Acetic acid is used for the titration of vitamin C (ascorbic acid) to create an acidic environment and prevent its oxidation. Vitamin C is easily oxidized in the presence of oxygen, which would affect the accuracy of the titration. The addition of acetic acid lowers the pH of the solution, creating an acidic environment that helps to protect the vitamin C from oxidation.

In the titration of vitamin C, an oxidizing agent, such as 2,6-dichlorophenolindophenol (DCPIP), is used. DCPIP changes colour from blue to colourless as it gets reduced by vitamin C. The volume of DCPIP solution required to completely reduce the vitamin C in the sample is measured, and from this, the concentration of vitamin C can be determined.

By maintaining an acidic environment with the addition of acetic acid, oxidation of vitamin C is minimized, ensuring accurate and reliable results in the titration. Additionally, the acidic conditions help to extract and dissolve vitamin C from the sample, aiding in the overall accuracy of the titration.