Henderson-Hasselbalch Equation

Henderson-Hasselbalch Equation

  • The Henderson-Hasselbalch equation is an expression which sets up a relationship between the pH or pOH of a solution and the pKa or pKb and the concentrations of an acidic or basic species.

  • The equation is especially useful when dealing with buffer solutions. Buffers are solutions that resist changes to the pH when small amounts of an acid or a base are added. Typically, these are a mix of a weak acid and its conjugate base or a weak base and its conjugate acid.

Understanding the Equation

  • The equation for acid-base solutions is given as follows: pH = pKa + log([A-]/[HA])

  • In this equation, pH is the negative logarithm (base 10) of the hydrogen ion concentration.

  • pKa is the negative logarithm of the ionisation constant of a weak acid (Ka). A smaller pKa value indicates a stronger acid.

  • [A-] represents the concentration (in mol/L) of the conjugate base form of the acid, while [HA] represents the concentration of the acid form.

Application of the Equation

  • This equation helps in determining the pH of a buffer solution and devising buffer systems.

  • By rearranging the Henderson-Hasselbalch equation, one can solve for the necessary concentrations of acid and/or conjugate base required to achieve a desired pH.

  • The equation can also be used to estimate the pH of a buffer solution or find the point of equivalence in a titration experiment.

Exceptions and Limitations

  • It’s important to realise that while the Henderson-Hasselbalch equation is very helpful, it does have some limitations. For example, it assumes that the concentration of the acid and the conjugate base remain constant, which may not be the case in a real-world laboratory scenario.

  • Also, it works best when the ratio of base to acid is between 10:1 and 1:10. Beyond these limits, the equation may not give accurate results. This is because the pH of the solution may be influenced by the water concentration, which is not taken into account by the Henderson-Hasselbalch equation.

Remember: The Henderson-Hasselbalch equation is a powerful tool to understand the behaviour of acids and bases in a solution. Knowing how to use it can greatly improve your problem-solving techniques in the lab.