[Tsuki]

The other day in the lab we were doing both forward and backward titrations and I am now writing up my lab report for it.

The forward titration was Vinegar with NaOH (measuring pH after adding certain volumes of NaOH) and after completion, it was left to the side while we did colour indicator changes with Vinegar again using Phenolpthalein and Bromophenol Blue.

After around 15 minutes, we went back to the beaker of our forward titration where we were to start a backward titration with HCl. The pH was measured again before adding any HCl and was higher than at the measurement of the end of the forward titration. It was hinted to us that because of the CO₂ produced by everyone in the room it will have been absorbed into the solution.

I have read online that CO₂ when absorbed into water forms an acidic solution, but the NaOH (+CO₂) forms sodium carbonate which is an alkaline solution.

I am being asked why the pH would have increased while left standing. Would I be right to say that it is because of the NaOH mixing with the CO₂ that causes the higher pH?

[Hunter2]

I would say no, because CO₂ decrease the pH, because its acidic in water. The Observation make no sense to me, after finished the titration you have an pH let say of 7. Why should it be different after 15 minutes. CO₂ would make it more lower but not higher.

[Tsuki]

After my beaker had been sitting the pH had increased though. It was 11.08 before adding any HCl when it was 10.83 when finishing titration.

We were titrating 15mL of NaOH into Vinegar, not trying to get to pH 7.

[Hunter2]

Where is the increase now. From 11.08 to 10.83 is a decrease after adding HCl. You didnt tell what was pH before and after adding Vinegar.

[Tsuki]

I am so sorry, I had the numbers around the wrong way.

I have discussed with other students and it seems my pH meter was not working as it should have been a decrease.

I knew that CO₂ and H₂O would make a weak acid which is what I could not understand.

[Hunter2]

But I think the goal here to understand a buffer.

Acetic acid and acetate forms a buffer. So with hydrochloric acid the pH  change is very slowly. The CO₂ in 15 min. has not much impact.