Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

Contrary to expectation, the title effects observed concurrently by potentiometry with ion-selective electrode and by sodium-23 NMR spectroscopy are completely different. The potentiometric stability constants for the sodium ion-18-crown-6 complex (K_ISE) exhibit a very pronounced variation with the counterion, K_ISE^NaSCN > K_ISE^NaCl > K_ISE^NaOH, as well as with the sodium salt concentration, the K_ISE^NaSCN/K_ISE^NaOH ratio being about 1.5, 2.0, 5.0 and 20 in the 0.01, 0.1, 0.5 and 1.0 mol l^-1 aqueous and about 2.0, 12, 600 and 2 500 in the 0.01, 0.1, 0.5 and 1.0 mol l^-1 methanolic solution, respectively. Whereas, the corresponding spectroscopic stability constants, K_NMR, are virtually insensitive to the changes of counterion as well as to the salt concentration the overall spread of the K_NMR values being less than 3. Resolution for the conflicting evidence is provided in terms of the activity coefficients γ_ML⁺, γ_M⁺, γ_L differentiating between the concentration (K_conc) and thermodynamic (K_th) stability constants in the relation K_conc = (γ_M⁺ γ_L : γ_ML⁺) K_th. At variance with the current opinion it is shown that only K_ISE are clean concentration constants subjected to the γ_ML⁺/γ_M⁺ γ_L variation, while K_NMR are hybrids between the concentration and thermodynamic constant leaning assumedly to the latter (K_th). Concerning the powerful effect of counterion and salt concentration of K_ISE, it is attributed to a large variation of γ_ML⁺. No satisfactory explanation can be, however, offered in customary terms of the Debye-Hückel theory.