JEE Advanced 2020 Full Test 3 Paper 2

Acid-base indicators are either weak organic acids or weak organic bases. Indicators change colour in dilute solution when the hydronium ion concentration reaches a particular value. For example, phenolphthalein is a colourless substance in any aqueous solution with pH less than 8.3. In between the pH range 8.3 to 10, transition of colour(colourless to pink) takes place and if pH of solution is greater than 10 solution is dark pink. Considering an acid indicator HIn, the equilibrium involving it and it's conjugate base(In') can be represented as:
‌

H

‌ acidic form ‌ln

⇌H++‌

ln−

‌ basic form ‌

pH of solution can be computed as: pH=pKln+log‌

[ln−]

[Hln]

In general, transition of colour takes place in between the pH range, pKln±1.