When does water act as a reducing agent?

Water can act as a reducing agent under specific electrochemical conditions. In this context, the standard reduction potential is a crucial factor. The standard reduction potential indicates how readily a species can be reduced: the higher the potential (measured in volts), the more likely it is to gain electrons and be reduced.

When we say that water acts as a reducing agent, we are referring to its ability to donate electrons to another species, thus facilitating the reduction of that species. The key threshold value here is 1.23 V, which is the standard potential for the reduction of water to hydrogen gas under standard conditions. If a reduction half-reaction has a standard reduction potential of 1.23 V or higher, it is energetically favorable for water to lose electrons to that species, which means water can effectively act as a reducing agent.

When the standard reduction potential is below this value, the reaction becomes less favorable for water to act as a reducing agent. Thus, water typically donates electrons (reduces other species) only when facing a half-reaction with a high enough reduction potential that allows for the spontaneous reaction to occur.