BL/CH 401 Lecture #3 - Acids, Bases, and Buffers

2. Titration of a Weak Acid illustrating its Ionization and Buffering Property:

{*Figure 5*}

All weak acids have titration curves like this one.

Bases (like ammonium, NH4+) are also weak acids and have similar titration curves.

The position where the Buffering zone is on the pH scale

is related to the chemical nature of the weak acid:

Acetic acid ionizes in the Acidic portion of the pH scale

{*Figure 6*}

Titration curves can also be put in the form of an equation:

{*Figure 7*}

The pK of a weak acid is the pH where [A-] = [HA]

At pH below the pK,

[HA] > [A-]

At pH above the pK,

[HA] <[A-]

Therefore the pK determines the buffering zone for a weak acid.

For example, Acetic Acid has a pK = 4.8 and a buffering zone from pH 3.8 to 5.8.

So a weak acid will be an effective buffer at pH = pK +/- 1 pH unit.

To buffer at a specific pH, use a weak acid with a pK near the pH you want to maintain.

©Wilbur H. Campbell, 1995, 1996; wcampbel@mtu.edu

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