Phenolphthalein is one of the most widely used acid–base indicators in titrations. To answer this, you simply need to recall the pH range over which it changes colour, which is a standard value worth memorising.
Step 1: What is phenolphthalein? Phenolphthalein is a pH indicator widely used in acid-base titrations. Its color change signals the endpoint of the titration.
Step 2: Recall its transition range. Phenolphthalein is colorless in acidic and neutral solutions and turns pink to magenta (deep pink) in alkaline solutions. This color change happens over a specific pH range that students must memorize.
Step 3: Evaluate the wrong options. pH 1.2 to 2.0 is the transition range of thymol blue (acid range). pH 3.1 to 4.4 is the range of methyl orange. pH 6.0 to 7.6 is the range of bromothymol blue. All of these are well below the alkaline region.
Step 4: Confirm the phenolphthalein range. Phenolphthalein changes color at pH 8.3 to 10.0. Below 8.3, it is colorless. Above 10.0, the color may fade again (turns colorless in strongly alkaline solutions). Its endpoint in a typical strong acid-strong base titration corresponds to approximately pH 8.3 to 10.0.
Step 5: Quick memory tip. Phenolphthalein transitions in the alkaline zone, and pH 8.3 to 10.0 is the standard value given in all pharmacopoeias.