The brown turbidity observed during the titration of potassium permanganate (KMnO4) with oxalic acid (H2C2O4) is due to the formation of manganese dioxide (MnO2). This occurs when the permanganate ions (MnO4-) are reduced by the oxalate ions (C2O42-) in an acidic solution.

The overall reaction for the titration can be represented as:

5 H2C2O4 + 2 KMnO4 + 3 H2SO4 → 10 CO2 + 2 MnSO4 + K2SO4 + 8 H2O

In this reaction, the oxalate ions act as a reducing agent, causing the manganese ions in the permanganate to be reduced from a +7 oxidation state to a +4 oxidation state. This reduction leads to the formation of manganese dioxide, which is a brown precipitate.

The brown turbidity can also be attributed to the formation of other manganese oxides, such as manganese(III) oxide (Mn2O3) and manganese(IV) oxide (MnO2). These oxides can form as intermediate products during the reduction of permanganate ions.

The presence of brown turbidity in the titration indicates that the endpoint of the titration has not yet been reached. To obtain accurate results in the titration, it is necessary to continue adding permanganate solution until the brown turbidity disappears and the solution remains clear.