SIR BUFFER: PROPERTIES OF
BUFFER SOLUTIONS
This SIR does even more than its title promises. It will
determine the equilibrium state of any solution containing an acid, a base, or an acid and
its conjugate base. It assumes ideality, but accounts for the water/H3O+/OH-
equilibrium.
The initial display gives you a choice of HCl, NaOH, five
conjugate weak acid - weak base pairs, and a "designer" acid and conjugate base
whose Ka you may set. You are allowed to put into your solution any acid and
its conjugate base, any acid and NaOH, any base and its conjugate acid, any base and HCl,
or any single acid or base.
Having chosen the system you are presented with a reaction table.
You may click on any starting concentration; a keypad will appear that lets you enter any
starting concentration within the range 10-6 M to 1 M. The computer will
compute the changes, the equilibrium concentrations and the pH. It also shows the pH which
would be obtained by using the standard weak acid, weak base or Henderson-Hasselbalch
approximation, whichever is appropriate. Quite often these approximations are considerably
in error; this SIR is a useful tool to explore their limitations.
If the resulting solution is a buffer you have a
further option: to plot the graph showing the effect on the pH of adding 1 M HCl or 1 M
NaOH to 100 mL of your solution. You may explore the graph with a set of crosshairs. You
may also put on the graph the (usually dramatic) effect of adding the strong acid and base
to a solution of the same pH as the buffer, but made up with HCl or NaOH alone. Another
option is to show the pH curve calculated using the Henserson-Hasselbalch equation.
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Updated July 17, 2000