The initial solution of acetic acid (HC2H3O2) is clear and colourless. A few drops of phenolphthalein indicator are added to each sample. A dilute solution of sodium hydroxide (concentration 2.50 x 10-4 mol/L) is used as the titrant. As the mixture reaches the endpoint, flashes of pink colour are seen and the titrant is added drop by drop. The endpoint is reached when one drop of titrant turns the mixture a pale pink colour that does not fade
The first small excess of NaOH greatly increases the concentration of OH–, concomitantly decreasing the H3O+ concentration, and causing the pH to continue to increase. Well past the equivalence point, the concentration of OH– becomes so large that only slight changes in pH are produced.