Standard Molal Research Articles

Standard ferric–ferrous potential and stability of FeCl 2+ to 90°C. Thermodynamic properties of Fe (aq)3+ and ferric-chloride species

Standard electrode potentials ( E T 0) for the reaction Fe 3+ (aq)+0.5H 2(g)=Fe 2+ (aq)+H + (aq) were derived from electrochemical measurements in a non-isothermal cell at temperatures from 20.8° to 90°C in HClO 4 solutions with ionic strength of 0.11–1.31 m. A value of E 298 0=0.770±0.002 V was obtained. An equation E T 0=0.6000–3.0961×10 −3 T+6.4351×10 −4 T×ln T, where T is temperature in K, based on the assumption of a constant heat capacity for the above reaction, Δ r C p 0=14.84 cal/mol K [Hovey, J.K., 1988. Thermodynamics of aqueous solutions. PhD Thesis, Univ. Alberta, Edmonton, USA.], fits experimental values of E T 0 from the present study together with data from Whittemore and Langmuir [Whittemore, D.O., Langmuir, D., 1972. Standard electrode potential of Fe 3++e −=Fe 2+ from 5–35°C. J. Chem. Eng. Data 17, 288–290.] (5°–35°C) with an uncertainty better than ±0.002 V. The standard thermodynamic properties of ferric–ferrous reaction at 25°C were calculated: Δ r G 0=−17.75±0.04 kcal/mol, Δ r H 0=−9.4±0.5 kcal/mol, Δ r S 0=28.0±1.5 cal/mol K. Using these values and thermodynamic data for Fe (aq) 2+ and H 2(g) from SUPCRT92 [Johnson, J.M., Oelkers, E.H., Helgeson, H.C., 1992. SUPCRT92: a software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species and reactions from 1 to 5000 bars and 0° to 1000°C. Computers and Geosciences 18, 899–947.], the standard thermodynamic properties for Fe (aq) 3+ ion at 25°C, 1 bar were generated: Δ f G 0=−4.12 kcal/mol, Δ f H 0=−12.65 kcal/mol, S 0=−68.9 cal/mol K. The equilibrium stability constants for the reaction Fe 3++Cl −=FeCl 2+ were generated from the comparison of ferric–ferrous electrode potentials in a non-complexing medium (HClO 4) with those in HCl solutions from 25° to 90°C. A value of log β 0=1.52±0.10 was obtained at 25°C, 1 bar. Experimental results were combined with the reaction enthalpies measured by Vasil'ev and Lobanov [Vasil'ev, V.P., Lobanov, G.A., 1967. Effect of temperature and ionic strength on heat of formation of hydrated FeCl 2+ in aqueous solution. Russ. J. Phys. Chem. 41, 1969–1973 [in Russian].] from 10° to 55°C, to describe the temperature dependence of the stability constant, enthalpy and heat capacity for this reaction: log β 1 0=0.018 T+420.12/ T−5.23; Δ r H 0 (cal/mol)=−1918+0.082 T 2; Δ r C P 0 (cal/mol K)=0.163 T. This data allows calculation of the standard Gibbs energy (−37.61 kcal/mol), enthalpy (−47.21 kcal/mol), entropy (−30.2 cal/mol K) and heat capacity (−5.7 cal/mol K) for FeCl 2+ using thermodynamic properties for Fe 3+ and Cl − aqueous ions from this study and SUPCRT92, respectively. Thermodynamic properties of Fe 3+ and FeCl 2+ generated in this study together with data for FeCl 2 + complex derived from the literature data (Δ f G 0=−71.27 kcal/mol, Δ f H 0=−81.84 kcal/mol, S 0=−5.9 cal/mol K, C p 0=7.0 cal/mol K at 25°C, 1 bar) allowed calculation of hematite solubility in HCl solutions from 25° to 200°C.

Thermodynamics of hydration of Na- and K-clinoptilolite to 300 °C

The hydration state of Na- and K-exchanged clinoptilolite from Castle Creek (Idaho, U.S.A.) has been measured by a pressure titration method to 300 °C and PH2O<30 bars. The water content of clinoptilolite can be predicted as a function of water activity and temperature with the equation: aH2O = [exp[[−ΔHh∘/nRT] + [ΔSh∘/nR] − 1/nRT· [W1Xh + W2Xh2]− ln(Xa/Xh)]]−1 where T is degrees in Kelvin, ΔHh∘ is the standard molal enthalpy of hydration, ΔSh∘ is the entropy of hydration, Xh and Xa are, respectively, the mole fractions of the hydrous and anhydrous components of the solid solution, W1 and W2 are interaction parameters, n is the maximum number of moles of H2O per formula unit (based on 12 oxygens), and R is the gas constant. This equation can be used to locate clinoptilolite-H2O isohydrons in aH2O-T space below the liquid-vapor equilibrium curve of water. The standard molal Gibbs free energy of hydration is −47.62 ± 5.52 kJ/mol H2O and −5.40 ± 2.71 kJ/mol H2O for the Na- and K-clinoptilolite, respectively. These standard-state thermodynamic properties of clinoptilolite hydration are in good agreement with previous data at low H2O pressures. The experiments indicate that clinoptilolite progressively dehydrates with increasing temperature at pressures along the liquid-vapor equilibrium curve. Kinetic data above 150 °C show that clinoptilolite dehydration and hydration reactions are fast and reversible and that steady-state hydration states are attained in minutes.

Calculation of the thermodynamic properties at elevated temperatures and pressures of saturated and aromatic high molecular weight solid and liquid hydrocarbons in kerogen, bitumen, petroleum, and other organic matter of biogeochemical interest

To supplement the relatively sparse set of calorimetric data available for the multitude of high molecular weight organic compounds of biogeochemical interest, group additivity algorithms have been developed to estimate heat capacity power function coefficients and the standard molal thermodynamic properties at 25°C and 1 bar of high molecular weight compounds in hydrocarbon source rocks and reservoirs, including crystalline and liquid isoprenoids, steroids, tricyclic diterpenoids, hopanoids, and polynuclear aromatic hydrocarbons. A total of ninety-six group contributions for each coefficient and property were generated from the thermodynamic properties of lower molecular weight reference species for which calorimetric data are available in the literature. These group contributions were then used to compute corresponding coefficients and properties for ∼360 representative solid and liquid high molecular weight compounds in kerogen, bitumen, and petroleum for which few or no experimental data are available. The coefficients and properties of these high molecular weight compounds are summarized in tables, together with those of the groups and reference species from which they were generated. The tabulated heat capacity power function coefficients and standard molal thermodynamic properties at 25°C and 1 bar include selected crystalline and liquid regular, irregular and highly branched isoprenoids, tricyclic diterpanes, 17α(H)- and 17β(H)-hopanes, 5α(H),14α(H)-, 5β(H),14α(H)-, 5α(H),14β(H)-, and 5β(H),14β(H)-steranes, double ether- and ester-bonded n-alkanes, and various polynuclear aromatic hydrocarbons, including methylated biphenyls, naphthalenes, phenanthrenes, anthracenes, pyrenes, and chrysenes. However, corresponding coefficients and properties for many more saturated and unsaturated high molecular weight hydrocarbons can be estimated from the equations of state group additivity algorithms. Calculations of this kind permit comprehensive thermodynamic description of the chemical evolution of organic matter with increasing depth in sedimentary basins.

Calculation of the Standard Molal Thermodynamic Properties of Crystalline, Liquid, and Gas Organic Molecules at High Temperatures and Pressures

Calculation of the thermodynamic properties of organic solids, liquids, and gases at high temperatures and pressures is a requisite for characterizing hydrothermal metastable equilibrium states involving these species and quantifying the chemical affinities of irreversible reactions of organic molecules in natural gas, crude oil, kerogen, and coal with minerals and organic, inorganic, and biomolecular aqueous species in interstitial waters in sedimentary basins. To facilitate calculations of this kind, coefficients for the Parameters From Group Contributions (PFGC) equation of state have been compiled for a variety of groups in organic liquids and gases. In addition, molecular weights, critical temperatures and pressures, densities at 25°C and 1 bar, transition, melting, and boiling temperatures (Tt,Pr, Tm,Pr, and Tv,Pr, respectively) and standard molal enthalpies of transition (ΔH°t,Pr), melting (ΔH°m,Pr), and vaporization (ΔH°v,Pr) of organic species at 1 bar (Pr) have been tabulated, together with an internally consistent and comprehensive set of standard molal Gibbs free energies and enthalpies of formation from the elements in their stable state at 298.15 K (Tr) and Pr (ΔG°f and ΔH°f, respectively). The critical compilation also includes standard molal entropies (S°) and volumes (V°) at Tr and Pr, and standard molal heat capacity power function coefficients to compute the standard molal thermodynamic properties of organic solids, liquids, and gases as a function of temperature at 1 bar. These properties and coefficients have been tabulated for more than 500 crystalline solids, liquids, and gases, and those for many more can be computed from the equations of state group additivity algorithms. The crystalline species correspond to normal alkanes (CnH2(n+1)) with carbon numbers (n, which is equal to the number of moles of carbon atoms in one mole of the species) ranging from 5 to 100, and 23 amino acids including glycine (C2H5NO2), alanine (C3H7NO2), valine (C5H11NO2), leucine (C6H13NO2), isoleucine (C6H13NO2), aspartic acid (C4H7NO4), glutamic acid (C5H9NO4), asparagine (C4H8N2O3), glutamine (C5H10N2O3), proline (C5H9NO2), phenylalanine (C9H11NO2), tryptophan (C11H12N2O2), methionine (C5H11SNO2), serine (C3H7NO3), threonine (C4H9NO3), cysteine (C3H7SNO2), tyrosine (C9H11NO3), lysine (C6H14N2O2), lysine:HCl (C6H15N2O2Cl), arginine (C6H14N4O2), arginine:HCl (C6H15N4O2Cl), histidine (C6H9N3O2), and histidine:HCl (C6H10N3O2Cl).1 The data for the latter compounds permit calculation of the standard molal thermodynamic properties of protein unfolding in biogeochemical processes (Helgeson et al 1998). The liquids and gases considered in the present study include normal alkanes (CnH2(n+1)) for carbon numbers ranging from 1 to 100, 2- and 3-methylalkanes (CnH2(n+1)) for 4 ≤ n ≤ 20 and 6 ≤ n ≤ 20, respectively, 2,3-dimethylpentane (C7H16), 4-methylheptane (C8H18), cycloalkanes (CnH2n) for 3 ≤ n ≤ 8, methylated benzenes (CnH2(n−3)) for 7 ≤ n ≤ 12, normal alkylbenzenes (CnH2(n−3)) for 6 ≤ n ≤ 20, normal 1-alcohols (CnH2(n+1)O) for 1 ≤ n ≤ 20, ethylene glycol (C2H6O2), glycerol (C3H8O3), normal 1-alkanethiols (CnH2(n+1)S) for 1 ≤ n ≤ 20, normal carboxylic acids (CnH2nO2) for 2 ≤ n ≤ 20, and the following miscellaneous species: 2-thiabutane (C3H8S), thiophene (C4H4S), thiophenol (C6H6S), acetone (C3H6O), 2-butanone (C4H8O), ethyl acetate (C4H8O2), pyridine (C5H5N), 3-methylpyridine (C6H7N), and quinoline (C9H7N). One additional liquid (2-methylthiacyclopentane (C5H10S)) was also considered along with crystalline and gaseous carbazole (C12H9N). The thermodynamic data and equations summarized below can be used together with the standard molal thermodynamic properties of high molecular weight organic compounds (Richard and Helgeson 1995, Richard and Helgeson 1998a,Richard and Helgeson 1998b) and minerals, inorganic gases, and aqueous species, including biomolecules (Johnson et al 1992; Shock 1992a, Shock 1994, Shock 1995; Shock et al 1997; Shock and Koretsky 1993, Shock and Koretsky 1995; Sassani and Shock 1992, Sassani and Shock 1994; Schulte and Shock 1993, Schulte and Shock 1995; Oelkers et al 1995; Amend and Helgeson 1997a,Amend and Helgeson 1997b,Amend and Helgeson 1997c, Amend and Helgeson 1998; Sverjensky et al 1997) to compute equilibrium constants and chemical affinities for a wide variety of organic-inorganic reactions in geochemical and biochemical processes at both high and low temperatures and pressures.

Group additivity equations of state for calculating the standard molal thermodynamic properties of aqueous organic species at elevated temperatures and pressures

Group additivity equations of state for aqueous organic molecules have been generated by combining the revised Helgeson-Kirkham-Flowers (HKF) equations of state ( Shock and Helgeson, 1988, 1990; Tanger and Helgeson, 1988; Shock et al., 1989, 1992) with experimental values of the standard molal properties of aqueous alkanes, alkanols, alkylbenzenes, car☐ylic acids, amides, and amines. Equations of state parameters for the groups represented by -CH 2−, -CH 3, -CHCH 3−, -C 6H 5, -CH 2OH, -COOH, -CONH 2, and -CH 2NH 2 were determined by regression of the experimental data. This procedure permits calculation of the standard molal thermodynamic properties of these groups at elevated temperatures and pressures. Although curves representing the apparent standard molal Gibbs free energies (Δ G°) and enthalpies (Δ H°) of formation, and the standard molal entropies ( S°) of the groups as a function of temperature and pressure are respectively similar for each of them, the temperature dependence of the standard molal heat capacities ( C p °) and volumes ( V°) of a number of the groups are quite different from one another. For example, the standard molal heat capacities of the hydrocarbon groups minimize with increasing temperature, but those of -CH 2OH and -CH 2NH 2 maximize. Computed values of Δ G°, Δ H°, S°, C p °, V°, and the equations of state parameters for the various groups were used together with group additivity relations to generate corresponding values of these properties for aqueous n-alkanes, 2-methylalkanes, n-alkylbenzenes, n-alkanols, n-car☐ylic acids, n-amides, and n-amines at temperatures ≤ 250°C and pressures ≤ 1 kbar. The validity and generality of the equations of state are supported by the fact that predicted equilibrium constants for liquid n-alkane solubility reactions in water compare favorably with experimental values reported in the literature for temperatures as high as 200°C. Furthermore, equilibrium constants for aqueous ethane coexisting with ethene at 325 and 350°C at 350 bars predicted from the equations of state are in close agreement with independently determined experimental values reported by Seewald (1994). The standard molal thermodynamic properties and equations of state parameters reported below provide the means to characterize the thermodynamic behavior of a wide variety of aqueous organic species involved in hydrothermal reactions at elevated temperatures and pressures.

Calculation of the standard molal thermodynamic properties of aqueous biomolecules at elevated temperatures and pressures Part 1L-α-Amino acids

Download options Please wait... Article information DOI https://doi.org/10.1039/A608126F Article type Paper Download Citation J. Chem. Soc., Faraday Trans., 1997,93, 1927-1941 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Calculation of the standard molal thermodynamic properties of aqueous biomolecules at elevated temperatures and pressures Part 1L-α-Amino acids J. P. Amend and H. C. Helgeson, J. Chem. Soc., Faraday Trans., 1997, 93, 1927 DOI: 10.1039/A608126F To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Jan P. Amend Harold C. Helgeson

Thermodynamics of NaCl in Aqueous Ethylene Glycol, Acetonitrile, and 1,4-Dioxane Mixtures from Emf Measurements at 25°C

The electromotive force of the amalgam cell {NaxHg1-x‖NaCl(m)‖AgCl‖Ag} has been measured at 25°C as a function of the mole fraction x of Na in amalgams and of the molality m of NaCl in (ethylene glycol + water), (acetonitrile + water), and (1,4-dioxane + water) mixed solvents, containing up to 0.8 mass fraction of the organic component, with relative permittivities e ≥ 27. The relevant standard molal electromotive forces \(E_{\text{m}}^ \circ \) have been determined, together with the mean molal activity coefficient γ± of NaCl as a function of its molality. In the case of (ethylene glycol + water) mixtures, a linear dependence of \(E_{\text{m}}^ \circ \) on the mass fraction of ethylene glycol is observed, which is quite unusual, The Debye-Huckel equation is applicable successfully over the whole range of molalities explored, which extends to the vicinity of the solubility limit of NaCl in each solvent. The dependence of the standard emf on the logarithm of the volume fraction of water in the aqueous-organic solvent mixtures have been analyzed in terms of Feakins and French's theory, leading to a primary hydration number 7.2 for NaCl, in good agreement with previous results employing different methods.

Solubilities of the common L-α-amino acids as a function of temperature and solution pH

Abstract

Organic Acids in Hydrothermal Solutions; Standard Molal Thermodynamic Properties of Carboxylic Acids and Estimates of Dissociation Constants at High Temperatures and Pressures

Organic Acids in Hydrothermal Solutions; Standard Molal Thermodynamic Properties of Carboxylic Acids and Estimates of Dissociation Constants at High Temperatures and Pressures

Stability relations of jarosite and natrojarosite at 150–250°C

The jarosite decomposition reaction KFe 3( SO 4) 2( OH) 6 = 1.5 Fe 2 O 3 + K + + 2 SO 4 2− + 3 H + + 1.5 H 2 O and the analogous natrojarosite reaction have been studied in hydrothermal experiments of 1 to 1 1 weeks duration at 150–250°C. For a log mK 2 SO 4 in the coexisting aqueous phase of between −0.4 and −1.2, the log mH 2 SO 4 required to stabilize jarosite decreases from −0.35 ± 0.05 at 250° to −0.58 ± 0.12 at 200°C. At higher K 2SO 4 concentrations the H 2SO 4 concentration required to stabilize jarosite increases at both temperatures, consistent with the position of the jarosite-hematite boundary predicted with PHRQPITZ. Natrojarosite could not be produced from hematite at 250°C. At 200°C, the natrojarositehematite boundary occurs at log mH 2 SO 4 of −0.17 ± 0.08 at log mNa 2 SO 4 of −0.2 to −1.0. At log mNa 2 SO 4 below about −1.0, natrojarosite is unstable with respect to the assemblage hematite +Fe(SO 4)(OH). Apparent standard molal Gibbs free energies computed from the experimental data are ΔG 0 jarosite,200°,100 bars = −3416.3 ± 1.7 kJ/mol and ΔG 0 natrojarosite, 200°, 100 bars = −3371.9 ± 2.0 kJ/mol. The estimated errors reflect a lack of complete reversibility in the experiments, but do not consider other, potentially greater, sources of uncertainty such as those associated with calculation of aqueous activity coefficients. Additional experiments were used to study the exchange reaction KFe 3( SO 4) 2( OH) 6 + Na + = NaFe 3( SO 4) 2( OH) 6 + K +. Ln K D for this reaction is −3.1 ± 0.5 at 250°C, −3.7 ± 0.4 at 200°C, and −4.9 ± 0.2 at 150°C. Ln K D does not show a systematic variation with X Na at either 250 or 200°C, which suggests that jarosite and natrojarosite may be modeled as an ideal solid solution at these temperatures. At 150°C In K D does not vary from X Na = 0.1 to 0.6, but good constraints on its value could not be obtained outside this compositional range. The experimental data therefore do not rule out a significant departure from ideality at higher X Na at 150°C.

SUPCRT92: A software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 1000°C

Recent advances in theoretical geochemistry permit calculation of the standard molal thermodynamic properties of a wide variety of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 1000°C. The SUPCRT92 software package facilitates practical application of these recent theories, equations, and data to define equilibrium constraints on geochemical processes in a wide variety of geologic systems. The SUPCRT92 package is composed of three interactive FORTRAN 77 programs, SUPCRT92, MPRONS92, and CPRONS92, and a sequential-access thermodynamic database, SPRONS92.DAT. The SUPCRT92 program reads or permits user-generation of its two input files, CON and RXN, retrieves data from the direct-access equivalent of SPRONS92.DAT, calculates the standard molal Gibbs free energy, enthalpy, entropy, heat capacity, and volume of each reaction specified on the RXN file through a range of conditions specified on the CON file, and writes the calculated reaction properties to the output TAB file and, optionally, to PLT files that facilitate their graphical depiction. Calculations can be performed along the liquid side of the H 2O vaporization boundary by specifying either temperature ( T) or pressure ( P), and in the single-phase regions of fluid H 2O by specifying either T and P, T and H 2O density, T and log K, or P and log K. SPRONS92.DAT, which contains standard molal thermodynamic properties at 25°C and 1 bar, equation-of-state parameters, heat capacity coefficients, and phase transition data for approximately 500 minerals, gases, and aqueous species, can be augmented or otherwise modified using MPRONS92, and converted to its direct-access equivalent using CPRONS92.

The influence of pressure on the activity coefficients of the solutes and on the solubility of minerals in the system Na-Ca-Cl-SO 4-H 2O to 200°C and 1 kbar and to high NaCl concentration

A model is presented which is used to calculate the effect of pressure on activity coefficients of aqueous solutes in the system Na-Ca-Cl-SO 4-H 2O to 200°C. Literature data for the density and compressibility of aqueous binary solutions of Na 2SO 4 and CaCl 2 to 200°C are used to calculate the first and second pressure derivatives of Pitzer's ion interaction model parameters, as well as the standard molal compressibility and volume of these two salts. Empirical correlations between the apparent molal volume and compressibility of the aqueous electrolytes are used to guide the choice of the temperature dependent expressions used for the numerical representation of the derivatives of Pitzer's parameters with respect to pressure. For sodium sulfate solutions, such correlations are used to extrapolate compressibilities to 200°C. The change in the thermodynamic properties of the-CaSO 0 4 ion pair with pressure is taken into account by the variation of its dissociation constant. The volumetric properties (partial molal volumes and compressibilities) of multicomponent solutions in the Na-Ca-Cl-SO 4-H 2O system can be predicted from the information generated here and the volumetric equations of Rogers and Pitzer (1982) for NaCl. This model is then combined with the high temperature model of Moller (1988) of the same system in order to calculate activity coefficients at high pressures to 200°C. The resulting model is validated by comparing calculated and measured solubilities of anhydrite and gypsum in pure water and in NaCl solutions up to 6 M. The agreement between the calculated and measured solubilities of the calcium sulfates is typically better than 10% up to 200°C and 1 kbar. The relevance of temperature and pressure corrections to the activity coefficients of aqueous solutes is discussed in regard to the assumed accuracy with which geochemical models are able to calculate mineral solubilities.

Triple-ion anions and polynuclear complexing in supercritical electrolyte solutions

Calculation of accurate mineral solubilities in hydrothermal solutions requires dissociation constants for the predominant aqueous complexes in solution, many of which may be present in appreciable concentrations only at high temperatures. Consideration of dissociation constants representing the standard molal Gibbs free energy changes resulting from the electrostatic interaction of aqueous species indicates that triple ions such as NaCl 2 − and Na 2Cl + may be major species in alkali metal halide solutions at high temperatures and low pressures where the dielectric constant of H 2O is ≲ 15. Molal triple ion dissociation constants were computed for temperatures from 400° to 800°C at pressures from 500 to 4000 bars using equations given by Fuoss and Kraus (1933b), together with values of the effective electrostatic radii of aqueous ions generated from algorithms developed by Tanger and Helgeson (1988) and Shock et al. (1990). The logarithms of the calculated second stepwise dissociation constants (log K 2) for 16 alkali metal and hydrogen halides typically decrease at 500 bars from ∼0 at 400° to ∼ −3 at 700°C, but they increase with increasing pressure from ∼ −3 at 500 bars to ∼0 at 3 kb and 700°C. Distribution of species calculations using these log K 2 values and dissociation constants for 1:1 ion pairs ( Oelkers and Helgeson, 1988), together with activity coefficients generated by adopting the revised HKF ( Helgeson, Kirkham, and Flowers, 1981) equation of state ( Tanger and Helgeson, 1988) to calculate extended term parameters in the Hückel equation ( helgeson et al., 1981) indicate that triple ions are the predominant species in low-pressure supercritical aqueous solutions at concentrations ≈ 0.8 m. Under these circumstances, polynuclear and mixed ligand complexes in natural electrolyte solutions such as KNaCl +, MgKCl 2 +, ZnNaCl 2 +, FeKCl 2 +, Na 2ClSO 4 −, etc. may have a substantial effect on the hydrothermal solubilities of minerals at high temperatures and low pressures where many geochemical processes occur.

An experimental investigation of heulandite-laumontite equilibrium at 1000 to 2000 bar P fluid

The univariant reaction governing the upper stability of heulandite (CaAl2Si7O18·6H2O), heulandite=laumontite+3 quartz+2H2O (1), has been bracketed through reversal experiments at: 155±6° C, 1000 bar; 175±6° C, 1500 bar; and 180±8° C, 2000 bar. Reversals were established by determining the growth of one assemblage at the expense of the other, using both XRD and SEM studies. The standard molal entropy of heulandite is estimated to be 783.7±16 J mol−1 K−1 from the experimental brackets. Predicted standard molal Gibbs free energy and enthalpy of formation of heulandite are −9722.3±6.3 kJ mol−1 and −10524.3±9.6 kJ mol−1, respectively. The reaction (1), together with the reaction, stilbite=laumontite+3 quartz+3 H2O, defines an invariant point at which a third reaction, stilbite=heulandite+ H2O, meets. By combining the present experimental data with past work, this invariant point is located at approximately 600 bar and 140° C. Heulandite, which is stable between the stability fields of stilbite and laumontite, can occur only at pressures higher than that of the invariant point, for \(P_{H_2 O}\)= Ptotal.These results are consistent with natural parageneses in low-grade metamorphic rocks recrystallized in equilibrium with an aqueous phase in which \(a_{H_2 O}\) is very close to unity.

Computation of the thermodynamic properties of reactions involving minerals and aqueous solutions with the aid of the personal computer

A BASIC program (EQCALC) for the calculation of the thermodynamic properties of reactions involving minerals, ions, and liquid water has been developed for the personal computer. EQCALC is written in “generic” BASIC in order to be as machine independent as possible. The program runs on the IBM PC, IBM compatibles, and many other microcomputers, requiring about 84K bytes for execution. The program outputs the standard molal volume, standard molal heat capacity, standard molal enthalpy, standard molal Gibbs free energy, and the logarithm of the equilibrium constant for the reaction.

Standard electrode potentials of the silver – silver phosphate and silver – silver arsenate electrodes in formamide at different temperatures

For the emf measurements of cells of the type Ag, AgCl/NaCl(m)|Na3X(m/3)/Ag3X, Ag where X is PO4 or AsO4, the standard potentials of the silver–silver phosphate and silver–silver arsenate electrodes have been determined in formamide at five different temperatures ranging from 5 to 45 °C. The standard molal potentials of the electrodes have been expressed through the equations[Formula: see text]The standard thermodynamic quantities, ΔG0, ΔH0, ΔS0 and ΔCp0 for the respective electrode reactions have been evaluated from the Em0's of the electrodes at these temperatures. The solubility products of silver phosphate and silver arsenate have also been calculated in formamide at these temperatures.

Chemical interaction of aqueous solutions with epidote-feldspar mineral assemblages in geologic systems; II, Equilibrium constraints in metamorphic/geothermal processes

Thermodynamic analysis of the system Na/sub 2/O-K/sub 2/O-CaO-FeO-Fe/sub 2/O/sub 3/-Al/sub 2/O/sub 3/-SiO/sub 2/-H/sub 2/O-HCl-CO/sub 2/ at pressures and temperatures to 5 kb and 600/sup 0/C affords quantitative description and interpretation of phase relations among epidote, garnet, plagioclase, and alkali feldspar solid solutions in hydrothermal systems. Comparison of computed and observed compositions of these minerals and coexisting fluids suggests that the calculations afford close approximation of equilibrium and mass transfer in metamorphic/geothermal processes. The standard molal enthalpies of decomposition reactions for plagioclase are of the order of -4 to -50 kcal mole/sup -1/, which requires the activity of the anorthite component of plagioclase coexisting with epidote solid solutions to increase with increasing temperature at constant pressure. In contrast, because the standard molal volumes of these reactions are also negative, a/sub CaAl/sub 2/Si/sub 2/O/sub 8// decreases with increasing pressure at constant temperature. Either stoichiometric epidote or clinozoisite is compatible with plagioclase, quartz, calcite, and an aqueous phase, but only at X/sub CO/sub 2// less than or equal to 0.2. The complex zoning commonly exhibited by both plagioclase and epidote solid solutions in geologic systems can be attributed to minor isothermal/isobaric changes in the composition of coexisting aqueous solutions. Logarithmic activity and fugacitymore » diagrams constructed with provision for solid solution permit documentation of such changes and facilitate thermodynamic interpretation of mineral and fluid compositions in metamorphic/geothermal systems.« less

Standard potentials of the silver—silver bromide electrode in dioxane—water mixtures and related thermodynamic functions

The standard potentials of AgAgBr electrode in eighteen dioxane + water solvent mixtures containing up to 90 wt % dioxane, have been evaluated from the emf measurements of the cell Pt, H 2 (g, 1 atm); HBr (m), X % dioxane, Y % water: AgBr, Ag at nine different temperatures in the range 15–55°C. The standard molal potentials in each solvent have been represented as a function of temperature. The standard thermodynamic functions for the cell reaction, the primary medium effects of various solvents upon HBr and the standard thermodynamic quantities for the transfer of one mole of HBr from water to the respective dioxane + water media have been computed and discussed in the light of ion—solvent interactions as well as the structural changes of the solvents.

Mercury-mercurous propionate electrode: standard potentials at different temperatures and related thermodynamic quantities in dioxan-water media

The standard molal potentials ( E° m /V) of the Hg/Hg 2(OPr) 2, OPr − electrode at 15, 20, 25, 30 and 35°C have been determined in dioxan-water media. The values of E° m as a function of the Celsius temperature, t, in different solvent composition may be represented by the following equations: ▪ The related thermodynamic quantities have been calculated. The dependence of the standard potentials at 25°C on the dielectric constant of the media (in the light of the Born equation), the volume fraction (Φ w ) and the mole fraction ( N w ) of water (based on Feakins-French relationship) have been examined. The variation of the thermodynamic quantities Δ G°, Δ S° and Δ H° with various parameters associated with solvent media have also been examined. The observations are similar to those reported in our previous work on mercury-mercurous acetate electrode in the same media. Δ C° p for the cell reaction has been calculated.

Mercury-mercurous propionate electrode: standard potentials at different temperatures and related thermodynamic quantities

The standard molal potentials E° m of the Hg/Hg 2(OPr) 2, OPr − electrode at 15°, 20°, 25°, 30° and 35° C have been determined. The E° m values obtained are 0.5114, 0.5072, 0.5031, 0.4988 and 0.4942 V respectively, which can be fitted to the equation Edg m/V = 0.5031 −8.56 × 10 −4 (itt/°C − 25)−3.0588 × 10 −6 ( t/ °C -25) 2. The changes in standard free energy, entropy and enthalpy for the cell reaction have been calculated.