The presence of which reactant is the best indicator of an oxidation-reduction reaction? KNO3 H2SO4 NaOH H2

The correct answer is H2.

An indicator of an oxido-reduction reaction can be defined as an indicator which undergoes a definite color change at a specific electrode potential. Almost all redox indicators with true organic redox systems involve a proton as a participant in their electrochemical reaction (f or example, methylene blue).