For many elements their chemistry changes greatly depending on their oxidation state. For example Fe(II) or Fe(III), Cu(I) or Cu(II), Mn(II) or Mn(IV), or sulfur may be S8, H2S, S2-, S2O32-, SO32-, or SO42- with S being in different oxidation states and each having very different physical and chemical properties.
Eh is the property that allows us to predict the stable oxidation state of an element and its compounds in the environment.
See the following pages:
What is redox potentialWhy do we need Eh ?Master VariablesWhat Controls Seawater Eh?How Do We Measure Eh?Importance of KineticsEh-pH diagramsEh-pH diagrams, cont.Summary