Themes > Science > Chemistry > Inorganic Chemistry > Acids and Bases > Acids and Bases Index > Choosing a Buffer


If you have to make up a buffer solution of a certain pH, it's fairly simple to decide on a weak acid to use to make the buffer. Consider the equation for the [H+] of a buffer

[H+] = Ka*(nHA/nA-)
There are two ways to adjust the pH of a buffer solution:
  1. Pick a weak acid with a Ka close to the [H+] you want.
  2. Adjust the HA to A- ratio to get the exact pH

Example: If you want to make a buffer of pH 4.5, how would you do it?

Solution: First, you need to pick a weak acid with a Ka about equal to the [H+] that you want. pH 4.5 gives a desired [H+] of 3.2*10-5 M. If you look over a table of weak acids, you see that acetic acid, Ka = 1.8*10-5 is a good choice.

Next, you need to adjust the ratio of acetic acid to acetate ion so as to get the exact pH. Use the above equation:

  • Ka*(nHA/nA-)
  • 3.2*10-5 = 1.8*10-5*(nHA/nA-)
  • nHA/nA- = 1.8
Thus, if you add acetic acid and sodium acetate to water in a ratio of 1.8 moles acid to 1.0 moles sodium acetate, you will get a buffer of pH 4.5.


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