorobenzoic acid which form a complex ion ( (HA2−) and hydrogen ion (H+), and the values of K` (where K` is the dissociation constant of the associated acid complex, H2A2) are given by the following equations K` = a2H+/m2 (4) = PaH + 1 log K1 − log m = pH - log γ ± (5) 2 K2 Food and Public Health 2012, 2(3): 65-68 67 K1 K2 = K `Kass (6) Where a is the activity and γ± is the activity coeffi- cient .The values obtained K1/K2, K` and Kass are reported in Table (3). Prediction of electrolyte activity coefficients is one of the classical problems in physical chemistry and is outlined in classical works.The defining characteristic of ions is that they carry a net charge and so the principle interaction be- tween ions are largest contribution to the activity coefficients are columbic .Debye and Hückel solved the problem for a system purely electrostatic interactions between point charges surrounded by a dielectric contnium.Therefore the extended Debye- Hückel equation was applied taking ac- count of the ion size From the activity coefficients ɣ± , calculated using Debye Hückel equation as explained in ref. 16 and from the molal solubility data ,values of K`, K1/K2 and Kass were evaluated following equations 4 – 6[11,12]. The maximum value of Kass was found to be by using Di where water is the least association parameters. Table 3. Log activity coefficients (log γ±), dissociation constants (K`) and association constants for O-chlorobenzoic acid saturated solutions in different solvents at 298.15 K. Solvent H2O Et DMSO AN Me Di DMF log γ± -0.0584 -0.7008 -0.5480 -0.3932 -0.6633 -0.8944 -0.8974 Pa H + 6.2984 2.8208 3.7980 3.5332 2.5533 9.9044 10.1774 K` 2.2361X105 4.1510 3.7980 34.276 2.2116 10.065 10.485 K1 K 2 7.0106X108 1.6097X106 5.4189X107 2.0600X103 6.1380X102 2.5054X1010 4.7288X1010 Kass 3.1352X103 3.8777X105 5.1574X106 60.099 277.536 2.4889X109 4.5100X109 The free energies of dissociation (∆Gd), free energies of association (∆GA), difference free energies (∆∆G) and free energies of solvation (∆Gs) for O-chlorobenzoic acid satu- rated solutions in various solvents were calculated by using the following equations and collected in Table (4). ∆Gd =-RT ln K` (7) ∆Gass = -RT ln Kass (8) ∆∆G = ∆Gass - ∆Gd (9) ∆Gs = -RT pKsp (10) pKsp = -2 log m -2 log γ± (11) Dissociation of electrolytes introduces two solutes, the anion and the cation into solution. In principle, these solutes have individual activity coeffients but, since there is no experiment that allows the measurement of thermodynamic properties of individual ions .The formation of complexes becomes more important at high concentrations of the com- plexing ion and is likely to be more extensive in non-aqueous solvents, partially in dipolar aprotic solvents whereas the solvation of anion is weaker leading to stronger complexa- tion. The solvation volumes for O-chlorobenzaoic acid were evaluated from the difference between Van der Waals of O-chlorobenzoic in various solvents. The Van der Waals volume of O-chlorobenzoic acid in solid state, was calculated from the Bondi method[19,20] and found to be 167.537 cm3/mole. Subtracting this value from Vw in solvents and dividing the results by the molar volumes of the solvents taken from refs.[19-21], n (the solvation numbers) were obtained and is given in Table (5). It was concluded that the solute-solvent interaction increased by increasing ∆∆G and ∆Gs due mainly to the increase of the association parameters in the corresponding solvents.Also it was observed that the association ,dissection and microscopic interactions between solute and solvent is important for any solvent individually, whether it is polar ,aprotic or amphiprotic . Increasing of the solvation numbers favor more solutesolvent interactions between orthocholrobenzic acid and solvents. Also small solvation numbers favour more solute solute interaction or ion pair formation resulting in the decrease of the solute – solvent interactions in the solvent under discussion. Big positive values for ∆∆G and big negative values for ∆Gs indicate also more solute –solvent interactions. Table 4. Free energies of dissociation (∆Gd), free energies of association (∆GA), difference free energies (∆∆G) and free energies of solvation (∆Gs) for O-chlorobenzoic acid saturated solutions in different solvents at 298.15 K (in k Joule/mole) Solvent H2O Et DMSO AN Me Di DMF ∆Gd -30.538 -3.5288 -5.8312 -8.7627 -1.9677 -5.7247 -5.8261 ∆GA -19.9590 -31.9034 -38.3192 -10.1549 -13.9481 -53.6389 -55.1127 ∆∆G 10.9480 -28.3746 -32.4880 -1.3922 -11.9804 -47.9142 -49.2866 ∆Gs -9.582 -2.0728 -2.3740 -3.0355 -4.4507 -6.8814 -6.9123 Table 5. Molar volumes of solvents (Vs), difference in different Van der Waals volumes (∆Vw) and solvation numbers (ns) for O-chlorobenzoic acid saturated solutions in different solvents at 298.15 K Solvent H2O Et DMSO AN Me Di DMF Vs 18.0724 58.6804 71.2995 52.8450 40.7322 86.3552 77.4118 ∆Vw 64.5649 58.7199 84.8629 41.7669 54.7479 75.7129 78.7589 ns 3.5725 1.0006 1.1902 0.7904 1.3441 0.8767 1.0174 REFERENCES  Apelblat , Alexander, .J. Molecular Liquids,95(2002)99-145.  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