Question

3. How could CO2 interfere with the titration of H3PO4?

Answer 1

During titration of phosphoric acid, after just boiling the sample, it is kept as such for 5-10 minutes. This helps in expelling the carbon dioxide which otherwise interferes with the titration.

CO2 (g) + H2O (l) -----> H2CO3 (aq)

Carbon dioxide causes interference as it dissolves in water to produce carbonic acid which influences the end-point of acid-base titrations. The large amount of carbon dioxide in sample increases its acidity with time causing fading of pink color as the time passes by. This may lead to addition of more amount of titrant. A little amount added will influence the end-point result.

To prevent this solvents muct be kept covered. The complete absence of carbon dioxide can be confirmed by using an indicator such as phenolphthalein.