Variation Of Activity Coefficients Research Articles

VLE measurement of binary systems 1-butyl-3-methylimidazolium-based ionic liquids and ethanol or water

Experimental vapor–liquid equilibrium (VLE) data for binary systems of ethanol or water with four ionic liquids (ILs) comprising 1-butyl-3-methylimidazolium cation and different anions were measured at atmospheric pressure within a broad region of IL concentrations, IL mole fraction ranging between 0 and 0.7 or even 0.87. Siwoloboff procedure of vapor–liquid phase transition temperature measurement was applied to estimate VLE of the binary mixtures. Obtained t–x data were fitted assuming ideal vapor and non-ideal liquid phase behavior, using the original NRTL equation to calculate component activity coefficients. VLE description results are presented in form of equilibrium diagrams and as the variation of activity coefficients with mixture composition. Quality of the VLE data fitting was assessed based on root mean square deviations and mean deviations in temperature. Some experimental binary t–x data sets were compared to those available in literature (measured in much narrower concentration range) or predicted from ternary VLE data. The selection of ILs in this study was related to the separation of ethanol−water mixture by extractive distillation. Principal IL selection criterium was its ability to affect the ethanol–water relative volatility. Thermodynamic description of the VLE data of IL-containing systems within a wide concentration range is an essential building block of the mathematical model of separation devices, especially in case of regeneration equipment where concentrated IL is present.

VLE measurement of binary systems containing imidazolium ionic liquids and water or ethanol

Experimental VLE data for binary systems of ethanol or water with three 1-ethyl-3-methylimidazolium ionic liquids (ILs) containing different anions were measured at normal pressure using the Siwoloboff procedure. ILs selected in this study were those that appear to be suitable for the ethanol−water mixture separation by extractive distillation. Measured vapor–liquid equilibria were described by the extended Raoult's law with components activity coefficients calculated using the NRTL equation. Detailed analysis of the VLE data is presented in form of equilibrium diagrams, variation of activity coefficients with mixture composition and thermodynamic consistency tests. Experimental t–x data were measured within a wider range of the binary mixture compositions and compared to those found in literature. Boiling point temperature measurement of binary mixtures (adapted Siwoloboff procedure) provided comparable results to those obtained applying conventional methods of VLE estimation (circulation method, ebulliometric measurement, etc.). Reliability and thermodynamic consistency of experimental data obtained using the adapted Siwoloboff procedure for high ILs’ content (high temperature) were affected by the accuracy of the temperature measurement. However, the trend of these experimental data is useful for VLE description in concentration regions where usually only coarse extrapolation is applied. Experimental data obtained for a wide binary mixture composition range (IL content of more than 70 mole%) seem to be more suitable for binary and ternary VLE description in separation equipment simulations, e.g. extractive distillation columns.

Solute–Solute and Solute–Solvent Interactions in the Ternary LiCl + Sucrose + Water System at 298.15 K

In this work, the potentiometric method was used to study the thermodynamic properties of the ternary LiCl + sucrose + water system, where the galvanic cell contained a solid-state chloride-selective electrode and a sodium-selective glass electrode. Measurements have been made for LiCl electrolyte concentrations ranging from 0.0046 up to about 3.800 mol·kg−1 in aqueous sucrose solutions whose molalities varied between mN = 0.1537 and mN = 5.4255 molal (65 wt%). Activity coefficients and standard Gibbs energies of transference of the electrolyte have been determined at 298.15 K. For describing the variation of activity coefficients of LiCl, Pitzer’s equation and a modified form of Pitzer’s equation were used and it was shown that there is a good agreement between them. Evaluation of activity coefficients and the optimized parameters of the two models made it possible to discuss the non-ideality behaviors of the systems in terms of solute–solute and solute–solvent interactions and of their variation according to medium effects. The interactions between electrolyte, sucrose and water were also interpreted in terms of the Gibbs energies of pairwise interaction gEN, for which the positive sign implies repulsive forces between the electrolyte and sucrose. Moreover, the estimated value of the primary hydration number of lithium chloride was within the range of those found by other researchers. Finally the calculated salting constant has confirmed that sucrose may be salted-out by the electrolyte and this fact is consistent with the literature.

Interaction of N-acetyl-l-cysteine with Na+, Ca2+, Mg2+ and Zn2+. Thermodynamic aspects, chemical speciation and sequestering ability in natural fluids

The estimation of thermodynamic parameters of N-Acetyl-L-cysteine (NAC) protonation were determined in NaCl(aq), (CH3)4NCl(aq), (C2H5)4NI(aq), employing various temperature and ionic strengths conditions, by potentiometric measurements. The interaction of NAC with some essential metal cations (e.g., Ca2+, Mg2+ and Zn2+) was investigated as well at 298.15 K in NaCl(aq) in the ionic strength range 0.1 ≤ I/mol dm−3 ≤ 1.0. The values of protonation constants at infinite dilution and at T = 298.15 K are: log K1H = 9.962 ± 0.005 (SH) and log K2H = 3.347 ± 0.008 (COO-H). In the presence of a background electrolyte, both log K1H and log K2H values followed the trend (C2H5)4NI ≥ (CH3)4NCl ≥ NaCl. The differences in the values of protonation constants among the three ionic media were interpreted in terms of variation of activity coefficients and formation of weak complexes. Accordingly, the determination of the stability of 4 species, namely: NaL−, NaHL0(aq), (CH3)4NL−, (CH3)4NHL0(aq) was assessed. In addition, as regards the interactions of Mg2+, Ca2+ and Zn2+ with NAC, the main species where the ML0(aq), ML(OH)−, and ML22−, that were found to be important in the chemical speciation of NAC in real multicomponent solutions. The whole set of the data collected may be crucial for the development of NAC-based materials for natural fluids selective decontamination from heavy metals.

The correlation and prediction of the temperature variation of infinite dilution activity coefficients of compounds in water

We list infinite dilution activity coefficients, γ∞, in water for 81 compounds from 273.15 to 373.15 K, and we use our linear free energy relationship to obtain equations for log γ∞ at 5 K intervals between 273.15 and 373.15 K. These equations, together with Abraham descriptors can then be used to calculate or predict the temperature variation of activity coefficient in water with an estimated error of 0.25–0.30 log unit. If an experimental value of log γ∞ is available at 298.15 K (or at any specific temperature) this can be used as a reference point to adjust our calculated values at any other temperature, leading to an estimated error for the temperature variation of around 0.1 log units. This procedure is especially useful in extrapolating limited data on log γ∞ over the entire range 273.15–373.15 K. The method requires no complicated software and calculations involve no more than simple arithmetic.

Experimental Researches Regarding the Thermodynamic Activities in the Binary Pb-Sb Alloy System

We have determined the thermodynamic activity of Pb and Sb in the Pb-Sb binary system. For the experiments, we used a galvanic cell in which the measuring electrode was made of Pb-Sb alloys with different chemical compositions. The reference electrode was manufactured from pure lead, and a mixture of salts (35% KCl, 17% NaCl and 48% PbCl2) was used as an electrolyte, which had a melting temperature of 399 °C (672 K). Tungsten wires were used for the contact conductors of the electrodes, and platinum wires were used for the electrical contacts between the measurement instrument and the contact conductors. The variations of activity coefficients depending on its atomic fraction were calculated for the Pb and Sb in the Pb-Sb binary alloy system. The experimental measurements were performed at a temperature of 650 °C (923 K). The obtained data are reliable and consistent, and now, based on these, thermodynamic functions can be obtained that can help the scientific community to correctly describe the behavior of the Pb-Sb binary alloy system.

Modeling of activity coefficients of amino acid and electrolyte in aqueous solutions

The amino acid exists in the zwitterion form with positive (amino group, −NH3+) and negative (carboxyl group, COO−) charge in water, and it is able to form the ion-pair complexes physically bonded with ions. The ion-pair complex between amino acid and ions has a strong effect on the variation of activity coefficient of amino acid in aqueous electrolyte solution. In this work, the PC-SAFT EOS combined with primitive mean spherical approximation (PM-MSA) model is applied to the 32 amino acid-electrolyte aqueous systems (a total of 1305 data points) for modeling the activity coefficients of amino acids and electrolyte in aqueous solutions. The ion-pair complex is considered as a cross-association between amino acid and ions, and this method accurately represents the activity coefficient of an amino acid and electrolyte over a wide range of concentrations. Based on the calculation of activity coefficient ratio of amino acid, the solubility of an amino acid in electrolyte-water system and density of solution are predicted, and the predicted results show qualitative agreement with experimental results. Therefore, the model proposed in this work is expected to be useful for describing activity coefficients of an electrolyte and amino acid in amino acid-electrolyte-water system.

Solubility, protonation and activity coefficients of some aminobenzoic acids in NaClaq and (CH3)4NClaq, at different salt concentrations, at T = 298.15 K

Protonation constants of 2-aminobenzoic acid, (anthranilic acid, vitamin L1) (1), 4-aminobenzoic acid (PABA) (2), and 2-aminoterephthalic acid (3) have been obtained from potentiometric measurements at 25°C (298.15K) and ionic strengths between 0 and 5mol⋅dm−3 NaCl and between 0 and 3mol⋅dm−3 (CH3)4NCl. Values for the step-wise protonation constants log K1 and log K2, extrapolated to zero ionic strength and infinite dilution, were determined as follows. For 2-aminobenzoic acid: 4.958±0.003 (error estimate at the 95% confidence interval) and 2.060±0.007; for 4-aminobenzoic acid: 4.868±0.008 and 2.420±0.004; for 2-aminoterephthalic acid: 5.089±0.004 and 3.594±0.006. Differences between protonation constants obtained using various concentrations of NaCl(aq) and (CH3)4NCl(aq) for ionic strength adjustment are interpreted in terms of formation of weak complexes between the ligands and the ions of the background electrolyte. Values of log K for the formation of 1:1 complexes of the aminobenzoate anion with the tetramethylammonium cation have been estimated. The dependence of the solubility was studied as a function of NaCl(aq) and (CH3)4NCl(aq) concentration. Similar behaviour was found for the three ligands. The total solubility increases with increasing NaCl(aq) concentration, whereas the opposite trend is observed in (CH3)4NCl(aq). Total and specific solubility was also determined, from which Setschenow coefficients and activity coefficients were deduced for the various species. Activity coefficient variation was examined in terms of both a Debye–Hückel type equation (DHt) and a SIT (Specific ion Interaction Theory) model.

Oxygen Nonstoichiometry and Thermodynamic Quantities of La2Ni0.95Al0.05O4.025 + δ

The oxygen nonstoichiometry of La2Ni0.95Al0.05O4.025 + δ (LNAO) is measured as a function of oxygen partial pressure (pO2) and temperature by coulometric titration method and thermogravimetric analysis (TGA) in 800°C–1000°C temperature range and 10−16‐1 atm pO2 range. The partial molar quantities for mixing of oxygen were calculated and results were compared with the literature results on La2NiO4 + δ (LNO). The variation in activity coefficient of holes versus oxygen nonstoichiometry illustrated an early positive deviation of the activity coefficient of holes from unity, leading to ≈ 7 at δ ≈ 0.08, which was lower than the literature value of ≈ 14 for La2NiO4 + δ at δ ≈ 0.08, indicating lesser deviation of LNAO from ideal solution behavior. The effective mass of holes () was 1.02–1.21 times the rest mass (mo), which indicated the band‐like conduction and allowed the effect of the small degree of polaron hopping to be ignored. The comparison of oxygen nonstoichiometry and partial molar quantities showed that incorporation of interstitial oxygen is less favorable in LNAO in comparison to LNO.

Partial Derivative Fitted Taylor Expansion: an efficient method for calculating gas/liquid equilibria in atmospheric aerosol particles – Part 2: Organic compounds

Abstract. A flexible mixing rule is presented which allows the calculation of activity coefficients of organic compounds in a multi-component aqueous solution. Based on the same fitting methodology as a previously published inorganic model (Partial Differential Fitted Taylor series Expansion; PD-FiTE), organic PD-FiTE treats interactions between binary pairs of solutes with polynomials of varying order. The numerical framework of organic PD-FiTE is not based on empirical observations of activity coefficient variation, rather a simple application of a Taylor Series expansion. Using 13 example compounds extracted from a recent sensitivity study, the framework is benchmarked against the UNIFAC model. For 1000 randomly derived concentration ranges and 10 relative humidities between 10 and 99%, the average deviation in predicted activity coefficients was calculated to be 3.8%. Whilst compound specific deviations are present, the median and inter-quartile values across all relative humidity range always fell within ±20% of the UNIFAC value. Comparisons were made with the UNIFAC model by assuming interactions between solutes can be set to zero within PD-FiTE. In this case, deviations in activity coefficients as low as −40% and as high as +70% were found. Both the fully coupled and uncoupled organic PD-FiTE are up to a factor of ≈12 and ≈66 times more efficient than calling the UNIFAC model using the same water content, and ≈310 and ≈1800 times more efficient than an iterative model using UNIFAC. The use of PD-FiTE within a dynamical framework is presented, demonstrating the potential inaccuracy of prescribing fixed negative or positive deviations from ideality when modelling the evolving chemical composition of aerosol particles.

Effects of Divalent Cations on the Single-Channel Conductance of the OmpF Channel: Linearity, Saturation and Blocking

Previous studies on the bacterial porin OmpF (Biophys. J. 96 (2009) 55-66) have shown that the channel moderate cationic selectivity of OmpF in NaCl and KCl solutions is reversed to anionic selectivity in solutions of CaCl2 and MgCl2. We study here the effect of salts of divalent cations on the channel conductance with a particular emphasis in dissecting the role of the electrolyte, the role of the counterion accumulation induced by the protein channel charges and any other effect not found in salts of monovalent cations. Single-channel conductance measurements are performed over a wide range of salt concentrations (up to 3 M).We find that the change of channel conductance with salt activity in bulk solution exhibits different features in salts of monovalent cations and in salts of divalent cations. In order to separate channel and electrolyte effects we analyse the correlation between channel conductance and bulk solution conductivity. While one scales with the other in solutions of NaCl and KCl over the whole concentration range studied, the conductance for CaCl2 and MgCl2 has two regimes. At salt concentrations below 1 M we found the same pattern as in solutions of NaCl and KCl. However, for higher concentrations such linearity between conductivity and conductance vanishes. Moreover, surprisingly, at high concentrations of MgCl2 and CaCl2 the conductance decreases as conductivity increases. Once, one accounts for the big variation of activity coefficients of divalent salts at high concentrations, the experimental results suggest that at low concentrations the pore conductance is controlled mainly by the electrolyte properties. The current recordings in salts of divalent cations reveal the existence of substates of lower conductance as one of the causes of the conductance decrease in the high concentration regime.

Soret separation of highly siderophile elements in Fe–Ni–S melts: Implications for solid metal–liquid metal partitioning

Soret separation of melts consisting of Fe (+ 1–10 wt.% Ni) with added C, Si or S and trace highly siderophile elements (HSE; Ru, Rh, Pd, Re, Os, Ir, Pt and Au) was investigated in experiments done at 2 GPa, T hot ~ 2000 °C and T hot−T cold ~ 250 °C. Experiments with added C and Si produced homogeneous samples, whereas those with S resulted in significant compositional gradients, with S enriched at the hot end of the sample, and Fe and HSE enriched at the cold end. The magnitude of the separation of the HSE is not the same for each, but varies in the order (smallest to largest) Pd~Au<Rh<Ru~Pt<Ir~Re~Os. We show that the observed distribution of HSE within experiments is consistent with the variation in activity coefficients for these elements as predicted by the non-metal avoidance model of Jones and Malvin (1990). Thus, our results indicate that the melt composition effect on solid metal–liquid metal partitioning can be captured in a single thermal diffusion experiment. The magnitude of the melt composition effect, as judged by the Jones–Malvin interaction parameter, β, varies with metal size in a manner similar to solid metal–liquid metal partition coefficients, suggesting like origins of the HSE “selectivity” for Fe-rich solid or liquid structures.

Organic particulate matter formation at varying relative humidity using surrogate secondary and primary organic compounds with activity corrections in the condensed phase obtained using a method based on the Wilson equation

Abstract. Secondary organic aerosol (SOA) formation in the atmosphere is currently often modeled using a multiple lumped "two-product" (N·2p) approach. The N·2p approach neglects: 1) variation of activity coefficient (ζi) values and mean molecular weight MW in the particulate matter (PM) phase; 2) water uptake into the PM; and 3) the possibility of phase separation in the PM. This study considers these effects by adopting an (N·2p)ζpMW,ζ approach (θ is a phase index). Specific chemical structures are assigned to 25 lumped SOA compounds and to 15 representative primary organic aerosol (POA) compounds to allow calculation of ζi and MW values. The SOA structure assignments are based on chamber-derived 2p gas/particle partition coefficient values coupled with known effects of structure on vapor pressure pL,io (atm). To facilitate adoption of the (N·2p)ζpMW,θ approach in large-scale models, this study also develops CP-Wilson.1 (Chang-Pankow-Wilson.1), a group-contribution ζi-prediction method that is more computationally economical than the UNIFAC model of Fredenslund et al. (1975). Group parameter values required by CP-Wilson.1 are obtained by fitting ζi values to predictions from UNIFAC. The (N·2p)ζpMW,θ approach is applied (using CP-Wilson.1) to several real α-pinene/O3 chamber cases for high reacted hydrocarbon levels (ΔHC≈400 to 1000 μg m−3) when relative humidity (RH) ≈50%. Good agreement between the chamber and predicted results is obtained using both the (N·2p)ζpMW,θ and N·2p approaches, indicating relatively small water effects under these conditions. However, for a hypothetical α-pinene/O3 case at ΔHC=30 μg m−3 and RH=50%, the (N·2p)ζpMW,θ approach predicts that water uptake will lead to an organic PM level that is more double that predicted by the N·2p approach. Adoption of the (N·2p)ζpMW,θ approach using reasonable lumped structures for SOA and POA compounds is recommended for ambient PM modeling.

Impact of dissolved sodium salts on kraft cooking reactions

Abstract This study examined the effects of inactive ions – ions not reacting with wood – during kraft cooking, which thus far have received modest attention. Six different sodium salts were added to kraft cooking liquors at two different levels of alkalinities. Delignification as well as the formation and degradation of hexenuronic acid (HexA) were observed of Scots pine sapwood meal. The delignification rate was greatly affected by the presence of additional anions. Chloride ions had the greatest retarding effect, while the addition of polyacrylate ions had almost no effect. When carbonate, sulphate and lactate ions were added to the liquors, the delignification rates were in-between the series with chloride and polyacrylates ions. We suggest that the anions affect the solubility of lignin fragments in analogy to the Hofmeister effect observed in various macromolecular systems in the presence of dissolved salts. When the reactions involving HexA were examined, the opposite results were obtained. In that case, the highest reaction rates were in the presence of chloride ions, and the lowest rates were obtained when no extra ions were added, and the second lowest rates were obtained in the presence of polyacrylate ions. As for delignification, the cooking series containing carbonate, sulphate and lactate ions had a reaction rate in-between the series with the highest and lowest rates. The differences obtained with various inorganic ions can be qualitatively explained by the Donnan equilibrium theory and by the variation in activity coefficients of hydroxide ions. Findings imply that knowledge of kraft cooking is far from complete. The effects of inactive ions on reaction kinetics should also receive more attention.

Activity coefficients at low dilution of CrO, NiO and CoO in melts in the system CaO–MgO–Al2O3–SiO2 at 1400°C: Using the thermodynamic behaviour of transition metal oxides in silicate melts to probe their structure

The activity coefficients at low dilution of NiO, CoO and CrO in ∼30 silicate melt compositions in the system CaO–MgO–Al2O3–SiO2 (CMAS) have been determined with a precision of 3% to 5%, one standard deviation, at 1400 °C by equilibration with the pure metal under controlled oxygen fugacity (Ni and Co) or with Cr2C3–graphite–CO. The activity coefficients vary by a factor of two over the range of melt compositions studied, but show no simple correlation with melt chemistry or melt structural descriptors. The variation in activity coefficients with melt composition among NiO, CoO and CrO is nevertheless highly correlated, as it also seems to be with the oxides of other divalent cations of similar ionic radius (e.g., Fe2+, Mg). In the case of NiO and CoO, the correlation is essentially perfect within the precision of the measurements, that is, the ratio γNiO/γCoO is the same for all melt compositions, implying that these two cations occupy a similar distribution of coordination environments in all the investigated CMAS melts. The deviations from a one-to-one correlation of the activity coefficient of CrO with those of NiO and CoO may be due to Cr2+ cations occurring in a distribution with more distorted coordination environments, due to the strong Jahn–Teller effect in this cation.

Activity Coefficients of NaCl in Fructose + Water at 298.15 K

The mean ionic activity coefficients of NaCl in fructose + water mixtures were experimentally determined at 298.15 K from electromotive force measurements of the following electrochemical cell containing two ion-selective electrodes (ISE): Na−ISE|NaCl(m),fructose(w),H2O(1−w)|Cl−ISE. The molality (m) varied between (0.005 and 3.5) mol kg-1, while the mass fraction of fructose in the mixture (w) was 0, 0.1, 0.2, 0.3, and 0.4. The emf values were analyzed using different models which employed extended Debye−Hückel equations and those of Pitzer, Scatchard, and Sen for describing the variations in activity coefficients with concentration. Results obtained with the different models were in good agreement. The interaction parameters of each model were comparatively analyzed with those of other similar systems reported in the literature, and discussion is presented concerning their variation with properties of the medium, particularly the dielectric constant. The free energy of transfer of the NaCl from the water to the mixed solvent was determined and its variation with the composition of the solvent is discussed in terms of ion−ion and ion−solvent interactions. Also determined was the primary hydration number of the NaCl, which was compared with results from the literature. Finally, the parameters of interaction gij and salting constant ks were estimated.

Hydrolysis and chemical speciation of dioxouranium(VI) ion in aqueous media simulating the major ion composition of seawater

The hydrolysis and chemical speciation of the dioxouranium(VI) ion at 25 °C was studied in a number of binary electrolytes (LiCl, NaCl, MgCl 2, CaCl 2, Na 2SO 4) and some mixtures (NaCl–Na 2SO 4, NaNO 3–Na 2SO 4, CaCl 2–MgCl 2) as well as artificial seawater (SSWE) as a function of ionic strength. The results in LiCl, CaCl 2 and MgCl 2 solutions confirmed the formation of (UO 2) 2(OH) 2 2+, (UO 2) 3(OH) 4 2+, (UO 2) 3(OH) 5 + and (UO 2) 3(OH) 7 − species (at I=0 mol l −1: log T β 22=−5.76, log T β 34=−11.82, log T β 35=−15.89 and log T β 37=−29.26). For NaNO 3, NaCl and artificial seawater the hydrolysis constant for the formation of the UO 2(OH) + species was also determined (at I=0 mol l −1: log T β 11=−5.19). The results in Na 2SO 4, Na 2SO 4/NaNO 3 and Na 2SO 4/NaCl required the formation of UO 2(SO 4) 0, UO 2(SO 4) 2 2−, UO 2(OH)SO 4 −, (UO 2) 2(OH) 2SO 4 0, (UO 2) 3(OH) 4SO 4 0 and (UO 2) 3(OH) 5SO 4 −, whose estimated values of each complex formation constant at I=0 mol (kg H 2O) −1 are (log T β pqr ±standard deviation, species in parenthesis): 3.32±0.02 [UO 2SO 4 0], 4.26±0.04 [UO 2(SO 4) 2 2−], −2.30±0.01 [UO 2(OH)(SO 4) −], −2.64±0.04 [(UO 2) 2(OH) 2(SO 4) 0], −8.45±0.04 [(UO 2) 3(OH) 4(SO 4) 0], −13.58±0.04 [(UO 2) 3(OH) 5(SO 4) −]. All the results were examined using the Pitzer model by considering the interactions of the cation hydrolytic species with Cl − and NO 3 − and anion hydrolytic species with Li +, Na +, Ca 2+ and Mg 2+ and, in addition, the “same sign” and “triple” interaction parameters. The resulting Pitzer parameters give an adequate representation of all the hydrolysis constants measured in the binary, ternary and artificial seawater solutions. Alternatively to the interpretation of the dependence of uranyl hydrolysis constants on ionic strength and on ionic medium in terms of variations of activity coefficients of ions, the formation of ion pairs was considered and some complex formation constants among dioxouranium(VI) species and different ions of background salts were calculated. Interactions of uranyl with major components of seawater were taken into account using the “single salt” BA approximation according to which SSWE is considered as a single sea salt (BA) where cation B and anion A, having charge z=± I/C BA =±1.117 , are representative of all major cations (Na +, K +, Ca 2+, Mg 2+) and anions (Cl − and SO 4 2−) of seawater, respectively. Pitzer parameters were also calculated for both the interactions of uranyl with B z+ and A z− ions and for the internal BA interactions. The last ones are: β (0)=0.1081 and β (1)=0.4238 for B 1.117+–A 1.117− interactions, and β (0)=−0.3134 and β (1)=1.5375 for H +–A 1.117− interactions. Literature data were collected carefully, and a critical analysis and accurate comparisons with results presented here were made.

Protonation equilibria and solubility of l-cystine

l-Cystine is the natural least soluble of the naturally occurring amino acids. Its solubility is a function of the hydrogen ion concentration depending on its protonation. This paper presents an investigation on properties of l-cystine in a constant ionic medium of NaCl at different concentrations (3.00, 2.00, 1.00, 0.500, 0.150 mol dm −3) and at 25 °C. In such experimental conditions, solubility and its changes as a function of hydrogen ion concentration were determined. Protonation of cystine was studied in acid and basic solution, by measuring the electromotive force of galvanic cell involving a glass electrode. Four protonation constants for l-cystine were determined in all the selected concentrations of ionic medium. The different values obtained for the constants in ionic medium of different concentration could be explained both with the variation of activity coefficients or with the complex formation between l-cystine and sodium ions. Variations of solubility for different concentration of NaCl were observed as well.

Role of basicity and tetrahedral speciation in controlling the thermodynamic properties of silicate liquids, part 1: the system CaO-MgO-Al2O3-SiO2

Activity coefficients of oxide components in the system CaO-MgO-Al2O3-SiO2 (CMAS) were calculated with the model of Berman (Berman R. G., “A thermodynamic model for multicomponent melts with application to the system CaO-MgO-Al2O3-SiO2,” Ph.D. dissertation, University of British Columbia, 1983) and used to explore large-scale relationships among these variables and between them and the liquid composition. On the basis of Berman’s model, the natural logarithm of the activity coefficient of MgO, ln(γMgOLiq), and ln(γMgOLiq/γSiO2Liq) are nearly linear functions of ln(γCaOLiq). All three of these variables are simple functions of the optical basicity Λ with which they display minima near Λ ∼ 0.54 that are generated by liquids with low ratios of nonbridging to tetrahedral oxygens (NBO/T) (<0.3) and a mole fraction ratio, XSiO2Liq/XAl2O3Liq, in the range 4 to 20. Variations in ln(γCaOLiq) at constant Λ near the minimum are due mostly to liquids with (XCaOLiq + XMgOLiq)/XAl2O3Liq < 1. The correlations with optical basicity imply that the electron donor power is an important factor in determining the thermodynamic properties of aluminosilicate liquids.For a constant NBO/T, ln(γCaOLiq/γAl2O3Liq) and ln(γMgOLiqγAl2O3Liq) form curves in terms of XSiO2Liq/XAl2O3Liq. The same liquids that generate minima in the Λ plots are also associated with minima in ln(γCaOLiqγAl2O3Liq) and ln(γMgOLiqγAl2O3Liq) as a function of XSiO2Liq/XAl2O3Liq. In addition, there are maxima or sharp changes in slope for NBO/T > 0.3, which occur for XSiO2Liq/XAl2O3Liq ranging from ∼0 to ∼6 and increase with increasing NBO/T. The systematic variations in activity coefficients as a function of composition and optical basicity reflect underlying shifts in speciation as the composition of the liquid is changed. On the basis of correlations among the activity coefficients, it is likely that the use of CaO, an exchange component such as SiMg−1 and two of MgO, CaAl2O4, or MgAl2O4 would yield significant savings in the number of parameters required to model the excess free energy surface of liquids over large portions of CMAS relative to the use of oxide end members.Systematic behavior of thermodynamic properties extends to small amounts of other elements dissolved in otherwise CMAS liquids. For example, ln(XFe2+Liq/XFe3+Liq) at constant oxygen fugacity is linearly correlated with ln(γCaOLiq). Similarly, ln(CS), where CS is the sulfide capacity is linearly correlated at constant temperature with each of the optical basicity, ln(aCaOLiq) and ln(γCaOLiq), although the correlation for the latter breaks down for low values of Λ. The well-known systematic behavior of sulfide capacity as a function of optical basicity for systems inside as well as outside CMAS suggests that ln(γCaSLiq) is also a simple function of optical basicity and that the relationships observed among the activity coefficients in CMAS may hold for more complex systems.

THERMODYNAMIC STUDY OF LIQUID-LIQUID EXTRACTION OF ARSENIC(V) BY NMPL FROM H2SO4 MEDIUM

The results obtained from the extraction of As(V) by 1-(2,6-dimethylheptyl-4-oxy)-3 methyl-1-oxo-Δ3-phospholen (NMPL) are presented in this paper. The conditions of extraction as a function of the extractant and H2SO4 concentrations have been determined. The influence of the extraction of water and sulfuric acid in the extraction process of As(V) has been analyzed. A model of extraction based on the Sergievskii-Dannus' equation has been developed. This model considers the variation of activity coefficients in the aqueous phase and in the organic phase. Thanks to this model its has been possible to predict the theo- retical curves of distribution of H2O, H2SO4 and H3AsO4, in accord with the experimental data.