Amino Acid Structure and Properties

Demonstration of pH effects on ionization

Notes on usage of pH/pK tutorial
This page Contains a JAVA program in the panel above. You must have Java enabled in your browser and it may take a while to start running.

The Java program above has three panels.
  1. The top depicts a graph of the % protonation of a weak acid or base as well as pie graphs of % protonated and % ionized.
  2. The middle panel provides numerical representation of the same data.
  3. The bottom panel provides some interaction.

    Sliders on the bottom panel allow you to alter the pH or pK independently to observe the effects on the amount of the acid or base is protonated and ionized. NOTE: pK is not an experimental variable. This is a property of a molecule and the envrionment that it is in. It is best if you set the pK to some value to represent a specific group (~ 4 for an organic acid (Glu or Asp) or ~9 for an amine (Lys))

    The Radio buttons on the right allow you to choose whether the ionizable molecule is an acid (GLU or Asp) or a base (Lys). In particular note what happens to the % ionized vs. the % protonated as you change from acid to base.

    pH on the other hand IS an experimental variable. You may add H+ to a solution as you want with addition of a strong acid - such as HCl; hydrochloric acid. (You can also remove H+ from an aqueous solution by adding OH- - as with NaOH; sodium hydroxide)

    The buttons on the left simulate the addition of H+ with a titration from pH 14 to pH 1 with the addition of a constant amount of a strong acid to the solution with each push of the lower button. Start the titration by pushing the "Start a Titration" button. The pH changes to 14. Then repeatedly push the "Titrate a Buffer" button. Observe where the pH changes rapidly and where the pH barely changes as you add more of the strong acid solution.

    Notes on the titration: The weak acid solution is 1 liter of a 50 mMolar solution. Every time you push the "Titrate a Buffer" button you "add" 1 mMole of strong acid to the solution.

    • Regardless of what pK you set... how many pushes of the button results in a pH that is equal to the pK?
    • How does this relate to the amount of substance in solution?
    • Why is this such a constant amount for these concentrations?