Dear Student,
Citric acid (C8H8O7) turns potassium permanganate from purple to colourless and act as reducing agent while KMnO4 act as oxidizing agent. It is an redox reaction in which MnO4- is reduced to MnO2 while citric acid is oxidised to CO2.
6KMnO4 + C6H8O7 + 3H2SO4 → 6CO2 + 6MnO2 + 7H2O + 3K2SO4
Regards!