Isopiestic Measurements of Water Activity for the H2SO4-H3PO4-H2O System at 298.15 K

Water activity for the binary H2SO4-H2O and H3PO4-H2O systems, as well as the ternary H2SO4-H3PO4-H2O system has been measured using an isopiestic method at 298.15 K. The results for binary systems obtained in this work agree well with literature data. The results show that the isopiestic composition lines of the H2SO4-H3PO4-H2O system obey Zdanovskii's rule at low concentrations, whereas at higher concentrations the isopiestic lines are observed to have positive deviations. The relative uncertainties of isopiestic molalities of parallel samples in each experimental run are better than 0.2 %. The measured water activities reported in this work can be used to parameterize thermodynamic models for chemical engineering of phosphorus and heavy metal hydrometallurgical processes.