Revised Helgeson Kirkham Flowers Research Articles

Tin transport and cassiterite precipitation from hydrothermal fluids

Cassiterite (SnO2) is the main ore mineral of tin in magmatic–hydrothermal tin deposits, but tin transport and precipitation mechanisms from hydrothermal fluids remain poorly understood. We critically evaluated aqueous tin speciation in hydrothermal fluids from extensive experimental data and thermodynamic modeling. Sn(II) chloride complexes in hydrothermal fluids exist mainly as SnCl+, SnCl2(aq), and SnCl3−. The revised Helgeson–Kirkham–Flowers model parameters of these three tin species and two tin ions (Sn4+ and Sn2+) were derived from the correlation algorithms among these parameters, and the standard molar properties of cassiterite were optimized to be internally consistent with the available thermodynamic dataset. These thermodynamic parameters, together with the available equilibrium constant equation of Sn(IV) chloride complexes, could reproduce the available solubility data of cassiterite in acidic solutions at 400–700 °C under oxygen fugacity (fO2) levels buffered by hematite–magnetite (HM) or nickel–nickel oxide (NNO). These comparisons allow modeling chemical systems of SnO2–NaCl–HCl–H2O (liquid phase) to examine tin transport and cassiterite precipitation mechanisms under tin-mineralizing conditions: 300–500 °C, 50–150 MPa, 2 molal NaCl, and fO2 levels from QFM (quartz–fayalite–magnetite) to HM. Sn(II) chloride complexes are commonly interpreted to dominate in aqueous tin speciation under fO2 = NNO, but our modeling results indicate that considerable contents of Sn(IV) chloride complexes also exist in those reduced fluids with high HCl contents, consistent with recent in situ high-temperature experiments and molecular dynamic simulations. The Sn(II)/Sn(IV) ratios in fluids depends on fO2, temperature, and HCl contents. A considerable amount of Sn(IV) possibly exist in an early mineralization stage even under fO2 = NNO; if so, redox reactions are unnecessary to precipitate cassiterite from these mineralizing fluids. We find that even if the fO2 levels are constant, simple cooling can alter mineralizing fluids to be more oxidized (e.g., from QFM to HM) and cause cassiterite precipitation, indicating that oxidizing agents are not necessary as previously thought. This explains why cassiterite can precipitate in host rocks (e.g., sandstone or quartzite) that do not provide oxidizing agents. A simple rise in fO2 levels and pH neutralization (e.g., greisenization) also cause cassiterite precipitation. Cassiterite solubility in oxidized acidic hydrothermal fluids (NNO < fO2≤HM) is high enough to account for the tin contents of fluid inclusions from typical tin deposits, but the mineralization potential of oxdized fluids is inferior to reduced fluids (fO2≤ NNO) under the same conditions.

Constructing the Pourbaix diagram of Fe-Cl−-H2O ternary system under supercritical water conditions

Supercritical water oxidation (SCWO) technology, which has outstanding performance, is impeded commercial development due to serious corrosion problems. In this work, the first Pourbaix diagrams of the Fe-Cl−-H2O ternary system under supercritical water conditions was established to predict the evolution process of metals in thermodynamics equilibrium. The fifteen species were rationally screened by the ion distribution coefficient, and thermodynamics parameters under supercritical conditions were calculated by the extrapolation method of the revised Helgeson-Kirkham Flowers (R-HKF) model. The results show that hematite and magnetite passivation film and ferrous iron are continuously eroded by FeCl3 (aq) corrosion zone with the increase of chloride concentration, which greatly expands the area. Interestingly, the range of the immunization zone is almost unchanged. On the other hand, most of the Fe(OH)- 3 corrosion zone, passivation zone, and immunization zone to be converted to ferrous iron caused by temperature rise when FeCl3 (aq) reaches a certain concentration. Metals often form a hematite and magnetite passivation film to reduce the dissolution rate of the substrate under alkaline conditions with relatively high potentials, so the pre-neutralization method can be used as an anti-corrosion measure.

Quartz solubility in sodium carbonate solutions at high pressure and temperature

We present new experimental data on the synthetic quartz (Qtz) solubility in the Na2CO3 aqueous solutions ranging in salt concentration from 0.3–3.5 m (mole/kg H2O). The data have been collected in the temperature (T) range 500–700 °C at pressure (P) 4 kbar and 600 °C/5 kbar in an internally heated pressure vessel, with phase assemblage bracketing as a solubility monitor in quench experiments: first disappearance of crystalline Qtz in a succession of runs with constant Na2CO3 concentration but decreasing the Qtz/solution weight ratio was taken for the solubility value. It was possible to “bracket” the solubility within about 1 wt% by carefully observing the presence or absence of crystalline Qtz in the run products. Qtz solubility varies systematically with P, T, and particularly, with the Na2CO3 molality, increasing from 0.7 m for the 0.3 m Na2CO3 to 4 m for the 3.5 m Na2CO3 (at 600 °C/4 kbar). In all cases the SiO2 molality in the solutions exceeds that of Na2CO3, which suggests the presence of polymerized silica species in the solutions. Quantum chemical computations showed the dimer H5Si2O7− as the most stable species, which corresponds to the dehydrated species Si2O4OH− in the framework of the revised Helgeson–Kirkham–Flowers equation of state (HKF). A complete set of thermodynamic quantities for Si2O4OH−, consistent with the revised HKF and the DEW model (Sverjensky et al., 2014) has been retrieved from the experiments. Calculations with the obtained set of thermodynamic parameters reproduce very well the Qtz solubility experiments performed in a simpler H2O-NaOH fluid at 600 °C/4 kbar. Model calculations of silica and Ca solubility in the 15 wt% (3 m) NaCl aqueous solution interacting with a siliceous carbonate rock (Qtz + Calcite saturated) along a 13°/km geotherm, which take into account Si2O4OH−, predict much higher bulk Si and Ca contents in the fluid phase than in equilibrium with the corresponding individual minerals for P above 4–5 kbar.

Thermodynamic modelling of fluids from surficial to mantle conditions

Carbon is subducted to depths where metamorphism liberates water-bearing fluids. The C-bearing fluids facilitate partial melting of the upper mantle, generating magmas that may erupt as arc volcanics. Degassing of the magmas releases CO 2 and other volatile species to the atmosphere. Over geological time, this process contributes to the composition of the atmosphere and planetary habitability. Here I summarize the background needed to carry out theoretical geochemical modelling of fluids and fluid–rock interactions from surficial conditions into the upper mantle. A description of the general criteria for predicting equilibrium and non-equilibrium chemical reactions is followed by a summary of how the thermodynamic activities of species are related to measurable concentrations through standard states and activity coefficients. Specific examples at ambient conditions involving dilute water are detailed. The concept of aqueous speciation and how it can be calculated arises from this discussion. Next, I discuss how to calculate standard Gibbs free energies and aqueous activity coefficients at elevated temperatures and pressures. The revised Helgeson–Kirkham–Flowers equations of state are summarized and the revised predictive correlations for the estimation of equation of state coefficients in the Deep Earth Water (DEW) model are presented. Finally, the DEW model is applied to the solubility and speciation of aqueous aluminium.

Thermodynamic Analysis of Catalyst Stability in Hydrothermal Reaction Media

Hydrothermal solutions are important media for the conversion of biomass-derived species to useful chemicals and the destruction of environmental pollutants. These solutions are aggressive and can degrade heterogeneous catalysts. This article describes a framework for understanding the hydrothermal stability of heterogeneous catalyst materials with respect to oxidation and dissolution. We applied the revised Helgeson–Kirkham–Flowers thermodynamic equation of state to determine the oxidation states and solubilities of metals and oxides in water at 150–550 °C and 22–50 MPa. Design criteria for catalyst compositions were determined through correlations between metal solubility and electronegativity and between oxide solubility and cation electronegativity, ionic–covalent parameter, and polarizing power. Design criteria for aqueous solution compositions were determined by constructing oxygen fugacity–pH diagrams, which illustrate material phase changes in response to changes in pH and the oxidative or reducti...

Geobiochemistry of metabolism: Standard state thermodynamic properties of the citric acid cycle

Integrating microbial metabolism into geochemical modeling allows assessments of energy and mass transfer between the geosphere and the microbial biosphere. Energy and power supplies and demands can be assessed from analytical geochemical data given thermodynamic data for compounds involved in catabolism and anabolism. Results are reported here from a critique of the available standard state thermodynamic data for organic acids and acid anions involved in the citric acid cycle (also known as the tricarboxylic acid cycle or the Krebs cycle). The development of methods for estimating standard state data unavailable from experiments is described, together with methods to predict corresponding values at elevated temperatures and pressures using the revised Helgeson–Kirkham–Flowers (HKF) equation of state for aqueous species. Internal consistency is maintained with standard state thermodynamic data for organic and inorganic aqueous species commonly used in geochemical modeling efforts. Standard state data and revised-HKF parameters are used to predict equilibrium dissociation constants for the organic acids in the citric acid cycle, and to assess standard Gibbs energies of reactions for each step in the cycle at elevated temperatures and pressures. The results presented here can be used with analytical data from natural and experimental systems to assess the energy and power demands of microorganisms throughout the habitable ranges of pressure and temperature, and to assess the consequences of abiotic organic compound alteration processes at conditions of subsurface aquifers, sedimentary basins, hydrothermal systems, meteorite parent bodies, and ocean worlds throughout the solar system.

Internally consistent thermodynamic data for aqueous species in the system Na–K–Al–Si–O–H–Cl

A large amount of critically evaluated experimental data on mineral solubility, covering the entire Na–K–Al–Si–O–H–Cl system over wide ranges in temperature and pressure, was used to simultaneously refine the standard state Gibbs energies of aqueous ions and complexes in the framework of the revised Helgeson–Kirkham–Flowers equation of state. The thermodynamic properties of the solubility-controlling minerals were adopted from the internally consistent dataset of Holland and Powell (2002; Thermocalc dataset ds55). The global optimization of Gibbs energies of aqueous species, performed with the GEMSFITS code (Miron et al., 2015), was set up in such a way that the association equilibria for ion pairs and complexes, independently derived from conductance and potentiometric data, are always maintained. This was achieved by introducing reaction constraints into the parameter optimization that adjust Gibbs energies of complexes by their respective Gibbs energy effects of reaction, whenever the Gibbs energies of reactant species (ions) are changed. The optimized thermodynamic dataset is reported with confidence intervals for all parameters evaluated by Monte Carlo trial calculations. The new thermodynamic dataset is shown to reproduce all available fluid-mineral phase equilibria and mineral solubility data with good accuracy and precision over wide ranges in temperature (25–800°C), pressure (1bar to 5kbar) and composition (salt concentrations up to 5molal). The global data optimization process adopted in this study can be readily repeated any time when extensions to new chemical elements and species are needed, when new experimental data become available, or when a different aqueous activity model or equation of state should be used. This work serves as a proof of concept that our optimization strategy is feasible and successful in generating a thermodynamic dataset reproducing all fluid-mineral and aqueous speciation equilibria in the Na–K–Al–Si–O–H–Cl system within their experimental uncertainties. The new dataset resolves the long-standing discrepancies between thermodynamic data of minerals and those of aqueous ions and complexes, by achieving an astonishing degree of consistency between a large number of fluid-mineral equilibrium data. All of this at the expense of changing the standard state properties of aqueous species, mainly the Gibbs energy of formation. Using the same strategy, the core dataset for the system Na–K–Al–Si–O–H–Cl can be extended with additional rock-forming elements such as Ca, Mg, Fe, Mn, Ti, S, C, B. In future, the standard-state properties of minerals and aqueous species should be simultaneously optimized, to create the next-generation of fully internally consistent data for fluid-mineral equilibria. Although we employ the widely used HKF equations for this study, the same computational approach can be readily applied to any other speciation-based equation of state for multicomponent aqueous solutions.

A suite of programs, OptimA, OptimB, OptimC, and OptimS compatible with the Unitherm database, for deriving the thermodynamic properties of aqueous species from solubility, potentiometry and spectroscopy measurements

This paper describes four programs for Windows, designed to obtain the thermodynamic properties of aqueous species from experimental data and reporting them in the Unitherm database format (HCh software package). Programs OptimA and OptimS allow users to derive the standard Gibbs free energies of aqueous species from the results of chemical experiments (e.g., potentiometry or solubility) and from ultraviolet–visible (UV–Vis) absorption spectra, respectively; programs OptimB and OptimC enable optimization of the parameters of the revised Helgeson–Kirkham–Flowers equation of state and the modified Ryzhenko–Bryzgalin electrostatic model, respectively, for an aqueous species from its standard Gibbs free energy or stability constant as a function of temperature and pressure.

Volumetric Properties of the Nucleoside Thymidine in Aqueous Solution at T = 298.15 K and p = (10 to 100) MPa

Sound speeds have been measured for aqueous solutions of the nucleoside thymidine at T = 298.15 K and at the pressures p = (10, 20, 40, 60, 80, and 100) MPa. The partial molar volumes at infinite dilution, $$ V_{2}^{\text{o}} $$ , the partial molar isentropic compressions at infinite dilution, $$ K_{S,2}^{\text{o}} $$ , and the partial molar isothermal compressions at infinite dilution, $$ K_{T,2}^{\text{o}} $$ $$ \{ K_{T,2}^{\text{o}} = - (\partial V_{2}^{\text{o}} /\partial p)_{T} \} $$ , have been derived from the sound speeds at elevated pressures using methods described in our previous work. The $$ V_{2}^{\text{o}} $$ and $$ K_{T,2}^{\text{o}} $$ results were rationalized in terms of the likely interactions between thymidine and the aqueous solvent. The $$ V_{2}^{\text{o}} $$ results were also compared with those calculated using the revised Helgeson–Kirkham–Flowers (HKF) equation of state.

Water in the deep Earth: The dielectric constant and the solubilities of quartz and corundum to 60 kb and 1200 °C

Comprehensive quantitative theoretical evaluation of water–rock interactions in the Earth has long been restricted to a pressure of 5.0kb – too low to address processes involving deep aqueous fluids. Yet such fluids are thought to play an important role in the long-term geologic cycling of many chemical elements. A reason for this restriction is the lack of information on the dielectric constant of water (εH2O) needed for the revised Helgeson–Kirkham–Flowers (HKF) equations of state for aqueous species. Equation of state coefficients are available for hundreds of aqueous species in SUPCRT92, but calculations using these species can only be made to 5.0kb (Shock et al., 1992).In the present study, the applicability of the revised HKF equations of state for aqueous species was extended to 60kb by developing estimates of (εH2O). We used a statistical mechanically-based equation for the dielectric constant of a hard-sphere fluid applicable to water (Franck et al., 1990). The equation was calibrated with experimental data, and data from a comprehensive analysis of the literature (Fernández et al., 1997), and then used to calculate (εH2O) to a density of 1.1gcm−3. The values of ln(εH2O) were found to be linear with ln(ρH2O) which enabled empirical extrapolation of (εH2O) to 60kb. Values of ρH2O were computed with a recent comprehensive evaluation consistent with experimental data and a molecular dynamics model for water (Zhang and Duan, 2005).The resulting dielectric constants were tested at 727°C and 58kb by comparison with the results of ab initio molecular dynamics calculations (Pan et al., 2013). Additional testing was carried out by computing standard Gibbs free energies of aqueous species using the new values of (εH2O) and ρH2O in the revised HKF equations to predict equilibrium constants which in turn enabled calculation of the solubilities of quartz and corundum for comparison with experimental measurements to 20kb and 1100°C. Our results strongly suggest that geochemically useful predictions can now be made that will facilitate analysis of water–rock interactions in the Earth at depths much greater than previously possible.

The speciation and transport of palladium in hydrothermal fluids: Experimental modeling and thermodynamic constraints

The solubility of PdO(cr) was measured in NaOH (to 0.1m, mol/kg H2O) solutions at 400°C, 1kbar, and the solubility of Pd(cr) was determined at 400–500°C, 1kbar in acidic chloride solutions (to 1.5m NaCl) buffered with respect to hydrogen. The Pd electrode potential Eo(PdCl42-)/Pd(cr) for the reaction PdCl42- + 2e−=Pd(cr) + 4Cl− was determined at 50 and 70°C in 1m chloride solutions. These data, together with reliable literature values, were used for calculation of the standard thermodynamic properties and the formation constants for Pd–OH, Pd–Cl, and Pd–S–HS complexes within the framework of the revised Helgeson–Kirkham–Flowers model. It was found that PdCl3- and PdCl42- become the most important Pd complexes in high temperature (t>300°C), chloride-rich fluids, and PdCl42- predominates at m(Cl)>0.1. The stability of Pd–Cl complexes increases sharply with increase in temperature. The near-neutral chloride–sulfide solutions (1m NaCl, <0.1m Stot) can transport Pd at ppm concentration levels at t⩾600°C, whereas decrease in temperature and increase in pH can lead to effective deposition of Pd minerals. The stability of Pd–S–HS complexes (Pd(HS)2°, Pd(HS)3- and PdS(HS)2−) decreases with increase in temperature. Therefore, the role of these complexes in hydrothermal transport of palladium is restricted to the low temperature solutions (t<100°C) and sulfur can be considered an efficient depositing agent for Pd. The calculated HKF Equation of State parameters were used to predict thermodynamic properties of Pd2+, Pd–OH, Pd–Cl, and Pd–S–HS complexes to 700°C, 2kbar. These parameters are incorporated into the FreeGs web-enabled database (http://www-b.ga.gov.au/minerals/research/methodology/geofluids/thermo/calculator/search.jsp) that can be used for geochemical application of thermodynamic data obtained in the present study.

Volumetric properties of the nucleosides adenosine, cytidine, and uridine in aqueous solution at T = 298.15 K and p = (10 to 120) MPa

Sound speeds have been measured for aqueous solutions of the nucleosides adenosine, cytidine, and uridine at T=298.15K and at the pressures p=(10, 20, 40, 60, 80, 100, and 120)MPa. Using methods described in previous work, these sound speeds were used to derive the partial molar volumes at infinite dilution, V2o, the partial molar isentropic compressions at infinite dilution, KS,2o, and the partial molar isothermal compressions at infinite dilution, KT,2o{KT,2o=-(∂V2o/∂p)T}, for the nucleosides in aqueous solution at elevated pressures. The V2o and KT,2o results were rationalized in terms of the likely solute–water interactions. The V2o results were also critically compared with those calculated using the revised Helgeson–Kirkham–Flowers (HKF) equation of state.

Bismuth speciation in hydrothermal fluids: An X-ray absorption spectroscopy and solubility study

The solubility of bismuth oxide (α-Bi2O3(s); bismite) in near-neutral sodium perchlorate solutions at 65 and 80°C, and pure water from 150 to 600°C, PSat to 800bar was studied using various batch-reactor techniques and in situ XAS spectroscopy. The solubility of Bi2O3(s) follows a similar trend to Sb2O3(s) (senarmontite), which has been interpreted in terms of a neutral Sb(OH)3(aq) complex. Thus a similar neutral complex, Bi(OH)3(aq), is inferred for Bi. XANES spectroscopy confirms that the Bi(OH)3(aq) complex carries a stereochemically active lone electron pair, and EXAFS data suggest that the geometry of the complex changes little over the temperature range 380–610°C at 800bar, with three oxygen neighbors at ∼2.08Å. The solubility data obtained in this study are used in conjunction with thermodynamic properties for α-Bi2O3(s) to obtain thermodynamic parameters for Bi(OH)3(aq) within the framework of the revised Helgeson–Kirkham–Flowers (HKF) equation of state. Speciation calculations using these new properties indicate that, similarly to arsenic and antimony, bismuth is transported predominantly as a neutral hydroxide complex in a wide range of temperature, pressure, and fluid compositions. In contrast to arsenic and antimony, bismuth is much less soluble in typical hydrothermal fluids in the form of hydroxide complexes, and high temperatures (⩾400°C) are required for significant Bi transport by aqueous fluids. These results are consistent with the common association between Bi mineralization and magmatism.

Pourbaix diagrams for the nickel-water system extended to high-subcritical and low-supercritical conditions

One of the Generation IV nuclear reactor concepts is the supercriticalwater-cooled reactor (SCWR). To assist with plant design and selection of potential construction materials, Pourbaix diagrams for the nickel water-system slightly below and above the critical temperature at a pressure of 25MPa have been developed. A revised Helgeson–Kirkham–Flowers model with parameters optimized to bunsenite solubility data was used to perform the necessary thermodynamic extrapolations. The third hydrolysis product (Ni(OH)3−) was included in this study after careful review of new data. The diagrams revealed the need for strict water chemistry control to prevent transitioning to regions of active corrosion.

Pourbaix diagrams for chromium, aluminum and titanium extended to high-subcritical and low-supercritical conditions

The supercritical water-cooled reactor (SCWR) is a Generation IV concept that will operate with water above its thermodynamic critical point as the coolant. To aid in material selection and reactor design, Pourbaix diagrams for the alloying elements chromium, aluminum, and titanium have been developed under high-subcritical and low-supercritical conditions. The necessary thermodynamic extrapolations were performed using a revised Helgeson–Kirkham–Flowers model. The diagrams reveal the necessity to examine the transient reactor operating conditions such as start-up and shut-down to avoid active corrosion or dissolution of these elements from the alloys.

Pourbaix diagrams for the iron–water system extended to high-subcritical and low-supercritical conditions

The supercritical water-cooled reactor (SCWR) is a Generation IV reactor concept that will operate at temperatures and pressures above water’s thermodynamic critical point. Pourbaix diagrams for the iron–water system at temperatures slightly below and above the critical point at 25MPa have been constructed to aid the evaluation and development of potential construction materials. High temperature data extrapolation was performed using a revised Helgeson–Kirkham–Flowers model and fit to data on magnetite and hematite solubility in high-temperature water. A low-concentration diagram at 350°C reveals the importance of water chemistry control to avoid transitioning to an active corrosion region.

Erratum to: Prediction of the thermodynamic properties of metal–arsenate and metal–arsenite aqueous complexes to high temperatures and pressures and some geological consequences

The standard thermodynamic properties at 25°C, 1 bar (ΔG f o , ΔH f o , S o, C P o , V o, ω) and the coefficients of the revised Helgeson–Kirkham–Flowers equations of state were evaluated for several aqueous complexes formed by dissolved metals and either arsenate or arsenite ions. The guidelines of Shock and Helgeson (Geochim Cosmochim Acta 52:2009–2036, 1988) and Sverjensky et al. (Geochim Cosmochim Acta 61:1359–1412, 1997) were followed and corroborated with alternative approaches, whenever possible. The SUPCRT92 computer code was used to generate the log K of the destruction reactions of these metal–arsenate and metal–arsenite aqueous complexes at pressures and temperatures required by the EQ3/6 software package, version 7.2b. Apart from the AlAsO 4 o and FeAsO 4 o complexes, our log K at 25°C, 1 bar are in fair agreement with those of Whiting (MS Thesis, Colorado School of Mines, Golden, CO, 1992). Moreover, the equilibrium constants evaluated in this study are in good to fair agreement with those determined experimentally for the Ca–dihydroarsenate and Ca–hydroarsenate complexes at 40°C (Mironov et al., Russ J Inorg Chem 40:1690, 1995) and for Fe(III)–hydroarsenate complex at 25°C (Raposo et al., J Sol Chem 35:79–94, 2006), whereas the disagreement with the log K measured for the Ca–arsenate complex at 40°C (Mironov et al., Russ J Inorg Chem 40:1690, 1995) might be due to uncertainties in this measured value. The implications of aqueous complexing between dissolved metals and arsenate/arsenite ions were investigated for seawater, high-temperature geothermal liquids and acid mine drainage and aqueous solutions deriving from mixing of acid mine waters and surface waters.

Prediction of the thermodynamic properties of metal–chromate aqueous complexes to high temperatures and pressures and implications for the speciation of hexavalent chromium in some natural waters

Abstract Log–log correlation plots between the dissociation constants of known metal–chromate complexes and those of corresponding metal–sulfate complexes at 25 °C, 1 bar were used to derive the standard partial molal Gibbs free energies of formation of unknown metal–chromate complexes involving either (i) monovalent cations, divalent cations, and trivalent lanthanides or (ii) trivalent cations (excluding those of rare earth elements, REE) and tetravalent cations. For each of these two classes of ionic associations, empirical relationships between the standard partial molal volumes, isobaric heat capacities and entropies of known metal–chromate complexes and the corresponding thermodynamic properties of metal ions have been found. These data were utilized to evaluate the solute-characteristic parameters of the revised Helgeson–Kirkham–Flowers equation of state and to compute the thermodynamic properties of the dissociation reactions of metal–chromate complexes at high temperatures and pressures. Speciation of Cr(VI) in seawater and in shallow groundwaters interacting with serpentinites and ultramafites shows the importance of the NaCrO 4 - , MgCrO 4 ° , and CaCrO 4 ° aqueous complexes.

Relation Between the SHE and the Internal Ag ∕ AgCl Reference Electrode at High Temperatures

This article reviews the electropotential relation between the internal reference electrode used in nuclear light water reactor (LWR) applications and the standard hydrogen electrode (SHE). The focus is on the formation of complexes and how these might affect the potential of the electrode. In LWR applications the internal reference electrode is subjected to temperatures up to above , depending on reactor design. At temperatures of , the solubility products governing the concentration of complexes and thus the concentration of free in solution are different from what is known at room-temperature conditions. The potential of the electrode is in turn dependent on the concentration of free . Therefore, it is likely that also the potential of the reference electrode is altered, compared to the value derived by ignoring the formation of complexes. The review is based partially on data from literature and partially on thermodynamic calculations. Thermodynamical calculations based on the revised Helgeson–Kirkham–Flowers model were used to derive the equilibrium concentration of free . The revised electropotential relation between the internal reference electrode and the SHE assigns the electrode a higher potential compared to currently used models.

A potentiometric study of the stability of aqueous yttrium–acetate complexes from 25 to 175 °C and 1–1000 bar

The stability of yttrium–acetate (Y–Ac) complexes in aqueous solution was determined potentiometrically at temperatures 25–175 °C (at P s) and pressures 1–1000 bar (at 25 and 75 °C). Measurements were performed using glass H +-selective electrodes in potentiometric cells with a liquid junction. The species YAc 2+ and YAc 2 + were found to dominate yttrium aqueous speciation in experimental solutions at 25–100 °C (log [Ac −] < −1.5, pH < 5.2), whereas at 125, 150 and 175 °C introduction of YAc 3 0 into the Y–Ac speciation model was necessary. The overall stability constants β n were determined for the reaction Y 3 + + n Ac - = YAc n 3 - n β n = YAc n 3 - n [ Y 3 + ] · [ Ac - ] n · γ YAc n 3 - n γ Y 3 + · γ Ac - n where terms in square parentheses stand for the molal concentrations [mol/kg H 2O], and γ i is the activity coefficient of the ith species. With temperature increase from 25 to 200 °C, the values of log β 1, log β 2 and log β 3 increase from 2.27 to 4.51, 3.86 to 7.86 and 4.63 to 10.24, respectively. Increase of pressure results in decrease of the stability of all three complexes: log β 1 decreases by 0.21 and 0.37, log β 2 by 0.40 and 0.73, and log β 3 by 0.57 and 1.01 when pressure increases from 1 to 1000 bar at 25 and 200 °C, respectively. However, the effect of pressure on Y speciation is much less pronounced than that of temperature and results only in a decrease of the predominance field of YAc 3 0 in high-pressure solutions. The calculated values of the partial molal thermodynamic functions for the overall formation reactions are: Δ r S ∘ = 23.4, 42.5, and 54.5 cal/mol K, Δ r C p ∘ = 73.2 , 122.2, and 182.5 cal/mol K, and Δ r V ∘ = 16.1, 30.9, 44.3 cm 3/mol for YAc 2+, YAc 2 + and YAc 3 0 , respectively. The revised Helgeson–Kirkham–Flowers (HKF) equation of state parameters were generated for Y–Ac complexes.