[gullie]

Hello,

I have an assignment due Thursday and I have a huge problem with it! See, I did an investigation a while back. To determine the ethanoic acid concentration, I did a titration on different vinegars. I used a pH sensor meaning I  have titration curves. The problem is, am I supposed to use the volume of NaOH needed to make the ethanoic acid reach pH7 or am I supposed to use the equivalence point to determine the ethanoic acid concentration of the vinegar? If I'm supposed to use the equivalence point, how can I find the equivalence point on a titration curve? Or am I supposed to use the fact that phenolphthalein's end point is pH 8.3 for determining the ethanoic acid concentration?

We havent learnt Kp values yet so I am not supposed to use them. So is there a way I could find the equivalence point, preferably without using calculus?

I've got titration curves with clear points, I'm pretty sure they're still save on the computer at school, too. Also, I have pH values for each 1mL of NaOH added to the different vinegars.

Help please!

Thank you! ^_^

gullie

[Borek]

You may try relatively simple graphical method, see plot below. You draw three lines - two tangent to titration curve before and after end point, one tangent to titration curve near endpoint. Endpoint pH is that in the half between intersections.

[gullie]

Do I draw three lines: get a point after the endpoint and get a point before the end point and draw tangents. A third line will be the tangent of a point near end point as in, the line that's vertical on the graph you've posted?

[Borek]

Do I draw three lines: get a point after the endpoint and get a point before the end point and draw tangents. A third line will be the tangent of a point near end point as in, the line that's vertical on the graph you've posted?

Something like that. For the first two points take those close to 50% and 150% titration.

[gullie]

How can I find the points where it is 50% and 150% titrated? What do you mean by those?
I've attached a titration curve to this post. I had to actually get my data points and enter them into Microsoft Excel because for some reason the computer didn't save the information for the three tests I conducted on the Balsamic Vinegar. So which points do you mean?

Thank you so much! I really do appreciate this!

gullie

[Borek]

You have ended a little bit too early for 150%, but 50% is around 10 mL. 50% means you have added half of the stoichiometric amount of titrant, 150% - you have titrated to the end point and you have added 50% excess. That's a very convenient way of expressing titration progress.

[gullie]

Would it be safer to use Graphical Method or to use pH8.3 (pheno) seeing as in all my graphs I'm pretty sure I stopped too early for 150% titration?
So for example for a different graph, is this what it would look like when I draw the lines (see below) and the 50% titration would be at 8.25mL?

On a side note, for extended lab reports, should I keep my raw data and data processing (such as graphs) in separate sections, even though I've put all the raw data into the Appendix?

[Borek]

Lines you have drawn are not tangent, they are either horizontal or vertical, they should show direction of the curve. You are right about 50% being at around 8.25 mL. Don't use phenolphthalein endpoint, as it is characteristic of the indicator, not of the titrated system (ie doesn't matter what system you titrate against phenolphthalein, endpoint will be always at the same pH; there can be some small differences due to dilution, but they can be neglected). Every acid has its own characteristic endpoint pH (which depends on the concentration and acid dissociation constant).

As for the lab report - there are no hard rules, stick to the ones given by your teacher.