Calculation of pH for High-Temperature Sulfate Solutions at High Ionic Strengths

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. This general method for calculating the pH of high ionic-strength aqueous solutions at elevated temperatures is an alternative to instrumental pH measurement techniques and is illustrated for the H₂O-Na₂SO₄-H₂SO₄ system. The pH is calculated from the partial dissociation of HSO₄⁻, while accounting for the buffering effects of completely dissociated Na₂SO₄ and H₂SO₄. Modern values for the temperature-dependent second dissociation constant of H₂SO₄, and the extended Debye-Huckel limiting law slopes are used in the calculations. Uncritical selection of thermodynamic data can result in differences in calculated pH that exceed 0.5 pH units in the temperature range 200 to 300°C.