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3.3 Acid-Base Equilibria

3.3 Acid-Base Equilibria

pKa and the Henderson-Hasselbalch Equation

For example: You are studying a bottle full of valine amino acids and you need to know if they are going to be ionized at pH 8.6 (because you plan on doing some charge-sensitive experiments with the valine). What do you do? Well, one thing you need to know is what state the a-NH3 groups are going to be in at pH 8.6. This is simple to calculate with the Henderson-Hasselbalch Equation. Find the pKa of the a-NH3 of valine on the pKa chart. Then calculate the fraction of valine a-NH3 ionized at pH 8.6 using the Henderson-Hasselbalch Equation:

1) substituting: 8.6 - 9.6 =

so: = 0.1

therefore, it is:

to a first approximation, a net charge of +0.9 is fully ionized, so we can draw this group as -NH3+.

If you either do some more problems similar to this, or simply inspect the Henderson-Hasselbalch Equation more critically, you will discover that there are some simple rules of thumb you can follow to save a lot of calculation time.

At any pH more than 1.0 unit different than the pKa, you can consider the molecule to be completely in one state. For instance, in the example above the pH was exactly 1.0 different from the pKa, giving an ionization of 90%, which we consider complete for the purposes of this class.

For practice, figure out what the state would be of the a-COOH groups in your batch of valine. What will be the total charge on each valine? Draw a valine molecule at pH 8.6.