We often have molecules like Methotrexate which has two carboxylic groups. These two similar substitution have different pKa. So, the ionization pH of each one is different. Also, they are weak acids and act like a buffering agent and make pH resistance during formulation preparation.
How can we calculate the amount of strong base like NaOH to adjust the pH?
How does each carboxyl acid substitution act during pH change?
Use the Henderson-Hasselbalch equation:
pH = pKa + log (conc Base/conc Acid). The two carboxylic acids have different pKa's and use the experimental one for your buffer. Remember that the pH of the buffer should be +/-1 pH unit from the pKa. Consider also the ionic strength influence on pKa.
Thanks for your kind help,
What about the pH which both of them are in ionized form?
To calculate the final pH and the amount of Acid and Base to adjust pH, What should i do? is Handerson- Hasselbalch equation applicable in calculation of pH?
If i should sum both carboxylic acid pKa together? or in special pH range , one acidic group ionizes and in another rage the other one has this condition? If i have two buffering points?
When both are in the ionized form, do the calculation for the strongest acid. In the middle of the pKas it is hard to adjust the pH. For theoretical purposed you can do a calculation based on the % ionized form that goes into the equation. If you need a pH the middle, chose another buffer.
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