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Acid-base indicators: what does "end-point colour" mean?

Example:

Phenolphthalein
Colour in acid: colourless
Colour in alkali: purple
End-point colour: pale pink

Methyl orange
Colour in acid: red
Colour in alkali: yellow
End-point colour: orange
Original post by Alpha-Omega
Example:

Phenolphthalein
Colour in acid: colourless
Colour in alkali: purple
End-point colour: pale pink

Methyl orange
Colour in acid: red
Colour in alkali: yellow
End-point colour: orange


When the acid/base have been neutralised.
Reply 2
Avatar for Alpha-Omega
Alpha-Omega
OP
12
Original post by thegodofgod
When the acid/base have been neutralised.


Is it when the pH is 7 (neutral)?


Or is it the colour change that we're looking for during acid-base titration?
(edited 12 years ago)
Original post by Alpha-Omega
Is it the colour change that we're looking for during acid-base titration?

Or is it when the pH is 7 (neutral)?


No, as GOG says it is the point when exactly all of the acid has been neutralised by the correct amount of base (or vice versa)

If you have chosen the correct indicator you will also see the colour change here. It is NOT pH 7 (unless you have strong acid and strong base)
Reply 4
Avatar for Alpha-Omega
Alpha-Omega
OP
12
Original post by charco
No, as GOG says it is the point when exactly all of the acid has been neutralised by the correct amount of base (or vice versa)

If you have chosen the correct indicator you will also see the colour change here. It is NOT pH 7 (unless you have strong acid and strong base)


Is the given end-point colour (e.g., pale pink for phenolphthalein) only true for an acid titrant? or is it also the same for a base titrant?

Edit: I've done a bit of googling, does the end-point colour show when the concentration of the unionised indicator is equal to the concentration of ionised indicator?


(edited 12 years ago)
Original post by Alpha-Omega
Is the given end-point colour (e.g., pale pink for phenolphthalein) only true for an acid titrant? or is it also the same for a base titrant?

Edit: I've done a bit of googling, does the end-point colour show when the concentration of the unionised indicator is equal to the concentration of ionised indicator?




Yes, but don't forget that in titration there is such a radical change of pH with tiny additions near the endpoint that it is virtually impossible to assure that you are exactly at the endpoint.

For example, with phenolphthalein titrating a base with an acid, you would look for the colour disappearing.

The other way round and you look for the colour appearing (the first permanent hint of pink).

Hence the equal concentration of both indicator forms being equal in concentration is a theoretical concept that is pretty unlikely in practise.

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