Question

The best indicator for the detection of endpoint in the titration of a weak acid and a strong base is:

• A. methyl orange (3 to 4)
• B. methyl red (5 to 6)
• C. bromothymol blue (6 to 7.5)
• D. phenolphthalein (8 to 9.6)

Answer 1

The correct option is D phenolphthalein (8 to 9.6)

Phenolphthalein is an indicator used for titrations of a weak acid and strong base, and itself is a weak acid. The weak acid is colourless and its ion is bright pink. It distinguishes the pH range from 8 to 9.6. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colourless. Adding hydroxide ions removes the hydrogen ions from the equilibrium which shifts to the right to replace them - turning the indicator pink.

Methyl orange, Methyl red are used for the titration of strong acid and strong base or strong acid and weak base while Bromothymol blue is used for weak acid and weak base titration indicator.