Effects of salt concentration and counterion on stability of alkali ion-18-crown-6 complexes in aqueous and methanolic solution: A conflicting evidence from potentiometric and sodium-23 NMR studies

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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 (KISE) exhibit a very pronounced variation with the counterion, KISENaSCN > KISENaCl > KISENaOH, as well as with the sodium salt concentration, the KISENaSCN/KISENaOH 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, KNMR, are virtually insensitive to the changes of counterion as well as to the salt concentration the overall spread of the KNMR values being less than 3. Resolution for the conflicting evidence is provided in terms of the activity coefficients γML+, γM+, γL differentiating between the concentration (Kconc) and thermodynamic (Kth) stability constants in the relation Kconc = (γM+ γL : γML+) Kth. At variance with the current opinion it is shown that only KISE are clean concentration constants subjected to the γML+/γM+ γL variation, while KNMR are hybrids between the concentration and thermodynamic constant leaning assumedly to the latter (Kth). Concerning the powerful effect of counterion and salt concentration of KISE, it is attributed to a large variation of γML+. No satisfactory explanation can be, however, offered in customary terms of the Debye-Hückel theory.

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