This document discusses the redox stability of water. It provides the redox potentials of water with respect to pH and defines the stability range of natural waters. It also discusses how water can act as both an oxidizing agent and reducing agent in redox reactions. For example, water is a poor reducing agent except towards strong oxidizing agents. However, metals with large negative standard potentials can reduce water to produce hydrogen gas. Understanding the redox stability of water is important for generating hydrogen fuel from water.

1. Redox stability of water
Dr. Mithil Fal Desai
Shree Mallikarjun and Shri Chetan
Manju Desai College Canacona Goa

2. Water as oxidising and reducing agent
H3O+
(aq) + e-  H2O(l) + ½ H2 (g)
H2O(aq)  ½ O2(g) + 2H+
2(aq) +e-
E° in V
0.0V
+1.23

3. Redox potentials of water v/s pH
(Pourbaix diagram)
0
-0.5
-1.0
+1.0
+0.5
+1.5
4 8 120
Neural
water
Potential(V)
pH
The sloping lines (white)
defining the upper and lower
values of thermodynamic
stability water with respect to
the potentials for the O2 /H2O
and H+/H2 redox couples,
respectively.
The sloping lines (orange)
lines represent overpotential.
The central blue zone
represents the stability range
of natural waters.

4. Oxidation by water
The strongly positive potential of the O2/H2O couple
(E° =+1.23 V) shows that acidified water is a poor
reducing agent except towards strong oxidizing agents
like
Co3+/Co2+ E° =+1.93 V
Ce4+/Ce3+ E° =+1.71 V
(MnO4)-/Mn2+ E° =+1.51 V
4Co3+
(aq) + 2H2O(l) → 4Co2+
(aq) + O2(g) +4H+
(aq)

5. Reduction by water
Metals with large negative standard potentials react
with aqueous acids produce H2 unless a passivating
oxide layer is formed.
M(s) + H2 O(l) +  M+
(aq) + ½ H2 (g) + OH-
(aq)
M(s) + H+
(aq) +  M+
(aq) + ½ H2 (g)
Above reactions are thermodynamically favorable
when M is an s-block metal, a 3d-series metal from
Group 3 or a lanthanoid.

6. The importance understanding
redox stability is not due to any
economic demand for oxygen
but because of the desire to
generate H2 (a ‘green’ fuel)
from water by electrolysis or
photolysis.
Importance