Computer estimation of dissociation constants. Part IV. Precision and accuracy of potentiometric determinations

The protonation constants logK _Hj of the acidH _J are determined by regression analysis of potentiometric titration data whencommon parameters (logK _Hj,j = 1,...,J) andgroup parameters (E ⁰,L ₀,H _T ) are refined. Two kinds of systematic error have been investigated: the error from an uncertainty of group parameters and the error from a computational strategy of the minimization algorithm used. An analysis of variance of logK _Hj matrix was made for 6 reproduced titrations and 7 computational strategies of 6 various regression programs. It was concluded that the influence of the program used is negligible. From two ways of calibration of the glass electrode cell,the internal calibration (performed during titration) was slightly more accurate thanthe external calibration (done separately). Of programs tested, the ESAB and ACBA are most powerful because they permit refinement of group parameters and internal calibration. D-tartaric acid was chosen as model substance.Previous Part III.: Iraqi J. Sci.1981,22, 67.     

Spectrophotometric study of the interaction of some triphenylmethane dyes and 1-carbethoxypentadecyltrimethylammonium bromide

The effect of the cationic surfactant 1-carbethoxypentadecyltrimethylammonium bromide at above and below its critical micelle concentration on the absorption spectra and the values of the apparent dissociation constant pK(2) of the dyes Phenol Red, Bromophenol Blue and Bromocresol Green has been studied, along with the influence of a strong electrolyte (NaCl) on the effects produced by the presence of surfactant micelles in the dye solution.     

Studies on Ground and Excited State Prototropic Equilibria Sodium 1,3-Dihydroxy-4-aminoanthraquinone-2-sulphonate

Abstract

The dependence of absorption and fluorescence spectra of sodium 1,3-dihydroxy-4-aminoanthraquinone-2-sulphonate on pH, has been studied. the ground state dissociation constants for the three successive equilibria have been evaluated by photometry and fluorimetry. Also, an estimation of the lowest excited singlet state (pK?₃) dissociation constant, obtained from fluorimetric titration measurements, has been performed.     

Synthesis and acid-base properties of phosphorylated diazacycloalkanes and their cyclic analogs

Novel cyclopendant organophosphorus complexing agents,viz. 1,5-bis(2-diphenylphosphorylmethyl (or ethyl))-1,5-diazacyclooctane (1, 2) and l-methyl-4-(2-diphenylphosphorylethyl) piperazine (3), were synthesized on the basis of 1,5-diazacyclooctane and piperazine. The protonation constants of the compounds synthesized and some of their analogs were determined by potentiometric titration in 70 % aqueous ethanol and in nitromethane. The nitrogen atoms of the ring are the protonation sites in all of the systems studied. The regularities of the variation of protonation constants have been explained by the formation of intramolecular hydrogen bonds. The conformational possibilities of the formation of H-bonds in the cations of the ligands have been examined by molecular mechanics.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2007–2012, November, 1994.The authors are grateful to T. V. Timofeeva and N. I. Raevskii for the help in coping with the MMX 88 calculation program and for the discussion of the results.This study was carried out with the financial support of the International Science Foundation (Grant No. MJN 000).     

A POTENTIOMETRIC STUDY OF METAL CHELATES WITH TETRAETHYLENEPENTAAMINEHEPTA-ACETIC ACID

Abstract

The protonation constants of tetraethylenepentaamineheptaacetic acid, TPHA, were determined by potentiometric titration in aqueous solution at an ionic strength of 0.10 M KNO₃ and at 25°C. The formation constants of various metal-TPHA complexes were also obtained by titrating mixtures of metal to ligand in molar ratios of 1 :1 and 2:1. Calculations were performed with the computer program BEST. Individual stability constants are reported for Co(II). Ni(II), Cu(II), Zn(II), Cd(II), Hg(II) and Pb(II) with TPHA as well as their related pro-tonated species. The stabilities of the 1:1 complexes parallel to those of similar complexes with DTPA and TTHA. However the 2: 1 complexes have significantly larger log K _ML's than their TTHA counterparts. The extra stability of the 2:1 metal-TPHA complexes is explained in terms of ligand denticity and steric effects. Mercury(II)-TPHA complexes exhibited the highest formation constants and the copper-TPHA complexes had slightly higher log K _ML's than those for Co(II), Ni(II), Zn(II), Cd(II) and Pb(II).     

Multiwavelength Spectrophotometric Determination of Dissociation Constants of Mercaptopyrimidines

Abstract

The dissociation constants of some 2- and 6-mercaptopyrimidine derivatives with amino or hydroxy substituents are reported. The constants have been determined at 25° C in 0.1 M KCl aqueous solution. The multiwavelength spectrophotometric data obtained have been 3mlyzed by means of the SQUAD program (Stability Quotients from Absorbance Data).     

Potentiometric Studies of Copper(II) Complexes of Some Substituted Benzylidene-2-Hydroxyanilines in Dioxane–Water Media

The formation and protonation constants of 17 Schiff bases-derived 2-hydroxyaniline with some substituted benzaldehydes, and the stability constants of Cu(II) complexes of these Schiff bases, have been determined potentiometrically in 20, 40, and 60% dioxane–water media. The data from the potentiometric titrations were evaluated with the BEST computer program. For all Schiff bases studied, it was observed that the log K_OH values related to the protonation equilibria of the phenolic oxygen are increased, and the log K_NH values related to the protonation equilibria of the azomethine nitrogen are decreased, as the dioxane content is increased. The variation of these constants is discussed on the basis of specific solute–solvent interactions and structural changes of Schiff bases from water to the dioxane–water media. Also, titrimetric-pH investigation of substituted benzilidene-2-hydroxyaniline systems has revealed the formation of stable mono-Schiff base complexes with the metal ion Cu(II).     

COORDINATION OF CU2± BY MONO-AMINOALKYLOXAMIDES IN AQUEOUS SOLUTION

Abstract

Complex formation equilibria of Cu^2± complexes of N-(2-aminoethyl)(oxamide, N-3-aminopropyl)oxamide, 1,8-diamino-3,6-diazaoctane-7,8-dione and 1,10-diamino-4,8-diazaoctane-9,10-dione in aqueous solution at 25°C ± 0.1°C and I = 0.1 mol dm^?3 (KNO₃) have been studied using potentiometric and spectrometric titrimetry. Mixed ligand titrations using 2,2′-bipyridyl as the second ligand have been added in order to obtain unambiguous results. The Cu^2± complexes of the monoalkyl substituted oxamides studied can be classified into three groups: (1) CuLH₁ and CuLH₂ complexes; these complexes have a single deprotonated oxamide group in a trans configuration; (2) a CuLH_?3 complex; this complex has a doubly deprotonated oxamide group in a cis configuration; (3) Cu₂LH_?2, Cu₃L₂H_?4 and Cu₃L₂H_?5 complexs; these polynuclear complexes have the doubly deprotonated oxamide group in a trans configuration. Deprotonation of the primary amide group in the Cu₂LH_?2 complex of these ligands occurs before pH = 5. This unprecedented deprotonation of a primary amide group under these conditions is due to the cooperation of both strong and optimally positioned coordinating groups. The concept of amide oxygen anchoring is introduced.     

Kinetic Studies of Ethyl N-Hydroxymethyl Carbamate

Abstract

Kinetics of the dissociation of ethy1 N-hydroxymethylcarbamate, and monomethylolethylcarbamate (MMEC) at various temperatures and pH values have been studied by measuring the formaldehyde concentration in solution via an electrochemical technique. Formaldehyde and ethyl carbamate (EC) are the products of the dissociation. Formaldehyde concentration as a function of time has been used in a computer program designed to calculate the rate constant, k_f, for the formation of MMEC form formaldehyde and EC and the rate constant, k_r, for the dissociation of MMEC.     

Stability Constants for Dimercaptosuccinic Acid with Bismuth(III), Zinc(II), and Lead(II)

Abstract

Stability constants for the complexation of zinc(II), lead(II), and bismuth(III) by the vicinal dithiolate chelating agent meso-dimercaptosuccinic acid (DMSA) have been determined by a combination of potentiometric titration and spectrophotometric competition at 25°C and 0.1 M ionic strength. The spectrophotometric studies use the shifts in the ultraviolet bands of the thiol groups to quantitate metal binding to DMSA in the presence of competitive aminocarboxylic acids. Bismuth(III) forms a bis(DMSA) chelate with an exceptionally high stability constant of 10^43,87. This complex undergoes a series of protonations over the pH range 10 to 2, but there appears to be no measurable dissociation of ligand over this pH range. The zinc-DMSA system is dominated by a Zn₂(DMSA)₂ dimer, which has a protonation constant of 10⁶ and dissociates completely at lower pH. No more than 20% of total zinc exists as a monomeric complex at any pH. Lead forms a 1:1 complex with a stability constant of 10^17,4. Insoluble protonated lead complexes precipitate at pH < 6.5. Speciation calculations have been used to evaluate the potential competition from serum zinc to the binding of Pb²⁺ and Bi³⁺ by DMSA. The results indicate that DMSA should be relatively effective for the in vivo chelation of both Bi³⁺ and Pb^{2 +}.     

Protonation acidity constants for benzotoluidides in sulfuric acid solutions

The protonation of o-, m-and p-benzotoluidide in sulfuric acid solutions is studied by UV spectroscopy in the 190–350 nm region. Principal component analysis is applied to estimate the contributions of the effect of protonation and the medium effect. For the substances studied in this work, the first principal component (PC) captures about 98 % of the variance and the second PC ∼100 % of the cumulative percentage variance in the 210–350 nm region. The same spectral region is used for calculation of the ionization ratio from the coefficients of the first PC and mole fractions of the base and its conjugate acid. Using these data and Hammett's equation (pK _{BH +} = H _X + log I), the pK _{BH +} values for the protonation reaction are obtained. The dissociation constants as well as the solvent parameters m* (∼0.43) and ϕ (∼0.60) are calculated using the Excess Acidity Method (-pK _{BH +}= 2.28–2.30) and the Bunnett-Olsen Method (-pK _{BH +}= 2.24–2.28). The probable sites of protonation are discussed.     

Synthesis of bis[(4,10-diaza-4,10-ditosyl-benzo-12-crown-4) 4′-yl] diaminoglyoxime and its complexes

A new macrocyclic ligand (HL) has been synthesised by reaction of 4-amino-4,10-diaza-4,10-ditosyl-benzo-l2-crown-4 with 1,2-dichloroethanedial-dioxime. Protonation constants of the ligand and overall formation constants have been calculated from potentiometric data using the program TITFIT. Both amino and hydroxyimino groups provide donor atoms together with hydroxyl ions at higher pH values. The order of formation constants of the mononuclear complexes are U(VI)O₂>Cu(II)>Ni(II)=Co(II)>Cd(II).Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August 1995.     

A POTENTIOMETRIC STUDY OF METAL CHELATES WITH PENTAETHYLENEHEXAAMINEOCTAACETIC ACID

Abstract

The protonation constants of pentaethylenehexaamineoctaacetic acid, PHOA, were determined by potentiometric titration in aqueous solution at an ionic strength of 0.10 M KNO₃ and at 25°C. The formation constants of various metal-PHOA complexes were also obtained by titrating mixtures of metal to ligand in molar ratios of 1:1 and 2:1. Calculations were performed with the computer program BEST. Individual formation constants are reported for Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Hg(II) and Pb(II) with PHOA as well as their related protonated species. The stabilities of the 1:1 and 2:1 complexes are similar in many respects to complexes formed with tetraethylenepentaamineheptaacetic acid (TPHA). The similarities in the stabilities of both the 1:1 and 2:1 complexes with PHOA and those with TPHA are explained in terms of ligand denticity and steric effects. Mercury(II)-PHOA complexes exhibited the highest formation constants, followed by copper(II)-PHOA complexes which had higher log K _ML's than those for Co(II), Ni(II), Zn(II), Cd(II) and Pb(II).     

A Branched Pore Model Analysis for the Adsorption of Acid Dyes on Activated Carbon

A new branched-pore adsorption model has been developed using an external mass transfer coefficient, K _f, an effective diffusivity, D _eff, a lumped micropore diffusion rate parameter, K _b, and the fraction of macropores, f, to describe sorption kinetic data from initial adsorbent-adsorbate contact to the long-term adsorption phase. This model has been applied to an environmental pollution problem—the removal of two dyes, Acid Blue 80 (AB80) and Acid Red 114 (AR114), by sorption on activated carbon. A computer program has been used to generate theoretical concentration-time curves and the four mass transfer kinetic parameters adjusted so that the model achieves a close fit to the experimental data. The best fit values of the parameters have been determined for different initial dye concentrations and carbon masses. Since the model is specifically applicable to fixed constant values of these four parameters, a further and key application of this project is to see if single constant values of these parameters can be used to describe all the experimental concentration-time decay curves for one dye-carbon system.The error analysis and best fit approach to modeling the decay curves for both dye systems show that the correlation between experimental and theoretical data is good for the fixed values of the four fitted parameters. A significantly better fit of the model predictions is obtained when K _f, K _b and f are maintained constant but D _eff is varied. This indicates that the surface diffusivity may vary as a function of surface coverage.     

Determination of Binding Constants of Polyethylene Glycol Vancomycin Derivatives to Peptide Ligands Using Affinity Capillary Electrophoresis

Vancomycin (Van) from Streptomyces orientalis has been derivatized with polyethylene glycol [PEG; PEG-550 (1), 750 (2), 1,100 (3), 2,000 (4), 5,000 (5), and 8,000 (6) g mol⁻¹] at the N-terminus of the glycopeptide backbone and their binding to d-Ala-d-Ala terminus peptides assessed using affinity capillary electrophoresis (ACE). Utilizing ACE, a plug of Van-PEG and non-interacting standards are injected and electrophoresed. Analysis of the change in the relative migration time ratio of the Van-PEG species, relative to the non-interacting standards, as a function of the concentration of peptide, yields a value for the binding constant (K _b). Values of K _b for N-acetyl-d-Ala-d-Ala, 7 to the Van-PEG derivatives are weaker than those for N _α,N _ε-diacetyl-Lys-d-Ala-d-Ala, 8 (for example, values of K _b for 7-1 and 8-1 are 1.8 and 47.7 × 10³ M⁻¹, respectively). These results demonstrate that derivatization of Van with PEG has little effect on the affinity of d-Ala-d-Ala peptide ligands to it. The findings further prove the versatility of ACE and its ability to estimate binding parameters of ligands to antibiotics.     

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.

---

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 and thermodynamic studies of 2-mercapto-5-(2-hydroxynaphthylideamino)-1,3,4-thiadiazole and its metal complexes

Summary Proton-ligand dissociation constant of 2-mercapto-5-(2-hydroxynaphthylideamino)-1,3,4-thiadiazole (MHT) and the stepwise stability constants of its metal complexes were determined potentiometrically in 40 mass/mass% ethanol-water mixture containing 0.1 M KCl. The stabilities of the complexes follow the order: Cu²⁺>Ni²⁺>Co²⁺>Mn²⁺. The dissociation constant (pK^H) of MHT and the stability constants (logK) of its metal complexes were determined at different temperatures and the corresponding thermodynamic parameters were calculated and discussed. The proton dissociation process is non-spontaneous, endothermic and entropically unfavoured. The formation of the metal complexes was found to be spontaneous, endothermic and entropically favoured.     

Biparameter Equations for Calculating Kovats Retention Indices of Hydrocarbons

Abstract

Biparameter equations, in which one parameter is the boiling point of the solutes and the other is any of their physicochemical properties able to account for dispersive solute-stationary phase interactions, are obtained by regression analysis. These equations permit the calculation of Kovats retention index (I_R ) for aliphatic and aromatic hydrocarbons in complex mixtures with standard deviations close to experimental error. The linear change of regression coefficients with the work temperature allow us to obtain equations suitable for calculating I_R at any temperature on a given stationary phase. Furthermore, accurate values for the magnitudes included in the equations can be obtained starting from I_R values.

When alkylbenzenes are separated on polar phases, such as Carbowax 20M, it is necessary to add a new parameter accounting for inductive interactions. Once again, regression coefficients, except that of the boiling point which remains constant, change linearly with the polarity of the phase (Tarjan's scale) enabling the obtaining of an equation for calculating I_R on any stationary phase at a given work temperature, although constant deviations between calculated and experimental I_R , on polymeric stationary phases, are found.     

Quantum-chemical study of the protonation of pyrrolo[2,1-b]thiazole and its selenium and tellurium analogs

The possibility of proton attack on various centers in pyrrolo[2,1-b]thiazole (1) has been evaluated. The results of semiempirical (MNDO, AM1, and PM3) andab initio (6-31G^*) calculations were compared. The MNDO and 6-31G^* methods give “chemically proper” and qualitatively coincident results. Analysis of the intramolecular (geometric and electronic) reorganization of molecule1, depending on the protonation center, has been carried out. The most probable attack centers, depending on the mechanism of electrophilic reaction, have been recognized. The energy parameters of intramolecular prototropic rearrangements in cation1 and the “blocking” factor value of methyl groups reducting the corresponding complex stability have been evaluated. It has been established that the relative stability of the protonated forms does not change on going to pyrrolo[2,1-b]selenium- and telluriumazoles, but the range of variations is considerably narrowed in the series S>Se>Te. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1707–1711, October, 2000.     

Novel Organostannanes with Assorted Drugs: Synthesis,Spectral, and Potentiometric Studies

Abstract

Here, we report on synthesis of some novel derivatives of dibutyltin dichloride/oxide with assorted drugs containing a labile proton of carboxylic acid or alcohol moiety. Ligands used in these reactions are ampicillin trihydrate (ABP), ibuprofen (BPPA), acetylsalicylic acid (ASA), and acetaminophen (APAP). Characterization and structure elucidation of the complexes was achieved by elemental analysis, infrared spectroscopy and multinuclear (¹H, ¹³C, and ¹¹⁹Sn) NMR spectroscopic studies. Their protonation behavior and stability constants were investigated and corresponding thermodynamic parameters were calculated using pH-potentiometric technique. It was suggested on basis of potentiometric studies that the complexes formed are invariably of 1:2 stoichiometry irrespective of ligand-to-metal ratio. The stability constants represented by log β values at 298 K were found to be in the order: ABP > APAP > BPPA > ASA. The free energy of formation of these complexes has a negative value, which shows that formation of the complexes is a spontaneous process. Also, the negative value of standard enthalpy change indicates that reaction of ligands with metal ion is exothermic and this validates the decrease in the value of log β with increasing temperature. Entropy change is positive which seems to be the driving force for complex formation.