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

1985 ◽  
Vol 50 (5) ◽  
pp. 1184-1193 ◽  
Author(s):  
Jiří Závada ◽  
Václav Pechanec ◽  
Jaroslav Zajíček ◽  
Ivan Stibor ◽  
Antonín Vítek
Keyword(s):  
Stability Constants ◽  
Salt Concentration ◽  
Satisfactory Explanation ◽  
Selective Electrode ◽  
Nmr Studies ◽  
Methanolic Solution ◽  
Conflicting Evidence ◽  
Hückel Theory ◽  
Thermodynamic Constant ◽  
Concentration Constants

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.

103Rh-NMR-Untersuchungen an Chloro-Bromo-Rhodaten(III) und Bestimmung der Stabilitätskonstanten im System [RhClnBr6-n]3-, n = 0-6 / 103Rh NMR Studies of Chloro-Bromo-Rhodates(III) and Determination of the Stability Constants in the System [RhCl„Br6-n]3-, n = 0-6

1989 ◽  
Vol 44 (11) ◽  
pp. 1402-1406 ◽  
Author(s):  
W. Kuhr ◽  
G. Peters ◽  
W. Preetz
Keyword(s):  
Nmr Spectroscopy ◽  
High Resolution ◽  
Stability Constant ◽  
Stability Constants ◽  
Trans Isomers ◽  
Nmr Studies ◽  
Downfield Shift ◽  
Overall Stability ◽  
The Stability

By 103Rh NMR spectroscopy the ten compounds of the system [RhCl„Br6_„]3-, n = 0-6 are identified by separate signals. A downfield shift of approximately 160 ppm is observed per substitution of Cl by Br, and the stereoisomers for n = 2, 3, 4 are separated by at least 4 ppm. From the relative intensities of the 103Rh signals in equilibrated solutions, whose total contents of Rh. Cl and Br are known, six individual stability constants are calculated. Their product gives the overall stability constant, indicating [RhBr6]3- to be 36,300 times more stable than [RhCl6]3-. On treatment of [RhBr6]3- with HCl cis/fac isomers are formed stereospecifically, whereas the reaction of [RhCl6]3- with HBr gives trans isomers, n = 2 and 4, containing 20—30% of the cis compounds; only mer-[RhCl3Br3] 3- is obtained pure. The high resolution spectra of [RhCl6]3- and [RhBr6]3- are exhibit five signals each, reflecting the intensity patterns of the most abundant isotopomers within [Rh35Cln37Cl6-n]3-, n = 2-6, and [Rh79Br„81Br6_„]3-, n = 1-5, respectively.

Determination of stability constants by pH titrations: a critical examination of data handling

1978 ◽  
Vol 56 (9) ◽  
pp. 1202-1211 ◽  
Author(s):  
Terrence B. Field ◽  
W. A. E. McBryde
Keyword(s):  
Data Analysis ◽  
Stability Constants ◽  
Input Data ◽  
Equilibrium Constants ◽  
Critical Examination ◽  
Data Handling ◽  
Random Errors ◽  
Selective Electrode ◽  
Methods Of Calculation

Data from simulated titrations (pH, titer, and pM) were generated from literature values of equilibrium constants, and then submitted to various published methods of calculation to regenerate the indicated equilibrium constants. These calculations were done with error-free input data, and the same with imposed random errors. In other experiments data from real titrations were subjected to the same analysis, and additional checks were made on the output of the calculations with an ion-selective electrode as a probe for pM. One method of data analysis was prone to cause errors in the finally recovered constants, and causes for this are suggested.

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