Proton Dissociation Constants Research Articles

Spectrophotometric investigations of the role of the organic solvent on the acid dissociation constants of some azo dyes derived from 2-aminobenzothiazole

Proton dissociation constants (p K a) of a set of hydroxyl azo dyes in various organic solvent + water mixtures have been determined. The results obtained were discussed in terms of the solvent characteristic. Effects of hydrogen-bonding interactions and solvent basicity on the ionization process have been also discussed. In a nutshell, the p K a values of the investigated compounds were found to be largely depending on both the ratio and the nature of organic cosolvent. In addition, it was very important to know with which structures we are dealing in liquid solutions and how these structures influence physicochemical properties of individual tautomers and their acidity constants. Thus, theoretical calculations have been done in order to obtain an insight into structure features and physicochemical properties of the compounds under study.

Mechanism of electrocatalytic hydrogen production by a di-iron model of iron–iron hydrogenase: A density functional theory study of proton dissociation constants and electrode reduction potentials

Simple dinuclear iron dithiolates such as (mu-SCH2CH2CH2S)[Fe(CO)3]2, (1) and (mu-SCH2CH2S)[Fe(CO)3]2 (2) are functional models for diiron-hydrogenases, [FeFe]-H2ases, that catalyze the reduction of protons to H2. The mechanism of H2 production with 2 as the catalyst and with both toluenesulfonic (HOTs) and acetic (HOAc) acids as the H+ source in CH3CN solvent has been examined by density functional theory (DFT). Proton dissociation constants (pKa) and electrode reduction potentials (E(o)) are directly computed and compared to the measured pKa of HOTs and HOAc acids and the experimental reduction potentials. Computations show that when the strong acid, HOTs, is used as a proton source the one-electron reduced species 2- can be protonated to form a bridging hydride complex as the most stable structure. Then, this species can be reduced and protonated to form dihydrogen and regenerate 2. This cycle produces H2 via an ECEC process at an applied potential of -1.8 V vs. Fc/Fc+. A second faster process opens for this system when the species produced at the ECEC step above is further reduced and H2 release returns the system to 2- rather than 2, an E[CECE] process. On the other hand, when the weak acid, HOAc, is the proton source a more negative applied reduction potential (-2.2 V vs. Fc/Fc+) is necessary. At this potential two one-electron reductions yield the dianion 2(2-) before the first protonation, which in this case occurs on the thiolate. Subsequent reduction and protonation form dihydrogen and regenerate 2- through an E[ECEC] process.

Quantum-chemical calculations, tautomeric, thermodynamic, spectroscopic and X-ray studies of 3-(4-fluorophenylhydrazone)pentane-2,4-dione

The structural and tautomeric characteristics of 3-(4-fluorophenylhydrazone)pentane-2,4-dione were studied using IR, 1H and 13C NMR spectroscopy, X-ray diffraction analysis and quantum-chemical calculations. Crystallographic data shows that the compound has a hydrazone structure that contains intramolecular H-bonds assisted by resonance, with a N–O distance of 2.5872 Å. The effective atomic charges in the tautomeric forms of the colorant (enol-azo, keto-azo and hydrazone), was calculated using an MO LCAO quantum-chemical method employing the Hückel approximation; the compound was stabilized in the hydrazone form and displayed high reactivity. The thermodynamic functions of the proton dissociation constant of the colorant were determined by potentiometry; the dissociation process was unspontaneous, endothermic and entropically unfavorable. The findings reveal that the colorant has the same structure in both DMSO solution and the solid state.

치환기를 가진 다섯 자리 질소-산소 (N2O3)계 리간드의 합성 및 전이금속(II) 이온과의 착물 안정도상수 결정

Hydrochloric acid salt of a new <TEX>$N_2O_3$</TEX> pentadentate ligand, N,N'-Bis(2-Hydroxybenzyl)-1,3-diamino-2-propanol(H-BHDP 2HCl) was synthesized. Br-BHDP 2HCl, CI-BHDP 2HCl, <TEX>$CH_3-BHDP$</TEX> 2HCl and <TEX>$CH_3O$</TEX>-BHDP 2HCl having Br, Cl, <TEX>$CH_3$</TEX> and <TEX>$CH_3O$</TEX> substituents at 5-position of the phenol group of H-BHDP 2HCl were also synthesized. The potentiometry study in aqueous solution revealed that the proton dissociations of the synthesized ligands occurred in four steps and their order of the calculated overall proton dissociation constants(<TEX>$log{\beta}_p$</TEX>) was Br-BHDP < Cl-BHDP < H-BHDP < <TEX>$CH_3O-BHDP$</TEX> < <TEX>$CH_3-BHDP$</TEX>. The order showed a similar trend to that of Hammett substituent constants(<TEX>${\delta}_p$</TEX>). The order of the stability constants(<TEX>$logK_{ML}$</TEX>) was Co(II) < Ni(II) < Cu(II) < Zn(II) < Cd(II) < Pb(II). The order in their stability constants (<TEX>$logK_{ML}$</TEX>) of each transition metal complex agreed with that of the overall proton dissociation constants (<TEX>$log{\beta}_p$</TEX>).

Accurate Aqueous Proton Dissociation Constants Calculations for Selected Angiotensin-Converting Enzyme Inhibitors

Studies that allow computing values of aqueous proton dissociation constants (pKa), gas phase proton affinities, and the free energy of solvation have been performed for six members of angiotensin-I-converting enzyme (ACE) inhibitor family (captopril, enalaprilat, imidaprilat, ramiprilat, perindoprilat, and spiraprilat). Density functional theory (DFT) calculations using PBE1PBE functional on optimized molecular geometries have been carried out to investigate the thermodynamics of gas-phase protonation. The conductor-like polarizable continuum model (CPCM) solvation method at various levels of theory was applied to calculate the free energy of solvation for the ACE inhibitors and their respective anions. The CPCM solvation calculations were performed on both gas-phase and solvent-phase optimized structures. The combination of gas-phase and solvation energies according to the thermodynamic cycle enabled us to compute accurate pKa values for the all studied molecules.

Synthesis, characterization, potentiometric and thermodynamic studies of transition metal complexes with 1-benzotriazol-1-yl-1-[( p -methoxyphenyl) hydrazono]propan-2-one†

A new series of Mn2+ and Co2+ complexes with 1-benzotriazol-2-yl-1-[(p-methoxyphenyl)-hydrazono]propan-2-one (BMHP) were synthesized and characterized by the elemental analysis, magnetic and different spectral techniques. Proton dissociation constant of the free ligand and the stepwise stability constants of its metal complexes were determined potentiometrically in 0.1 M KC1 and 40% (v/v) ethanol–water. The dissociation constants of the free ligand and its metal complexes were determined at different temperatures and the corresponding thermodynamic parameters were calculated and discussed. The dissociation process was found to be non-spontaneous, endothermic and entropically unfavorable. The changes in the standard ΔG 0 and ΔH 0 accompanying complexation were decreased with increasing metal ionic radius but increased with increasing electronegativity, ionization enthalpy, and hydration enthalpy of the metal ion. The values of (−ΔG 0) and (−ΔH 0) were in the order: Mn2+ < Co2+ < Ni2+ < Cu2+, in accord with the Irving-Williams order. The complexes were found to be stabilized by both enthalpy and entropy changes and the results suggest that complexation is an enthalpy-driven process. The distribution diagrams of the complexes in solution were evaluated. †Presented at the 37th International Conference of Coordination Chemistry (37th ICCC), Cape Town, 13–18 August, 2006.

Spectral and equilibrium studies on some new derivatives of 4-amino-5-phenyl-3-mercapto-1,2,4-triazole

4-Amino-5-phenyl-3-mercapto-l,2,4-triazole (APMT), 4-(4'-methoxy)benzylidineamino-5-phenyl-3-mercapto-l,2,4-triazole (PMBPMT), 4-benzylidineamino-5-phenyl-3-mercapto-1,2,4-triazole (BPMT), 4-(2'-hydroxy)benzylidineamino-5-phenyl-3-mercapto-l,2,4-triazole (HBPMT) and 4-amino-5-(4'-nitro)phenyl-3-mercapto-1,2,4-triazole (ANPMT) were synthesized and characterized by elemental analyses, IR, 1 H NMR, 13 C NMR, DEPT and Mass spectral studies. Proton dissociation constants of these compounds in 70% v/v dioxan-water medium at 303 K and 0.1 M (KNO 3 ) ionic strength were measured. The order of the pK a corresponding to mercapto group follows the sequence : PMBPMT > BPMT > HBPMT > APMT > ANPMT.

Ionization and Solubility of Chitosan Solutions Related to Thermosensitive Chitosan/Glycerol-Phosphate Systems

Chitosan is a linear cationic biopolymer composed of glucosamine and N-acetyl-glucosamine that is only soluble in acidic aqueous solutions and precipitates when neutralized. However, it was recently discovered that chitosan dissolved in solutions containing glycerol phosphate was soluble at near neutral pH and produced a sol-gel transition when heated. Understanding this unique thermogelling system requires improved characterization of the ionization and solubility behaviors of chitosan, in particular dependencies on temperature, salt, chitosan concentration, and fD, where fD is the fraction of glucosamine monomers (deacetylated monomers) in chitosan. In the current study we performed temperature-controlled titration and dilution experiments on chitosan solutions with fD of 0.72, 0.85, and 0.98 at concentrations ranging from 1.875 to 30 mM of its glucosamine monomer and with 0 to 150 mM added salt. Light transmittance measurements were performed during titration to indicate precipitation. We found the apparent proton dissociation constant of chitosan, pKap, to (1) decrease strongly with increased temperature, (2) increase strongly with increased salt, (3) increase strongly with increased chitosan concentration in low-salt conditions, and (4) decrease weakly with increasing fD. All of the above influences on chitosan pKap were accurately predicted using a mean-field Poisson-Boltzmann (PB) cylindrical cell model where the only adjustable parameter was the temperature-dependent chitosan intrinsic monomeric dissociation constant pK0(T). The resulting chitosan pK0 values at 25 degrees C were in the range from 6.63 to 6.78 for all chitosans and salt contents tested. The temperature dependence of chitosan ionization was found to strongly reduce pK0(T) by 0.023 units per degrees C, for example, resulting in a reduction of chitosan pK0(T) from 7.1 at 5 degrees C to 6.35 at 37 degrees C for fD of 0.72 in 150 mM salt. A similar temperature-dependent reduction of the pKa of the glucosamine monomer was found (-0.027 units per degrees C) while the pKa of glycerol phosphate did not change significantly with temperature. The latter result suggested that chitosan solutions heated in the presence of glycerol phosphate will become partly neutralized by transferring protons to glycerol phosphate and thereby allow attractive interchain forces to form a physically cross-linked gel under the appropriate conditions. Additionally, the degree of ionization of chitosan when it precipitates upon addition of a strong base was measured to be in the range from 0.25 to 0.55 and was found to (1) be insensitive to temperature, (2) increase strongly with increased salt, and (3) increase strongly with fD. The salt effect was accounted for by the PB model, while the influence of fD appeared to be due to acetyl groups impeding attractive chain-to-chain association to increase solubility and require reduced ionization levels to precipitate.

중금속 이온 분리를 위한 새로운 네 자리 N2O2계 리간드의 합성 및 착 화합물의 안정도상수 결정

Hydrochloride acid salts of new <TEX>$N_2O_2$</TEX> tetradentate ligands containing amine and phenol N,N'-bis(2-hydroxybenzyl)-o-phenylenediamine(H-BHP), N,N'-bis(5-bromo-2-hydroxybenzyl)-o-phenylenediamine(Br-BHP), N,N'-bis(5-chloro-2-hydroxybenzyl)-o-phenylene-diamine(Cl-BHP), N,N'-bis(5-methyl-2-hydroxybenzyl)-o-phenylene-diamine (Me-BHP) and N,N'-bis(5-methoxy-2-hydroxybenzyl)-o-phenylenediamine(MeO-BHP) were synthesized. The ligands were characterized by elemental analysis, mass and NMR spectroscopy. The elemental analysis showed that the ligands were isolated as dihydrochloride salt. The potentiometry study revealed that the proton dissociation constants<TEX>$(logK_n{^H})$</TEX> of ligands and stability constants <TEX>$(logK_{ML})$</TEX> of transition and heavy metals complexes. The order of the stability constants of each metal ions for ligands was Br-BHP < Cl-BHP > H-BHP < MeO-BHP < Me-BHP.

Relationship between quantum-chemical descriptors of proton dissociation and experimental acidity constants of various hydroxylated coumarins. Identification of the biologically active species for xanthine oxidase inhibition

Quantum-chemical descriptors related to proton dissociation constants of a set of coumarins hydroxylated in various positions have been computed and related to the experimental p K a values. An excellent correlation was found between the computed deprotonation energies of hydroxycoumarins in water and their experimental p K a values, and the results were used to predict the p K a of other hydroxycoumarins. Then, predicted and experimental p K a values were used as a basis for interpreting and discussing the variation of xanthine oxidase inhibitory activities within a subset of coumarins, with the aim of identifying the molecular species most relevant for enzyme inhibition.

Gallium uptake by transferrin and interaction with receptor 1

The kinetics and thermodynamics of Ga(III) exchange between gallium mononitrilotriacetate and human serum transferrin as well as those of the interaction between gallium-loaded transferrin and the transferrin receptor 1 were investigated in neutral media. Gallium is exchanged between the chelate and the C-site of human serum apotransferrin in interaction with bicarbonate in about 50 s to yield an intermediate complex with an equilibrium constant K (1) = (3.9 +/- 1.2) x 10(-2), a direct second-order rate constant k (1) = 425 +/- 50 M(-1) s(-1) and a reverse second-order rate constant k (-1) = (1.1 +/- 3) x 10(4) M(-1) s(-1). The intermediate complex loses a single proton with proton dissociation constant K (1a) = 80 +/- 40 nM to yield a first kinetic product. This product then undergoes a modification in its conformation which lasts about 500 s to produce a second kinetic intermediate, which in turn undergoes a final extremely slow (several hours) modification in its conformation to yield the gallium-saturated transferrin in its final state. The mechanism of gallium uptake differs from that of iron and does not involve the same transitions in conformation reported during iron uptake. The interaction of gallium-loaded transferrin with the transferrin receptor occurs in a single very fast kinetic step with a dissociation constant K (d) = 1.10 +/- 0.12 microM and a second-order rate constant k (d) = (1.15 +/- 0.3) x 10(10) M(-1) s(-1). This mechanism is different from that observed with the ferric holotransferrin and suggests that the interaction between the receptor and gallium-loaded transferrin probably takes place on the helical domain of the receptor which is specific for the C-site of transferrin and HFE. The relevance of gallium incorporation by the transferrin receptor-mediated iron-acquisition pathway is discussed.

새로운 N,N,O계 세 자리 리간드의 합성과 전위차적정법에 의한 전이금속 착물의 안정도상수의 결정

Hydrobromic acid salts of new N, N, O tridentate ligands containing phenol, 2-[(2-Methylamino- ethyl- amino)-methyl]-phenol(H-MMP. 2HBr), 5-Bromo-2-[(2-Methylamino-ethylamino)-methyl]-phenol (Br- MMP. 2HBr), 5-Chloro-2-[(2-Methylamino-ethylamino)-methyl]-phenol(Cl-MMP. 2HBr), 5-Methyl-2-[(2-Methylamino-ethylamino)-methyl]-phenol(Me-MMP. 2HBr), 5-Methoxy-2-I(2-Methylamino-ethylamino)- methyl]-phenol(MeO- MMP. 2HBr) and. 1-[(2-Methylamino-ethylamino)- methyl]-naphthalen-2-ol(Nap- MMP. 2HBr) were synthesized. The synthesized ligands were confirmed by C. H. N. atomic analysis, UV-visible and IR spectroscopies, <TEX>$^1$</TEX>H NMR, <TEX>$^{13}$</TEX>C NMR and mass analysis. The potentiometry study revealed that the proton dissociation constants(logK<TEX>$_n^H$</TEX>) of the synthesized ligands and stability constants (logK<TEX>$_{ML}$</TEX>, logK<TEX>$_{LM2}$</TEX>) of transition metal complexes of Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) ions occurred in three steps and the order of the calculated overall proton dissociation constants(log<TEX>$\beta_p$</TEX>) and stability constants (logK<TEX>$_{ML}$</TEX>) of ligands was Br-MMP. 2HBr < Cl-MMP 2HBr < H-MMP. 2HBr < Nap-MMP. 2HBr < Me-MMP. 2HBr < MeO-MMP. 2HBr. The order showed a similar trend to that of Hammett substituent constants(<TEX>$\delta_p$</TEX>). The synthesized ligands usually form 2:1(ML<TEX>$_2$</TEX>) complexes with transition metal ions. The order of the stability constants of each transition metal ions was Co(II) < Ni(II) < Cu(II) ;> Zn(II) ;> Cd(II) ;> Pb(II).

Comparison of Starch Hydrolysis Activity and Thermal Stability of Two Bacillus licheniformis α-Amylases and Insights into Engineering α-Amylase Variants Active under Acidic Conditions

Bacillus licheniformis alpha-amylase (BLA) is widely used in various procedures of starch degradation in the food industry, and a BLA species with improved activity at higher temperature and under acidic conditions is desirable. Two BLA species, designated as PA and MA, have been isolated from the wild-type B. licheniformis strain and a mutant strain, respectively. In this study, their starch-hydrolysis activity and thermal stability were examined. MA showed higher activity than PA, especially at acidic pH (pH 5.0-5.5), and even after 1 h of treatment at 90 degrees C. MA was active in the range of pH 4.0-8.0, which is much wider than that (pH 4.5-7.5) of PA. It was shown that the proton dissociation constants on the acidic and alkaline sides (pKa1 and pKa2) were shifted to more acidic and basic values, respectively, by the mutation of PA to MA. The activation energy and thermodynamic parameters for their thermal inactivation indicate that MA is more thermally stable and catalytically active than PA, suggesting that MA could be useful for glucose-production process coupled with reactions catalyzed by beta-amylase.

A manganese porphyrin superoxide dismutase mimetic enhances tumor radioresponsiveness

To determine the effect of the superoxide dismutase mimetic Mn(III) tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP(5+)) on tumor radioresponsiveness. Various rodent tumor (4T1, R3230, B16) and endothelial (SVEC) cell lines were exposed to MnTE-2-PyP(5+) and assayed for viability and radiosensitivity in vitro. Next, tumors were treated with radiation and MnTE-2-PyP(5+)in vivo, and the effects on tumor growth and vascularity were monitored. In vitro, MnTE-2-PyP(5+) was not significantly cytotoxic. However, at concentrations as low as 2 mumol/L it caused 100% inhibition of secretion by tumor cells of cytokines protective of irradiated endothelial cells. In vivo, combined treatment with radiation and MnTE-2-PyP(5+) achieved synergistic tumor devascularization, reducing vascular density by 78.7% within 72 h of radiotherapy (p < 0.05 vs. radiation or drug alone). Co-treatment of tumors also resulted in synergistic antitumor effects, extending tumor growth delay by 9 days (p < 0.01). These studies support the conclusion that MnTE-2-PyP(5+), which has been shown to protect normal tissues from radiation injury, can also improve tumor control through augmenting radiation-induced damage to the tumor vasculature.

Hydrogen ion titration of alkyldimethylamine oxides by 13C and 1H NMR and conventional methods

In the hydrogen ion titration of micelles, the degree of ionization of the micelle at a given pH has to be evaluated to obtain a p K a value of micelles ( K a being the proton dissociation constant) at the pH. We compared the degree of ionization obtained from 13C and 1H NMR spectra with that obtained from the stoichiometric method. We used dodecyldimethylamine oxide (C12DMAO) and hexyldimethylamine oxide (C6DMAO) to examine the titration behavior of micelles and monomers, respectively. We determined p K a values of amine oxides both in H 2O and D 2O. As to the monomer (C6DMAO), the degree of ionization from NMR, α NMR, coincided with that from the conventional stoichiometric method α. The difference of p K 1 of amine oxide monomer between D 2O and H 2O was about 0.5: p K 1(D) ≈ p K 1(H) + 0.5. The difference was about the same as that for carboxylic acids. As to the C12DMAO micelle, α NMR did not coincide with α over a considerable range of α. The NMR chemical shift might be influenced by micellar structure changes induced by the ionization, such as the sphere-to-rod transition. The intrinsic logarithmic dissociation constants of the micelle were 5.9 ± 0.1 for H 2O, and 6.5 ± 0.1 for D 2O.

Mechanism of Formation of the Complex between Transferrin and Bismuth, and Interaction with Transferrin Receptor 1

The kinetics and thermodynamics of Bi(III) exchange between bismuth mononitrilotriacetate (BiL) and human serum transferrin as well as those of the interaction between bismuth-loaded transferrin and transferrin receptor 1 (TFR) were investigated at pH 7.4-8.9. Bismuth is rapidly exchanged between BiL and the C-site of human serum apotransferrin in interaction with bicarbonate to yield an intermediate complex with an effective equilibrium constant K(1) of 6 +/- 4, a direct second-order rate constant k(1) of (2.45 +/- 0.20) x 10(5) M(-1) s(-1), and a reverse second-order rate constant k(-1) of (1.5 +/- 0.5) x 10(6) M(-1) s(-1). The intermediate complex loses a single proton with a proton dissociation constant K(1a) of 2.4 +/- 1 nM to yield a first kinetic product. This product then undergoes a modification in its conformation followed by two proton losses with a first-order rate constant k(2) = 25 +/- 1.5 s(-1) to produce a second kinetic intermediate, which in turn undergoes a last modification in the conformation to yield the bismuth-saturated transferrin in its final state. This last process rate-controls Bi(III) uptake by the N-site of the protein and is independent of the experimental parameters with a constant reciprocal relaxation time tau(3)(-1) of (3 +/- 1) x 10(-2) s(-1). The mechanism of bismuth uptake differs from that of iron and probably does not involve the same transition in conformation from open to closed upon iron uptake. The interaction of bismuth-loaded transferrin with TFR occurs in a single very fast kinetic step with a dissociation constant K(d) of 4 +/- 0.4 microM, a second-order rate constant k(d) of (2.2 +/- 1.5) x 10(8) M(-1) s(-1), and a first-order rate constant k(-d) of 900 +/- 400 s(-1). This mechanism is different from that observed with the ferric holotransferrin and implies that the interaction between TFR and bismuth-loaded transferrin probably takes place on the helical domain of the receptor which is specific for the C-site of transferrin and HFE. The relevance of bismuth incorporation by the transferrin receptor-mediated iron acquisition pathway is discussed.

질소-산소계 시프염기 리간드의 합성과 전이금속(II) 착물의 안정도상수결정

N,N-bis(2-salicylaldehyde)dipropylenetriamine(5- Hsaldipn), N,N-bis( 5-bromosalicyl-aldehyde) dipropylenetriamine (5-Brsaldipn), N,N-bis(5-chlorosalicy laldehyde )dipropylene-triamine(5-Clsaldipn), N,N-bis(2-hydroxy- <TEX>$5-methoxy-benzaldehyde)dipropylenetriamine(5-OCH_3saldipn)$</TEX> and N,N-bis (2-hydroxy-5-nitrobenzaldehyde)dipropylenetriamine <TEX>$(5-NO_2saldipn)$</TEX> were synthesized and characterized by elemental analysis, infrared spectrometry, NMR spectrometry and mass spectrometry. Their proton dissociation constants were determined in 70% dioxane/30% water solution by potentiometric. Stability constants of the complexes between these ligands and the metal ions such as Cu(II), Ni(II) and Zn(II) were measured in dimethyl sulfoxide by a polarographic method. Stability constants for the ligands were in the order of <TEX>$5-OCH_3$</TEX> > 5-H > 5-Br > 5-Cl > <TEX>$5-NO_2$</TEX> saldipn. Enthalpy and entropy changes were obtained in negative values.

Thermodynamic study of the protonation of dimethyldodecylamine N-oxide micelles in aqueous solution at 298 K. Establishment of a theoretical relationship linking critical micelle concentrations and pH.

Dodecyldimethylamine N-oxide (DDAO) is a zwitterionic surfactant with acid-base properties. The proton dissociation constant of this surfactant was determined by a novel potentiometric method at "controlled chemical potential" of the proton using a classical pH-glass electrode. When the DDAO was in its monomeric form, the pKa was about 5, consistent with the value commonly reported in the literature. However, a unique proton dissociation constant specific to the micellar form of this surfactant could not be obtained. We found that the acid-base behavior of the DDAO micelles depended on their environment. Indeed, we were able to establish thermodynamic relations linking the critical micellar concentration to the degree of protonation of the micelles. The experimental values were in good accordance with this model.

Gibbs–Donnan and specific-ion interaction theory descriptions of the effect of ionic strength on proton dissociation of alginic acid

The apparent proton dissociation constants of a commercial alginic acid have been obtained in KNO 3 and NaCl at concentrations ranging between 0.01 and 2 mol L −1. An analysis of the dependence on the ionic strength at a constant value of the degree of dissociation was done by means of empirical functions derived from the Gibbs–Donnan formalism for polyelectrolytes and a specific-ion interaction theory (SIT). Both functions were able to fit the experimental data, although SIT yielded rather high errors in the fitted parameters due to a problem of multicollinearity, in contrast to the function derived from the Gibbs–Donnan approach.

Potentiometric and Thermodynamic Studies of 5-(4′-Sulfonylazidophenylazo)-3-phenyl-2-thioxothiazolidin-4-one and Its Metal Complexes

The proton dissociation constant of 5-(4′-sulfonylazidophenylazo)-3-phenyl-2-thio- xothiazolidin-4-one (SPT) and the stability constants of its complexes with some metal ions were calculated potentiometrically in 0.1 M KCl and 40% (v/v) ethanol–water mixture. The order of stability was found to be Mn2+ < Co2+ < Ni2+ < Cu2+ < Zn2+. The effect of temperature on the dissociation of SPT and the stability of its complexes were studied. The corresponding thermodynamic functions were derived and discussed. The dissociation process is unspontaneous, endothermic, and entropically unfavorable. The formation of the metal complexes was found to be spontaneous, endothermic and entropically favorable.