A flow-through electrical conductance instrument for dilute aqueous solutions: Measurements of 1:1 electrolytes to 656 K and 28 MPa

Knowledge of thermodynamic properties of aqueous solutions at high temperatures and pressures is fundamental to the understanding of geologic and industrial processes. This is important to the ever increasing needs for power and raw materials in our progressing society. This is also of interest because of the information obtained about water, perhaps the most important substance in the universe. Specifically, the electrical conductance of dilute aqueous electrolyte solutions yields equilibrium constants and information about solvent-ion interactions. Because of this, a flow instrument was built and used to measure the electrical conductivity of dilute aqueous solutions in the vicinity of the critical point of water. The precision of the instrument was estimated to be about 0.1% even for very dilute solutions (2 {dollar}times{dollar} 10{dollar}sp{lcub}-5{rcub}{dollar} mol/dm{dollar}sp3{dollar} at 656 K and 28 MPa). The ability to achieve such high precision was because of the use of flow techniques which considerably minimized errors due to corrosion and adsorption compared to the static methods used by previous investigators. The overall accuracy at the highest temperature and pressure was estimated to be better than 0.4% for the equivalent conductance at infinite dilution and better than 0.2% for the equilibrium dissociation constant, which was found using the Fuoss-Hsia-Fernandez-Prini conductance theory (Fernandez-Prini, 1969). The Shedlovsky equation (1938) used previously by many investigators at high temperature and pressure was found to have systematic deviations in the residuals at high temperature and pressure. Solutions of NaCl, LiCl, NaBr and CsBr were measured. No critical point phenomena were seen in the measurements at the highest temperature and pressure of 28 MPa of solutions at the lowest density. The differences between the mobilities of the different salts at high temperature and pressure appear to be decreasing in comparison with their mobilities at room temperature. There is little difference between the equilibrium constants of the different salts. This is further experimental evidence that 1:1 aqueous electrolytes may behave similarly around the critical point as already theorized.