Question

Explain the action of phenolphthalein indicator in the titration of acetic acid versus sodium hydroxide.

Answer 1

Titration is a chemical analysis to find out the concentration of unknown solution by adding a solution of known concentration. The known solution is known as titrant and the unknown solution is known as analyte. Phenolphthalein is an indicator which is colorless for acidic medium and turns its color to pink for basic medium.

Complete answer:
Acetic acid is a weak acid which means that it does not dissociate completely into its ions. Sodium hydroxide is a strong base; it will neutralize the hydrogen ions from acetic acid during the process of titration. The concentration of hydrogen ions is less as compared to hydroxyl ions because acetic acid is a weak acid due to which its dissociation is less and sodium hydroxide being a strong base dissociates more. The pH value tells us about the power of hydrogen or simply the concentration of hydrogen ions in the solution. The range of the pH scale is from 0-14, the scale shows less than 7 in acidic medium and greater than 7 in basic medium.
Phenolphthalein itself is a weak acid. It is colorless below pH 8.5 and changes into pink color after this value. When titration of acetic acid and sodium hydroxide takes place, due to less number of hydrogen ions than hydroxyl ions the pH of the solution increases abruptly. It attains the pH value greater than 8.5 due to which phenolphthalein undergoes a color change it changes from colorless to pink and thus serves as an indicator.
The dissociation of acetic acid into its ions- $C{{H}_{3}}COOH\to C{{H}_{3}}CO{{O}^{-}}+{{H}^{+}}$
Dissociation of sodium hydroxide- $NaOH\to N {{a} ^ {+}} + O {{H} ^ {-}}$

Note:
Indicator is added to know the equivalence point for the titration. It is that point at which the concentration of titrant being added is enough to completely neutralize the analyte solution. The indicator shows a color change according to its properties and thus the equivalent point is determined. Phenolphthalein turns pink at the equivalence point for the above given titration thus indicating us that the neutralization has been completed. The chemical formula for phenolphthalein is ${{C} _{20}} {{H} _{14}} {{O}_ {4}}$; it is an organic compound and is a weak acid.