Patrick;

Generally, any reaction product in chemistry will retard any further reaction to some extent. So, the graph is perfectly reasonable. That is what happens with oxidized HQ which turns into HQ-mono sulfonate. The quinone vanishes from the equation and therefore the reaction speeds up.

The conversion of oxidized ascorbic acid back to ascorbic acid is through an electron transfer that oxidizes the sulfite to sulfate. These two types of reaction are not identical and yet do involve oxidation and reduction reactions.

PE