Electrochemical processes in high-temperature aqueous solutions

Understanding the behavior of high-temperature aqueous solutions represents a new frontier in electrochemical studies that is both technically challenging and technologically important. Interest in this field has increased significantly over the last decade, mainly due to the many important electrochemical processes that take place in high-temperature aqueous environments. Water is the most ubiquitous of solvents and by virtue of its extraordinary physicochemical and transport properties, it forms the medium in which diverse processes occur from biochemistry to geochemistry. However, there is a scarcity of electrochemical studies at temperatures above 100 degree Celsius. This chapter provides a background of the techniques available at elevated temperatures, particularly as related to measurements of pH, and serves as a critical review of their application to electrochemical studies of high-temperature aqueous systems. For reliable high-temperature potentiometric experiments, the measured open circuit potential should be stable and reproducible. For assessing the viability and accuracy of high-temperature potentiometric measurements, reference systems should be used with a known activity of H⁺ (aq), a_H+. A great deal of effort has been expended in recent years to develop a reliable and stable reference electrode suitable for measurements in high-temperature aqueous solutions. Two approaches employed are the use of an internal reference electrode operating within the high-temperature environment, and use of an external reference electrode working at room temperature, but connected to the high-temperature environment. The first approach requires solving the well-known problem of the diffusion potential, whereas the latter involves solving the problems of the thermal-liquid junction and thermoelectric potentials.