Question

will the pH at the half equivalence point always equal the pKa...what about at the second or third half equivalence points?

Answer 1

In lab we ran a titration with a .1 M unknown triprotic acid. We found the first endpoint but ran out of time before getting to the third. There were only 2 acids that had Ka values that even remotely fit with our data would it be ok to use the known ka2 and ka3 values to estimate the second half and third half equiv points to eliminate one of those acids?

Answer 2

pH = pKa - log[b/a]
at half equivalence point, b = a
log[1] = 0 so pH = pKa

No idea about second half and third half though.

Answer 3

shoot I have the known ka values for phosphoric and citric acid in the list of possible unknowns. and while both give nearly the same value for the first 1/2 equiv point the second and third ka's are quite different.

Answer 4

phosphoric 7.5x10^-3 , 6.2x10^-8 and 3.6x10^-13
citric 7.4x10^-3 , 1.7x10^-5 and 4.0x10^-7

would using (pKa1+pKa2)/2 work well enough to show that the pH for citric acid does not match our data well enough (our first equivalence point was at about pH 4.7-5.0)