Trace analysis of acids and bases by conductometric titration with multiparametric non-linear regression

A chemometric method for analysis of conductometric titration data was introduced to extend its applicability to lower concentrations and more complex acid-base systems. Auxiliary pH measurements were made during the titration to assist the calculation of the distribution of protonable species on base of known or guessed equilibrium constants. Conductivity values of each ionized or ionizable species possibly present in the sample were introduced in a general equation where the only unknown parameters were the total concentrations of (conjugated) bases and of strong electrolytes not involved in acid-base equilibria. All these concentrations were adjusted by a multiparametric nonlinear regression (NLR) method, based on the Levenberg-Marquardt algorithm. This first conductometric titration method with NLR analysis (CT-NLR) was successfully applied to simulated conductometric titration data and to synthetic samples with multiple components at concentrations as low as those found in rainwater (∼10 μmol L⁻¹). It was possible to resolve and quantify mixtures containing a strong acid, formic acid, acetic acid, ammonium ion, bicarbonate and inert electrolyte with accuracy of 5% or better.     

Determination of acidic sites and binding toxic metal ions on cumin surface using nonideal competitive adsorption model

A fundamental study of the application of cumin biomass in the recovery of Cu and Zn metal ion uptake from food and drinks is carried out at different pH's and at fixed ionic strength. The chemical characteristics of protein in cumin seeds were investigated. Results showed that cumin contains 18.25% crude protein, which includes 18 amino acids. The main reactive groups on protein cumin are amino and carboxylic groups of dicarboxylic amino acids, leading to a pH-dependent charge. Therefore, the cumin surface is considered as a heterogeneous system. To describe protonation behavior in a heterogeneous cumin biomass (cumin/0.1 M NaNO(3)) system, acid-base titrations have been performed with conductometric and potentiometric titration. Measurement of the reactivity of cumin surface in the adsorption of Cu and Zn metal ions and determination of metal binding at different pH's were also carried out. To solve broad and ill-defined titration curves, a simplified version of nonideal competitive analysis (NICA) by Plette et al.) was applied. The results show that the sorption of the bivalent metal ions onto the whole surface of cumin could be attributed to a monodentate binding to one site mainly carboxylic-type site.     

The influence of water-acetone solvents on the acid-base properties of glycylglycine

The protonation and dissociation constants of glycylglycine in variable-composition water-acetone solvents were determined potentiometrically at the ionic strength of solutions 0.1 M (NaClO₄) and 298 K. Reagent solvation contributions to shifts of acid-base equilibria are analyzed.     

Isothermal titration calorimetric studies of the acid-base properties of poly(N-isopropylacrylamide-co-4-vinylpyridine) cationic polyelectrolyte colloidal microgels

Isothermal titration calorimetry (ITC) and potentiometric titration were used to study the protonation properties of the 4-vinylpyridine (4-VP) moiety in cationic poly(N-isopropylacrylamide-co-4-vinylpyridine) colloidal microgels [poly(NIPAM-co-4-VP)]. Calorimetric pH titrations were performed using microgels of different 4-VP content and the influence of ionic strength and counter ions have been examined. The calorimetric titration output consists of several thermal contributions reflecting the complex nature of the interactions in the aqueous microgel dispersions. In contrast to the potentiometric results, the calorimetric titration data could not be completely described by a theoretical model solely taking into account protonation equilibria. Deviations from the proposed model correlate with swelling or shrinking of the gel particles. The calorimetric results also reveal a pronounced counter-ion effect of perchlorate compared to chloride ions. In the presence of perchlorate ions, small secondary thermal effects accompany protonation of the 4-VP moiety due, in part, to kinetically limited conformational changes in the co-polymer microgel.     

Instrumental measurement of bitter taste in red wine using an electronic tongue

An electronic tongue (ET) based on potentiometric chemical sensors was assessed as a rapid tool for the quantification of bitterness in red wines. A set of 39 single cultivar Pinotage wines comprising 13 samples with medium to high bitterness was obtained from the producers in West Cape, South Africa. Samples were analysed with respect to a set of routine wine parameters and major phenolic compounds using Fourier transform infrared-multiple internal reflection spectroscopy (WineScan) and high-performance liquid chromatography, respectively. A trained sensory panel assessed the bitterness intensity of 15 wines, 13 of which had a bitter taste of medium to high intensity. Thirty-one wine samples including seven bitter-tasting ones were measured by the ET. Influence of the chemical composition of wine on the occurrence of the bitter taste was evaluated using one-way analysis of variance. It was found that bitter-tasting wines had higher concentrations of phenolic compounds (catechin, epicatechin, gallic and caffeic acids and quercetin) than non-bitter wines. Sensitivity of the sensors of the array to the phenolic compounds related to the bitterness was studied at different pH levels. Sensors displayed sensitivity to all studied compounds at pH 7, but only to quercetin at pH 3.5. Based on these findings, the pH of wine was adjusted to 7 prior to measurements. Calibration models for classification of wine samples according to the presence of the bitter taste and quantification of the bitterness intensity were calculated by partial least squares-discriminant analysis (PLS-DA) regression. Statistical significance of the classification results was confirmed by the permutation test. Both ET and chemical analysis data could discriminate between bitter and control wines with the correct classification rates of 94% and 91%, respectively. Prediction of the bitterness intensity with good accuracy (root mean square error of 2 and mean relative error of 6% in validation) was possible only using ET data.     

Comparative acid-base properties of the surface of components of the CdTe-ZnS system in series of substitutional solid solutions and their analogs

The acid-base properties of the surface of solid solutions and binary components of the CdTe-ZnS system are studied by hydrolytic adsorption, nonaqueous conductometric titration, mechanochemistry, IR spectroscopy, and Raman scattering spectroscopy. The strength, nature, and concentration of acid centers on the original surface and that exposed to CO are determined. The changes in acid-base properties in dependence on the composition of the system under investigation in the series of CdB₆, ZnB₆ analogs are studied.     

Coulometric titrations in microscale study of ionic equilibria

Summary Coulometric titration, a well established technique in research and in routine analysis, can successfully be applied to the study of ionic equilibria in solution. Processes, where either lyate or lyonium ions serve as titrants, for determination of acids and bases, respectively, were investigated. Among the systems studied are: autoprotolytic equilibria of various amphiprotic solvents, acid-base equilibria in water, organic and mixed solvents and complexation reactions. In all these examples the titration was followed by a potentiometric pH-indicator electrode. In nonaqueous solvents coulometry overcame the limited solubility of electrolytes, the difficulty of standardization on solutions and the necessity of working in isolated systems to avoid contamination of the sample with water vapour or with carbon dioxide.Because volumes below 1 ml and concentrations in the order of 0.001 mol/l can be used, this technique can be successfully applied to the investigation of micromole amounts of compounds, when protolytic properties of substances available in minute quantities should be evaluated. Furthermore this technique was applied to the evaluation of protonation constants of biologically important buffers and the stability constants of their calcium complexes.     

Solvent Effects on Tautomeric and Microscopic Protonation Constants of Glycine in Different Aqueous Solutions of Methanol and Ethanol

The acid-base equilibria of glycine have been studied in different aqueous solutions of methanol and ethanol (0?C45?% v/v) using a potentiometric method. In this study, the macro and micro protonation constants of the amino acid and its tautomeric constant have been determined at 25?°C and constant ionic strength (0.1 mol?dm^?3 NaClO₄). The protonation and the tautomeric constants of glycine in different binary mixtures were analyzed in terms of the Kamlet, Abboud and Taft (KAT) parameters. Single-parameter correlations of the constants versus ?? (hydrogen-bond donor acidity), ?? (hydrogen-bond acceptor basicity) and ?? ^? (dipolarity/polarizability) are poor in all solutions. Multi-parameter correlations show better results, but dual-parameter correlations represent significant improvements with regard to the single- and multi-parameter models. Linear correlation is observed when the experimental protonation constant values are plotted versus the calculated ones when the KAT parameters are considered. Finally, the results are discussed in terms of the effect of the solvent on protonation and tautomeric constants.     

Protolytic properties of polyamine wasp toxin analogues studied by 13C NMR spectroscopy

Acid-base properties of the natural polyamine wasp toxin PhTX-433 (1) and seven synthetic analogues [PhTX-343 (2), PhTX-334 (3), PhTX-443 (4), PhTX-434 (5), PhTX-344 (6), PhTX-444 (7), and PhTX-333 (8)], each having four protolytic sites, were characterized by 13C NMR spectroscopy. Nonlinear, multiparameter, simultaneous fit of all chemical shift data obtained from the NMR titration curves yielded macroscopic pKa values as well as intrinsic chemical shift data of all differently protonated macrospecies. Analyses of the chemical shift data demonstrated strong interactions between all four sites and provided information about complex relationships between chemical shift values and protonation state. Deprotonation of fully protonated forms starts at the central amino group of the polyamine moiety, and the extent of this trend depends on the distance to the flanking, protonated amino groups. The pKa1 values of 1-8 are in the range 8.2-9.4. Hence, some of the toxins are incompletely protonated at the pH and ionic strength conditions used for assessment of their interactions with ionotropic glutamate and nicotinic acetylcholine receptors, and the degree of protonation is expected to have pharmacological importance in the ion-channel binding event.     

Acid-base equilibria in solutions of cephotaxime and cephtriaxone

The concentration constants of acid-base equilibria in aqueous solutions of cephotaxime and cephtriaxone were determined pH-metrically at 20°C against the background of 0.1, 0.4, 0.7, and 1.0 M solutions of KCl and KNO₃. The thermodynamic constants of these equilibria were calculated by the extrapolation of the concentration constants to zero ionic strength according to the Hückel equation. Cephotaxime and cephtriaxone ionic form sizes were calculated by the semiempirical PM3 method for energetically favorable conformations.     

Thermodynamic Constants of Acid-Base Equilibria in Solutions of Penicillins

The constants of acid-base equilibria in solutions of benzylpenicillin, carbenicillin, and ampicillin at 20°C were determined by pH-metric titration (supporting electrolytes 0.1, 0.4, 0.7, and 1.0 M KCl and KNO₃). The sizes of the ionic species of the penicillins in their energetically favorable conformation were determined by the PM3 semiempirical method. The thermodynamic equilibrium constants were estimated by extrapolation of the concentration constants to zero ionic strength by the Huckel equation, taking into account the calculated sizes of the ions.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 6, 2005, pp. 1049–1054.Original Russian Text Copyright © 2005 by Alekseev, Demskaya, Dodonova.     

Some studies on the determination of indicator transition and its use in the titration of weak basic samples

In the titration of a weak basic sample, especially in the dilute solution, the transition color of the indicator is not sharp at the equivalence point. In order to avoid the indicator error, the color transition near the equivalence point must be known accurately. The color transition of an acid-base indicator can be calculated from complementary tristimulus data. To ensure accuracy, however, the chemical stoichiometric relationships about the acid-base equilibria are considered. A general expression for determination of the equivalence point is presented and factors influencing the titration are discussed.     

Distinguishing polyfunctional from monofunctional acids and bases by acid-base titrimetry, multiparametric curve-fitting, and deviation-pattern recognition

A new technique for distinguishing diacidic from monoacidic weak bases (or dibasic from monobasic weak acids) is based on fitting the data obtained in a potentiometric acid-base titration to theoretical equations for the titration of a monoacidic base (or monobasic acid). If the substance titrated is not monofunctional the best fit to these equations will involve systematic deviations that, when plotted against the volume of reagent, yield a "deviation pattern" with a shape characteristic of polyfunctional behaviour. Ancillary criteria based on the values of the parameters obtained from the fit are also described. There is a range of uncertainty associated with each of these criteria in which the ratios of successive dissociation constants are so close to the statistical values that it is impossible in the face of the errors of measurement to decide whether the substance is monofunctional or polyfunctional. If the data from one titration prove to lie within that range, the decision may be based on the results of a second titration performed at a different ionic strength. Further fitting to the equations describing more complex behaviour provides a basis for distinguishing non-statistical difunctional substances from trifunctional ones, trifunctional ones from tetrafunctional ones, and so on.     

31P nuclear magnetic resonance titrations: Simultaneous evaluation of all pH-dependent resonance signals

The simultaneous evaluation of ali pH-dependent resonance signals (or multiplets) of an NMR titration offers a substantially increased accuracy and significance. The number of linearly independent titration equilibria is determined by graphical matrix rank analysis. The chemical shifts of all pH-dependent resonance lines are plotted against each other (chemical shift or CS diagrams) indicating whether a single or more titration equilibria are NMR spectrometrically observable and how far they overlap with each other. An iterative curve-fitting program allowing the simultaneous evaluation of all (pH) curves is available, from which pK values and chemical shifts of all species can be calculated. The starting pK values for the iteration need only be estimated very approximately (accuracy ±1–2 units). The titration end-points do not have to be experimentally accessible. The different methods for the simultaneous evaluation of all pH-dependent NMR signals are exemplified in the ³¹P NMR titration of thiamine pyrophosphate. In this case either the observed resonance lines (two doublets in a broad band proton decoupled spectrum) or the calculated chemical shifts for this AB system can be evaluated. A titration of sodium pyrophosphate was performed and evaluated for comparison.     

氨基酸质子化反应的滴定量热测定

在298．2K和0．1mol•dm－3（KNO3）离子强度的实验条件下，用常规滴定量热法测定了八种氨基酸羧基质子化反应的平衡常数和反应焓变；用扩展的滴定量热法测定了这些氨基酸氨基质子化反应的平衡常数和反应焓变；并讨论了各热力学性质之间的关系．     

Estimation of Pitzer's ion interaction parameters for electrolytes involved in complex formation using a chemical equilibrium model

Pitzer's ion interaction model has been widely accepted for calculating the thermodynamic properties for electrolytes at high ionic strength. For weak electrolytes, a better estimation can be obtained by combining the model with chemical equilibria. The method of calculation is to treat each individual species as a single, separated ion. The concentration of each ion will be constrained by the mass balance equation and its activity will be guarded by the stability constants. Including chemical equilibria in Pitzer's model provides not only a better estimation of the thermodynamic properties of weak electrolytes but also a better understanding of the equilibrium among the complexes. The results may be used for correcting the effect from high ionic strength solution when determining the stability constants. When considering chemical equilibria, some of the parameters reported by Pitzer may have to be reestimated. The method of estimation and comparison between final results are presented. The binary system of HF, and the ternary systems of CuCl₂ in NaCl and in HCl are used for demonstration.     

Multi-wavelength spectrophotometric determination of the protolytic constants of tetracycline hydrochloride in some nonaqueous-water mixed solvents: a solvatochromism study

Annihilation of the contribution of one chemical component from the original data matrix is a general method in rank annihilation factor analysis (RAFA). However, RAFA is not applicable for studying the protonation equilibria of multiprotic acids but in this study two-rank annihilation factor analysis (TRAFA) was used as an efficient chemometrics algorithm for determination of the protolytic constants (pKa) of tetracycline hydrochloride (TCHC) in some nonaqueous-water mixed solvents such as acetonitrile (AN)-water and methanol (MeOH)-water from the spectral pH-absorbance data. The spectral data was obtained from spectrophotometric acid-base titrations of different solutions of TCHC at (25.0±0.10)°C and an ionic strength of 0.10 M. In TRAFA algorithm the pKa values were obtained with relationship between residual standard deviation (R.S.D.) and hypothetical pKa values. In the case of TCHC, the spectra were divided in two consecutive subdivisions according to their pH range having two pKa and TRAFA was run twice. The validity of the obtained pKa values was checked with well-known chemometrics algorithms such as DATAN, EQUSPEC, SPECFIT/32 and SQUAD. The effects of changing solvent composition on the protolytic constants were explained by linear solvation energy relationships (LSER) utilizing solvatochromic parameters.     

The effect of surfactants on the dissociation constants of phenothiazine derivatives Alkalimetric determination of diethazine and chlorpromazine

The effect of a cationic, an anionic and a non-ionic surfactant on the acid-base equilibria of the phenothiazine derivatives, diethazine hydrochloride and chlorpromazine hydrochloride, has been studied. It has been found that the presence of cationic and non-ionic surfactants strongly enhances the dissociation of the two derivatives, whereas the anionic surfactant decreases the dissociation constant. These effects are in agreement with a theory based on a pseudophase, ion-exchange model of micelles. From the dissociation-constant values as a function of the surfactant concentration, the binding constants for diethazine and chlorpromazine with the surfactants Septonex and sodium dodecylsulphate have been calculated. The ability of cationic surfactants to solubilize the free bases of the phenothiazine derivatives and to increase their dissociation constants has been utilized to develop a new method for alkalimetric determination of the derivatives in a micellar medium. The method has been applied to determination of the content of the active component in pharmaceutical preparations.     

Thermodynamic Characteristics of Acid-Base Reactions of Aromatic Derivatives of α-Aminopropionic Acid in Aqueous Solution

The thermal effects of acid-base reactions of phenylalanine and tryptophan in aqueous solutions (ionic strength 0.5, 1.0, and 1.5; supporting electrolyte KNO₃) at 298.15 K were determined calorimetrically. The thermodynamic characteristics of stepwise dissociation of the amino acids in aqueous solution were calculated. The influence of the structure of the amino acids on the thermodynamic parameters of the protolytic equilibria was examined.