Claudia Foti

A new approach in the use of SIT in determining the dependence on ionic strength of activity coefficients. Application to some chloride salts of interest in the speciation of natural fluids

Abstract This paper describes a modified version of the SIT (Specific ion Interaction Theory) method and its use in determining the dependence on ionic strength of activity coefficients. In the new approach the interaction coefficients (ε) are not constant but depend on ionic strength (I /mol kg-1) according to the simple relationship: ε = ε∞+ (ε0 - ε∞) / (l + 1) where ε0 and are true constants for I→ 0 and l→ ∞, respectively. To check the two parameter SIT equation, we calculated ε0 and for the activity coefficients of HCl, LiCl, NaCl, KCl, MgCl2, CaCl2 and SrCl2, in a wide ionic strength range (0.1 ≤ l/mol kg-1 ≤ 4.5, for KCl; 0.1 ≤ l/mol kg-1 ≤ 6, for HCl, LiCl, NaCl; 0.3 ≤ l/mol kg-1 ≤ 12, for SrCl2; 0.3 ≤ l/mol kg-1 ≤ 15, for MgCl2; 0.3 ≤ l/mol kg-1 ≤ 18, for CaCl2). Results show that the γ values calculated using this approach fit quite well over the whole I-range for all the electrolytes considered. Comparison is made with the analogous one parameter SIT equation. The temperature coefficients of interaction coefficients were also calculated using (HCl) in the range 0 ≤ t/°C ≤ 60.

Solubility and Activity Coefficients of Acidic and Basic Nonelectrolytes in Aqueous Salt Solutions. 2. Solubility and Activity Coefficients of Suberic, Azelaic, and Sebacic Acids in NaCl(aq), (CH~3)~4NCl(aq), and (C~2H~5)~4NI(aq) at Different Ionic Strengths and at t = 25 ^oC

The solubility of three dicarboxylic acids, (COOH)-(CH 2 ) n -(COOH) (n = 6, 7, 8), was determined in NaCl(aq) (0.5 to 4.5) mol·L -1 , (CH 3 ) 4 NCl(aq) (0.25 to 3) mol·L -1 , (C 2 H 5 ) 4 NI (0.25 to 1) mol·L -1 , and in pure water at t = 25 °C. The solubility trend is (C 2 H 5 ) 4 NI > (CH 3 ) 4 NCl > NaCl. The Setschenow constants and activity coefficients of neutral species were calculated from these data. To fully characterize the acid-base behavior of above dicarboxylic acids, the protonation constants were determined in the same conditions. From these data, together with solubility ones, Pitzer and SIT parameters were calculated.

The interaction of amino acids with the major constituents of natural waters at different ionic strengths

Abstract The interaction of amino acids with the major constituents of natural waters has been studied potentiometrically by determining protonation constants at different ionic strengths (e.g., I ≤5.6 mol (kg H 2 O) −1 (NaCl)) and in artificial seawater (containing Na + , K + , Ca 2+ , Mg 2+ , Cl − and SO 4 2− ) at different salinities. For glycine determinations in mixed NaCl–MgCl 2 , electrolyte solutions were also performed. The data included in this work, together with some already published, make it possible to calculate parameters for dependence on ionic strength using different models, i.e. an extended Debye–Huckel type equation and Pitzer equations. The results can be interpreted both in terms of specific interaction and in terms of complex formation. The formation constants of Mg 2+ –glycine complexes have been calculated from protonation data in mixed Na + –Mg 2+ solutions at high ionic strength: a simple method for taking into account the different composition, at constant ionic strength, is reported. The overall complexing ability of major seawater components towards amino acids has been calculated from potentiometric data in artificial seawater using the single salt approximation. The relevance of the data reported in this work is discussed in connection with speciation problems. Particular attention has been paid to finding some useful predictive relationships.

Thermodynamic parameters for the binding of inorganic and organic anions by biogenic polyammonium cations

Thermodynamic parameters for the interaction of protonated biogenic polyamines with inorganic or organic polyanions were studied potentiometrically (H(+)-glass electrode) and calorimetrically, at 25 degrees C. No background salt was used in the measurements to avoid interferences, and the formation constants and formation enthalpies were extrapolated to zero ionic strength. Species formed are ALH(r) [L=Cl(-), SO(4)(2-), HPO(4)(2-), P(2)O(7)(4-) and P(3)O(10)(5-); tartrate, malate, citrate, glutamate, 1,2,3-propanetricarboxylate, 1,2,3,4-butanetetracarboxylate], with r=1,2...(n+m-2) and r=1,2...(n+m-1) for inorganic and organic ligands, respectively (n, m=maximum degree of protonation of amine and ligand, respectively). The stability of the various species formed is a function of charges involved in the formation reaction. DeltaH(0) values are generally positive, and therefore these complexes are entropically stabilized. Results are discussed in connection with several previously reported data on similar systems. DeltaG(0) and TDeltaS(0) follow a linear trend as a function of polyammonium cation and inorganic or carboxylic anion charges. DeltaG(0) and TDeltaS(0) charge relationships are reported. In particular, mean values of DeltaG(0) and TDeltaS(0) for single interaction were calculated: DeltaG(0)=7.0 kJ mol(-1) n(-1), TDeltaS(0)=9.1 kJ mol(-1) n(-1) and DeltaG(0)=5.7 kJ mol(-1) n(-1) and TDeltaS(0)=8.7 kJ mol(-1) n(-1), for the species of inorganic and organic polyanions, respectively (n=number of possible salt bridges). A linear relationship was also found for TDeltaS(0) versus DeltaG(0), whose equation is TDeltaS(0)=-7-1.39 DeltaG(0) (with r=0.9409; r, correlation coefficient). The body of correlations found for these thermodynamic parameters shows quite good predictive value.

Salt effects on the protonation and on alkali and alkaline earth metal complex formation of 1,2,3-propanetricarboxylate in aqueous solution

The protonation of 1,2,3-propanetricarboxylate (tricarballylate, tca) was studied in LiCl, NaCl, KCl, MgCl(2), CaCl(2) and tetraethylammonium iodide (Et(4)NI) aqueous solutions, at 25 degrees C, in the ionic strength range 0 < I < 1M, using the pH-metric technique. The differences between protonation constants determined in Et(4)NI and those determined in the other background salts were interpreted in terms of complex formations. Least squares calculations are consistent with the formation of MLH(j) (j = 0,1.2), M(2)LH(i) (i = 0,1,2), M(2)L species, when M = Mg(2+), Ca(2+). Potentiometric measurements performed in mixed NaClKCl, NaClCaCl(2) and MgCl(2)CaCl(2) solutions showed the formation of mixed metal complexes NaKL, NaKHL, NaCaL and CaMgL. The dependence on ionic strength of protonation and complex formation constants was evaluated using a simple Debye-Hückel type equation.

Chelating Agents for the Sequestration of Mercury(II) and Monomethyl Mercury(II)

Both mercury(II) and monomethyl mercury(II) poisonings are of great concern for several reasons. As it happens for other metals, chelation therapy is the most indicated treatment for poisoned patients. The efficacy of the therapy and the reduction of side-effects can be sensibly enhanced by an accurate knowledge of all the physiological mechanisms involved in metal uptake, transport within and between various tissues, and (possibly) clearance. All these aspects, however, are strictly dependent on the chemical speciation (i.e., the distribution of the chemical species of a component in a given system) of both the metal and the chelating agent in the systems where they are present. In this light, this review analyzes the state of the art of research performed in this field for mercury(II) and methylmercury(II). After a brief summary of their main sources, the physiological patterns for the treatment of mercury poisoning have also been considered. The binding ability of various chelating agents toward mercury has been then analyzed by modeling the behavior of the main classes of ligands present in biological fluids and/or frequently used in chelation therapy. Their sequestering ability has been successively evaluated by means of a semiempirical parameter already proposed for its objective quantification, and the main characteristics of an efficient chelating agent have been evaluated on this basis.

The single salt approximation for the major components of seawater: association and acid–base properties

In this note we propose the use of the single salt approximation for the composition of seawater. This approximation has been applied to a synthetic seawater. Association and acid-base properties h...

Ion-selective electrode measurements for the determination of formation constants of alkali and alkaline earth metals with low-molecular-weight ligands

Formation constants of acetate, hydrogencarbonate, malonate, citrate and 1,2,3-propanetricarboxylate complexes with Na+, K+ and Ca2+ were determined potentiometrically using sodium, potassium and calcium selective electrodes, at 25 °C and at different ionic strengths, in the range 0 <I ≤ 1 M. Formation constants obtained by ion-selective electrode (ISE) measurements were compared with those obtained by different techniques. It has been found that the use of ISEs gives reliable results in the study of weak complexes, also under non-constant ionic strength conditions.

Thermodynamic data for Pb2+ and Zn2+ sequestration by biologically important S-donor ligands, at different temperatures and ionic strengths

Thermodynamic parameters for the interactions of cysteine (cys) and penicillamine (psh) with Pb(II) and glutathione (gsh) with Pb(II) and Zn(II) were determined in NaNO3 or NaCl aqueous solution by potentiometry, at different ionic strengths (0 < I ≤ 1 mol L−1) and temperatures (15 ≤ t ≤ 45 °C). For Pb2+ systems, the formation of PbL, PbLH, PbLH2 and PbL2 species was found, together with PbLOH for cys and psh, and PbL2H and PbL2H2 for gsh. The speciation models for Pb2+–psh and –gsh systems were confirmed by UV spectrophotometric measurements. For the Zn2+–gsh system, a more complex speciation model was obtained with the formation of ZnL, ZnLH, ZnLH2, ZnL2, ZnL2H, ZnL2H2, ZnLOH and ZnL2OH species. From the dependence of formation constants on temperature, rough ΔH values were evaluated: the main contribution to the complexation free energy is entropic in nature, with small enthalpic values. Moreover, from the dependence on ionic strength, formation constants extrapolated at I = 0 mol L−1 were obtained. The sequestering ability of the ligands towards Pb2+ and Zn2+ was evaluated by determining the pL0.5, i.e. the −log of the concentration of the ligand able to complex half of the metal ion fraction. All these ligands show good sequestering ability. For Pb2+–cys, –psh and –gsh systems, pL0.5 reaches the value of 8.2, 9.0 and 5.9, respectively, at pH = 7, I = 0.1 mol L−1 and t = 25 °C. Under the same experimental conditions, the sequestering ability of gsh towards Zn2+ is lower, with pL0.5 = 4.1.

Inorganic speciation of organotin(IV) cations in natural waters with particular reference to seawater

Abstract This paper examines the inorganic complexing capacity of seawater, where chloride and sulfate ions are present in high concentration, towards mono- di- and tri-organotin(IV) cations which show a different trend of acidity, depending on cation charges, and a corresponding tendency to hydrolysis. By considering hydrolytic species and chloride and sulphate complex formation, a basic inorganic speciation model of organotins in synthetic seawater (Na+, K+, Ca2+, Mg2+, Cl−, SO42−) has been built up. The model has been extended to also consider interactions of organotins with carbonate and fluoride ions, which are other important components of seawater. Because of the strength of hydrolysis processes, the main complexes formed are in general mixed hydroxo-species. No species are formed by organotin cations and/or their hydroxo-species with fluoride owing to their very low concentration in fluoride, in comparison to the other components of seawater. In order to simplify calculations and to establish a cumulative inorganic binding capacity for seawater, we applied a chemical complexation model, according to which the major inorganic components of seawater are considered as a single salt BA.