pKa Calculator
Determine pKa values using the Henderson-Hasselbalch equation or from the acid dissociation constant (Ka)
🧪 Calculation Method
📚 Common Acids Reference
| Acid | Formula | pKa |
|---|---|---|
| Acetic acid | CH₃COOH | 4.76 |
| Formic acid | HCOOH | 3.75 |
| Hydrochloric acid | HCl | -7.0 |
| Carbonic acid | H₂CO₃ | 6.35 |
| Phosphoric acid | H₃PO₄ | 2.16 |
| Citric acid | C₆H₈O₇ | 3.13 |
📊 pKa Result
Calculation Steps
Henderson-Hasselbalch Equation
Rearranged to solve for pKa:
Using the provided values:
🔍 pKa Interpretation
What is pKa?
The pKa value is the negative base-10 logarithm of the acid dissociation constant (Ka). It indicates the strength of an acid in solution.
pKa Scale
• pKa < 0: Strong acids (completely dissociate)
• pKa 0-4: Moderately strong acids
• pKa 4-10: Weak acids (partially dissociate)
• pKa > 10: Very weak acids
Henderson-Hasselbalch Equation
This equation describes the relationship between pH, pKa, and the ratio of conjugate base to weak acid concentrations in a solution.
🧪 Acid Dissociation Constant (Ka)
The acid dissociation constant (Ka) measures the strength of an acid in solution. It is the equilibrium constant for the dissociation reaction:
Where [HA] is the concentration of the acid, and [A⁻] is the concentration of its conjugate base.
📈 pKa and pH Relationship
When pH = pKa, the concentrations of the acid and its conjugate base are equal.
• pH < pKa: The acidic form (HA) predominates
• pH > pKa: The basic form (A⁻) predominates
This relationship is crucial for understanding buffer solutions and acid-base equilibria in biological systems.
💡 Practical Applications
pKa values are essential in:
- • Buffer preparation and selection
- • Pharmaceutical drug design
- • Biochemical research
- • Analytical chemistry methods
- • Environmental science
Understanding pKa helps predict how substances will behave in different pH environments.