How does CO2 affect titration?

1 Answer
Apr 30, 2014

CO₂ from the air dissolves in water and forms carbonic acid, H₂CO₃. This can affect the end-point of acid-base titrations.

This is important if you are using sodium hydroxide as the base and phenolphthalein as the indicator.

Excessive carbon dioxide in the sample makes it more and more acidic as time passes. The pink colour fades after a short time.

Your instructor often says, "Titrate to the first permanent pink colour". In this case, "permanent" means "30 s".

The pink colour may fade after 30 s. That will tempt you to add another drop of titrant. That is too much.

As long as the end-point colour persists more than 30 s, the end-point is good.