Microcomputer-controlled polarographic instrumentation and its use in the determination of stability constants of crown ether complexes

A computer-controlled polarographic system is described, based on a commercially available polarograph interfaced to a microcomputer. Experiments are controlled and monitored entirely from software, including automatic evaluation of the Tast polarograms and addition of solutions to the polarographic cell from a motor burette. The program was written in FORTH, a computer language especially apt for laboratory automation. The system is used in the determination of stability constants of crown ether complexes.     

A polarographic study of the pyridine-copper-chloride complexes in methanol and determination of the stability constants

Copper(I) chloride has been shown to form two complexes with pyridine in anhydrous methanol, CuCiPy and CuClPy(2). Formation constants have been determined and found to be log K(1) = 7.8 +/- 0.3 and log K(2) = 9.45 +/- 0.3. The role of the complexes in the catalytic oxidation of xylenol is discussed.     

A Method for the polarographic determination of the stability constants of mixed complexes

A new method for determining the stability constants of mixed complexes is proposed which is especially appropriate when only the mixed complex 1:1:1 is formed. In order to verify this method, the coordinate system Pb(II)-SCN^?-NO^?₃ has been studied polarographically in aqueous medium of a constant ionic strength of μ = 1.0 M. Using our knowledge of the stability constants of simple complexes and applying this method we obtain β₁₁ = 8 for the mixed complex.     

Stability constants for some divalent metal ion/crown ether complexes in methanol determined by polarography and conductometry

Stability constants in methanol at 25.0°C were evaluated for the complexes of the divalent cations Ca²⁺, Ni²⁺, Zn²⁺, Pb²⁺, Mg²⁺, Co²⁺ and Cu²⁺ with the macrocyclic polyethers 15-crown-5 (15C5), 18-crown-6 (18C6), dicyclohexyl-18-crown-6 (DC18C6) and dibenzo-24-crown-8 (DB24C8). The log K values of the 1:1 complexes were generally in the range 2.1–4.2, which is low in comparison to the values of the corresponding crown ether/alkali metal ion complexes. M₂L complexes were observed for the systems Pb²⁺/18C6, Pb²⁺/DC18C6, Ca²⁺/DC18C6 and Cu²⁺/D18C6, whereas ML₂ complexes were found for Ca²⁺/18C6 and Cu²⁺/18C6. Within the series of complexes studied, there was no clear relationship between cation diameter and hole size.     

Structure and stability of supramolecular crown ether complexes

Despite the fact that the complexation of ammonium cations with ionophores like crown ethers plays an important role in biological and industrial processes, there is still a lack of theoretical methods to reproduce or even predict the host–guest complex structures or their thermodynamic stabilities in an accurate manner. Hence, the development of ionophores has often relied on a trial‐and‐error approach and the synthetic efforts associated with this have been enormous, so far. Therefore, theoretical methods for the reliable prediction of binding affinities of crown ether derivatives with ammonium ions would be an indispensable tool for the rational design of new receptors with tailored properties. Here, we suggest a computationally efficient but still accurate theoretical approach. It is tested for a model system consisting of 18‐crown‐6 ether and an ammonium cation, but is invented for application to much larger complexes. The accuracy of various approximate quantum‐chemical methods, based on density functional theory (DFT) and many‐body perturbation theory, is evaluated against the gold standard CCSD(T) in the basis set limit as internal reference. An important aspect is the consideration of dispersion interactions in DFT methods, for which the dispersion‐correction by Grimme was employed. For all selected methods, the basis‐set dependence of calculated interaction energies was investigated. © 2015 Wiley Periodicals, Inc.     

Studies of crown ether complexes aryldiazonium ion complexes

DNMR studies show that for complexation of aryldiazonium salts 21-crown-7 is the preferred host.     

Affinity capillary electrophoresis employed for determination of stability constants of antamanide complexes with univalent and divalent cations in methanol

Affinity capillary electrophoresis (ACE) and pressure‐assisted ACE were employed to study the noncovalent molecular interactions of antamanide (AA), cyclic decapeptide from the deadly poisonous fungus Amanita phalloides, with univalent (Li⁺, Na⁺, K⁺, and NH₄⁺) and divalent (Mg²⁺ and Ca²⁺) cations in methanol. The strength of these interactions was quantified by the apparent stability constants of the appropriate AA‐cation complexes. The stability constants were calculated using the nonlinear regression analysis of the dependence of the effective electrophoretic mobility of AA on the concentration of the above ions in the BGE (methanolic solution of 20 mM chloroacetic acid, 10 mM Tris, pH_MeOH 7.8, containing 0–50 mM concentrations of the above ions added in the form of chlorides). Prior to stability constant calculation, the AA effective mobilities measured at actual temperature inside the capillary and at variable ionic strength of the BGEs were corrected to the values corresponding to the reference temperature of 25°C and to the constant ionic strength of 10 mM. From the above ions, sodium cation interacted with AA moderately strong with the stability constant 362 ± 16 L/mol. K⁺, Mg²⁺, and Ca²⁺ cations formed with AA weak complexes with stability constants in the range 37–31 L/mol decreasing in the order K⁺ > Ca²⁺ > Mg²⁺. No interactions were observed between AA and small Li⁺ and large NH₄⁺ cations.     

The simultaneous determination of the stability constants of protonated and unprotonated complexes in solution

A method is given for the simultaneous calculation of the stability constants of complexes of the type, MA, MA₂, MHA, given the overall composition of the solution and the pH. Iterative calculation of activity effects enables constants to be obtained when the ionic strength is not kept constant.     

Precise polarographic determination of the stability constants of cadmium and lead with oxalate and sulphate

Precise polarographic determinations of the stability constants of some complexes of lead and cadmium with oxalate and sulphate were made using differential-pulse polarography, normal- pulse polarography and direct-current polarography at three different concentrations of the metal ions in the range 1 × 10^?4 to 9 × 10^?8 M. The stability constants, evaluated with the DeFord-Hume procedure, at the different metal ion concentrations and using the different techniques are in good agreement with each other and with literature values. In the calculations corrections were made for changes in liquid-junction potentials and the effect of alterations in the diffusion coefficient of the reactant was also considered for diffusion towards a spherical electrode. For metal ion concentrations above approximately 10^?6 M the precision of the determinations of the half-wave potentials and of the calculations of the stability constants approaches that commonly obtained in potentiometric measurements on similar systems.     

Radiometric determination of stability constants of mercury species complexes with L-cysteine

Liquid-liquid distribution method has been applied for the determination of the composition and stability constants of mercury species with L-cysteine /H₂L/. In the pH range 2–8, mercury /II/ forms a complex Hg/HL/₂ with very high stability constant /lg ₂ = 40.0/, whereas methylmercury and phenylmercury form complexes CH₃HgHL /lg ₁ = 15.56/ and C₆H₅HgHL /lg ₁ = 16.5/. At pH13, anionic complexes HgL ₂ ^2– /lg ₂ = 42.7/, CH₃HgL^– /lg ₁ = 16.58/ and lg ₁ = 17.7 are formed.     

Catalytic-thermometric determination of stability constants of complexes using the iodine-azide reaction

A catalytic-thermometric method for the determination of the stoichiometry and stability constants of complexes with sulphur-containing ligands is proposed. The method is based on the inhibition of the catalytic activity of the ligands on the iodine-azide reaction when a metallic ion is present. A study of the errors associated with the method has been performed and used to improve experimental design. The conclusions have been applied to the determination of the stoichiometry and the stability constants of the Ni(II)-DDTC and Ni(II)-PDTC complexes.     

Computerized conductometric determination of stability constants of complexes of crown ethers with alkali metal salts and with neutral molecules in polar solvents

A computerized conductometric procedure for the determination of stability constants of the complexes of crown ethers (15-crown-5, benzo-15-crown-5 and 12-crown-4) with alkali metal salts in polar solvents is described, based on a microcomputer-controlled titration system. For the control of the experiments from software, a modular computer program was written in FORTH computer language. The procedure is especially suitable for the study of 1:2 metal ion/ligand complexes, which occur frequently with the compounds used. For the study of the interaction between crown ethers and neutral molecules, an indirect procedure is outlined.     

Ultrafiltration and determination of Zn- and Cu-humic substances complexes stability constants

This study exhibits that size fractionation of humic substances (HS) and their metal complexes by ultrafiltration is an efficient procedure for simultaneous determination of stability constants. Using sequential-stage ultrafiltration and a radiotracer technique the HS–Cu and HS–Zn complexes studied can gently be size-fractionated and their free metal fractions simply be discriminated. The conditional stability constants Ki obtained for size fractions of these HS metal complexes exhibit a clear molecular size dependence. Accordingly, the highest Ki values (6.6 for Zn and 6.4 for Cu) are found in the HS fractions of >105 kDa. Moreover, the overall stability constants K found for Cu (log K=5.5) and Zn complexes (log K=4.5) of the aquatic HS complexes studied are quite comparable to those reported in the literature.     

The determination of the ionisation constants of polymaleic acid and the stability constants of its complexes with copper

The ionisation constants of unfractionated polymaleic acid, H₃A, a synthetic analogue of fulvic acid, and the stability constants of its copper complexes were determined by differential pulse polarography. The ionisation constants are: pK₁ 5.16, pK₂ 7.04 and pK₃ 9.82 at 20° C and ionic strength 0.12 M. The stability constant of the CuHA complex, which is the predominant species at pH 5–9, lies in the range log (CuHA) = 7.15–8.7; the mean value is 8.17.     

Spectrophotometric determination of sodium by ion-pair extraction with crown ether complexes and monoanionic dyes

A highly selective photometric procedure for the determination of sodium in blood serum is proposed, based on the ion-pair extraction of a sodium—cryptand complex. A comparative study of cryptands with various cavity sizes, different pairing anions and extraction solvents is described. The use of cryptand 211, picrate and toluene, as ligand, pairing anion and extraction solvent, respectively, allows sodium to be determined at the μg ml^?1 level. A 350-fold amount of potassium (by weight) does not interfere, nor do other alkali metals. Possible interferences by multivalent cations are masked by EDTA.     

Data analysis in the determination of stoichiometries and stability constants of complexes.

The determination of stoichiometries and stability constants of complexes by means of UV-visible spectrophotometry applying traditional methods does not evaluate the quality of the values obtained, since the classic application of these methods does not provide coverage intervals. However, the use of chemometric techniques in different steps of the application of these methods makes it possible to obtain not only a real value of the characteristics of the complexes but a validation of such a value. In this paper a methodology is proposed that combines some traditional methods, three different regression models (LMSR, LSR and LSPR) and a small number of mathematical algorithms. This methodology is able to estimate, in a simple and rigorous way, the stoichiometry and stability constant of a complex and its corresponding uncertainties.     

The stability constants of TAR and PAR complexes

A discussion is given of the problem of ascribing dissociation constants in the hypothetical situation in which the protons of the hydroxy groups of TAR and PAR dissociate in the reverse of the normal order, and it is suggested that the o-hydroxy group should be regarded as intrinsically as acidic as the p-hydroxy group, and that the change in pK due to the internal hydrogen 'bonding is probably less than one unit.     

Spectrophotometric determination of the dissociation constants of crown ethers with grafted acridone unit in methanol based on Benesi-Hildebrand evaluation

As potential chromo- and fluorophores for optical sensors a number of acridone grafted crown ethers containing NO2 group(s) and/or Br or Cl atom(s) in the aromatic rings (compounds) were studied by spectrophotometric method. In the first step of this work the acid-base and complexing properties of these compounds as well as those of the acridone, thioacridone and 4,5-dinitroacridone were investigated. Compounds proved to be very week acids and therefore the conventional spectrophotometric method based on the measurement of the ratio of the protonated/deprotonated forms of the compounds was not applicable for the determination of the dissociation constants (pKa values). Thus, a new spectrophotometric approach was elaborated for the pKa determination of these compounds, which is based on spectrophotometric titration in methanol with strong base and the titration results were evaluated by the Benesi-Hildebrand method. (In the studies with TEAOH, TMAOH and LiOMe, the complex formation between the ionophores and the cations of the bases could be excluded.) As it was expected, the experimentally determined pKa values depended on the nature of the substituents of the acridone moiety and pKa values ranged between 12.6 and 14.9. The lower pKa value of thioacridone compared to the acridone can be explained by the larger size of the sulfur atom. The outstandingly larger pKa value for 4,5-dinitroacridone can be attributed to the formation of intramolecular hydrogen-bonds between the ortho-position nitro group and the NH proton. As a trend, the strong electron withdrawing nitro substituents decreased considerably the pKa values of compounds and compared to that of the halogen (Cl and Br) atoms (for example compounds and) of weaker withdrawing effect.