Re: Why does titration of H2O2 with 0.02M KMnO4 not work at high conc,ie.1.0 M?

Sharon,

This is an excellent experimental question. If I understood your experiment correctly, you are titrating 0.02M KMnO₄ into 0.05M H₂O₂, and that this does not work at very high concentrations of peroxide. Furthermore, I assume that the source of the pink color was from Mn²⁺ (which forms Mn(H₂O)₆²⁺ in water, with a light pink color) and not from pH indicator dye.

I think there are 2 basic possibilities as to why your experiment does not work.

- There is still an operational error. You repeated the experiment several times, which is good, but it is possible that you are unknowingly committing the same operational error each time. For example, perhaps the concentrations of the solutions are not what you think they are. Or, perhaps there is an impurity in the solutions that is affecting the titration.

- If there's nothing wrong with the operation of the experiment, then I would suppose that there are secondary reactions that either produce something other than Mn²⁺ or use up Mn²⁺. This may be a mass-action effect related to the enormous excess of H₂O₂. I can't be very specific without knowing more about the exact experimental procedure or other observations, though.

I suggest that you take another careful look at your experimental procedure. First, look for operational errors and don't assume anything. Then, if you don't find anything, repeat the experiment and write down any and all observations. (For example, does some pink form initially, then go away? How long does it take?, etc.) This may give clues as to what is happening. Discuss it with your chemistry teacher, or submit a follow-up question to MadSci with your new data. (If you do submit a follow-up, I'd be happy to see what turns up; just provide more details and tell them to forward the question to me.)

Good luck and stay curious!

Your MAD Scientist,
Raymond Cheong