Science Buddies: "Ask an Expert"

I have completed the project on "Determining the amount of acetic acid in vinegar" and I have also finished with analyzing data......

But I have a few doubts...........
1.) Why are we supposed to use phenolphthalein indicator which turns pink after pH 8.3, and not that one which would indicate the end point of titration at pH 7........because at pH 8.3 there would be more of OH- ions then needed to neutralize the H+ ions of acetic acid........and thus the reading for the amount of NaOH used will be wrong.

2.) Acetic acid is a weak acid........so if I dilute vinegar in more amount of water then the amount of NaOH used will be more because more H+ ions from acetic acid will be formed..........and in less of amount of water less H+ ions will be formed.............shouldn't we then titrate the vinegar without diluting in water for exact results.........

I know I should have asked these questions before starting with the project !!!!!!!!!!!!!!!!

I am really very thankful to this site as I got the idea of my project from the "science fair project ideas" section !!!!!!!!!!!!!

Thank you

Hi,

Great questions! With this experiment, you are titrating acetic acid, which contains one carboxyl (COOH) group and is therefore a weak acid. Here is a site that shows the titration curve (change in pH) for a weak acid.

http://www.chemguide.co.uk/physical/aci ... urves.html

Read the background information and then look at the fifth graph (running alkali into a weak acid). Notice that there is hardly any change in the pH until about 25 ml of alkali is added to the sample; then the pH goes straight up from 7 to 12-13. If the phenolphalein indicator were being used instead of a pH meter for this graph, the end point would be the virtually the same for a pH 7 end point as for pH 8.3 end point.

However, scientifically, you are right. It would best to use a pH meter and titrate to pH 7, rather than to the pH 8.3 end point.

You are correct; acetic acid is a weak acid, which means that only a small percentage of the carboxyl groups are ionized in pure water. However, during the titration, the protonated (hydrogen-containing) carboxyl groups will continue to exchange hydrogen ions for the sodium ions, until all of the hydrogen ions on the carboxyl groups have been replaced with sodium. The hydrogen ions are being neutralized with the NaOH to form water, and the endpoint occurs when all of the hydrogen ions have been replaced. Notice the difference in the weak acid titration curve with the second graph in the reference, which shows the titration of a strong acid.

Please let me know if you need additional explanation.


Donna Hardy

Oh thank you.........i got it..............

Hi,

Thanks for letting me know that you understand the explanation. Be sure to include a summary of this discussion on your science board if you have room.

Donna Hardy