Electrode Concentration Cell Research Articles

Experimental determination of the solubility product of dolomite at 50–253 °C

The ‘dolomite problem’, the scarcity of present-day dolomite formation near the Earth’s surface, has attracted much attention over the past century. Solving this problem requires having reliable data on the stability and kinetics of formation of this mineral. Toward this goal, the solubility of natural dolomite (CaMg(CO3)2) has been measured from 50 to 253 °C in 0.1 mol/kg NaCl solutions using a hydrogen electrode concentration cell (HECC). The obtained apparent solubility products (Kapp-sp-dol), for the reaction: CaMg(CO3)2 = Ca2+ + Mg2+ + 2CO32−, were extrapolated to infinite dilution to generate the solubility product constants for this reaction (Ksp°-dol). The derived equilibrium constants were fit and can be accurately described by log10Ksp°-dol = a + b/T (K) + cT (K) where a = 17.502, b = −4220.119 and c = −0.0689. This equation and its first and second derivatives with respect to T were used together with corresponding aqueous species properties to calculate the revised standard state thermodynamic properties of dolomite at 25 °C and 1 bar, yielding a Gibbs energy of formation (ΔfG298.15∘) equal to −2160.9 ± 2 kJ/mol, (log10Ksp°-dol = −17.19 ± 0.3); an enthalpy of formation (ΔfH298.15∘) of −2323.1 ± 2 kJ/mol, an entropy (S298.15∘) of 156.9 ± 2 J/mol/K and heat capacity (Cp298.15∘) of 154.2 ± 2 J/mol/K (uncertainties are 3σ). The dolomite solubility product derived in this study is nearly identical to that computed using SUPCRT92 (Johnson et al., 1992) at 200 °C, but about one order of magnitude higher at 50 and 25 °C, suggesting that dolomite may be somewhat less stable than previously assumed at ambient temperatures.

Experimental determination of the solubility product of magnesite at 50 to 200 °C

Accurate knowledge of magnesite thermodynamic properties over a wide range of temperatures is crucial for characterizing mineral stabilities in the system MgO–CO 2–H 2O and for modeling carbon dioxide fate in important natural and industrial processes. However, available databases, especially for its solubility product, are sparse and contradictory, leading to considerable uncertainties in the calculation of chemical equilibria and phase transformations among carbonates. In this study, the solubility of synthetic magnesite was investigated from 50 to 200 °C in 0.1 mol kg − 1 NaCl solutions and in some cases under constant CO 2 partial pressure (4–30 bars) both by means of a hydrogen electrode concentration cell (HECC) and a traditional batch Ti-reactor. The obtained apparent solubility products ( Q sp-mgs) were extrapolated to infinite dilution to generate the solubility products ( K sp°-mgs), allowing calculation of the thermodynamic properties of magnesite. Of all the temperature functions tested, the equation giving a reliable fit of our data in the investigated temperature range (50–200 °C) has the following form: log 10 K sp°-mgs = a + b / T (K) + cT (K) with: a = 7.267, b = − 1476.604 and c = − 0.033918. Based on this equation and its first and second derivatives with respect to T, we were able to derive values at 25 °C, 1 bar for magnesite thermodynamic functions: Δ f G 298.15 o = (− 1026.48 ± 2) kJ mol − 1 (log 10 K sp°-mgs = − 7.80 ± 0.3), Δ f H 298.15 o = (− 1111.75 ± 2) kJ mol − 1 , S 298.15 o = (60.00 ± 2) J mol − 1 K − 1 , and C p 298.15 o = (75.91 ± 2) J mol − 1 K − 1 (uncertainties are 3 σ).

Formation Quotients of Aluminum Sulfate Complexes in NaCF3SO3 Media at 10, 25, and 50 °C from Potentiometric Titrations Using a Mercury/Mercurous Sulfate Electrode Concentration Cell

Mercury/mercurous sulfate electrode concentration cells (with liquid junction) are employed in this study to determine the formation constants of aluminum sulfate complexes, with the principal advantage that the change in the free sulfate concentration is measured directly without the need to know the standard potential of the electrode. Potentiometric titrations were conducted at temperatures of 10, 25, and 50 degrees C and ionic strengths of approximately 0.3, 0.5, and 1.0 molal in aqueous solutions of the inert 1:1 electrolyte sodium trifluoromethanesulfonate (NaTr). Stoichiometric molal formation quotients Q1 and Q2, respectively, for the reactions Al3+(aq) + SO4(2-)(aq) AlSO4+(aq) and Al3+(aq) + 2SO4(2-)(aq) <= >Al(SO4)2-(aq) were determined. The values of log Q1 obtained from this work in NaTr media at ionic strengths of 0.3 and 1.0 mol x kg(-1) and 50 degrees C (1.72 +/- 0.08 and 1.35 +/- 0.06, respectively) are in excellent agreement with the values (1.71 +/- 0.2 and 1.32 +/- 0.1) determined in NaCl media from the recent potentiometric study conducted in the same laboratory using a hydrogen electrode concentration cell by Ridley et al. (Ridley, M. K.; Wesolowski, D. J.; Palmer, D. A.; Kettler, R. M. Geochim. Cosmochim. Acta 1999, 62, 459-472). The value of log Q2 (2.05 +/- 0.05) in 1.0 mol x kg(-1) from this work is smaller than the value reported by Ridley et al. (2.6 +/- 0.5) but within the combined experimental error. Empirical isothermal equations are presented to permit calculation of the equilibrium quotients as a function of ionic strength (0-1 mol x kg(-1)), giving log K1 and log K2 values at 25 degrees C and infinite dilution of 3.84 +/- 0.12 and 5.58 + 0.09, respectively. The value for log K1 obtained in this study at 25 degrees C is bracketed within experimental uncertainty by values reported by Kryzhanovskii et al. (Kryzhanovskii, M. M.; Volokhov, Y. A.; Pavlov, L. N.; Eremin, N. I.; Mironov, V. E Zh. Prikl. Khim. 1971, 44,476-479) and Nishide and Tsuchiya (Nishide, T.; Tsuchiya, R. Bull. Chem. Soc. Jpn. 1965, 38, 1398-1400), namely, 3.89 and 3.73, respectively. All other literature values for the first aluminum sulfate association constant are considerably lower than these results, which is also true for the second association constant, although there are few experimental data available for the latter. Empirical equations are also presented for calculating values of log Q1 and log Q2 from 0 to 1 molal ionic strength and from 10 to 125 degrees C, spanning the range of most environmental conditions at which these reactions are important.

Measurement of bacterial surface protonation constants for two species at elevated temperatures

This study reports the first potentiometric titrations of bacterial surfaces at elevated temperatures. We used a hydrogen electrode concentration cell to examine the protonation behavior of Bacillus subtilis (an aerobic, gram-positive species present in a wide variety of low-temperature, near-surface environments) at 30, 50, and 75°C. We also investigate the protonation of TOR-39 (an anaerobic, Fe-reducing, thermophylic bacteria isolated from a deep sedimentary basin) at 50°C and compare its surface properties to those of B.subtilis. Using a surface complexation approach that has been previously applied only to room temperature experiments involving bacteria, we determine the number of functional group types present on each cell wall, their acidity constants, and their site concentrations. We also characterize the temperature dependence of the acidity constants for B.subtilis. Our results indicate that the surface acidity can be successfully described using site-specific reactions involving discrete surface functional groups.Our results indicate that the acidity constants for the surface functional groups on B. subtilis do not change significantly over the temperature interval studied, and that the surface protonation at each temperature can be estimated using a single set of acidity constants and site densities. The best fitting model for B. subtilis is a three- site model with pKa values of 4.1, 5.8, and 7.7. Average site abundances for the three dominant functional groups on B. subtilis were determined to be 9.5 · 10−5, 1.0 · 10−4, and 8.8 · 10−5 moles of sites/gm. For TOR-39, the best fitting model was also a 3-site model with pKa values of 4.5, 5.8 and 8.2, with site abundances of 6.0 · 10−5, 5.0 · 10−5 and 3.0 · 10−5. This study suggests that bacterial surface site protonation reactions are not strongly temperature dependent, at least to 75°C. The lack of significant temperature dependence could greatly simplify the task of modeling bacteria-water-rock interactions at elevated temperature.

Thermodynamic Properties of the Ionization of Morpholine as a Function of Temperature and Ionic Strength

A hydrogen−electrode concentration cell was used to monitor pH in a study of morpholine ionization in aqueous solutions to temperatures of 290 °C and to ionic strengths of 1 m, maintained with either sodium trifluoromethanesulfonate or sodium chloride. The resulting hydrolysis quotients were combined with values from two previous potentiometric investigations of this reaction, as well as existing heat capacity and apparent molar volume data, to provide a global fitting equation. The computed thermodynamic parameters for the acid dissociation of the morpholinium ion at 25 °C and infinite dilution are log K = (−8.491 ± 0.003); ΔH = (39 ± 1) kJ·mol-1; ΔS = (−31 ± 1) J·K-1·mol-1; ΔCp = (40 ± 7) J·K-1·mol-1; and ΔV = (3 ± 1) cm3·mol-1. At low ionic strengths, including values at infinite dilution, the log Q data were found to be near linear with respect to the reciprocal temperature in Kelvin over the measured temperature range of 0 °C to 300 °C.

Magnetite surface charge studies to 290°C from in situ pH titrations

The proton-induced surface charge of magnetite was investigated in 0.03 and 0.30 molal sodium trifluoromethanesulfonate solutions from 25°C to 290°C by potentiometric titrations using a stirred hydrogen electrode concentration cell. Pure magnetite with excellent crystallinity was produced by reaction with the Ni/NiO/H 2O hydrogen fugacity buffer at 500°C. Inflection points in the 0.03 molal proton sorption isotherms (pH infl) at 6.50, 6.24, 5.65, 5.47, 5.31 and 5.55 at temperatures of 50°C, 100°C, 150°C, 200°C, 250°C and 290°C, respectively, were used as estimates of the pristine point of zero charge (pH ppzc) for modeling purposes. These pH infl values parallel 1/2 p K w and agree within the assigned uncertainty (±0.3 pH units) at all temperatures with independent estimates of the pH ppzc calculated from an extension of 88the revised MUSIC model. The surface charging can be adequately described by a one-p K model with a surface protonation constant fitted to the pH infl values, and giving the standard state thermodynamic properties log K H,298=7.00, Δ H 298°=−32.4±0.8 kJ/mol and constant Δ C p=128±16 J K −1 mol −1, with Δ S 298° assumed to be equal to that of rutile protonation (25.5±3.4 J K −1 mol −1. The 0.03 and 0.30 molal proton sorption isotherms also exhibit pHs of common intersection (pH cip) at 6.33, 5.78, 5.37, 4.82, 4.62 and 4.90 at 50°C, 100°C, 150°C, 200°C, 250°C and 290°C, respectively. The difference between the pH cip and pH ppzc≅pH infl values can be related to specific binding of Na + on the negatively charged surface, which increases with increasing temperature, although the pH cip values may also be affected by dissolution of the solid. The electrical double layer model includes a basic Stern layer capacitance, with specific cation and anion binding at the Stern layer, and a fixed diffuse layer capacitance computed from Guoy–Chapman theory. To fit the steepness and asymmetry of the charging curves above the pH ppzc, an additional cation binding constant was invoked, which allows the cation to experience the surface potential. Significant kinetically controlled dissolution of magnetite was observed below the pH ppzc, which may be a result of leaching of Fe 2+ from the surface, to produce a magnetite+hematite assemblage, despite the high hydrogen partial pressures (ca. 10 bars) used in these experiments.

Thermodynamic equations for a model lithium-ion cell

Fundamentals of classical thermodynamics of electrochemical systems have been employed to formulate mathematical equations for a model lithium-ion and lithium electrode concentration cell. Equations have been formulated to determine the energetic lithium interaction coefficients in the solid lithium-ion electrode phases and the salt mean activity coefficient in the electrolytic solution used in the model cell from the measured experimental data. In addition, a mathematical equation to predict the open-circuit voltage of the model cell for the case of differing electrolyte compositions in its porous electrodes has been developed.

The solubility of zinc oxide in 0.03 m NaTr as a function of temperature, with in situ pH measurement

The solubility of zincite (ZnO) has been measured in noncomplexing solutions over a wide range of pH m (4–11), and temperature (75–200°C) at 0.03 mol · kg −1 ionic strength in NaTr media (sodium trifluoromethanesulfonate, a noncomplexing 1:1 electrolyte), in a hydrogen electrode concentration cell (HECC), which provided continuous in situ measurement of hydrogen ion molality. Total zinc content was analyzed by atomic absorption using graphite furnace, flame, and inductively coupled plasma (ICP) spectrometers. The direction of approach to the equilibrium saturation state was varied to demonstrate that the system was reversible thermodynamically. Separate experiments were performed in alkaline solutions (0.03 mol · kg −1 NaOH) at 25 and 50°C in polypropylene syringes, and between 50 and 290°C in a Teflon-lined pressure vessel. The aim of these experiments was to reach higher pH m (>8 depending on the temperature) to determine the thermodynamic properties of the negatively charged species, Zn(OH) 3 −. A least-squares regression of the results obtained at this ionic strength was used to determine the molal solubility products (Q sn) of zincite. The solubility products (Q sn) were extrapolated to infinite dilution (K sn), permitting calculation of the thermodynamic properties of aqueous species of zinc for comparison with previous work.

Potentiometric Titrations of Rutile Suspensions to 250°C

A stirred hydrogen electrode concentration cell was used to conduct potentiometric titrations of rutile suspensions from 25 to 250°C in NaCl and tetramethylammonium chloride media (0.03 to 1.1m). Hydrothermal pretreatment of the rutile improved titration reproducibility, decreased titration hysteresis, and facilitated determination of the point of zero net proton charge (pHznpc). These pHznpc values are 5.4, 5.1, 4.7, 4.4, 4.3 (±0.2 pH units), and 4.2 (±0.3 pH units) at 25, 50, 100, 150, 200, and 250°C, respectively. The difference between these pHznpc values and 12 pKw(the neutral pH of water) is rather constant between 25 and 250°C (−1.45 ± 0.2). This constancy is useful for predictive purposes and, more fundamentally, may reflect similarities between the hydration behavior of surface hydroxyl groups and water. A three-layer, 1pKa surface complexation model with three adjustable parameters (two capacitance values and one counterion binding constant) adequately described all titration data. The most apparent trend in these data for pH values greater than the pHznpc was the increase in proton release (negative surface charge) with increasing temperature. This reflects more efficient screening by Na+relative to Cl−. Replacing Na+with the larger tetramethylammonium cation for some conditions resulted in decreased proton release due to the less efficient screening of negative surface charge by this larger cation.

Use of Alkali Glasses for the Preparation and Pressure Measurement of Alkali Metals, and Cesium Plate-out Levels on Transition Metals

We have prepared pure Li, Na, and Cs films in ultrahigh vacuum apparatus by the electrolysis of alkali-oxide-containing glasses. Details of the procedure which enables precise amounts of alkali metal to be metered out are given. The alkali ions electrolyzed out of the glass cannot be replaced and the glass becomes progressively weakened. The construction and use of an electrochemical Cs pressure gauge is described. The gauge is an electrode concentration cell in which the electrodes display the activity differences. The electrolyte is any alkali-oxide-containing glass. When the Cs pressure to be measured is zero the gauge gives a base EMF which is different for each glass electrolyte used. Pyrex glass (4% Na 2O) works better than Cs 2O glass as the electrolyte because the base EMF for Pyrex is high. The EMF-pressure relationship is given by the Nernst equation and Cs pressures down to 10 -17 Torr can be measured. The gauge has been used to assess Cs plate-out levels (i.e., equilibrium amounts deposited) on clean Ni at 0°C from Cs vapor at pressures below 10 -13 Torr. Ni is an effective Cs getter, leaving little Cs in the vapor phase. Alkali-oxide-containing glasses approximate to Ohmic ionic conductors with a limited degree of electronic conduction. The glass characteristics alter with passage of current and some decomposition occurs with the formation of silica.

Base strength of amines at high temperatures. Ionization of cyclohexylamine and morpholine

A potentiometric technique using a hydrogen electrode concentration cell with flowing solutions was employed to measure the ionization equilibria of cyclohexylamine and morpholine over the temperature range 50–295°C in dilute KCl media. Also, the isothermal pressure coefficients were determined to 250°C in the same media. Smoothing functions are presented which fit these and other selected data, and thermodynamic quantities for the ionization process were derived. Nearly linear variation of ΔH vsT ΔS was observed for this process as well as other similar reactions, and the significance is discussed. Differences in basicities of amines are found to decrease at elevated temperatures, and reversals in relative base strengths are seen.

The ionization of aqueous ammonia to 300�C in KCl media

A potentiometric technique using a hydrogen electrode concentration cell with flowing solutions was employed to study the ionization of ammonia over the temperature range 50–295°C in KCl media from 0.04 to 3.3m. The isothermal pressure coefficient of the ionization reaction was obtained at pressures up to 100 bars and temperatures to 250°C in these same media, and was found to vary as the square root of the ionic strength within the experimental error. Smoothing functions have been presented which also fit selected published data on the ammonia ionization reaction with a standard error of fit of 1.8 times the assigned uncertainties. Thermodynamic parameters for the reaction have been tabulated at rounded temperatures, and ionic strengths at the saturation vapor pressure. A limit is given for the association quotient of HCl at 300°C in 3m KCl based on these data.

Acidity measurements at elevated temperatures. VII. Dissociation of water

The self-dissociation of water has been studied over the temperature range from 0 to 300°C and in KCl media from 0.02m to 2.7m. Also, isothermal pressure coefficients of the dissociation quotients have been obtained in these same media up to 250°C. A potentiometric method employing a hydrogen electrode concentration cell with flowing solutions was employed. The estimated accuracy of logQw values up to 250°C is 0.02 log units and at 300°C is 0.04 log units. Smoothing functions have been found which fit these data along with the precise potentiometric data of Harned and co-workers at low temperatures, the existing calorimetric data up to 55°C and the recent conductimetric measurements of pure water up to 271°C by Bignoldet al., within about 1.5 times the estimated errors. Thermodynamic quantities for the dissociation reaction have been tabulated for rounded values of temperature and ionic strength at the saturation pressure of water. The isothermal pressure coefficients of log Qw varies approximately linearly with the square root of the ionic strength. This and the dependence of logKw on the density of the water is consistent with the assumption that the molal volumes of aqueous ions vary linearly with the compressibility coefficient of water. The heat for the dissociation reaction at infinite dilution is also shown to be strongly dependent on density.

熔融Fe-Mn-C合金のMnおよびCの活量

The solubility of carbon in Fe-Mn-C melts is determined at 1530±10°C in the range of Mn=40%.By construction of the following electrode concentration cell, the electromotive force corresponding to the change in Mn and C in Fe-Mn-C melts up to 36% of Mn content was measured at about 1540°C.Fe, Mn, C|SiO2, CaO, MnO, Al2O3|Fe, Mn, (36%), Csat.From this experiment, it was shown that Fe-Mn melts obeyed the Raoult's law but showed a gradually negative deviation from that law on addition of C. The effect of C on the activity coefficient of Mn was discussed by using the interaction coefficient γ(C)Mn. In addition, the effect of Mn on the activity coefficient of C was also discussed.Comparing with data of Fe-Cr-C melts already reported, it was shown that the effect of Mn on the activity coefficient of C was about one third of that of Cr.Furthermore, values of the interaction parameter, ε(X)C, which showed the effect of a number of dissolved elements on the activity coefficient of C were obtained in Fe-X melts saturated wit carbon.

熔融Fe-Si-C系のSiおよびCの活量について

By constructing the following electrode concentration cell, the author measured the electromotive force corresponding to the change in Si and C in molten Fe-Si-C alloys: ⊕ Fe, Si, C|silicate slag|Si \ominusThe following equations were derived from this experiment in the range up to NSi=0.25 at 1,540°: & logγ_Si’=-2.5(1-N_Si)^2+0.13(1-N_Si)^-3& logγ_Si=-2.5(1-N_Si-N_C)^2+0.13(1-N_Si-N_C)^-3where γSi′ and γSi are the activity coefficients in Fe-Si and Fe-Si-C alloys respectively. Then the interaction coefficients, γSiC, and γCSi are discussed from the experimental results and theoretical calculations. Furthermore, the iso-activity lines of Si and C have been established in such alloys.