Acid-Base titration

One colorless liquid is added gradually to another colorless liquid, and at one moment the solution becomes pink

Scientific name: Titration of NaOH with HCl using phenolphthalein


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Wear eye protection and lab gloves. Avoid contact of acid and base with skin.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

H+ + OH- → H2O

H2Ind → Ind2-

Step-by-step instruction

  1. Prepare an acid (i.e., HCl) solution of unknown concentration.
  2. Pour this solution into a conical flask. Add a few drops of the phenolphthalein solution.
  3. Fill a buret with a base (i.e., NaOH) solution of known concentration.
  4. Add the base solution drop by drop into the flask. The solution becomes bright purple, when the equivalence point is obtained.

Scientific background

The neutralization reaction between base and acid occurs in the solution. Concentration of hydrogen ions is changed - pH increases.

Phenolphthalein presence in the acidic solution is in the colorless molecular form H2Ind. When the base is added, phenolphthalein turns to the ion form Ind2-. When all the acid is neutralized, and pH becomes about 8.2, the solution becomes pink.


Published on 03 May 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst