Drug Pathways and Chemical Concepts  Prof. Sally Boudinot  11.
Spectrophotometric Determination of pKa of Bromothymol Blue
Please remember these concepts:    1. Many processes can be at
equilibrium.   But with changes in condition - concentration,
temperature -  the system will no longer be at equilibrium and
will adjust to try to get there again.   2. The equilibrium
concentrations of H3O+ and OH- are vanishingly small in pure
water.    3. A weak acid or a weak base drug, in water, will
disassociate to some extent.  The pH of the drug solution  will
depend upon the pKa.   4. Buffers stabilize pH.  This stabilized
acidity determines the form of drug disassociation in systems. 
The Henderson-Hasselbach equation conveniently handles drug
ionization questions for buffered systems like the body. Lab
Guide:  This is the sequence of steps in this laboratory
determination:    1. Prepare buffer phosphate solutions with
varying pH.   2. Use an indicator (bromothymol blue) as the weak
acid whose pKa we will determine.