Donald A. Palmer

Thermodynamics of aqueous association and ionization reactions at high temperatures and pressures

Electrochemical and electrical conductance cells have been widely used at ORNL over the years to quantitatively determine equilibrium constants and their salt effects to 300°C (EMF) and 800°C (conductance) at the saturation pressure of water (EMF) and to 4000 bars (conductance). The most precise results to 300°C for a large number of weak acids and bases show very similar thermodynamic behavior, which will be discussed. Results for the ionization constants of water, NH3(aq), HCl(aq), and NaCl(aq), which extend well into the supercritical region, have been fitted in terms of a model with dependence on density and temperature. The entropy change is found to be the driving force for ion-association reactions and this tendency increases (as it must) with increasing temperature at a given pressure. Also, the variation of all thermodynamic properties is greatly reduced at high fixed densities. Considerable variation occurs at low densities. From this analysis, the dependence of the reaction thermodynamics on the P-V-T properties of the solvent is shown, and the implication of large changes in hydration for solutes in the vicinity of the critical temperature will be discussed. Finally, the change in the molar compressibility coefficient for all reactions in water is shown to be the same and dependent only on the compressibility of the solvent.

Thermal decarboxylation of acetate. Part I. The kinetics and mechanism of reaction in aqueous solution

Abstract In an effort to understand the kinetics of the thermal decarboxylation of acetate and the role of catalysis, a series of laboratory experiments were conducted to measure the rate constants for the decomposition of acetate (acetic acid and sodium acetate) in the presence of titanium, silica, stainless steel, gold, and magnetite. Activation energies for decarboxylation of acetic acid and acetate ion range from about 8 kcal mol −1 in stainless steel vessels to 69 kcal mol −1 in silica tubes. Extrapolated rate constants at 100°C for acetic acid differ by more than fourteen orders of magnitude between the experiments conducted in stainless steel and the catalytically least active titanium vessels. Gold and titanium were the least active catalysts for the acetic acid substrate, while stainless steel, silica, and magnetite showed marked catalytic effects. Methane and carbon dioxide were the predominant reaction products of most of these experiments, although mass spectrometric analyses of the gas phase revealed concentrations of carbon monoxide and hydrocarbons (apparent mass range from 29 to 56) amounting to as much as 55 mole percent of the total volatile products, depending on the catalyst. The reactions were generally first order in acetic acid or acetate ion, except for those involving the acid over silica and magnetite which were zero order. These results and the observed effects of variations in surface area are rationalized in terms of changes in the mode of surface catalysis. The mechanistic assignment is simplified by the existence of three unique straight lines on an isokinetic plot ( i.e ., activation enthalpy versus activation entropy) which fit all the respective first- and zeroorder reactions. The results described here provide the nucleus for the discussion in Part II of the role of acetate in the primary migration of methane and the transportation of metals in hydrothermal solutions.

Aluminum speciation and equilibria in aqueous solution: V. Gibbsite solubility at 50°C and pH 3–9 in 0.1 molal NaCl solutions (a general model for aluminum speciation; analytical methods)

This study reports 184 new measurements of the solubility of gibbsite at 50°C and 0.1 molal ionic strength in NaCl solutions of acetate, bistris, and tris buffers with hydrogen ion concentrations ranging from 10−3to 10−9molal. Samples collected at 35, 63, 66, 120 and 144 days show no detectable difference in the total aluminum at similar pH values. Correction of the measured solubilities for com- plexation reactions involving Al3+ with acetate and Al(OH)4− with bistris gives the solubility curve due to Al(OH)y3−y species alone, which is smooth and continuous, with a minimum near 10−8 molal (0.3 ppb) and pH 5.5. The corrected solubilities are shown to be in complete agreement with measurements of the same material in more strongly acidic (palmer and wesolowski, 1992) and basic solutions (wesolowski, 1992) and with the formation constant for Al(OH)2+determined potentiometrically by palmer and wesolowski (1993). An additional species, Al(OH)+2, was introduced in order to explain the solubility at pH values around 5.5, and the molal formation quotient for the reaction Al(OH)3,cr + H+ ⇆ Al(OH)2+ + H2O was determined to be 10−3.04 ± 0.05 at 50°C and 0.1 molal ionic strength. The results of this study were combined with our previous results and the new boehmite solubility data of castet et al. (1993) to provide a consistent model for the distribution of monomeric aluminum hydrolysis species and the solubility of gibbsite in 0–5 molal NaCl brines in the 0 to 100°C range. Salinity is shown to be a major factor controlling the solubility of aluminum minerals in solutions 1 to 2 units more acidic than the neutral pH at temperatures of 0 to 100°C. Acetate complexation is modeled from the results of this study and palmer and bell (1994), and is shown to enhance the solubility of gibbsite by more than an order of magnitude in mildly acidic brines containing a few thousand parts per million total acetate, in the absence of competition by other metal ions. A model is also presented for the aluminum hydrolysis constants at higher temperatures at infinite dilution which is quantitatively consistent with the low temperature data. Detailed aluminum analysis techniques employing ion chromatography are discussed in the Appendix.

Electrical conductivity measurements of aqueous sodium chloride solutions to 600°C and 300 MPa

AbstractElectrical conductance measurements of dilute (<0.1 mol-kg−1) aqueous NaCl solutions were made primarily to quantify the degree of ion association which increases with increasing temperature and decreasing solvent density. These measurements were carried out at temperatures from 100 to 600°C and pressures up to 300 MPa with a modified version of the apparatus used previously in the high temperature study in this laboratory. Particular emphasis was placed on conditions close to the critical temperaturelpressure region of water, i.e., at 5° intervals from 370 to 400°C. The results verify previous findings that the limiting equivalent conductance Ao of NaCl increases linearly with decreasing density from 0.75 to 0.3 g-cm−1 and also with increasing temperature from 100 to 350°C. Above 350°C. Ao is virtually temperature independent. The logarithm of the molal association constant as calculated exclusively from the data≥400°C is represented as a function of temperature (Kelvin) and the logarithm of the density of water (g-cm−3) as follows:

Thermal decarboxylation of acetate. Part II. Boundary conditions for the role of acetate in the primary migration of natural gas and the transportation of metals in hydrothermal systems

log K_m = 0.997 - 650.07/T - (10.420 - 2600.5/T)log\rho _w

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

Note that this function also provides a good representation of the log Km values obtained from 350 to 395°C at densities greater than ca. 0.6 g-cm−3. More precise conductance data now available in the literature suggest a systematic error of unknown origin may exist in the data obtained at lower densities in this region. The relevant thermodynamics quantities derived from differentiation of this equation with respect to temperature and pressure are listed in the text.

Ion association of dilute aqueous sodium hydroxide solutions to 600°C and 300 MPa by conductance measurements

The equilibrium quotient for the formation of triiodide was studied as a function of temperature, 3.8–209.0°C, and ionic strength, 0.02–6.61. The best-fit value for the molal equilibrium constant at 25°C is 698±10 and the corresponding partial molal enthalphy, entropy, and heat capacity of formation are: ΔHo=−17.0±0.6 kJ-mol−1, ΔSo=−0.6±0.3 J-K−1-mol−1, and ΔCpo=−21±8 J-K−1-mol−1. Activity coefficients of iodine were determined as a function of ionic strength (NaClO4) at 25°C and conclusions are drawn as to the corresponding ionic strength dependence of the triiodide anion.

Aqueous high-temperature solubility studies. I. The solubility of boehmite as functions of ionic strength (to 5 molal, NaCl), temperature (100 -290°C), and pH as determined by in situ measurements

Acetate can mediate the primary migration of natural gas from tight, water-laden source rocks by acting as a mobile precursor that is expelled during compaction to resevoir rocks where subsequent decarboxylation yields methane. The viability of this mechanism is demonstrated by integrating, via a computer model, the experimental kinetic rate data for acetate decarboxylation that are presented in Part I with the thermal-temporal-spatial relationships of sediment and fluid in the upper portions of actively forming sedimentary basins. Specifically, this analysis indicates that when the enthalpy of activation is between 32 and 42 kcal · mol−1, and the temperature is between 80 and 130°C, acetate can survive the thermal regime of a typical sedimentary basin long enough to migrate out of the source rocks and yet decarboxylate to form a natural gas deposit within the time frame imposed by the age of the basin sediments. The results suggest that this migration mechanism for natural gas may be predominant within a significant geological and chemical window. A similar analysis has shown that acetate can survive moderate hydrothermal temperatures (<300°C) long enough to promote the mobility of metals as acetate complexes. Ongoing experimental studies now show that metal-acetate complexes are stronger than the analogous chloro-complexes and may account for a large proportion of the metal in hydrothermal solutions with access to organic material.