The colour of phenolphthalein indicator in alkaline solution at the end-point of an acid-base titration is?

Analyze the question You are asked for the colour of phenolphthalein in alkaline solution (the base medium). The wording is a little tricky, because it mentions the "end-point." We need to be clear about what colour is seen in the alkaline solution.

Concept Phenolphthalein is a weak-acid indicator that works in the pH range of about 8.3 to 10.0.

• In acidic solution (pH below 8.3), it is colourless.
• In alkaline solution (pH above 10), it is pink (or pinkish-red).
• At the end-point of a titration of an acid with a base (base added from the burette), the colour changes from colourless to faint pink.

Solution

1. The question asks for the colour in alkaline solution.
2. In alkaline medium, phenolphthalein ionises to give its pink form.
3. So the colour is pink.
4. Options A (colourless) is for acid medium, B (orange) and D (yellow) are for methyl orange (orange at the end-point, yellow in alkali), not phenolphthalein.

Answer: C. pink

Tip Remember "Phenolphthalein = Pink in base." For methyl orange, think "Yellow in alkali, Red in acid," and orange at the end-point (the neutral point). Many students lose marks mixing up these two indicators, so memorise both together.