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General method for determining macrodissociation constants of polyprotic, amphoteric compounds from solubility
Journal of Pharmaceutical Sciences
|January 1, 1978
Summary
This study presents a new method using solubility and pH measurements to estimate macrodissociation constants (pKa) for weak acids and bases, including complex polyprotic substances. The approach accurately determines thermodynamic pKa values, validated with real-world data.
Area of Science:
- Analytical Chemistry
- Physical Chemistry
- Biochemistry
Background:
- Determining dissociation constants (pKa) of poorly soluble compounds is challenging.
- Existing methods may not adequately address polyprotic or amphoteric substances.
- Accurate pKa values are crucial for understanding chemical behavior and biological interactions.
Purpose of the Study:
- To develop a general method for estimating macrodissociation constants (pKa) of weak acids and bases.
- To extend solubility-pH-pKa relationships to polyprotic and amphoteric compounds.
- To provide a reliable estimation procedure for thermodynamic pKa values.
Main Methods:
- Utilizing equilibrium solubility and pH measurements.
- Deriving extended equations relating solubility, pH, and macroscopic pKa.
- Implementing a subroutine for approximating thermodynamic pKa values.
Main Results:
- A general method for estimating pKa values from solubility and pH data was successfully developed.
- The method was applied to polyprotic and amphoteric substances.
- Thermodynamic pKa estimates for tyrosine and 2,3-dihydroxyadenine closely matched values obtained by other established methods.
Conclusions:
- Equilibrium solubility and pH measurements offer a viable approach for determining macrodissociation constants.
- The developed method provides accurate thermodynamic pKa estimates for challenging compounds.
- This technique enhances the characterization of weak acids and bases, particularly those with low solubility.