# Finding the percentage unionised of a compound with 3 pKa

• August 1st, 2017, 12:56 AM
rin
Finding the percentage unionised of a compound with 3 pKa
Hi!

My compound, Tacrolimus, has 3 pKa (2.94, 9.95 and 14.07). Even based on the structure, I am not entirely sure if it is acidic or basic (though I am leaning towards acidic as it has ketone groups). I am not sure how to use the Henderson-Hasselbalch equation to calculate the percentage unionised at pH 4.5 considering that it has 3 pKa. Does anyone know how to calculate?

Thank you!
• August 1st, 2017, 05:59 AM
PhDemon
OK, it's been a while but let's see if I remember how to do this!

For the first dissociation step, pKa = 2.94

As pH = -log[H+]

we can say

So the ratio of conjugate base to undissociated acid is 36.3.

This calculation assumes that at pH 4.5 the second and third dissociation steps are negligible (which looking at the pKa values they will be).
• August 2nd, 2017, 10:08 PM
rin
Quote:

Originally Posted by PhDemon
OK, it's been a while but let's see if I remember how to do this!

For the first dissociation step, pKa = 2.94

As pH = -log[H+]

we can say

So the ratio of conjugate base to undissociated acid is 36.3.

This calculation assumes that at pH 4.5 the second and third dissociation steps are negligible (which looking at the pKa values they will be).

Hi!

Thank you for the response! So it seems that Tacrolimus will be mostly ionised at pH 4.5. I assume that means that if I were to try and extract Tacrolimus from a medium that is at pH4.5 using an organic solvent, it would not be possible? Even though Tacrolimus is more soluble in organic solvents than aqueous.
• August 3rd, 2017, 06:36 AM
PhDemon
Not sure, it might be one of those try it and see if it works times ;)
• August 4th, 2017, 03:38 AM
rin
Quote:

Originally Posted by PhDemon
Not sure, it might be one of those try it and see if it works times ;)

I shall adjust the pH and try. Thanks!!! :)