Lecture 5: Eh-pH Diagrams - Fundamental Aspects
NPTEL Web Course
Lecture 5
Eh-pH Diagrams – Fundamental Aspects
Keywords: Eh-pH Relationship, Water Stability, Oxidation, Reduction.
Eh and pH as environmental parameters in the electrochemical equilibrium diagram.
Eh-pH diagrams showing reactions and products at electrochemical equilibrium are
often referred to as Pourbaix diagrams. As shown in Fig. 5.1 below, there are four
regions in the diagram corresponding to oxidizing (acidic), oxidizing (alkaline),
reducing (acidic) and reducing (alkaline) environments.
The basic diagram for aqueous environment involves upper and lower, stability
limits for water, represented by the oxygen (universal oxidizing agent) and hydrogen
(universal reducing agent) reactions.
Fig. 5.1 Basic regions in a Eh – pH diagram
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 5: Eh-pH Diagrams - Fundamental Aspects
NPTEL Web Course
Stability limits of water
a) O2 + 4H+ + 4e = 2H2O
E0 = +1.23V
Eh = 1.23 – 0.059 pH (at po2 =1)
b) 2H+ + 2e = H2
E0 = 0.00V
Eh = 0-0.059 pH (at pH2 = 1)
These equilibria are plotted in Fig. 5.2. Above the oxygen line, oxygen liberation
occurs.
Below the hydrogen line, hydrogen liberation occurs.
Water is stable
between the two lines.
In neutral or alkaline solutions, the following reactions hold good.
2H2O + 2e = H2 + 2OHO2 + 2H2O + 4e = 4OHElectrochemical evolution of hydrogen represents water decomposition. At more
positive potentials, oxygen reduction or water oxidation takes place. Slope of both
lines correspond to 59 mV/pH.
Fig 5.2. Stability limits of water
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 5: Eh-pH Diagrams - Fundamental Aspects
NPTEL Web Course
Three types of lines in the diagram
a) Those depending only on Eh, but independent of pH (Horizontal to the Xaxis).
b) Those dependent only on pH, but independent of Eh (Vertical to the Xaxis).
c) Those dependent on Eh and pH (Slanted with definite slopes).
The above types of reactions and general effects of Eh and pH on redox reactions
(oxidation, reduction) are illustrated in Fig. 5.3.
Fig 5.3. Effects of changes in Eh and pH
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 5: Eh-pH Diagrams - Fundamental Aspects
NPTEL Web Course
The following aspects are noteworthy:
As the potential increases in the positive (noble) direction
Loss of electrons is favoured (oxidation).
Metal dissolution is favoured.
The system becomes more oxidizing.
Ratio of
Ox
increases.
Re d
When the potential decreases in the negative direction
Gain of electrons favoured (reduction).
The system is more reducing.
Metal deposition (plating) favoured.
Ratio of
Ox
decreases.
Re d
Increasing pH favours metal hydroxide precipitation. Still higher pH may lead to
solubilisation again (eg: dihypoferrite and aluminate)
Consider two redox reactions:
OX1 + n1e= Red1
OX2 + n2e = Red2
OX1 + Red2 = OX2 + Red1
As shown in Fig. 5.4, through Eh – pH diagrams, one can predict oxidizability and
reducibility of different reactants.
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 5: Eh-pH Diagrams - Fundamental Aspects
NPTEL Web Course
Fig 5.4 Role of oxidising and reducing agents
Oxidant OX1 can oxidise Red2 to OX2 while OX1 get reduced to Red1.
Relative positions of various oxidation and reduction reactions in the diagram
indicate possibilities of cell reactions as shown above. Oxidizability of various
metals such as gold, silver, copper, nickel, cobalt and iron by oxidants such as
oxygen, hydrogen peroxide, halides (chloride, bromide, iodide), permanganate and
dichromate can be predicted. Similarly, possibility of using hydrogen (and other
reducing agents) to reduce and precipitate metal ions such as Ag+, Cu++, Ni++, Zn++
and Fe++ can also be predicted based on relative positions of respective lines in the
diagram. For example, all metal-metal ion redox lines which are placed above the
hydrogen line can be thermodynamically reduced by hydrogen.
Iron can be used to displace copper from acidic solutions (cementation).
Cu++ + Fe = Cu + Fe++
E0 for Cu++ / Cu is + 0.34V, while E0 for Fe / Fe++ is -0.44V.
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore