Potentiometric determination of the protonation constants of some phenols in 1.0 mol/L NaCl at 25°C

The potentiometric determination of the protonation constants of phenol, 2-chlorophenol, 2-nitrophenol, 2,4-dichlorophenol and 2-methylphenol in 1.0 mol/L NaCl at 25°C is presented. An automated system has been used in the experiments. The determination of the constants has been carried out using both graphical and numerical methods.     

Potentiometric study of the protonation equilibrium of Tris(Hydroxymethyl) aminomethane in aqueous sodium perchlorate solutions at 25 degrees C: construction of a thermodynamic model

The protonation equilibrium of the Tris(Hydroxymethyl)aminomethane (TRIS) has been studied using an automated potentiometric system. The temperature was kept constant at 25 degrees C and the ionic strength was 0.1, 0.5, 1.0, 2.0 and 3.0 mol dm(-3) in NaClO(4). The experimental constants, obtained at different ionic strengths, were correlated by means of the modified Bromley methodology (MBM) and the thermodynamic protonation constant found to be log (0)beta = 8.07 +/- 0.01 . Those values together with some others for NaCl medium were used to construct a thermodynamic model on both molal and molar scales for the protonation equilibrium of TRIS.     

A comparative study of the application of the method of least-squares in the potentiometric determination of protonation constants

Methods of simple and multiple linear regression applied to the potentiometric determination of protonation constants of diprotic and triprotic acids are studied critically. The best way of fitting the data, according to the order of magnitude of the constants, is established. The conclusions are checked by calculating the protonation constants of succinic and citric acids.     

The thermodynamic model of inorganic arsenic species in aqueous solutions Potentiometric study of the hydrolitic equilibrium of arsenic acid

Protonation constants of arsenic acid were determined at different ionic strengths in NaClO(4) (0.1, 0.5, 1.0, 3.0 mol dm(-3)), NaCl (0.5 and 1.0 mol dm(-3)) and KCl (0.5, 1.0 and 3.0 mol dm(-3)) ionic media by means of a potentiometric study. The distribution of arsenate species was defined depending on two important variables in natural environments: pH and composition. All the experimentation was performed at 25 degrees C. The differences found in the protonation constants for different medium compositions, were explained by the different behaviour of the interaction parameters of the species considered in the different media and ionic strengths. These parameters were reported for all hydrolitic As(V) species and were calculated using the Modified Bromley's Methodology (MBM). The corresponding thermodynamic stepwise formation constants were also determined (log degrees K(1)=11.58+/-0.01, log degrees K(2)=7.06+/-0.01, log degrees K(3)=2.25+/-0.01). All the results obtained showed not only the importance of the ionic strength but also of the composition of the ionic medium on the distribution of the acid-base species of As(V) as a function of pH in natural waters.     

Potentiometric studies of complex formation between methylmercury(II) and EDTA

The protonation of EDTA (ethylenediaminetetra-acetic acid) and its complex formation with methylmercury(II) ions was studied by a potentiometric method at 25 degrees in 1.0M NaNO(3). The protonation constants of EDTA and the equilibrium constants for the complexation reactions between methylmercuric ions and EDTA were evaluated by analysing the experimental data with the ETITR version of the general error-minimizing program LETAGROP. The results were used to develop an analytical method, based on complexometric titration and ion-exchange separation, for the determination of inorganic and organic mercury in the presence of one another.     

Potentiometric study of the distribution equilibria of phenols between 1.0 moldm−3 NaCl and organic solvents at 25 °C

The potentiometric determination of the distribution coefficient of phenol, 2-chlorophenol, 2-nitrophenol, 2,4-dichlorophenol and 2-methyl-phenol between 1.0 mol dm^–3 NaCl and five different organic solvents at 25 °C is presented. An automated potentiometric system was used and the determination of the constants was carried out using both graphical and numerical methods. The relevance of this study for the development of supported liquid membrane (SLM) recovery systems is discussed.     

Protonation of polyamines in NaCl aqueous solution and binding of Cl by polyammonium cations

Equilibrium constants relative to the binding of Cl⁻ by nine open chain polyammonium cations (di, tri and tetra) were determined by potentiometric measurements (H⁺-glass electrode), at T=25°C. To this end the protonation constants of these amines were measured in NaCl aqueous solutions, in the ionic strength range 0.1<I≤1 mol dm⁻³. The different amines (some of which are N-alkyl substituted) were chosen in order to consider several factors affecting the values of protonation constants, the chloride complex formation constants and the dependence on ionic strength of apparent protonation constants. As concerns these last two points, it was found that fully N-alkyl substituted amines behave in a very similar way, with respect to partially or non-substituted ones. Simple linear relationships are reported involving chloride formation constants, parameters for the dependence on ionic strength of protonation constants and charges in polyammonium cations. The complexes formed by two linear polyamines with NO₃⁻ have also been studied for comparison. Literature data are examined.     

Complexation of N-bis[2-(1,2-dicarboxyethoxy)ethyl]aspartic acid with Fe(II), Co(II), and Ni(II) in aqueous solution

In a search for environment-friendly metal chelating ligands for industrial applications, the protonation and complex formation equilibria of N-bis[2-(1,2-dicarboxyethoxy)ethyl]aspartic acid (BCA6) with Fe(II), Co(II), and Ni(II) ions in aqueous 0.1 M NaCl solution were studied at 25°C by potentiometric titration. The model for complexation and the stability constants of different complexes were determined for each metal ion using SUPERQUAD. In all cases, complex formation was dominated by stable ML^4? complexes.     

Protonation States of Glufosinate in Aqueous Solution

Glufosinate, an analogue of glutamic acid (also known as phosphinothricin), is an important herbicide. The protonation constants of glufosinate were determined by means of potentiometric titrations at variable temperatures with ionic strength 0.5 mol·L^?1 (NaCl solution). The heat effects of the protonation reactions of glufosinate were measured by direct calorimetry. NMR spectroscopies have demonstrated that the first protonation site occurs on the nitrogen atom in the amino group, followed by one of the oxygen atoms in the phosphono group, and finally the carboxyl oxygen atom. This trend is in good agreement with the enthalpy of protonation and crystal structure results. These data will help to predict the speciation of glufosinate in physiological systems.     

Thermodynamic Protonation Parameters of some Sulfur-Containing Anions in NaCl<Subscript>aq</Subscript> and (CH<Subscript>3</Subscript>)<Subscript>4</Subscript>NCl<Subscript>aq</Subscript> at <Emphasis Type="Italic">t</Emphasis>=25 °C

Protonation constants of one thiocarboxylate (thioacetate) and four sulfur-containing carboxylates (2-methylthioacetate, thiolactate, thiomalate, 3-mercaptopropionate) were determined by potentiometric measurements in a wide ionic strength range [0≤I≤5 mol⋅L⁻¹ in NaCl and 0 ≤I≤3 mol⋅L⁻¹ in (CH₃)₄NCl] at t=25 °C. For two of these ligands (2-methylthioacetate and thiolactate), the protonation enthalpies were also determined by calorimetric measurements in NaCl ionic medium [0 ≤I≤5 mol⋅L⁻¹] at t=25 °C. Individual UV spectra of the protonated and unprotonated 3-mercaptopropionate species, together with values of the protonation constants, were obtained by spectrophotometric titrations. Results were analyzed in terms of their dependence on the ionic medium by using different thermodynamic models [Debye-Hückel type, SIT (Specific ion Interaction Theory) and Pitzer’s equations]. Differences among protonation constants obtained in different media were also interpreted in terms of weak complex formation.     

Study on the interaction of lanthanum(III) with adrenaline

Lanthanum(III) equilibria in the presence of adrenaline have been investigated by potentiometric titration under physiological conditions (37°C and an ionic strength of 0.15?M NaCl). The interaction of lanthanum(III) with adrenaline has also been studied using an ab initio method. The complex species in the lanthanum(III)–adrenaline system have been ascertained and the protonation constants for adrenaline and the stability constants for lanthanum(III) complexes with adrenaline have been obtained. Adrenaline can form stable lanthanum(III) complexes with the phenolic hydroxyl group of adrenaline as the binding site of lanthanum(III).     

Solute–Solvent Interaction Effects on Protonation and Aggregation Constants of TTMAPP in Different Aqueous Solutions of Methanol

The protonation constants of 5,10,15,20-tetrakis(4-trimethyl-ammonio-phenyl)-porphine tetratosylate (TTMAPP) were determined in water–methanol mixed solvents, using a combination of spectrophotometric and potentiometric methods at 25 °C in 0.1 mol·dm^?3 sodium perchlorate. Two protonation constants, K ₁ and K ₂, were characterized and analyzed in various media in terms of the normalized polarity parameter ( $ E_{\text{T}}^{\text{N}} $ E T N ). A linear correlation is observed when the experimental log₁₀ K ₁ and log₁₀ K ₂ values are plotted versus the calculated ones over the range of 40–90 % (v/v) methanol. The self aggregation of TTMAPP was observed from acidic media (pH ? 3) to alkaline pH, where it reached its highest intensity, when methanol is lower than 40 % in solution. The formation of aggregate species prevents a quantitative analysis of titration curves and thus, the determination of the protonation constants of TTMAPP. Therefore, to evaluate the protonation constants of TTMAPP in low or zero percent of methanol, the Yasuda–Shedlovsky extrapolation approach has been used.     

Complexation of some environmentally friendly carboxylic acid ligands with La3+ ion in aqueous solution

In search for environmentally friendly metal chelating ligands for industrial applications, protonation, and complex-formation equilibria of iminodisuccinic acid (IDS), 3-hydroxy-2,2′ -iminodisuccinic acid (HIDS), N,N′-ethylenediaminedisuccinic acid (EDDS), and oxydisuccinic acid (ODS) with La³⁺ in aqueous 0.1?mol?L^?1 NaCl solution were studied at 25°C by potentiometric titration. The model for complexation and the stability constants of the different complexes were determined using the computer program SUPERQUAD. Complex formation was dominated by stable LaL^? complexes for all tested ligands.     

Solute-Solvent Interaction Effects on Protonation Equilibrium of Some Water-Insoluble Flavonoids

The protonation constants of three different flavonoids (naringenin, chrysin, and daidzein) were determined in water–DMSO mixed solvents using a potentiometric method at 25.0 (±0.1) °C and 1.00 mol⋅dm⁻³ tetra-n-butylammonium chloride as supporting electrolyte. The protonation constants were characterized and were analyzed in various solvent media in terms of the Kamlet, Abboud and Taft (KAT) parameters. Single-parameter correlations of the protonation constants versus α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity) or π ^∗ (dipolarity/polarizability) are poor for all compounds, but the dual-parameter α and π ^∗ correlation presents significant improvement with regard to the single- and multi-parameter models. A linear correlation is observed when the logarithm of the experimental protonation constants is plotted versus the calculated ones when the KAT parameters are considered. The protonation constants of the flavonoids in water were also calculated by the Yasuda-Shedlovsky extrapolation method at zero percent organic solvent. Finally, the results are discussed in terms of the effect of solvent composition on the protonation constants.     

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).     

Determination of Protonation Constants of O-Phospho-<Emphasis Type="SmallCaps">l</Emphasis>-serine in Aqueous Solution: Potentiometry,Microcalorimetry, NMR Spectroscopy and Quantum Chemical Calculations

O-Phospho-l-serine is one of the naturally occurring phosphorylated amino acids, having important pharmacological activity and bioactivity. The protonation constants of O-phospho-l-serine were determined by means of potentiometric titrations at 25 °C and ionic strength of 0.5 mol·L^?1 (NaCl). The heat effects of the protonation reaction of the O-phospho-l-serine were measured by direct calorimetry. NMR spectroscopy has demonstrated that the first protonation site occurs at the nitrogen atom in the amino group, followed by one of the oxygen atoms in the phosphono group, and finally the carboxyl oxygen atom. This trend is in good agreement with the enthalpy of protonation and quantum chemical calculations. These data will help to predict the speciation of O-phospho-l-serine in physiological systems.     

Reliability of protonation constants of SNAZOXS oligomers

Concentration protonation constants of variously protonated of oligomers of sulphoazoxine SNAZOXS were determined by regression analysis of potentiometric titration curves. The group and common parameters were estimated using different computational strategies of three regression programs, MINIQUAD, MIQUV and PSEQUAD. ANOVA proved that six various computational strategies of three regression programs have no significant influence on reliability of protonation constants estimated in comparison with the reproducibility of the titration. Chemical model of protonation equilibria L(2)H(5-), L(2)H(4-)(2), L(2)H(3-)(3), and L(2)H(2-)(4) and reaction scheme of oligomers protonation for SNAZOXS was found.     

Equilibrium studies of organotin(IV) complexes with vitamin B(6)

The complex-formation equilibria of dimethyltin(IV), trimethyltin(IV) and tributyltin(IV) with pyridoxamine were investigated in dioxane-water mixtures and at different temperatures using a potentiometric technique. The stepwise formation constants of the complexes formed in solution were calculated using the non-linear least-square program MINIQUAD-75. The effect of dioxane as a solvent on the protonation constants of pyridoxamine and the formation constants of organotin(IV) complexes was discussed. The thermodynamic parameters DeltaH degrees and DeltaS degrees calculated from the temperature dependence of the equilibrium constants were investigated. The concentration distribution of the various complex species was evaluated as a function of pH.     

Unexpected dependence of the protonation constant of 2,2'-bipyridyl on ionic strength

The protonation constants of 2,2'-bipyridyl and ammonia have been determined by pH titration at 25 degrees , at ionic strengths of 0.1, 0.2, 0.5, 1.0, 1.5 and 2.0M obtained by using LiNO(3), NaNO(3), KNO(3), LiClO(4) and NaClO(4) as background electrolytes. The protonation constants generally change by about 0.3-0.4 log units for both ligands in nitrate media. A similar change in the protonation constant of ammonia was observed in perchlorate media. There is, however, a change of about 0.8-0.9 log units in the protonation constant of bipyridyl in the perchlorate media. This phenomenon is interpreted by postulating ion-pair formation between perchlorate and the protonated form of bipyridyl, HBp(+) + ClO(4)(-) rlharr2; HBp(+).ClO(4)(-) with formation constants of 0.54 in 2M lithium nitrate and 0.45 in 2M sodium nitrate.     

Limitations of potentiometric studies to determine the surface charge of gibbsite gamma-Al(OH)3 particles

Surface charges of gibbsite particles were probed by potentiometric titration and subsequently analyzed to estimate intrinsic proton affinity constants of OH surface groups. A detailed spectroscopic characterization of the molecular structure of surface OH groups yielded estimates of bond lengths and bond valences of OH surface sites. Based on these results, the effects of the setting parameters of a MUSIC calculation have been shown in comparison with previous predictions yielding higher pKa values (2 < -pKa < 4) for the protonation of basal doubly coordinated OH surface groups and lower pKa values (7.9 < -pKa < 9.9) for the protonation of lateral singly coordinated OH surface groups. Comparison with experimental data is complicated by reproducible hysteresis between acid and base addition in optimal raw potentiometric titration curves at different ionic strengths. Such effects prevented the determination of a univocal intersection point to provide the global point of zero charge of gibbsite particles, even though the ionic strength dependence of the point of zero net proton charge and the different crossovers between curves indicated that the point of zero charge could be estimated between 8.1 and 9.6, in relative agreement with the lateral affinity constant calculated with the MUSIC model. Still, two main drawbacks remained to differentiate the reactivity of lateral singly and basal doubly coordinated surface groups. First, significant kinetic effects observed in acidic media indicated a dissolution process and/or protonation of basal surface groups. Second, the choice of specific surface areas, especially for a heterogeneous sample, led to several cases for the calculation of the absolute surface charge of particles. Therefore, our results demonstrated the heterogeneous reactivity of gibbsite particles and that the prediction and the experimental determination of respective surface groups are still complex even if some trends emerge.