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Well, I have a small difficulty in one of my project.
I am working with the solubility and bioavailability enhancement of
As Rifampicin is having the two pKa values i.e 1.7 due to its
napthelene -OH groups and another pKa value of 7.9 due to nitrogen of the
Drug behaves as amphoteric.
If taken in to consideration, various pH solutions and its solubility, how
to calculate the % drug ionized for this drug.
i.e. at one pH solution e.g. pH 8, how do we calculate the total % drug
Can we use dirctly the Handerson Hassalbatch equation. Or we need to think
the protonation or deprotonation of the particular functional group at
that particular pH.
Kindly convey your comments or suggestions!
Have a nice day!
Mr. Kole Prashant L.,M.Pharm (PhD) * Res.Adddress:
Faculty, 75/4, Pooja Marg,
Pharmacy Group, Vidya Vihar,
Faculty Division III, BITS, Pilani
Birla Institute of Technology and Science, Rajasthan 333031
Email: plkole.at.bits-pilani.ac.in, prashantkole.-a-.rediffmail.com
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Since the pKa of your ionizable groups are separated by more than 2 orders
of magnitude, the ionization of any one group should not affect the other.
In such a case using the Henderson-Hasselbach equation to determine the
degree of ionization at the pH of the solution will be adequate.
If, however the groups were interacting, ie the pKa's were separated by less
than 2 units then a separate treatment of the data (based on and derived
from the HH equation should be used). Treatment of such ionizations have
been published and you may benefit from doing a literature search for author
= Schulman S.(Univ. of Florida). You will find various papers published in
J.Pharm Sci in the early 80's on doxorubicin and other anthracycline pka
determinations. If you are unable to do so you can E-Mail me a request and I
will forward the exact reference (I'll have to dig it out from my files and
I am just too lazy to do it right now).
Anup Zutshi Ph.D.
Global Drug Metabolism
Mail Stop: 7260-300-104.1
Kalamazoo, MI 49001
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