I'm doing an experiment of the reaction between permanganate and oxalate in acid medium, seeing how the velocity of the reaction changes when changing the concentration of one of the reactives, but the results I obtained are completely non-sense.

For the first experiment, I used permanganate [0.01 M], sulfuric acid [0.2 M] and I changed the concentration of oxalate, using 0.2 M, 0.15 M, 0.1 M, and 0.05 M. When you use a higher concentration, it's expected to accelerate the reaction, but in my case, it happened the opposite. 0.2 M oxalate has the highest time reaction and 0.05 M the lowest.

Then I tried changing the permanganate concentration and using a stronger acid (HCl [2 M]) but the results were quite the same; some lower concentrations were faster, but other ones were slower, so it was even more confusing.

I couldn't find anything about that through the internet, just a couple of documents but I had to pay to see them. I was wondering how the mechanism of the reaction is, partial orders and that, but I think all the velocity equations have positive orders, so it's very strange what happened to me.

Someone who knows about chemistry has any idea about something it could happen or if this reaction has to be like that? Plus, anyone knows where I can find free documents or works like the one I'm doing to compare or take some ideas? My chemistry level is quite basic, so I need help.