Question

Why is phenolphthalein a good indicator for acid-base titrations?

Answer 1

The phenolphthalein will change colour .

Explanation:

As NaOH (Sodium hydroxide) is a alkaline substance so,it changes colour from colourless to PINK .

Answer 2

The equation is `"HIn" + "NaOH" → "In"^"-" + "H"_2"O"`.

Explanation:

Phenolphthalein is a weak acid that has different colours in its acid and base forms.

I have indicated the acidic proton by a red arrow.

(Adapted from slideplayer.com)

Because it is an indicator, we often write its equilibrium as

`underbrace("HIn")_color(red)("colourless") + "H"_2"O" ⇌ underbrace("In"^"-")_color(red)("deep pink") + "H"_3"O"^"+"`

`"p"K_text(a) = 9.7`, so it is 50 % ionized (pink) at pH 9.7.

That's why it is an excellent indicator for strong acid-strong base titrations.

The reaction with `"NaOH"` is

`"HIn + NaOH" → "In"^"-" + "H"_2"O"`.