The observation that macrocyclic polyethers form stable complexes with alkali and alkaline earth metal cations has stimulated a great deal of interest in these crown compounds for their possible applications in various branches of chemistry and biology [1,2]. Extensive thermodynamic data [3,4] suggest that the stability of macrocyclic complexes depends on the relative cation and ligand cavity size, the number and spacial arrangements of the ligand bonding sites, the substitution on the macrocyclic ring and the solvent effects.The cyclic polyether 18-crown-6 has six oxygen atoms in the polyether ring, that are bond places of this ionophore [1,5].The aim of the present communication was to determine especially the stability constant of the complex species PbL 2+ , where L is 18-crown-6, in nitrobenzene saturated with water.
Experimental18-crown-6 was supplied by Merck, Darmstadt, Germany. Cesium dicarbollylcobaltate (CsDCC) was synthesized in the Institute of Inorganic Chemistry, ®eþ, Czech Republic. The radionuclide 85 Sr (DuPont, Belgium) was of standard radiochemical purity. Nitrobenzene and all other chemicals (analytical grade, Lachema, Brno, Czech Republic) were used without further purification.In order to obtain the solution of HDCC in nitrobenzene, the solution of CsDCC in this medium (0.2 mol dm -3 ) was twice equilibrated with equal volumes of 15% (v/v) n-propanol in diluted H 2 SO 4 of the concentration of about 1 mol dm -3 . H 2 SO 4 and n-propanol were removed from the organic phase by a ten-fold equilibration with the solution of H 2 SO 4 PbL 2+ (nb) + Sr 2+ (aq) taking place in the two -phase water -nitrobenzene system (L = 18-crownin distilled water (about 1 mol dm -3 ) of an equal volume, followed by two equilibrations with distilled water. The concentration of HDCC in the nitrobenzene solution was determined by neutralization titration (NaOH, bromocresol green) after a ten-fold dilution with ethanol or acetone. The equilibration of the nitrobenzene solution of HDCC with stoichiometric Sr(OH) 2 , which was dissolved in the aqueous solution of Sr(NO 3 ) 2 (0.2 mol dm -3 ), yields the corresponding Sr(DCC) 2 solution in nitrobenzene.The extraction experiments were carried out in 10 cm 3 glass test-tubes fitted with polyethylene stoppers.2 cm 3 of the aqueous solution of Pb(NO 3 ) 2 of the concentration in the range from 2×10 -4 to 2 × 10 -3 mol dm -3 and microamounts of 85 Sr were added to 2 cm 3 of the nitrobenzene solution of 18-crown-6 and Sr(DCC) 2 , whose initial concentrations varied from 1× 10 -4 to 1× 10 -3 mol dm -3 (in all experiments, the initial concentration of 18-crown-6 in nitrobenzene, C L in,nb , was always equal to the initial concentration of Sr(DCC) 2 in this medium, C Sr(DCC) in,nb 2 ). The test-tubes filled with the solutions were shaken for two hours at a temperature of 25 ± 2 °C using a laboratory shaker; under these conditions the equilibrium in the system under study has been established already after approximately 20 minutes of shaking. Then the phases were separated by ce...