Archive of SID Journal of Applied Chemical Research, 9, 7-12 (2009)
Determination of Water-Ethanol Mixtures Autoprotolysis Constants and Solvent Effect M. Faraji*1, A. Farajtabar2, F. Gharib3 1 Department of Chemistry, Islamic Azad University, Babol branch, Babol, Iran. 2 Department of Chemistry, Islamic Azad University, Jouybar branch, Jouybar, Iran. 3 Department of Chemistry, Shahid Beheshti University, Tehran, Evin, Iran. *
[email protected] (Received 5 Dec. 2008; Final version received 9 March. 2009) Abstract The autoprotolysis constants (pKap) of water/ethanol mixtures containing 0-90 % v/v of ethanol have been determined at 25 oC and 0.1 M Ionic strength using potentiometric method. Ionic strenght of each mixture was maintained by NaClO4 solution. The results indicate that water-ethanol mixtures are more basic media than pure water and the pKap value of this media increases with addition of ethanol. The influence of the dielectric constant of investigated mixture solvent on the autoprotolysis constant was described employing Yasuda-Shedlovsky procedure. Keywords: Water-ethanol mixture, Solvent, Autoprotolysis constant, Dielectric constant.
Introduction The aqueous organic solvent mixtures such as water-ethanol, has proven appropriate reaction media in different fields of chemistry due to specific properties and better ability to dissolve more compounds than pure solvents [1]. The availability and diversity of these reaction media is storngly increased from the combination of pure water and alcohol solvents in binary mixtures. Among the thermodynamics properties of solvent, autoprotolysis constant (pKap) is one of the most important properties and knowledge of this parameter is considered as fundamental concept in application of the solvent. The pKap value of solvent indicates the range of pH of media and its importance for standardization of pH measurements have been described for both organic solvents and aqueous organic solvent mixtures [2,3]. The autoprotolysis constant, is a particularly important criterion for the solvent selection, since determines the acidic or basic region available in the solvent used. The smaller the autoprotolysis constant, the greater the range of acid or base strengths that can exist in a solvent and the greater the likelihood that it will be a differentiating solvent. Hence, acid-base titrations are best carried out in solvents with small autoprotolysis constant values.
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M. Faraji et al., J. Appl. Chem. Res., 9, 7-12 (2009)
Different methods were employed for the determination of pKap. Among them, potentiometric titration is fast, reproducible and accurate method for measuring autoprotolysis constant. This method is based on the measurement of e.m.f data of acidic and basic region of potentiometric titration process [2, 4-6]. Therefore, pKap can be calculated, considering standard potential of both acidic and basic region of titration curve. In mixtures of water and a miscible organic solvent such as ethanol, the dielectric constant of mixed solvent changes considerably with the proportion of ethanol [7]. In repect to electrostatic interactions, the influence of dielectric constant of solvent can be used for elucidation of solvent composition effect on chemical equilibrium such as autoprotolysis constant. The objective of this study is the determination of autoprotolysis constant of different waterethanol mixtures involving 0-90 valume percent of ethanol using potentiometric method. Relationship between the autoprotolysis constants and dielectric constant of mixtures, over the whole experimental range studied, were examined and Yasuda-Shedlovsky plot [8, 9] was used to correlate these parameters. Experimental Ethanol was purchased from Merck and was used as recieved without further purification. Stock solutions of NaOH and HCl were prepared from Titrazol solution (Merck) and its concentration was determined by titrations. Double-distilled water with conductivity equal to 1.3 +_ 0.1 mW-1 cm-1, was used. Analytical reagent grade sodium perchlorate was supplied from Merck and used without further purification. Working solutions of HCl and NaOH were prepared in water-ethanol mixtures. All mixtures were prepared by volume and concentrations of HCl and NaOH were kept constant at 0.01 M and 0.1 M respectively. Potentiometric measurements was carried out in a double-walled thermostated reaction vessel at 25 °C and the ionic strength of mixtures was maintained at 0.1 M with sodium perchlorate. A Jenway research potentiometer, model 3520, with a combined pH electrode was used for e.m.f measurement in potentiometric titrations of acidic solution mixtures. For each experiment, double-walled reaction vessel, was charged with 2 ml of stock solution of hydrochloric acid, and the required amount of ethanol and sodium perchlorate were diluted with double-distilled water to 20 ml. The solution was titrated with small addition of the sodium hydroxide solution with same proportion of ethanol and the same ionic strength. E.m.f readings of solution were taken after each addition of titrant when stabilization of solution potential was achieved. This stabilization criterion was 0.2 mV within at least 2 min. duration. e.m.f data versus added volume of titrant in both acidic and basic region of titration were used for the
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determination of autoproyolysis constant of water-ethanol mixtures. Results and Discussion The ionization process in mixed solvent such as water-ethanol mixture can by presented by RH + RH
RH2+ + R-
(1)
In equation (1), RH2+ and R- are solvated proton and produce ions of solvent respectively. Therefore the conditional or stoichiometric autoprotolysis constant of water-ethanol mixture obtained from potentiometric titration will be according to equation(2): Kap = [RH2+][R-]
(2)
Kap, [RH2+] and [R-] are the stoichiometric autoprotolysis constant, the concentration of solvated proton and the concentration of lyate ion, respectively. In potentiometric determination of the stoichiometric autoprotolysis constant, the titration curve can be divided into two acidic and basic regions. In acidic region, the solution potential at 25°C is given by following equation. E = E°acidic – 59.16 log gRH2+ -59.16 log [RH2+]
(3)
Where E°acidic is the specific constant of the potentiometric cell in the acidic range that includes the standard potential of the glass and reference electrodes and liquid junction potential. gH+ is activity coefficient of solvated proton in solution. It is assumed that at constant ionic strength of the solution under titration, activity coefficient of the solvated proton remains unchanged. Therefore E´°acidic can be easily calculated from measured e.m.f and concentration of solvated proton in every titration point of acidic region using linear regression. In basic region of titration, by eliminating [RH2+], and taking into account equation (2), the function for e.m.f takes the form of : E = E´°basic + 59.16 log [R-]
(4)
Where E´°basic is the specific constant of the potentiometric cell in the basic range that includes standard potential of the glass electrode, standard potential of the reference electrode, liquid junction potential and the activity coefficients. Similar to acidic region, the value of [R-] can be calculated from known total analytical concentration of hydrochloric acid in vessel solution and the known total analytical concentration of added sodium hydroxide solution. Considering equation (4), E´°basic can be simply calculated from measured e.m.f and concentration of lyate ion in every titration point of basic region using linear regression. At the end, the stoichiometric outoprotolysis constant of water-ethanol mixtures at 25°C can be obtained by means of the known relation [10]. pKap = (E´°basic - E´°acidic)/59.16
(5)
The autoprotolysis constant values of water-ethanol mixtures involving 0 to 90 volume percent
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of ethanol, expressed in log unit, are summurized in Table1 and plotted versus volume percentage of ethanol in Figure 1.
17.0 16.5
pKap
16.0 15.5 15.0 14.5 14.0 13.5 0
20
40
60
80
100
Volume percentage of ethanol
Figure 1. Illustration of the experimental values of pKap versus the volume percentage of ethanol in different mixed solvents.
Relationships between pKap of water-ethanol solutions and dielectric constant ( er ) of the solvent were investigated using Yasuda-Shedlovsky approach [7, 8]. Yasuda-Shedlovsky approach is extrapolation method based on Born equation [11] and Bjerrum’s theory [12] of ion salvation. This extrapolation method was successfully used to determine the aqueous dissociation constant of low miscible compounds in water where the success of potentiometric method to direct determination of aqueous pKa is sometimes dubitable by poor aqueous solubility [13]. Considering this technique, the plot of pK + log [H2O] against A + B/er leads to a straight line where empirical parameters A and B are the intercept and the slope of the plot respectively. In present work, dielectric constant values for different aqueous mixtures of ethanol (from reference 7) are shown in Table 1. Table 1. Autoprotolysis constants and the dielectric constants of different water-ethanol solution mixtures in 25 °C.
a
ethanol % (v/v)
mole fraction of ethanol
pKap
era
0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00
0.00 0.03 0.07 0.12 0.17 0.24 0.32 0.42 0.55 0.74
13.78 14.02 14.21 14.35 14.50 14.65 14.87 15.23 15.86 16.62
78.56 73.95 69.05 63.85 58.36 52.62 46.71 40.73 34.84 29.19
The values of er were obtained from ref [7].
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The pKap + log [H2O] values were plotted against the reciprocal of the dielectric constant of solvent mixture in Figure 2. As the Figure 2 shows, a linear relationship with correlation coefficients of more than 0.98 is observed pKap + log [H2O] = (14.635 ± 0.08) + (78.183 ± 3.78)*(1/er)
(6)
Analysis of the equation 6 shows that the pKap of mixture solvent decreases with increasing ethanol concentration. The linearity of the relationship indicates that, the electrostatic interaction in dielectric constant form is an important parameters for elucidation of solvent effect over the whole range of this experimental solvent composition.
pKap + log[H2O]
17.50
17.00
16.50
16.00
15.50 0.01
0.015
0.02
0.025
0.03
0.035
0.04
1/H r
Figure 2. Illustration of pKap + log [H2O] values versus the reciprocal of the dielectric constant in different mixed solvents.
The autoprotolysis constants of media in different water-ethanol mixture were measured by potentiometric method. These results indicate that pKap of media increase as the percentage of ethanol increased. When Yasuda-Shedlovsky plot was used, significant linear relationship between pKap and dielectric constant of solvents was reached. Therefore for each water-ethanol mixture in the experimental range of 0-90 volume percentage of ethanol, the derived equation 6 can be effectively used to calculate pKap values. Acknowledgements The authors gratefully acknowledge the financial support form the Research Council of Islamic Azad University, Babol branch. References [1] L.Z. Benet, J.E. Goyan, J. Pharm. Soc., 56, 665 (1967). [2] E.P. Serjeant, Potentiometry and Potentiometric Titrations, Wiley, New York (1984). [3] T. Mussini, A.K. Covington, P. Longhi, S. Rondinini, Pure Appl. Chem., 57, 865 (1985). [4] K. Izutsu, Acid-Base Dissociation Constants in Dipolar Aprotic Solvents, IUPAC Chemical
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M. Faraji et al., J. Appl. Chem. Res., 9, 7-12 (2009)
Data Series No. 35. Blackwell Scientific Publications, Oxford (1990). [5] S. Rondinini, P. Longhi, P.R. Mussini, T. Mussini, Pure Appl. Chem., 59, 1693 (1987). [6] K. Izutsu, Electrochemistry in Nonaqueous Solutions, Wiley, New York (2002). [7] G. Akerlof, J. Am. Chem. Soc., 54, 4125 (1932). [8] M. Yasuda, Bull. Chem. Soc. Jpn., 32, 429 (1959). [9] T. Shedlovsky, In: B. Pesce (Ed.), Electrolytes, Pergamon, New York (1962). [10] J. Tencheva, G. Velinov, O. Budevsky, J. Electroanal. Chem., 68, 65 (1976). [11] M. Born, Z. Phys., 1, 45 (1920). [12] N. Bjerrum, E. Larsson, Z. Phys., 127, 358 (1927). [13] A. Avdeef, et al, J. Pharm. Biomed. Anal., 20, 631 (1999).
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