EhpHdiagram

An EhpHdiagram is a thermodynamic plot that represents the stability of aqueous species as a function of redox potential (Eh) and pH. The diagram is equivalent to the Eh–pH diagram, often called a Pourbaix diagram, and is used to predict which species will dominate under specified environmental conditions. The axes are Eh (volts) and pH (log scale). Boundaries separate regions where different phases or ions are thermodynamically favored.

Construction involves gathering thermodynamic data for relevant species—dissolved ions, solid phases, and complexes—and drawing lines for each redox or acid–base couple where the two sides have equal chemical potential. Each line follows a Nernst-type relation, and the slope reflects the transfer of protons. The resulting map partitions the pH–Eh plane into stability areas for distinct species or phases.

Interpretation: locating a condition within a region indicates the most stable form at that pH and Eh. For example, oxidizing conditions at near-neutral pH may favor insoluble oxides, while reducing conditions can promote dissolved species. The diagrams support qualitative predictions about corrosion risk and material compatibility.

Limitations: EhpHdiagram is a thermodynamic tool that neglects kinetic barriers, complex speciation, and non-ideal activities. It is temperature dependent and requires accurate data; reliability decreases in highly multi-component or poorly characterized systems.