Estimation of Association Constants of Macrocyclic Ethers by Potentiometry. Part V. Na þ Binding in Dioxane/Water (80/20) U ¨ MIT C ¸ AKIR a, *, H. I ˙ BRAHIM UG ˘ RAS ¸ a and C ¸ AKIL ERK b a University of Balikesir, Faculty of Art and Science, Department of Chemistry, 10100, Balikesir, Turkey; b Istanbul Technical University, Faculty of Science, Department of Chemistry, Maslak, 34469, Istanbul, Turkey Received (in Austin, USA) 16 July 2002; Accepted 7 November 2003 The various association constants, b nm and K nm , of NaCl and NaClO 4 with [12]crown-4, [15]crown-5, [18]crown-6, benzo[15]crown-5, benzo[12]crown-4, dibenzo[18]- crown-6 and dinaphtho[18]crown-6 have been estimated in a mixture of dioxane/water (80/20) using a sodium ion selective electrode (ISE). The constants have been estimated using 1=ðK nm ½L o  n1m21 Þ 5 ð1 2 nP 0 Þ n  ð1 2 mP 0 Þ m =P 0 ; where P 0 , the mole fraction of complexed Na 1 in the presence of identical cation/ligand concen- trations, is obtained for different aggregation– dissociation possibilities. The association constants of the complexes are reported and discussed in terms of the multiple composition due to strong dioxane/water binding. The binding mechanism is shown to depend on the free cation and counter ion that are formed by different ratio macrocycle–metal complexes in dioxane/water (80/20) solutions. Keywords: Macrocycles; Ion binding; Association constants; Potentiometry; ISE; Na þ INTRODUCTION The cation association of macrocycles in solution has been widely investigated [1,2]. However, the cationic interactions of macrocycles are still open to further investigation with respect to thermodynamic para- meters and the binding mechanisms of coordination tendency and the radii of the cation as well as the size and conformational ability of the macrocycle [3 – 7]. We have been working on the cationic recognition of macrocycles using different methods [8–10]. In recent studies we have reported Na þ and K þ complexing of some crown ethers in 50/50 dioxane/water using a sodium ion selective electrode (ISE) [11 – 15]. In the present work we have studied the association constants of NaCl and NaClO 4 complex- ing with [12]crown-4, [15]crown-5, [18]crown-6, benzo[15]crown-5, benzo[12]crown-4, dibenzo[18]- crown-6 and dinaphtho[18]crown-6 in a dioxane/ water (80/20) solvent mixture using a sodium ISE. The binary water/dioxane mixture as solvent is used to enhance the power of macrocycle–cation inter- actions with the maximum solubility of the aromatic macrocyclics in the presence of the smallest amount of water. Previously reported studies on the association constants of macrocycles with oxygen donor com- plexes used various experimental and calculation methods in different solvents [1–7,16–21]. We recently estimated the various association constants, b nm , in an equilibrium with an n/m ratio of a macrocycle ether [L] and a cation [A þ ] to form a complex [A n þ L m ] in solutions with identical initial concentrations, ½A þ o ¼½L o ; using the linear regression analysis of an equation depending on the mole fraction, P 0 , of the complex ½A n þ L m ; see Eqs (1) and (2) [8–15]. nA þ þ mL * ) A þ n L m ð1Þ b nm ¼½A þ n L m =½A þ  n ½L m ð2Þ The mole fraction of a complexed ligand P ¼½A þ n L m ={½A þ  2 ð1 2 mÞ½A þ n L m } is, in general, given by P 0 ¼ P=½1 þðm 2 1ÞP for simplicity [11–14]. Accordingly, the multiple association con- stants, b nm , of a complex of n/m stoichiometry in solution can be obtained from the slope, 1/b nm , of the plot of 1=½A þ o  nþm21 versus ð1 2 nP 0 Þ n ð1 2 mP 0 Þ m =P 0 ; Eq. (3). However, if n ¼ 1; m ¼ 1 and P ¼ P 0 ; then ISSN 1061-0278 print/ISSN 1029-0478 online q 2004 Taylor & Francis Ltd DOI: 10.1080/10610270310001645841 *Corresponding author. E-mail: ucakir@balikesir.edu.tr Supramolecular Chemistry , April 2004, Vol. 16 (3), pp. 193–197