Competitive Potentiometric Study of Complexation of Some Organoammonium Ions with Selected Crown Ethers in Ethanol Solution Using Ag + Ion as a Probe Hamid Reza Pouretedal † and Mojtaba Shamsipur* ,‡ Department of Chemistry, Shiraz University, Shiraz, Iran, and Department of Chemistry, Razi University, Kermanshah, Iran The complexation of NH 4 + , CH 3 NH 3 + ,C 2 H 5 NH 3 + ,C 6 H 5 NH 3 + (anilinium), and C 5 H 5 NH + (pyridinum) ions with 18-crown-6, dicyclohexyl-18-crown-6, and 15-crown-5 in ethanol solution has been investigated by a competitive potentiometric method using Ag(I) ion an auxiliary ion. The enthalpy and entropy of the complexes studied were determined from the temperature dependence of the stability constants. For all cations used, the stability of the resulting complexes varies in the order dicyclohexyl-18-crown-6 > 18- crown-6 > 15-crown-5, while, for each crown ether studied, the stability order of the resulting organic ammonium ions is NH 4 + > CH 3 NH 3 + > C 2 H 5 NH 3 + > C 6 H 5 NH 3 + > C 5 H 5 NH + . In all cases studied, the complexes are enthalpy-stabilized but entropy-destabilized. Introduction Owing to their many similarities to cyclic antibiotics and biological transport agents, macrocyclic polyethers have been extensively used as interesting model compounds for the study of molecular effects on membrane permeability (Ovchinnikov et al., 1974). Because of the fundamental role of ammonium ion in different biological processes (Izatt et al., 1978; Salisbury and Ross, 1985), the interaction between organic and biogenic ammonium ions and mac- rocyclic ligands has received considerable attention during the past two decades (Izatt et al., 1985, 1991). We have recently reported the thermodynamics of com- plexation of the ammonium ion and several other proto- nated organic amines with a variety of crown ethers and cryptands in nonaqueous and mixed solvents (Ganjali and Shamsipur, 1995; Hasani and Shamsipur, 1993, 1994; Shamsipur and Ganjali, 1997). In this paper we report the thermodynamic study of complexation of ammonium (NH 4 + ) methylammonium (CH 3 NH 3 + ), ethylammonium (C 2 H 5 - NH 3 + ), anilinium (C 6 H 5 NH 3 + ) and pyridinium (C 5 H 5 NH + ) ions with crown ethers 18-crown-6 (18C6), dicyclohexyl- 18-crown-6 (DC18C6), and 15-crown-5 (15C5) in absolute ethanol by a competitive potentiometric method using Ag + ion as a suitable auxiliary ion. Experimental Section Reagent-grade tetraethylammonium perchlorate (TEAP) and silver nitrate (both from Fluka) were used without any further purification except for vacuum-drying over P 2 O 5 . Perchlorate salts of NH 4 + , CH 3 NH 3 + ,C 2 H 5 NH 3 + ,C 6 H 5 - NH 3 + , and C 5 H 5 NH + ions were prepared from the 1:1 interaction of reagent-grade perchloric acid with ammonia, methylamine, ethylamine, aniline, and pyridine, respec- tively. The resulting perchlorate salts were recrystallized three times from triply distilled deionized water and vacuum-dried at 50 °C over P 2 O 5 for 72 h. Macrocyclic ligands 18C6 and DC18C6 (Merck) were purified and dried using previously reported methods (Hasani and Shamsipur, 1993, 1994). Reagent-grade 15C5 (Merck) was used as received. Absolute ethanol (Merck) was used as solvent. The water content was found to be <50 ppm. An Ag + /Ag concentration cell was used to monitor the concentration of silver ion during the potentiometric titra- tion of Ag + ion with a solution of crown ether in the presence and absence of the protonated amines used (Gutknecht et al., 1978). The concentration of free silver ion was measured with a silver electrode, potentials being measured with a digital voltmeter (model 624 Metrohm). The reference electrode was an Ag + /Ag electrode, immersed in a known solution of AgNO 3 (1.0 × 10 -4 M) in ethanol and separated from the test solution by a salt bridge containing 0.05 M TEAP in the same solvent. In all experiments, the cell was thermostated at the desired temperature (0.05 °C, using a Lo-Temprol 154 precision scientific thermostat. All titrations were carried out using a Mettler electronic buret with a precision of (0.001 mL. In all experiments, the ionic strength was kept constant at I ) 0.05 M using TEAP as supporting electrolyte. Results and Discussion To evaluate the stability constants, the following proce- dure was conducted. The silver electrode was placed in the reaction vessel containing 23 mL of 0.05 M TEAP in ethanol. The reference compartment contained 0.05 M TEAP and 1.0 × 10 -4 M silver nitrate in the same solvent. First, a concentrated silver nitrate solution was added gradually, and the cell potential was measured. Plots of emf vs log [Ag + ] were strictly linear with slopes of (60 ( 1) mV/decade. The same solution was then back-titrated with a crown solution, in the presence and absence of the protonated amines, in the same solvent (Figure 1). The stability constants of Ag + complexes, K Ag , with various crown ethers (eq 1) were determined by simple potentiometric titration of Ag + ion with a solution of the ligand. † Shiraz University. ‡ Razi University. Ag + + crown 9 8 K Ag Ag + -crown (1) 742 J. Chem. Eng. Data 1998, 43, 742-744 S0021-9568(97)00241-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/28/1998