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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:

for

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 above the pKa, the molecule will be more negatively charged.
- At any pH below the pKa, the molecule will be more positively charged.
- A simple graphical way to remember these points is:

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.