An autocatalyst is

• Auto catalyst: In certain reactions, one of the product acts as a catalyst. In the initial stages, the reaction is slow but as soon as the products come into existences the reaction rate increases. This type of phenomenon is known as auto-catalysis.

• In the permanganate titration of oxalic acid in the presence of H2​SO4​ (acid medium);
• There is a slow discharge of the colour of permanganate solution in the beginning but after some time, the discharge of the colour becomes faster.

Molecular equations

\(\rm 2KMnO_4 \ + \ 3H_2SO_4 \longrightarrow \ K_2SO_4 \ + \ 2MnSO_4 \ + \ 3H_2O \ + \ 5[O]\)

\(\begin{array}{*{20}{c}} {\rm COOH}\\ {| \ \ \,\,\,\,\,\,\,\,\,\,\,}\\ {\rm COOH} \end{array} .\rm 2H_2O \ + \ [O] \ \xrightarrow{60 - 70^\circ C} \ 2CO_2 \ + \ 3H_2O] \ \times \ 5\)

\(\rm 2KMnO_4 \ + \ 2H_2SO_4 + 5 \begin{array}{*{20}{c}} {\rm COOH}\\ {| \ \ \,\,\,\,\,\,\,\,\,\,\,}\\ {\rm COOH} \end{array} . \rm 2H_2O \xrightarrow{ \ \ \ \ \ \ \ \ \ \ \ } \ K_2SO_4 \ + \ 2MnSO_4 \ + \ 18H_2O \ + \ 10CO_2\)

• This is due to the formation of MnSO4​ during the reaction which acts as a catalyst for the same reaction.

So, Mn⁺² is acting like auto catalyst for this reaction.