[lingo_belle]

Hi all,

Am doing titration using potassium permanganate. according to procedure, to standardized KMnO4, it was done with drawing 25ml of 0.1M Na2C2O4, 2.5ml acid and 75ml of water into a conical flask.

24ml of KMnO4 was added and suppose to turn the solution colorless. after which, it was heated to abt 65 degrees Celsius. Further titration with KMnO4 will turn the solution pale pink and remain so for 30secs. Volume used of KMnO4 will be used to determine the conc of KMnO4. 

But I'm unable to observe so.  after adding 24ml of KMnO4, it turn from purple into maroon....somewhat brownish..any ideas why? any remedies? BTW, any idea why the addition of water too....

Thx very much

belle

[Borek]

Have you remembered to add strong acid?

[lingo_belle]

Yes, I did. 2.5ml of 98% H2SO4.

[Borek]

Brown substance, like the one you have observed in your solution, is most likely MnO₂.

Depending on pH of the solution MnO₄^- gets reduced to MnO₄^2- (in high pH), MnO₂ (neutral), Mn²⁺ (low pH). That's not a hard rule, but it works most of the time. That's why IMHO your observations suggest neutral solution. But if you are absolutely sure you have added acid - I have no other simple ideas.