pKa Calculator
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Common pKa Values
Below is a table of common acids and their pKa values:
| Acid | Formula | pKa | Acid Strength |
|---|---|---|---|
| Hydroiodic acid | HI | -10 | Strong |
| Hydrobromic acid | HBr | -9 | Strong |
| Hydrochloric acid | HCl | -7 | Strong |
| Sulfuric acid (1st) | H₂SO₄ | -3 | Strong |
| Nitric acid | HNO₃ | -1.3 | Strong |
| Hydronium ion | H₃O⁺ | 0 | Strong |
| Sulfuric acid (2nd) | HSO₄⁻ | 1.9 | Weak |
| Phosphoric acid (1st) | H₃PO₄ | 2.1 | Weak |
| Formic acid | HCOOH | 3.8 | Weak |
| Acetic acid | CH₃COOH | 4.8 | Weak |
| Carbonic acid (1st) | H₂CO₃ | 6.4 | Weak |
| Phosphoric acid (2nd) | H₂PO₄⁻ | 7.2 | Weak |
| Hydrogen sulfide (1st) | H₂S | 7.0 | Weak |
| Ammonium ion | NH₄⁺ | 9.2 | Weak |
| Carbonic acid (2nd) | HCO₃⁻ | 10.3 | Weak |
| Phosphoric acid (3rd) | HPO₄²⁻ | 12.3 | Weak |
| Phenol | C₆H₅OH | 10.0 | Weak |
| Hydrogen sulfide (2nd) | HS⁻ | 12.9 | Weak |
| Water | H₂O | 15.7 | Very Weak |
| Ethanol | C₂H₅OH | 16.0 | Very Weak |
| Ammonia | NH₃ | 38 | Extremely Weak |
| Methane | CH₄ | 50 | Extremely Weak |
What is pKa?
The pKa value is a measure of the strength of an acid in solution. It represents the negative logarithm (base 10) of the acid dissociation constant (Ka):
pKa = -log₁₀(Ka)
The pKa value tells us how easily an acid gives up a proton (H⁺). The lower the pKa value, the stronger the acid.
The Significance of pKa
- Acid Strength: Acids with lower pKa values are stronger (donate protons more readily)
- pH Calculations: pKa is crucial for calculating pH in buffer solutions
- Drug Development: pKa affects drug absorption, distribution, and interaction with biological systems
- Chemical Reactions: pKa helps predict reactivity and equilibrium positions
The Henderson-Hasselbalch Equation
This equation relates pH, pKa, and the concentrations of an acid (HA) and its conjugate base (A⁻):
pH = pKa + log₁₀([A⁻]/[HA])
Buffer Solutions
A buffer solution contains a weak acid and its conjugate base. The best buffers work when:
- The desired pH is within ±1 unit of the pKa value
- The concentrations of acid and conjugate base are similar
Temperature Effects
pKa values typically change with temperature. This calculator can account for temperature variations using the van't Hoff equation, which relates the equilibrium constant to temperature.