JEE Main · 2020 · 09 Jan 2020 · Shift 1 · Redox Reactions and Electrochemistry

A species can act as both oxidising and reducing agent only when its key atom sits at an intermediate oxidation state, free to move either way. In ${\mathrm{H}}_2{\mathrm{O}}_2$ oxygen is at $-1$, in ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_3$ sulfur is at $+4$, and in $\mathrm{HN}{\mathrm{O}}_2$ nitrogen is at $+3$, all intermediate values. In ${\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_4$, however, phosphorus already sits at its highest state $+5$, so it can only be reduced and cannot serve as a reducing agent.