Hi all, I have a little doubt.
Suppose we have a buffer solution HA/A- at pH=2 and we insert an acid drug HD with pKa = 5. From the Henderson-Hasselbalch it is possible to obtain all the relative quantities of the acid and conjugate base of the drug present in the buffer :
pH=pKa + log ([D-]\[HD]) <=> [HD] = 1000*[D-], so D- is practically negligible.
However, referring to the classic exercises on buffers, in theory adding the HD acid drug to the buffer it should react with A- of the buffer itself and, thinking in terms of moles, the HD acid drug is completely consumed and converted into the corresponding base -:
HD + A- --> HA + D-
So, in solution it would remain D-. So how does HD get 1000 times higher than D-?
Thanks in advance.