[Nana00]

Hello,

Can anyone help me with this question or point to where I could obtain an answer...

1.   When performing diprotic acid titrations, you may see two clear endpoints or one may not be unclear.
a.   When do the unclear endpoints occur?
b.   In which acids from the list below would you expect the endpoint to be unclear? Explain.
Acid   FW   pKa1   pKa2
Malonic acid   104.06   2.85   5.70
Maleic acid   116.07   1.92   6.23
Fumaric acid   116.07   3.02   4.38
Succinic acid   118.09   4.21   5.64
Itaconic acid   130.10   3.85   5.45
Glutaric acid   132.13   4.31   5.42

Thank you!

[Cavillus]

Hello,

Can anyone help me with this question or point to where I could obtain an answer...

1.   When performing diprotic acid titrations, you may see two clear endpoints or one may not be unclear.
a.   When do the unclear endpoints occur?

Well, in the cases I examined, the second endpoint isn't visible is K_a2 is comparable with K_w, for example considering the third dissociation of H₃PO₄.
Another case happens if the two K_a aren't so different, as in the case of K_a1 and K_a2 of EDTA.

b.   In which acids from the list below would you expect the endpoint to be unclear? Explain.
Acid   FW   pKa1   pKa2
Malonic acid   104.06   2.85   5.70
Maleic acid   116.07   1.92   6.23
Fumaric acid   116.07   3.02   4.38
Succinic acid   118.09   4.21   5.64
Itaconic acid   130.10   3.85   5.45
Glutaric acid   132.13   4.31   5.42

Thank you!

Given the rules I wrote before, I can assess that, for sure, Fumaric acid has only one endpoint:

http://www.demochem.de/chembox_diprotic_tit-e.htm

As an analogy, Succinic, Glutaric and Itaconic acid will have only one endpoint.

Probably, Malonic acid has two endpoints, even if they won't be so far.