Potentiometric determination of aminal stability constants

Potentiometric titration was used to determine the logarithms of the stepwise equilibrium constants for the species formed between morpholine and formaldehyde in aqueous solution, ionic strength 0.5 and 2.5M (KCl) at 25 degrees C. The instrumental and computational techniques developed for metal-ligand stability constant determination were applied. Formaldehyde is equivalent to the metal-ion and is represented by M while neutral morpholine is equivalent to the ligand and is represented by L. The stability constants of the following equilibria were determined by non-linear regression (figures in parentheses are at ionic strength 2.5 M KCl): M + L left arrow over right arrow ML (hemi-aminal) logK(1) = 2.90 +/- 0.02 (2.980 +/- 0.004); ML + L left arrow over right arrow ML(2) (bis-aminal); log K(2) = 1.3 +/- 0.2 (1.41 +/- 0.07); MLH left arrow over right arrow ML + H(+) (protonated hemi-aminal) pK(a) = 5.87 +/- 0.01 (6.411 +/- 0.005); ML(2)H left arrow over right arrow ML(2) + H(+) (protonated bis-aminal) pK(a) = (7.6 +/- 0.2). the pK(a) of the protonated bis-aminal could only be determined at the higher ionic strength. The results are in good agreement with reported values determined using the classic formol titration. The automated titration system acquired the full time course of the pH change upon each titrant addition allowing a kinetic analysis to be performed as well as an equilibrium analysis. The forward and reverse rate constants for M + L left arrow over right arrow ML were 0.77M(-1) sec(-1) and 8.1 x 10(-4) sec(-1). respectively.     

Potentiometric determination of successive stability constants of ethylenediamine complexes of several metals in dimethylsulphoxide

The stability constants of the complexes of silver(I), zinc(II), cadmium(II) and manganese(II) with ethylenediamine in dimethylsulphoxide at 25 degrees and ionic strength 0.1Mhave been determined potentiometrically. The stability constants are consistently larger in DMSO than in water as expected from the difference in dielectric constant for the two solvents.     

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.     

Potentiometric determination of stability constants of Lanthanon(III) complexes with some Schiff bases and benzothiazolines

The protonation constants of five Schiff base and two benzothiazoline type ligands and stability constants of their complexes with six lanthanide ions were determined by potentiometrically in ethanol-water solution (1:1, v/v) at 25 +/- 0.1 degrees C. The Schiff base-type ligands were salicylidene 2-iminopyridine (SAPy), salicylidene-5-methyl-2-iminopyridine (SAPyMe), salicylidene-5-chloro-2-iminopyridine (SAPyCl), 2-(2-pyridylmethyleneamino) phenol (PyOH), 2-(2-quinolylmethyleneamino) phenol (QuOH) and the benzothiazoline-type ligands were pyridine-2-car-bozaldehydebenzothiazoline (PyS) and quinoline-2-carboxaldehydebenzothiazoline (QuS). The order of stability constants was found to be for metal ions La(III) < Pr(III) < Nd(III) < Eu(III) < Ho(III) < Yb(III), and for ligands SAPyCl < SAPy < QuS < QuOH < PyS < PyOH < SAPyMe. The FORTRAN programs PKAS and BEST were used for the calculation of protonation constants and stability constants, respectively.     

Potentiometric determination of the dissociation constants of an asymmetric sorbent containing l-proline, and the stability constants of its Cu(II) complexes

The dissociation constants of the carboxyl groups (pK(a1) = 2.2, n = 1.8) and amino groups (pK(a2) = 9.5, n(2) = 1.6) of a sorbent prepared by reacting l-proline with a cross-linked chloromethylated styrene polymer have been determined by potentiometric titration. The potentiometrically measured stability constants of the Cu(II) complexes of the resin (logbeta(1) = 6.9 and log beta(2) = 12.4) were found to be close to the values for the Cu(II) complexes of N-benzyl-l-proline. For complexed resins of alpha-amino-acid type the pH-values of decomplexation do not appear to be directly correlated with the stability constants.     

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.     

Potentiometric determination of boron in graphite microspheres for nuclear engineering

The well-known acidimetric determination of boron via the boric acid-mannitol complex is applied to the analysis of boron-loaded graphite microspheres used in nuclear engineering. After the sample is accurately ground, it is fused with an oxidizing alkaline flux. The melt is dissolved with warm water, the solution is acidified to completely remove carbon dioxide and the boric acid is titrated potentiometrically with carbonate-free sodium hydroxide after its complexing with mannitol.The determination of at least 0.2% boron in graphite is carried out with an average error of less than 1%.     

Redox method for the determination of stability constants of some trivalent metal complexes

Stability constants determination of very stable metal complexes using the redox method, based on the equilibrium of Fe (3+)Fe (2+) followed by a couple of platinum/reference electrodes, was undertaken and tested to complexes of some trivalent metal ions with well known polyaminopolycarboxymethylated linear ligands (edta, nta, cdta, dtpa and ttha) and also to some new macrocyclic ligands. SUPERQUAD program was used for the calculations, after adaptation of the experimental data. The method proved to be very useful for Fe(3+) and In(3+) complexes, if no polynuclear complexes are formed or/and if the kinetics of the complexation reaction is not very slow. However, for the Ga(3+) complexes the applicability of this method is very limited and the competition with OH(-) using the displacement reaction which occurs at pH higher than 6 with formation of Ga(OH)(4)(-) seems to give more accurate results. A complete data of stability constants for the case of the complexes of ttha with In(3+) is given.     

Potentiometric determination of autoprotolysis constants of solvents

A new method is proposed for determination of autoprotolysis constants by titration of a weak acid with strong base or strong acid with weak base, and applied to water and ethylene glycol (EG). The constants found were pK(w) = 13.90-13.97, and pK(EG) = 15.4 +/- 0.2.     

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.     

Use of ion exchange for the determination of stability constants of metal-humic substance complexes

Humic substances (HSs) occur throughout the ecosphere in soils, waters and underground systems. The strong complexation of HSs is of importance in the migration of radionuclies in geological media. Renewed interest in stability constants of complexes of radioelements and radionuclides with humic and fulvic acids has been generated by problems associated with the nuclide migration in the environment. Use of the ion exchange method for the determination of conditional stability constants of metal-HS complexes was examined and reviewed. The complexation of HS to metal ions cannot be described in rigorous mathematical terms because of the ill-defined nature of HSs in contrast with the complexation of single ligands. Furthermore, the advantages and disadvantages of Schubert's and, Ardakani-Stevenson's, curve fitting methods were discussed. The great stabilities of HS complexes to rare earths (Yb(III), Tb(III), Eu(III), Gd(III)), americium(III), cobalt(II), uranyl(VI) and thorium(IV) were revealed.     

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.     

Potentiometric determination of the stability constants of a model (Na + K)ATPase complex

Formation constants for the binary and ternary Mn(II)-nitrilotriacetic acid (NTA) and adenosine triphosphate (ATP) which model the action of (Na⁺ + K⁺) ATPase have been determined at 25°C and I = 150 mmole dm^-3 NaCl. The results are interpreted in terms of the known stabilities of the enzyme complexes and it is concluded that metal-ion chelation of ATP alone is not enough for hydrolysis to occur. A substantial stabilisation of the ternary complex occurs, possibly through bridging sodium ions.     

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 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.     

Potentiometric and spectrophotometric determination of the dissociation constants of cefetamet

Summary The dissociation constants of cefetamet-Na have been determined using potentiometric titrations and spectrophotometry. The investigations were carried on in water solutions at constant temperature and ionic strength, and at differentH ₀ andpH values. Potentiometric investigations were performed at three different temperatures and ionic strengths. The concentration dissociation constants and the corresponding thermodynamic dissociation constants were calculated by a computer program. The mixed dissociation constants (pK) of cefetamet-Na have been determined spectrophotometrically in theH ₀ range from -5.80 to 0.00 and atpH values from 0.00 to 12.70 and are in good agreement with values achieved by graphical methods as well as with those obtained by potentiometric methods. Based on the determined values, the thermodynamic parameters (G, H, S) were calculated atI=0.1M.

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Potentiometrische und spektrophotometrische Bestimmung der Dissoziationskonstanten von Cefetamet

Zusammenfassung Die Dissoziationskonstanten von Cefetamet-Na wurden mittels potentiometrischer Titrationen und auf spektrophotometrischem Weg bestimmt. Die Untersuchungen wurden in wäßriger Lösung bei konstanter Temperatur und Ionenstärke und verschiedenenH ₀- undpH-Werten durchgeführt. Potentiometrische Messungen wurden bei drei verschiedenen Temperaturen und Ionenstärken vorgenommen. Die stöchiometrische Dissozation und die entsprechenden thermodynamischen Dissoziationskonstanten wurden mit Hilfe eines Computerprogramms berechnet. Die gemischten Dissoziationskonstanten (pK) wurden spektrophotometrisch imH ₀-Bereich von -5.80 bis 0.00 und impH-Bereich von 0.00 bis 12.70 bestimmt und stimmen sowohl mit Werten, die mit Hilfe der graphischen Methode erhalten wurden, als auch mit potentiometrisch bestimmten gut überein. Aus den ermittelten Werten der Dissoziationskonstanten wurden die thermodynamischen Parameter (G, H, S) fürI=0.1M berechnet.

     

Potentiometric determination of boron in silicon with an ion-selective electrode

A potentiometric method is proposed for the determination or boron in silicon, based on dissolution of silicon by treatment with hydrofluoric acid and ammonium fluoride in the presence of hydrogen peroxide, and conversion of boron to fluoroborate ion. The fluoroborate activity is measured with the Orion fluoroborate-selective electrode. Some important points, such as the effect of various interfering ions and the hydrolysis of fluoroborate ion, are described in detail. The method is applied to the determination of boron in silicon containing at least 10 p.p.m. of boron, with a relative standard deviation of ±4%.     

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.     

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.