Liliana N. Trevani

High-temperature wall-tube cell

A high-temperature wall-tube electrode cell (WTE) for electrochemical studies in hydrothermal systems is described. The cell contains a jet impinging on a wall (electrode) where the hydrodynamics in the stagnation region is well defined and the WTE has uniform accessibility and is equivalent to a rotating disc electrode (RDE). The construction and testing of the high-temperature WTE is described and its performance tested using different redox couples such as hexacyanoferrate(II/III) and quinone/hydroquinone. Diffusion coefficients of iron(II)/iron(III) are reported at temperatures up to 142 °C in sulfate solutions and are analysed taking into account the speciation in the solution. The application of the Stokes–Einstein equation to describe the temperature dependence of the diffusion coefficients in systems with strong ionic association is discussed for iron(II)/iron(III) and other species.

Transport properties in high temperature and pressure ionic solutions

Publisher Summary This chapter is focused on the study of transport properties in aqueous ionic solutions at elevated temperatures and pressures. The transport coefficients are defined in connection with the phenomenological laws that describe the transport of charge, mass or momentum in electrolyte solutions. The chapter summarizes the laws and the main characteristics of the transport parameters, which include electrical conductivity, transport numbers, diffusion, limiting laws, viscosity, and thermal conductivity. The most successful experimental methods and devices used to measure transport properties in high-temperature and -pressure aqueous solutions containing ionic solutes are High-Temperature Electrical Conductivity Cells and Electrochemical Methods. The simplest apparatus to measure the viscosity of electrolyte solutions is the rolling-ball viscometer, consisting of an inner tube, a ball, and an optical detector. The most precise method for measuring the viscosity of corrosive hydrothermal fluids is the oscillating-disk viscometer. The presence of ionic solutes generates new diffusion coefficients and also modifies to some extent the transport properties of water in the solution. This chapter deals mainly with those transport properties that are a direct consequence of the presence of ionic solutes, i.e., electrical conductivity and solute diffusion. The electrical conductivity, and the diffusion of salts and their ionic components are analyzed as a function of temperature and density (pressure) over the entire concentration range, from infinite dilution to very concentrated solutions.

Ionic association in asymmetric electrolytes

The cluster theory of electrolytes previously developed for symmetric salts has been extended to the asymmetric 1 : 2 electrolytes. The intracluster contribution to the configurational Helmholtz energy was calculated for different cluster diameters in order to include this parameter in a Helmholtz-energy minimization procedure. We have calculated the speciation for model asymmetric electrolytes as a function of the strength of the interparticle interactions and compared the results with those for equivalent 2 : 2 model electrolytes. The calculated activity coefficients agree reasonably well with Monte Carlo simulation results for the primitive model. Theoretical predictions for the temperature dependence of the speciation and activity coefficients of 1 : 2 electrolytes are compared with experimental results for MgCl2 and Na2SO4.

Apparent Molar Volumes and Standard Partial Molar Volumes of Aqueous Sodium Phosphate Salts at Elevated Temperatures

Densities and apparent molar volumes of aqueous NaH2PO4 (0.1 to 2.1) mol·kg−1, Na2HPO4 (0.1 to 0.5) mol·kg−1 and Na3PO4 (0.4 to 0.6) mol·kg−1 have been determined using a platinum vibrating tube densimeter at temperatures from 473 K to 598 K, 473 K to 570 K, and 373 K to 497 K, respectively, and a pressure of 15 MPa. For the monosodium and disodium phosphate salts, the Pitzer ion-interaction model was used to extrapolate the apparent molar volumes to infinite dilution and to obtain standard partial molar volumes, V°, of NaH2PO4(aq) and Na2HPO4(aq). These new results show that predicted values from the Helgeson−Kirkham−Flowers model overestimate V° for these two species. For the monosodium salt, the difference ranged from 5 cm3·mol−1 at 473 K to 24 cm3·mol−1 at 573 K, and for the disodium salt, the difference was 6 cm3·mol−1 at 473 K and 16 cm3·mol−1 at 570 K. The values for disodium phosphate show evidence of significant contributions from an ion pair, probably NaHPO4−(aq). The apparent molar volumes of N...