Teresa Vilariño

Effect of ionic strength on the formal potential of the glass electrode in various saline media

We examined the variation with ionic strength (I, adjusted with KCl, KNO(3), KBr, NaCl or NaClO(4)) of the formal potential (E(const)) for glass electrodes exhibiting a Nernstian response (i.e. E(cell)=E(const)-slog[H(+)]). For this purpose, we investigated the different factors included in the formal potential, so we obtained reported values for the liquid junction potential as a function of ionic strength and determined the logarithm of the activity coefficient for the proton in various saline media, using Pitzer equations.

Full description of copper uptake by algal biomass combining an equilibrium NICA model with a kinetic intraparticle diffusion driving force approach

In this work kinetic and equilibrium studies related to copper binding to the protonated macroalga Sargassum muticum are reported. An intraparticle-diffusion linear driving force (LDF) model has been chosen for the quantitative description of the kinetics at several initial metal concentrations. Copper intraparticle homogeneous diffusion coefficient (D(h)) obtained is in the range 0.2-0.9×10(-10) m(2) s(-1). NICA isotherm is demonstrated to constitute a substantial improvement with respect to a simpler Langmuir competitive equation. The binding parameters were chosen to provide the best simultaneous description of the equilibrium experiments. Values of log K(Cu) (4.3), n(Cu) (1) and p (0.31) in NICA isotherm, and log K(Cu) (3.5-5) in Langmuir competitive model, have been obtained. These parameters have been also used to predict the competition between copper and cadmium for binding sites. Two acids, HNO(3) and HCl, have been tested to evaluate their effectiveness to release copper from the metal-laden biomass.

Effect of ionic strength on the protonation of various aminoacids analysed by the mean spherical approximation

The dependence of the ionic strength of stoichiometric protonation constants, obtained potentiometrically, of several amino acids has been analysed by the mean spherical approximation (MSA), a mathematically simple statistical mechanical treatment based on integral equations. Unlike the Debye–Huckel equation, which overestimates the effects of ion–ion interactions as the sizes of all ions are neglected, the MSA represents the electrolytes as a mixture of hard spheres immersed in a dielectric continuum which represents the solvent. This approach fits experimental data using concentration, electrical charge and diameter concepts and is an alternative to semiempirical models based on different ionic strengths with coefficients that defy theoretical interpretation. Analysis of data using both semiempirical models and a quasi-lattice model, not containing Debye–Huckel terms, is also included.

The salting coefficient and size of alkylamines in saline media at different temperatures: estimation from Pitzer equations and the mean spherical approximation

Abstract The renewed theoretical interest in the proton transfer associated to the amino group together with the scarcity of acid-base studies of amines in moderate to concentrated saline media focussed our attention on the study of the basicities of some alkylamines, namely monomethyl, dimethyl and trimethylamine, in aqueous saline solutions of KCl at various temperatures. A non-conventional analysis of stoichiometric equilibrium constants versus ionic strength data is carried out. On one hand, Pitzer’s model is easily applied to calculate the salting coefficient and the thermodynamic equilibrium constant of the alkylamines. On the other hand, the mean spherical approximation has the advantage over the Debye–Huckel based theories that it can account for effects produced by species of different sizes. Here, it is applied to predict the dependence of the salting behavior on the size of the alkylamines.

Theoretical calculations of the ionic strength dependence of the ionic product of water based on a mean spherical approximation

A modified version of the restricted primitive model for electrolyte solutions based on the mean spherical approximation (MSA) is applied to estimate the ionic strength dependence of the ionic product of water in aqueous solutions containing different salts, which are commonly used as background electrolytes (NaCl, KCl, KNO3, and NaC104). The modification involves the use of permittivity of the solvent as concentration-dependent parameter and a single average effective diameter. This is a way of including effects originated from the solvent which do not exist in the primitive model. In the case of potassium nitrate and sodium perchlorate, a complete methodology to calculate the effective diameter and density dependence of the dielectric constant has been proposed and developed. Fits between calculated and experimental pKw values are possible over wide concentration ranges using a single adjustable parameter, namely, the average hard core diameter of water.

The mean spherical approximation methodology applied to the acid–base equilibria of glycine in artificial seawater

We present results obtained by use of a statistical mechanics approximation based on integral equations, the mean spherical approximation, to predict equilibrium constants for glycine in artificial seawater and to illustrate the effect of the ionic medium. A relationship between the dissociation constants of glycine and the molar concentration of the seawater solution is derived in the range of salinities varying from 0.5 to 4%. The MSA is a simple analytical theory for electrolytes that is very useful in representing the thermodynamic properties of ionic solutions over a wide range of concentrations in terms of one simple parameter, Γ, and the charges, diameters, and concentrations of the ions. A concentration dependent dielectric constant rather than the pure solvent permittivity together with the concentration-dependent diameters of the cations are incorporated in the calculations. This is a way of including the effect of soft repulsions and attractions and the effect of solvation. In calculations, an optimized diameters for the protonated glycine ion and the proton, both of which are involved both in the equilibria of glycine, have been obtained.

The mean spherical approximation and the prediction of the size of the species involved in an ionization equilibrium in saline media

The unrestricted version of the mean spherical approximation is applied to describe the ionization equilibrium of benzoic acid, as a typical simple system for carboxylic acids, in the presence of an excess of background electrolyte and variable ionic strength. When the activity coefficients are theoretically determined by use of the mean spherical approximation (MSA), the thermodynamic properties of the ionization equilibrium in 1:1 electrolytes for ionic strengths up to 2.5 M can be accurately described, provided different sizes are assumed for each species in solution. The results of this study show that the application of MSA allows one to estimate the size of the species involved in an ionization process from experimental data of stoichiometric equilibrium constants, an alternative normally not explored. However, results suggest that it is necessary to include more sophisticated models of MSA in order to develop this approach with more confidence.

Effect of Ionic Strength on the Kinetics of the Oxidation of Ascorbic Acid by Hexacyanoferrate(III): Comparison between Specific Interaction Theories and the Mean Spherical Approximation

The influence of ionic strength on the rate constant of the oxidation reaction of ascorbic acid by hexacyanoferrate(III) in acidic medium is investigated. The dependence of the rate constant on ionic strength is discussed according to different specific interaction theories and the mean spherical approximation.

A Systematic Analysis and Review of the Fundamental Acid-Base Properties of Biosorbents

A broad variety of materials of biological origin have been successfully used in recent decades for the removal of pollutants from solution. These biosorbents present a range of natural polymers that play a key role on their adsorption capacity. It is therefore critical to understand the physicochemical properties of the chemical groups that form these polymers. According to bibliography, less than 3% of biosorption papers include studies on proton binding. The acid-base properties of biomass are affected by pH, ionic strength and medium composition. Nevertheless, these crucial parameters are not always considered during biosorption studies. This review outlines the major advances on proton binding data interpretation and modelling on biosorbents. In addition, we propose some experimental considerations that cover all issues raised in this review concerning the acid-base properties of biosorbents. Only 30% of the reviewed papers that study algae, agricultural wastes or lignocellulosic materials use Donnan or double-layer surface models to account for electrostatic interactions on proton binding. Expressions for activity coefficients, such as Debye-Huckel or Pitzer equations, are shown only in c.a. 15% of these papers. Moreover, studies investigating a range of ionic strengths represent a 40%, while this variable is not even considered in 20% of the papers. We could not find any biosorption study related to specific salt or Hofmeister effects. Moreover, in 6 out of 10 papers there is important experimental information missing such as the calibration of the electrodes. We consider therefore that there is an important need for reviewing the role of proton binding on biosorption studies.

Acid-Base Equilibria in Saline Media: Application of the Mean Spherical Approximation

The simplest mathematical formulation of the hypothesis that chemical equilibrium exists in the bulk of a saline solution is based on the equilibrium constant, K T = K * Q(γ i ), where K T is the thermodynamic equilibrium constant, K * the stoichiometric constant and Q(γ i ) the ratio of activity coefficients (γ i ) associated with the equilibrium. Studies on the effect of ionic strength on stoichiometric constants are based on modelling of the Q(γ i ) term (Sastre de Vicente 1997; Daniele et al.1997).