Fig: the Henderson-Hasselbalch equation

The Henderson-Hasselbalch equation allows us to calculate the pH of a solution when we know the pKa value of either the acid or base used in the buffer solution... A table of pKa values for common acids and bases can be found here, as well as the concentration of the acid and base used in solution.

Below are two examples of how to calculate the initial pH of both an acid and a base buffer solution:

• Example 1
  Consider a base buffer solution containing 0.2M NH₃ and 0.2M NH₄Cl. Calculate the pH that the solution will buffer at:
  1. Our first task is to find out the pKa value of the base used NH₃. We can find this by looking at the pKa values from the table. The pKa of Ammonia NH₃ is 9.25.
  2. Then we simply put the values into the Henderson-Hasselbalch equation:
     
     pH = pKa - log₁₀([Acid]/[Base])
     pH = 9.25-log₁₀(0.12/0.2)
     pH = 9.25 - log₁₀0.6
     pH = 9.25 - ^-0.22185
     pH= 9.25 + 0.22185
     pH = 9.47
  
  
• Example 2
  Now consider an acid buffer solution containing 0.3M CH₃COOH and 0.17M CH₃COONa. What will the initial pH of this buffer solution be?
  1. Again find the pKa value of the acid used, in this case it is acetic acid (ethanoic acid). Its value is 4.75.
  2. Now put into the Henderson-Hasselbalch equation:
     pH=pKa - log₁₀([Acid]/[Base])
     pH = 4.75 - log₁₀ (0.3/0.17)
     pH= 4.75 - log₁₀1.765
     pH = 4.75 - 0.2467
     pH = 4.50

     The calculation of the initial pH of a buffer solutuion is a simple task, it is a little to harder to calculate the new pH of a buffer solution after a small amount of acid or base has been added to it. You can find demonstrations of how to calculate this upon the following page.

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Author: Christopher Saywell (document modification date: 28th April 2004)