Ion-interaction Equations Research Articles

Volumetric Properties and Ion Interactions for Sodium Hypophosphite Aqueous Solution from 283.15 to 363.15 K at 101.325 kPa

Densities of sodium hypophosphite aqueous solution (NaH2PO2) with the molality varied from 1.019143 to 10.43887 mol kg–1 at temperature intervals of 5 K range from 283.15 to 363.15 K at 101.325 kPa were measured by a precise Anton Paar Digital vibrating-tube densimeter. From the density data, the thermal expansion coefficients, apparent volume and partial molar volumes were obtained. According to the Pitzer ion-interaction equation of the apparent molar volumes, the Pitzer single-salt parameters and their temperature-dependent correlation for NaH2PO2 were firstly obtained by the least-squares method. The model shown that apparent molar volumes agree well with the experimental values, which indicated the single salt parameters and the temperature-dependent formula are reliable.

Volumetric Properties in the NaAsO2 + H2O System at Temperature from 283.15 to 363.15 K and Atmospheric Pressure

Densities of sodium arsenite (NaAsO2) aqueous solution with the molality varied from 0.19570 to 1.94236 mol·kg−1 at temperature intervals of 5 K from 283.15 to 363.15 K and 101 ± 5 kPa were measured by a precise Anton Paar Digital vibrating-tube densimeter. Apparent molar volumes (VΦ) and thermal expansion coefficient (α) were obtained on the basis of experimental data. The 3D diagram of apparent molar volume against temperature and molality and the diagram of thermal expansion coefficient against molality were generated. According to the Pitzer ion-interaction equation of the apparent molar volume model, the Pitzer single-salt parameters (βM,X0υ, βM,X1υ, βM,X2υ, and CM,Xυ, MX = NaAsO2) and their temperature-dependent correlation F(i, p, T) = a1 + a2ln (T/298.15) + a3(T − 298.15) + a4/(620 − T) + a5/(T − 227) (where T is temperature in Kelvin and ai are the correlation coefficients) for NaAsO2 were obtained for the first time. The predictive apparent molar volumes agree well with the experimental values, and those results indicated that the single-salt parameters and the temperature-dependent formula are reliable.

Study of Ion-Pairing and Thermodynamic Properties of Sodium Fluoride in Aqueous Solutions at Temperatures from 298.15 to 353.15 K

In this investigation, the water activity and osmotic coefficients of the binary solution NaF–H2O were determined using the hygrometric method, from dilution to saturation and from 298.15 to 353.15 K. The hygrometric measurement results obtained have been combined with NaF(cr) solubility data in water to construct a chemical model that calculates solute and solvent activities from 298.15 to 353.15 K and from dilute to saturated solution concentration [up to m(NaF) = 1.2 mol·kg−1 at 353.15 K], and solid–liquid equilibria in the NaF–H2O system. The solubility modelling approach, based on the fundamental Pitzer ion interaction equations, was employed. It was found, that the standard approach for 1–1 electrolytes, with three β (0) , β (1), and C ϕ single electrolyte ion interaction parameters, gives a very good agreement with osmotic coefficients data for unsaturated solutions, and with NaF(cr) equilibrium solubility data, used in parameterization. The raw experimental osmotic coefficients (ϕ) and model calculated mean activity coefficients γ(NaF) have been compared with reference experimental ϕ data, and with γ estimations made by using different calculation approaches. The comparison shows a good agreement between ϕ data at 298.15 K. There is a disagreement between γ(NaF) calculations made in this study, and calculations based on Scatchard equations, and an extended Debye–Huckel (DH) model. The Pitzer based model constructed here has been also used to calculate the thermodynamic solubility product $$ \left( {K_{\text{sp}}^{\text{o}} } \right) $$ of NaF(cr) from 298.15 to 353.15 K. The solubility isotherm m s for NaF–H2O and thermodynamic solubility product [given as ln $$ K_{\text{sp}}^{\text{o}} $$ (NaF(cr))] are presented. In the saturated solution of NaF(aq), from experimental data and Fuoss approach using the obtained parameters, the experimental and calculated association constants K ass of the ion pairs were estimated at temperatures ranging from 298.15 to 353.15 K. The results are in good agreement.

Series approach to modeling of amino acid and electrolyte mixtures in the aqueous media

In this work, the electrolyte, amino acid and water solutions are modeled using activity coefficient (AC) approach. Two different models, modified Pitzer ion interaction equation proposed by Esteso (PE) and the Pitzer–Simonson–Clegg model (PSC) are used in their original and modified forms to correlate the activity coefficient and osmotic coefficient data for binary and ternary systems containing water, amino acid/peptide and electrolytes. The considered aqueous systems contain one of the eight amino acids: alanine, glycine, serine, threonine, β-alanine, methionine, α-amino n-butyric acid and α-amino butyric acid. Also, glycylglycine is used as one peptide. The salts are NaCl, NaBr, NaNO3, KCl and KBr.The results obtained from the studied models are compared to those of experimental data. The comparison shows that the PE model can correlate the experimental AC data of ternary systems more accurate than the other PSC. The equations are tested against the large number (30) of available experimental data sets.

Application of the Pitzer Model to Assignment of pH to Phthalate Standard Buffer Solutions

Stoichiometric ionization constants of phthalic acid have been determined in potassium chloride solutions at temperatures from 10 to 40 °C, with molalities up to 3 mol⋅kg−1, from potentiometric titrations of potassium hydrogen phthalate. The procedure described by Gans and O’Sulliven (Talanta 51:33–37, 2000) was followed to calibrate the electrochemical cell in terms of hydrogen ion concentration and the SUPERQUAD program was used for the calculations. Pitzer’s ion-interaction equation was applied to this system for the calculation of activity coefficients and a good fit was observed at all temperatures. From these results and from phthalic acid solubilities, Pitzer model coefficients were determined which enabled the evaluation of the activity coefficient of each chemical species, including that of the chloride ion, that are necessary for the establishment of phthalate pH standards at ionic strengths higher than 0.1 mol⋅kg−1(the upper limit of applicability of the Bates-Guggenheim convention).

NH4Cl + NH4NO3 + H2O Ternary Mixed Electrolyte System: Thermodynamic Modeling Based on Potentiometric Measurements

This work reports the results of the thermodynamic modeling of NH4Cl salt in the ternary NH4Cl (m1) + NH4NO3 (m2) + H2O mixed electrolyte system based on potentiometric measurements. The electrochemical measurements were performed, at similar constant ionic strengths, in different series of mixed salt solutions, each characterized by a fixed salt molal ratio r (where r = m1/m2 = 0.05, 1, 5, 10, and m1, m2 are the molalities of NH4Cl and NH4NO3, respectively). The galvanic cell used for collecting the potentiometric data consisted of a solvent polymeric (PVC) NH4+ selective membrane electrode (NH4+ ISE) and an Ag/AgCl electrode. Pitzer ion-interaction equations for mixed salts were used for modeling the nonideal behavior of the system, over the ionic strength ranging from 0.01 mol·kg−1 up to about 6 mol·kg−1, at 298.15 K. On the basis of the obtained Pitzer ion-interaction parameters, the osmotic coefficients, solvent activities, and excess Gibbs free energies were also determined for these investigated te...

Thermodynamics of the In|In+3 Electrode in HCl + InCl3 Solutions

Electromotive force measurements have been made using the cell $$\mbox{In(s)}|\mbox{HCl }(m_{\mathrm{A}}),\mbox{InCl}_{3}(m_{\mathrm{B}}),\mbox{H}_{2}\mbox{O}|\mbox{AgCl, Ag}$$ in the ionic strength range of I=0.05, 0.1, 0.25, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol⋅kg−1 at 25 °C. The value of E o, the standard potential of the In/In3+ electrode, has been determined at 25 °C. Our value of E o (−0.3371 V) at 25 °C obtained from our measurements is in good agreement with −0.336 (Hakomori, J. Am. Chem. Soc. 52: 2372–2376, 1930) and −0.3382 V (Covington et al., J. Chem. Soc. 4394–4401, 1963). The activity coefficients of InCl3 as well as Harned interaction coefficients have been determined at 25 °C for each of the experimental ionic strengths at ionic strength fractions of 0.1, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 of HCl. Harned’s rule for the salt is obeyed at I=0.05,0.1 and 0.25 mol⋅kg−1 but the quadratic terms are needed for higher ionic strengths. These data, together with others for the activity coefficient of HCl in the same solutions, have been treated by the ion-interaction (Pitzer, Activity Coefficients in Electrolyte Solutions, CRC Press, 1991) equations in a previous publication.

The speciation equilibrium coupling with phase equilibrium in the H 2O–CO 2–NaCl system from 0 to 250 °C, from 0 to 1000 bar, and from 0 to 5 molality of NaCl

A model is developed for the calculation of the speciation equilibrium of H +, OH −, HCO 3 −, CO 3 2−, and CO 2(aq) in aqueous solutions coupled with liquid-vapor phase equilibrium in the CO 2–H 2O–NaCl system from 273 to 523 K, from 0 to 1000 bar (possibly to 2000 bar) , and from 0 to 5.0 molality ( m) of NaCl. The first and second ionization constants of carbonic acid, pH values in aqueous solutions, and their dependence on temperature ( T), pressure ( P) and NaCl concentrations can be calculated with accuracy close to those of experiments in the stated T– P– m range. Using the specific ion-interaction equations of Pitzer to calculate activity coefficients, the model is also able to accurately predict the difference between dissociation constants and apparent dissociation constants, and the concentrations of H +, OH −, HCO 3 −, CO 3 2−, and CO 2(aq) at a given T– P– m condition. Compared to previous models, this model covers larger T– P– m space and much more closely reproduces experimental results, especially in the elevated pressure region. Even without fitting to the experimental data for pH values, dissociation constants and apparent dissociation constants at pressures above the saturation pressures of pure water, this model can accurately predicted these properties up to high pressures. A computer program based on this model is developed and online calculation is made available at: www.geochem-model.org/models.htm.

Measurement and Calculation of Solubilities in the Ternary System NaCH3COO + NaCl + H2O from 278 K to 323 K

Solubility determinations in the ternary system NaCH3COO + NaCl + H2O were performed from 278.15 K to 323.15 K. The new measurements were combined with previous measurements of osmotic coefficients at 298.15 K and activity coefficients from 298.15 K to 318.15 K. The measurements were fitted with an extended Pitzer type model to generate equations for the prediction of solubilities and activities in mixed solutions of the ternary system. The model yields accurate representation of the experimental data. Although it was found that the values of the mixing parameters in the ion interaction equations are significantly different from zero, their influence is only small. Hence, extrapolation to temperatures outside of the fitted range appears to be possible. However, due to limitations of the binary model equations for NaCl (aq) and NaCH3COO (aq) there is an upper ionic strength limit of about (15 to 18) mol·kg-1. Hence, solubility calculations are limited to temperatures below about 328 K.

Thermodynamic properties of aqueous mixed ternaries { y NH 4Cl+(1− y) BaCl 2 } (aq) and { y NH 4Cl+(1− y) CaCl 2 } (aq) at 298.15 K

Hygrometric measurements of the water activity are reported for the thermodynamic properties of the mixed aqueous electrolyte systems of ammonium–calcium chloride and ammonium–barium chloride at 298.15 K. The measurements were made at a large number of total molalities, varying from 0.4 mol kg −1 to saturation at three ionic-strength fractions ( y ) of NH 4Cl for the systems NH 4Cl–CaCl 2 (aq) and NH 4Cl–BaCl 2 (aq) with y = 0.20 , 0.50 and 0.80. The experimental data are reported as water activities and osmotic coefficients at various molalities, along with derived values of the activity coefficients of both solutes. The ranges of water activities a w are 0.800–0.989 for ammonium–barium chlorides and a w = 0.560 – 0.989 for ammonium–calcium chlorides. The activity coefficients were calculated by means of a thermodynamic model based on a variant of the Pitzer ion-interaction equations with extended binary parameters. However, the results were also compared for the precision of the presentation of the osmotic coefficients using an extended model with the calculations of the Zdanovskii–Stokes–Robinson, Kusik and Meissner, Robinson and Stokes, Lietzke and Stoughton, Reilly, Wood and Robinson, and Pitzer models.

Thermodynamics of electrolytes in mixed solvent media — Application of the Pitzer ion interaction approach

Although for a long time considerable attention has been paid to the application of the Pitzer ion interaction approach to describe the thermodynamic properties of a variety of aqueous electrolyte solutions, such studies in mixed solvent media are still scarce. The present paper discusses the application of the Pitzer ion interaction approach in the case of electrolyte solutions in mixed solvent media. In particular, a comprehensive equation for the thermodynamic properties of hydrochloric acid in dioxane–water mixtures within the temperature range 273.15 to 323.15 K has been generated in the ion interaction (Pitzer) equation form on the basis of electrochemical cell measurements of the hydrochloric acid available in the literature. It was necessary to set varying values of the exponential coefficients α1 and α2 of the standard Pitzer model in different mixed solvent media to achieve a satisfactory fit. The present model quantitatively reproduces the experimental cell electromotive forces over the entire concentration and temperature range used for the experiments. The Pitzer ion interaction approach is, thus, proved to be a very valuable method for the correlation and prediction of thermodynamic properties of electrolytes in mixed solvent media. The implications of the Pitzer ion interaction parameters (β(0), β(1) β(2), and Cϕ) have also been discussed in terms of interionic forces. The present study also stresses the need for additional emf measurements to help interpolation through the dioxane composition. Key words: Pitzer's equations, Pitzer ion interaction parameter, activity coefficients, emf, hydrochloric acid.

Geochemical modeling of evaporation processes on Mars: Insight from the sedimentary record at Meridiani Planum

New data returned from the Mars Exploration Rover (MER) mission have revealed abundant evaporites in the sedimentary record at Meridiani Planum. A working hypothesis for Meridiani evaporite formation involves the evaporation of fluids derived from the weathering of martian basalt and subsequent diagenesis. On Earth, evaporite formation in exclusively basaltic settings is rare. However, models of the evaporation of fluids derived from experimentally weathering synthetic martian basalt provide insight into possible formation mechanisms. The thermodynamic database assembled for this investigation includes both Fe2+ and Fe3+ in Pitzer's ion interaction equations to evaluate Fe redox disequilibrium at Meridiani Planum. Modeling results suggest that evaporation of acidic fluids derived from weathering olivine-bearing basalt should produce Mg, Ca, and Fe-sulfates such as jarosite and melanterite. Calculations that model diagenesis by fluid recharge predict the eventual breakdown of jarosite to goethite as well as the preservation of much of the initial soluble evaporite component at modeled porosity values appropriate for relevant depositional environments (< 0.30). While only one of several possible formation scenarios, this simple model is consistent with much of the chemical and mineralogical data obtained on Meridiani Planum outcrop. B) 2005 Elsevier B.V. All rights reserved.

Crystal growth in porous materials—I: The crystallization pressure of large crystals

A critical review of the existing literature on the pressure exerted by growing crystals in porous materials reveals that a number of different equations are in use. A derivation of an equation for the crystallization pressure based on the chemical potentials of the loaded and the unloaded faces of a growing crystal is provided. The equation obtained is compared to other equations available in the literature and the different approaches are discussed in detail. The treatment also includes the non-ideal behavior of the liquid phase using the ion interaction approach (Pitzer equations) which is well-established in solution thermodynamics. Incorporating the ion interaction equations in the crystallization pressure equation yields a quite simple expression that appears to be more convenient than previous treatments. The equation is applied to calculate crystallization pressures for supersaturated solutions of aqueous NaCl, NaNO 3, Na 2SO 4, and MgSO 4 including the various hydrated forms of these salts. Depending on the nature of the salt, neglecting the non-ideal behavior may cause considerable error in crystallization pressure calculations. Finally, it is emphasized that the basic assumption of non-uniform pressure is fundamental to understand the dynamics of crystallization pressure evolving in porous materials.

Vapor pressures and isopiestic molalities of concentrated CaCl 2(aq), CaBr 2(aq), and NaCl(aq) to T = 523 K

The Oak Ridge National Laboratory high-temperature isopiestic apparatus was outfitted with precise pressure gauges to allow for direct vapor pressure measurements. Vapor pressures over concentrated solutions of CaCl 2(aq), and CaBr 2(aq) were measured at temperatures between (380.15 and 523.15) K in the range of water activities between 0.2 and 0.85. Isopiestic molalities were used to determine osmotic coefficients at the conditions where NaCl reference standard solutions remained undersaturated. The main goal of this work was to improve the accuracy of isopiestic comparisons based on the calcium chloride reference standard. Osmotic coefficients for CaCl 2(aq) and CaBr 2(aq) calculated from both isopiestic and direct vapor pressure results were combined with the literature data and used to build general thermodynamic models based on a variant of extended Pitzer ion-interaction equations and valid at the saturation pressure of water. While these empirical models approach the accuracy of the experimental data in a wider range of concentrations and temperatures than any previously published equations, considerable amounts of accurate data and a substantial effort are required in order to obtain a satisfactory representation using power series-based virial equations. The effect of experimental uncertainties on the accuracy of the direct vapor pressure results is discussed, including in particular the error caused by the presence in the apparatus of a small amount of CO 2. The substantial decrease of the solubility product of CaCO 3 in concentrated chloride solutions at temperatures above 423 K is a serious defect of calcium chloride as a water activity reference standard.

Apparent molar volumes and apparent molar heat capacities of Pr(NO 3) 3(aq), Gd(NO 3) 3(aq), Ho(NO 3) 3(aq), and Y(NO 3) 3(aq) at T = (288.15, 298.15, 313.15, and 328.15) K and p = 0.1 MPa

Relative densities and relative massic heat capacities have been measured for acidified solutions of Y(NO 3) 3(aq), Pr(NO 3) 3(aq), and Gd(NO 3) 3(aq) at T = (288.15, 298.15, 313.15, and 328.15) K and p = 0.1 MPa. In addition, relative densities and massic heat capacities have been measured at the same temperatures and pressure for Y(NO 3) 3(aq) and Ho(NO 3) 3(aq) solutions without excess acid (n.b. measurements at T = 328.15 K for Ho(NO 3) 3(aq) were not performed due to the limited volume of solution available). Apparent molar volumes and apparent molar heat capacities for the aqueous salt solutions have been calculated from the experimental apparent molar properties of the acidified solutions using Young’s rule, whereas the apparent molar properties of the solutions without excess acid were calculated directly from the measured densities and massic heat capacities. The two sets of data for the Y(NO 3) 3(aq) systems provide a check of the internal consistency of the Young’s rule approach we have utilised. The concentration dependences of the apparent molar volumes and heat capacities of the aqueous salt solutions have been modelled at each investigated temperature using the Pitzer ion interaction equations to yield apparent molar properties at infinite dilution. Complex formation within the aqueous rare earth nitrate systems is discussed qualitatively by probing the concentration dependence of apparent molar volumes and heat capacities. In spite of the complex formation in the aqueous rare earth nitrate systems, there is a high degree of self-consistency between the apparent molar volumes and heat capacities at infinite dilution reported in this manuscript and those previously reported for aqueous rare earth perchlorates.

Thermodynamic Properties of Dilute NaCl(aq) Solutions near the Critical Point of Water

Completely dissociated aqueous solutions of alkali halides are successfully modeled to about 500−600 K, for instance, by the ion-interaction equations. At higher temperatures, the ions associate to neutral molecules and clusters. In the more concentrated solutions, some “effective chemical continuum” treatment may be applied. Neither approach is, however, suitable for the intermediate region of near-critical temperatures and low solution concentrations, where the ions are only partially associated and thermodynamic properties are subject to rapid changes. A Helmholtz energy model coupled with chemical association was proposed for these systems and used for simultaneous correlation of experimental apparent molar volumes, apparent molar heat capacities, and association constants of dilute NaCl(aq) solutions at temperatures from 573 to 723 K and at concentrations where the solutions are partially associated. Conversion from a MacMillan−Mayer to a Lewis−Randall reference state was included in the calculations. Performance of the model for data description is good, suggesting its application for other associating electrolytes in a wide range of state conditions.

Apparent molar heat capacities and apparent molar volumes of Pr(ClO 4) 3(aq), Gd(ClO 4) 3(aq), Ho(ClO 4) 3(aq), and Tm(ClO 4) 3(aq) at T=(288.15, 298.15, 313.15, and 328.15) K and p=0.1 MPa

Acidified aqueous solutions of Pr(ClO 4) 3(aq), Gd(ClO 4) 3(aq), Ho(ClO 4) 3(aq), and Tm(ClO 4) 3(aq) were prepared from the corresponding oxides by dissolution in dilute perchloric acid. Once characterized with respect to trivalent metal cation and acid content, the relative densities of the solutions were measured at T=(288.15, 298.15, 313.15, and 328.15) K and p=0.1 MPa using a Sodev O2D vibrating tube densimeter. The relative massic heat capacities of the aqueous systems were also determined, under the same temperature and pressure conditions, using a Picker Flow Microcalorimeter. All measurements were made on solutions containing rare earth salt in the concentration range 0.01 ⩽ m/(mol · kg −1) ⩽ 0.2. Relative densities and relative massic heat capacities were used to calculate the apparent molar volumes and apparent molar heat capacities of the acidified salt solutions from which the apparent molar properties of the aqueous salt solutions were extracted by the application of Young's Rule. The concentration dependences of the isothermal apparent molar volumes and heat capacities of each aqueous salt solution were modelled using Pitzer ion-interaction equations. These models produced estimates of apparent molar volumes and apparent molar heat capacities at infinite dilution for each set of isothermal V φ,2 and C p φ,2 values. In addition, the temperature and concentration dependences of the apparent molar volumes and apparent molar heat capacities of the aqueous rare earth perchlorate salt solutions were modelled using modified Pitzer ion-interaction equations. The latter equations utilized the Helgeson, Kirkham, and Flowers equations of state to model the temperature dependences (at p=0.1 MPa) of apparent molar volumes and apparent molar heat capacities at infinite dilution. The results of the latter models were compared to those previously published in the literature. Apparent molar volumes and apparent heat capacities at infinite dilution for the trivalent metal cations Pr 3+(aq), Gd 3+(aq), Ho 3+(aq), and Tm 3+(aq) were calculated using the conventions V 2 ∘(H +(aq)) ≡ 0 and C p2 ∘(H +(aq)) ≡ 0 and have been compared to other values reported in the literature.

Densities and apparent molar volumes of HClO 4(aq) and Yb(ClO 4) 3(aq) at elevated temperatures and pressures

Relative densities have been measured for acidified aqueous solutions of ytterbium perchlorate {Yb(ClO 4) 3} at approximately T=(348.15, 373.15, 398.15, and 423.15) K and p=(10.0, 20.0, and 30.0) MPa over the concentration range 0.01624⩽ m 2/(mol · kg −1) ⩽ 0.2531 using an optically coupled vibrating tube densimeter (OCVTD). Experimental apparent molar volumes have been calculated from the density measurements, and apparent molar volumes for the aqueous perchlorate salt have been calculated using Young's rule. The application of Young's rule requires apparent molar volumes for aqueous perchloric acid (HClO 4) solutions over extended temperature and pressure ranges. These values were calculated from densities for aqueous HClO 4 solutions that were measured using the OCVTD at the same temperatures and pressures as those used to investigate the density surface of the acidified aqueous Yb(ClO 4) 3 solutions. The temperature, pressure, and composition surfaces of the apparent molar volumes for Yb(ClO 4) 3(aq) and HClO 4(aq) have been modelled using Pitzer ion-interaction equations. Apparent molar volumes at infinite dilution obtained from these models have been compared to those which can be calculated using the semi-empirical Helgeson, Kirkham, and Flowers equations of state. Values for the apparent molar volume at infinite dilution of the ytterbium trivalent cation have also been calculated using simple additivity principles.

Mathematical model for exergy in the system NaCl-H2O

In this work mathematical models for the exergy and relative enthalpy of the water solution of sodium-chloride and the pure crystalline sodium-chloride where derived. The environment was defined with the temperature 20 ?C, pressure 101,325 kPa and with liquid water and crystalline sodium-chloride, both having at the defined environmental conditions the value of exergy equal zero. The obtained mathematical model is valid in the temperature interval from 0 to 170?C and for the all compositions of solution, until to saturation. The derived mathematical model for the exergy is based on the Pitzer's ion interaction equation which satisfactory represents the dependence of the osmotic coefficient and the mean ionic activity coefficient on the composition of solution. The calculated values of exergy using the mathematical model were compared with the appropriate diagram for the exergy constructed using the table values of activity coefficients, osmotic coefficients and partial molar enthalpies.

( p, Vm, T, x) measurements for aqueous LiNO 3 solutions

( p, V m, T, x) properties of four aqueous LiNO 3 solutions (0.181, 0.526, 0.963, and 1.728) mol · kg −1 H 2O were measured in the liquid phase with a constant-volume piezometer immersed in a precision liquid thermostat. Measurements were made for 10 isotherms between (298 and 573) K. The range of pressure was from (2 to 40) MPa. The total uncertainty of density, pressure, temperature, and concentration measurements were estimated to be less than 0.06 %, 0.05 %, 10 mK, and 0.014 %, respectively. The values of saturated density were determined by extrapolating experimental ( p, ρ) data to the vapor-pressure at fixed temperature and composition using an interpolating equation. A polynomial type of equation of state for specific volume was obtained as a function of temperature, pressure, and composition by a least-squares method from the experimental data. The average absolute deviation (AAD) between measured and calculated values from this polynomial equation for density was 0.02 %. Measured values of solution density were compared with values calculated from Pitzer’s ion-interaction equation. The agreement is within (0.2 to 0.4) % depending of concentration range.