Weak-Acid Buffer Quadratic

Weak acids only partially dissociate in water. When a weak acid (HA) is mixed with its conjugate base (A⁻), they form a buffer solution that resists changes in pH. The common Henderson-Hasselbalch approximation simplifies the calculation by assuming initial concentrations equal equilibrium concentrations. However, a more accurate model uses the mass action equation and the quadratic formula to find the exact equilibrium concentrations, especially useful when the approximation fails (like highly dilute solutions or strong acids).

experiment

You can investigate the accuracy of the Henderson-Hasselbalch approximation by exploring the simulation Weak-Acid Buffer Quadratic module:

1. Set the pKa to exactly 5.
2. Set the Initial acid M to exactly 0.01.
3. Set the Base acid ratio to exactly 1. Observe the small difference between the equilibrium model pH and the Henderson Hasselbalch pH.
4. Change the Base acid ratio to exactly 0. Notice how the Henderson Hasselbalch pH becomes undefined (Unavailable), while the equilibrium model still correctly calculates the pH of the pure weak acid solution.