pKa of a Buffer

Over the years, I have alternated this exercise with a glycine titration described elsewhere. The main purpose of this exercise is to review the concept of buffers and pKa. Students are given 75 mL of a 0.2 M solution of the base form of a buffer, which they will titrate with 0.5 M HCl. Any commonly-used buffers can be used; I use ethanolamine (pKa = 9.5), diethanolamine (pKa = 8.9), triethanolamine (pKa = 7.8), monohydrogen phosphate (effective pKa = 6.9) and morpholinoethanesulfonate (MES, titrated with an equivalent of NaOH). Using a pH meter, they titrate their solutions with 0.5 M HCl in 1-mL portions. From their tabulated and graphed data, they are asked to identify their buffer by its pKa and to indicate the pH range over which it would be an effective buffer.

 

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Revised 2/21/08