Determination of Activity Coefficients, Osmotic Coefficients, and Excess Gibbs Free Energies of HCl in N,N-Dimethylformamide-Water Mixed Solvent Systems by Potentiometric Measurements Farzad Deyhimi,* ,† Ali Ebrahimi, ‡ Hosein Roohi, ‡ and Khorshid Koochaki § Department of Chemistry, Shahid Beheshti University, Tehran-Evin 19839, Iran, Department of Chemistry, Sistan & Balouchestan University, Zahedan, Iran, and Department of Chemistry, Graduate Faculty Studies, Arak Islamic Azad University, Arak, Iran Mean activity coefficients (γ ( ), osmotic coefficients (), and excess Gibbs free energies (G E ) of HCl in mixed-solvent systems containing (0, 0.1, 0.2, 0.3, and 0.4) mass fractions (w) of N,N-dimethylformamide (DMF) in water were determined at 25 °C using the Pitzer ion-interaction model. These parameters were obtained from the experimental potentiometric data of the galvanic cell Ag|AgCl|HCl(m), DMF(w), H 2 O(1 - w)|H + (glass membrane ISE) over the molality range from dilute solution to about 4 mol/kg in these mixed-solvent systems. Introduction The prediction of thermodynamic properties of various electrolytes in water-organic solvent mixtures is of interest in many chemical applications in industrial and in envi- ronmental processes. The Pitzer ion-interaction model has been successfully used both in aqueous and in mixed- solvent systems for the determination of various thermo- dynamic properties of concentrated pure and mixed- electrolyte solutions. 1-9 In this work, using the Pitzer ion- interaction model, the activity and osmotic coefficients along with the excess Gibbs free energies of HCl in mixtures containing successively (0, 0.1, 0.2, 0.3, and 0.4) mass fractions of DMF in H 2 O were determined at 25 °C. The determination was based on a potentiometric tech- nique using both a pH glass membrane electrode and a Ag/ AgCl electrode. The electrolyte concentration was changed using a standard addition technique over the molality range up to about 4 mol/kg in these aqueous and mixed- solvent systems. Experimental Section Reagents. All reagents were of analytical grade from Merck (Germany) or Fluka (Switzerland). Bidistilled water with an electrical conductivity of less than 1.5 µS‚cm -1 was used for the preparation of electrolytes in pure- and mixed- solvent systems. DMF of analytical grade from Fluka (mass % > 99.5) was fractionally distilled, and only the interme- diate fraction was used. HCl of analytical grade (mass % > 99.5) was also used for the preparation of different primary concentrated stock solutions in doubly distilled water or in DMF-water mixtures. The concentration of HCl in the prepared primary concentrated stock solution was checked by titration with NaOH (“Titrizol” Merck standard). Potentiometric Measurements. The potentiometric cell data were measured using a high input impedance (>1TΩ) Topward multimeter (model 1304, Taiwan, Korea) with (0.01-mV resolution. All measurements were performed under stirring conditions, and the temperature was kept constant at (25 ( 0.1) °C by employing a thermostated bath (Thelco, Precision Scientific Co.) connected to a double- walled electrochemical cell. The electrodes and a glass thermometer ((0.05 °C) were immersed in the solution through a lid preventing solvent evaporation. The pH (glass membrane) electrode was from Metrohm (model 6.0238.000, Switzerland). The Ag/AgCl electrodes were prepared es- sentially as described elsewhere by the electrolysis of Ag wire in 0.1 M HCl versus a Pt electrode. 10 Both the pH and Ag/AgCl electrodes were conditioned overnight in the appropriate (DMF + water) mixed solvent before each series of measurements. In each mixed-solvent system and for each molality, the corresponding stabilized potential value was recorded after about 20 min. Method For a single M ν+ X ν- electrolyte, Pitzer equations for the excess Gibbs free energy (G E ), osmotic coefficient (), and mean ionic activity coefficient (γ ( ) are as follows: 1-2,9 γ ( is the molality-scale mean ionic activity coefficient of the electrolyte M ν+ X ν- ; z + and z - are the respective charges of the ions in electrostatic units; n w is the number of kilograms of solvent; m is the molality of the electrolyte; * Corresponding author. E-mail: f-deyhimi@cc.sbu.ac.ir. Phone: +9821- 2401765. Fax: +9821-2403041. † Shahid Beheshti University. ‡ Sistan & Balouchestan University. § Islamic Azad University. G E n w RT ) f Gx + m 2 (2ν + ν - )B Gx + m 3 [2(ν + ν - ) 3/2 ]C Gx (1) φ - 1 ) |z + z - |f φ + m ( 2ν + ν - ν 29 B φ + m 2 ( 2(ν + ν - ) 3/2 ν 29 C φ (2) ln γ ( ) |z + z - |f γ + m ( 2ν + ν - ν 29 B γ + m 2 ( 2(ν + ν - ) 3/2 ν 29 C γ (3) 1185 J. Chem. Eng. Data 2004, 49, 1185-1188 10.1021/je034184j CCC: $27.50 © 2004 American Chemical Society Published on Web 07/28/2004