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

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Answer 1 · 2017-11-16T15:40:31

The phenolphthalein will change colour .

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

Answer 2 · 2017-11-16T19:46:30

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

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.

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.

www.titrations.info

The reaction with $"NaOH"$ is

$"HIn + NaOH" → "In"^"-" + "H"_2"O"$ .