In an experiment testing catalyst influence on reaction, is NaOH a reactant or a catalyst?
In basic solution the half reactions are:

Reduction: 

$\ce{H2O2 + 2 e- -&gt; 2 OH-}$

Oxidation: 

$\ce{I- + 6 OH- -&gt; IO3- + 3 H2O + 6 e-}$

So the overall reaction is

$\ce{3 H2O2 + I- -&gt; IO3- + 3 H2O}$

thus $\ce{OH-}$ is overall neither a reactant nor a product. Hence it is either a catalyst, an inhibitor, or a spectator ion.