What would an overall reaction order of zero mean?
The overall order in this case is not that meaningful. If your reaction is really (-2) order in hydroxide, it could be a reaction that requires two protons in steps leading up to the rate-determining step. Then, if you add hydroxide, your [H+] concentration goes down and your reaction gets slower. In other words, hydroxide is a second order *inhibitor*. To say that the reaction is zero order overall and therefore the rate is independent of both reactants is totally wrong. Let’s say you have to protonate crystal violet in a preliminary equilibrium step. Then, the protonated crystal violet reacts in a rate-determining, biomolecular step (it reacts with itself somehow, or it dimerizes and then reacts). That would explain what you’re seeing.
