Hydrogen-electrode Concentration Cell Research Articles

Association quotients of aluminum sulphate complexes in NaCl media from 50 to 125°C: Results of a potentiometric and solubility study

The speciation and molal formation quotients for the complexation of aluminum with sulphate were measured based on potentiometric and solubility experiments. Potentiometric titrations, utilizing a hydrogen-electrode concentration cell, were performed from 50 to 125°C at ionic strengths of 0.1, 0.3 and 1.0 molal in aqueous NaCl media. Two aluminum-sulphate species, AlSO 4 +and Al(SO 4) 2 −, were identified from the titration data and the formation quotients for these species were modeled by empirical equations to describe their temperature and ionic strength dependencies. Thermodynamic parameters for the complexation reactions were obtained by differentiating the empirical equations with respect to temperature. The thermodynamic quantities obtained for the formation of AlSO 4 + at 50°C and infinite dilution are: logK 1 = 3.7 ± 0.4, ΔH 1° = −10 ± 30 kJ · mol −1, ΔS 1° = 40 ± 100 J · K −1 · mol −1 and ΔC° p 1 = 1900 ± 800 J · K −1 · mol −1; whereas the values for Al(SO 4) 2 − are: logK 2 = 5.6 ± 0.7, ΔH 2° = 10 ± 50 kJ · mol −1, ΔS 2° = 100 ± 100 J · K −1 · mol −1 and ΔC° p 2 = 2800 ± 800 J · K −1 · mol −1. A solubility study, which was undertaken to verify the 50°C potentiometric data, was performed by reacting powdered gibbsite (Al(OH) 3) with sulphate solutions at 10 −3.5 and 10 −4 molal H +, total sulphate concentrations from 0.005 to 0.080 molal, and 0.1 and 1.0 molal ionic strength in aqueous NaCl media. The results of the solubility study are in good agreement with the potentiometric data and establish that Al-sulphate complexation substantially enhances the equilibrium solubility of gibbsite.

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.

The aqueous chemistry of aluminum. A new approach to high-temperature solubility measurements

Predictions of changes in geothermal reservoir permeability and porosity during exploitation and reinjection, as well as aluminosilicate scale formation in wells and plant equipment, are currently limited by inaccuracies and discrepancies in our knowledge of the aqueous speciation of aluminum and the solubilities of aluminosilicates in high-temperature brines. To address this problem, the solubility of pure synthetic boehmite (AlOOH) has been measured in noncomplexing solutions over a wide range of pH (2–10), temperature (100–290°C), and ionic strength (0.03–1 mol·kg −1 NaCl) in a hydrogen-electrode concentration cell (HECC) that provided continuous, in situ measurement of hydrogen ion molality. This represents the first such study ever reported of a pH-dependent mineral solubility profile across the entire pH range of natural waters at temperatures above 100°C. Samples of the solution were withdrawn after the pH reading stabilized for analysis of total aluminum content by ion chromatography. Acidic or basic titrants could then be metered into the cell to affect a change in the pH of the solution. The direction of approach to the equilibrium saturation state could be readily varied to ensure that the system was reversible thermodynamically. A least-squares regression of the results obtained at low ionic strength was used to determine the molal solubility products ( Q s0to Q s4) of boehmite, which allowed comparison with those obtained from two recently-reported high-temperature studies of boehmite solubility, which relied on the conventional batch technique. Comparisons are also made with the low-temperature (<90°C) hydrolysis constants for aluminum obtained from solubility measurements with gibbsite as the stable phase. Based on these results, it is possible to draw some general conclusions concerning the relative importance of the aluminum species in solution and to reduce significantly the number of experiments needed to define this complex system. Finally, the application of this new technique to the study of the kinetics and thermodynamics of the dissolution and formation of more complex aluminosilicate minerals is discussed.

Dissociation quotients for citric acid in aqueous sodium chloride media to 150°C

The three molal dissociation quotients for citric acid were measured potentiometrically with a hydrogen-electrode concentration cell from 5 to 150°C in NaCl solutions at ionic strengths of 0.1, 0.3, 0.6, and 1 molal. The molal dissociation quotients and available literature data at infinite dilution were fitted by empirical equations in the all-anionic form involving an extended Debye-Huckel term and up to five adjustable parameters involving functions of temperature and ionic strength. This treatment yielded the following thermodynamic quantities for the first dissociation equilibrium at \(\begin{gathered} 25^ \circ {\text{C: log }}K_{1{\text{a}}} = - 3.127 \pm 0.002{\text{, }}\Delta H_{{\text{la}}}^{\text{o}} = \hfill \\ 4.1 \pm 0.{\text{2 kJ - mol}}^{{\text{ - 1}}} {\text{, }}\Delta S_{{\text{1a}}}^{\text{o}} = - 46.3 \pm 0.{\text{7 J - K}}^{{\text{ - 1}}} - {\text{mol}}^{{\text{ - 1}}} {\text{, and }}\Delta Cp_{{\text{1a}}}^{\text{o}} = - 162 \pm \hfill \\ {\text{7 J - K}}^{{\text{ - 1}}} - {\text{mol}}^{{\text{ - 1}}} ; \hfill \\ \end{gathered} \) for the second acid dissociation equilibrium at \(\begin{gathered} 25^ \circ {\text{C: log }}K_{{\text{2a}}} = - 4.759 \pm 0.001{\text{, }}\Delta H_{{\text{2a}}}^{\text{o}} = \hfill \\ 2.2 \pm 0.1,{\text{ }}\Delta S_{{\text{2a}}}^{\text{o}} = - 83.8 \pm 0.4{\text{, and }}\Delta Cp_{{\text{2a}}}^{\text{o}} = - 192 \pm 15 \hfill \\ \end{gathered} \), and for the third dissociation equilibrium at \(\begin{gathered} 25^ \circ {\text{C: log }}K_{{\text{3a}}} = - 6.397 \pm 0.002{\text{, }}\Delta H_{{\text{3a}}}^{\text{o}} = \hfill \\ - 3.6 \pm 0.2,{\text{ }}\Delta S_{{\text{3a}}}^{\text{o}} = - 134.5 \pm 0.7{\text{, and }}\Delta Cp_{{\text{3a}}}^{\text{o}} = - 231 \pm 7 \hfill \\ \end{gathered} \).

Hydrolysis of magnesium(II) at elevated temperatures

A potentiometric titration technique utilising a hydrogen-electrode concentration cell was applied to study the hydrolytic behaviour of the magnesium(II) ion in 0.10 and 1.0 mol kg–1 sodium chloride at 60, 100, 150 and 200 °C. Data analysis indicates the presence of Mg2+(aq) and the solid species Mg(OH)2(s), only. Independent measurements have confirmed that [Mg(OH)]+(aq) is a negligible ionic species under the prevailing experimental conditions. Experimental evidence indicated that the solid species formed both rapidly and reversibly. The equilibrium quotients for brucite formation were determined for each temperature and ionic strength investigated and the corresponding enthalpy and entropy of formation calculated. Formation constants have also been calculated at zero ionic strength using the Debye–Huckel equation, and these data, together with some other literature data, comply with the equation log β12s0= 2.49 –(5847/T) where log β12s0 is the logarithm of the formation constant of brucite at zero ionic strength and T is the temperature in Kelvin.

Hydrogen ion adsorption at the rutile-water interface to 250°C

A stirred hydrogen-electrode concentration cell was used to follow hydrogen ion adsorption by the rutile surface in NaCl media (0.01–1.0 m) between 25 and 250°C. The pH of zero net surface charge (pHzpc) decreases to 200°C and then appears to increase. Away from the pHzpc, negative surface charge is screened more efficiently by Na+ ions than positive charge is by Cl− ions and this effect increases with temperature. Thus, Na+ moves closer to the rutile surface with increasing temperature relative to Cl−. Finally, [pHzpc − 12pKw] is approximately constant (− 1.1 ± 0.2) for this rutile sample to 250°C and is also equivalent to the equilibrium constant for an isocoulombic form of the “IpKa” surface ionization model. Thus, the constancy of [pHzpc − 12pKw] may also be useful for extrapolating rutile pHzpc values to higher temperatures and pressures than those studied here, and may also apply to many other oxide surfaces as well.

Dissociation quotients of D-galacturonic acid in aqueous solution at 0.1 MPa to 1 molal ionic strength and 100�C

The molal dissociation quotients of D-galacturonic acid were measured potentiometrically in a newly-designed, hydrogen-electrode concentration cell from 5 to 100°C at four ionic strengths ranging from 0.1 to 1.0 mol-kg−1 using sodium trifluoromethanesulfonate (NaF3CSO3) as the supporting electrolyte. These quotients were fitted in the all anionic (isocoulombic) form by an empirical equation incorporating three adjustable parameters. When combined with the known dissociation quotient for water in the same medium, this treatment yielded the following thermodynamic quantities for the acid dissociation equilibrium at 25°C and infinite dilution: log KH=−3.490±0.011, ΔHH0=0.4±0.2 kJ-mol−1, ΔSH0=−65±1 J-mol−1-K−1, and ΔCp, H0=−231±8 J-mol−1-K−1. Comparisons are made with the corresponding results of a limited number of previous studies carried out near ambient conditions.

Aluminum speciation and equilibria in aqueous solution: III. Potentiometric determination of the first hydrolysis constant of aluminum(III) in sodium chloride solutions to 125°C

The first molal hydrolysis quotient of aluminum(III) was measured potentiometrically from 25 to 125°C at 25° intervals at ionic strengths of 0.1, 0.3, 1.0 and 5 mol · kg −1 with sodium chloride as the supporting electrolyte. The experimental method involved using a hydrogen-electrode concentration cell modified to compensate for any intrinsic potential offset between the two electrodes. The initial concentration of Al 3+ was varied to test for the presence of multinuclear aluminum species while being kept to a maximum of 10 −3 mol · kg −1 to minimize their occurrence. Similarly, the maximum degree of hydrolysis of Al 3+ reached in each titration was ca. 30%, after which polymerization and/or precipitation became apparent. The equilibrium quotients obtained in this study and selected values from the literature were fitted by an empirical equation incorporating a linear dependence of log K 1,1 on the reciprocal temperature (Kelvins) over the range 10–200°C and three ionic-strength-dependent parameters. Comparisons are made between the results of this study and the literature values.

Dissociation constant of bisulfate ion in aqueous sodium chloride solutions to 250.degree.C

The molal equilibrium quotients for the protolytic dissociation of bisulfate ion (HSO{sub 4}{sup {minus}}) were measured potentiometrically in aqueous sodium chloride media by using a hydrogen-electrode concentration cell with liquid junction. The ionic strength was varied from 0.1 to 5.0 mol kg{sup {minus}1} at temperatures ranging from 50 to 250{degree}C. Equilibrium quotients obtained from the measured potentials were combined with published values of the corresponding equilibrium constants, enthalpy, and heat capacity changes at infinite dilution and ambient temperatures, and fitted to an empirical equation containing the Pitzer form of the extended Debye-Hueckel term and nine adjustable parameters.

Ionization equilibriums of silicic acid and polysilicate formation in aqueous sodium chloride solutions to 300.degree.C

The ionization behavior of silicic acid and polysilicate formation in basic solutions have been studied by precise potentiometry using titration techniques in a hydrogen-electrode concentration cell. Polysilicate formation was studied in 1 m NaCl solutions at temperatures from 60 to 290/sup 0/C and at Si(IV) concentrations 0.005 to 0.05 m. At the lowest silica concentration only mononuclear species occur over wide temperature and pH ranges. At hydroxyl numbers from about 0.7 to 1.0 (average charge per silicon) small polysilicates which equilibrate rapidly occur at higher Si(IV) concentrations. Polysilicate formation decreases with increasing temperature. The equilibrium quotient for the most significant reaction, Si(OH)/sub 4/(aq) + OH/sup -/ broken arrows SiO(OH)/sub 3//sup -/ + H/sub 2/O, has been precisely determined from 0.1 to 5.0 m NaCl and to about 300/sup 0/C. Values of the logarithm of the equilibrium quotient for the reaction are 3.96 and 2.20 at 50 and 300/sup 0/C in 1 m NaCl and 4.32 and 2.26 at the same two temperatures in 5 m NaCl. An analytical expression from which the thermodynamic quantities for the reaction can be computed is presented. The small effect of salt concentration is interpreted as evidence for little or no sodium ion complexing.

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.

The ionization of water in NaCl media to 300°C

The apparent ionization quotient for water has been measured potentiometrically near the saturation pressure from 25 to 295°C in 1 and 3m NaCl using a previously described hydrogen-electrode concentration cell. The results are presented in terms of a modification of the Bronsted-Guggenheim treatment for activity coefficients of ions. The mathematical form of the temperature dependence for the interaction coefficients was indicated by the more extensive data on \(\gamma _{{\rm H}^ + } \gamma _{{\rm O}{\rm H}^ - } \) in KCl media. From a least-squares analysis of these data in NaCl along with the very precise data from the literature for NaCl media from 0 to 50°C, the following expression for the effect of salt concentration and temperature on logQw′ is obtained $$\begin{gathered} log Q'_W = log K_W + 2.0AI^{1/2} /(1 + I^{1/2} ) - [p_1 + p_2 /T + p_3 T^2 + p_4 F(I)]I \hfill \\ - 0.0157\phi m_{NaCl} \hfill \\ F(I) = [1 - (1 + 2I^{1/2} - 2I) exp( - 2I^{1/2} )]/4I \hfill \\ \end{gathered} $$

Phosphoric acid dissociation equilibria in aqueous solutions to 300°C

The dissociation equilibria of phosphoric acid were studied potentiometrically using a hydrogen-electrode concentration cell to 300°C at KCl concentrations up to 1.0m. Least-squares analysis of data atP(V) concentration of 0.008 and 0.045m indicate the absence of significant amounts of polyphosphates up to 200°C.P(V) data at concentrations less than 0.01m extending to 300°C were analyzed in terms of the species H n PO 4 (−3) . The equilibrium quotients expressed for neutralization reactions have been fitted with a nine-parameter expression for the first stepwise neutralization and 11 parameters for the second. Fewer data were obtained for the third neutralization since the HPO2− becomes a strong base above about 150°C in water. Thermodyanamic parameters have been derived for these equilibria, and comparisons have been drawn with other similar data recently reported. There is evidence from the salt effects that the extent of association of HPO4 2− or H2PO 4 − with K+ is not great in 1m KCl.

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.