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PublisherNACE InternationalLanguageEnglish
Document ID 92010035
Content TypeJournal Paper
TitleCalculation of pH for High-Temperature Sulfate Solutions at High Ionic Strengths
AuthorsB.R. Staples, G.R. Holcomb, and S.D. Cramer
JournalCorrosion
VolumeVolume 48, Number 01
DateJanuary, 1992
Copyright1992. NACE International.
KeywordspH, high temperature, ionic strength, sulfate, Debye-Huckel
PreviewABSTRACT
Hydrogen ion activity is the most important variable in water chemistry.1 The absence of reliable and widely applicable methods for measuring or estimating pH at elevated temperatures is a major obstacle in determining or interpreting physicochemical processes in high-temperature aqueous solutions.2 The difficulty is particularly true for high ionic-strength solutions, such as those encountered in geothermal processes, in pressure leaching and other hydrometallurgical processes, and in corrosion, particularly localized corrosion. Corrosion studies at the U.S. Bureau of Mines on stainless steels in acid sulfate solutions prompted the development of procedures to calculate the pH for solutions at temperatures to 300°C and high ionic strengths, that is, above 0.3m (mol. kg ­1). Pourbaix has shown that the corrosion performance of metals can be correlated with the thermodynamic stability of the metal, its oxides, and other metal compounds in aqueous solutions.3 Potential-pH diagrams representing equilibrium conditions at elevated temperatures have been computed for a number of metal/solution systems.4-8 Solution pH is one of the critical factors that must be defined in all corrosion kinetics and mechanism studies. It is a factor that must be considered in the evaluation of corrosionresistant materials for industrial applications. Progress has been made over the past decade in measuring the pH of solutions at elevated temperatures.1,2,9-13 MacDonald, Wentrcek, and Scott used electrodes of Pd black deposited on Pd, Pt, and Au for pH measurements from 25 to 275°C.2 The calibration was carried out using B(OH)3 and LiOH solutions. The pH was calculated from the equilibrium constants at temperature and computed liquid junction potentials and reported as a function of molality and temperature. Problems were encountered from non-Nernstian responses. Deviations from earlier measurements were due to differences in reported equilibrium partial pressures. Tsuruta and MacDonald investigated the pH 35
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