Topic 8,18: Buffers  Maintaining pH
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/buffer12.swf
Introduction:
*If a small volume of a strong acid or base is added to water, the pH of the water will change significantly.
e.g. adding  2 drops of 1.0 M HCl(aq) to a litre of water will change the pH from ____ to ____.
*When a solution is __________, its solutes protect it against large changes in pH even when ________ volumes
of ___________ acids or bases are added. (i.e. they buffer the system against a pH)
*Every life form is extremely sensitive to slight changes in pH. The pH of blood is _______. If it were to change to
7.00 or 8.00, for example, you would _________. Blood is a ___________ solution.
How do Buffers Work?
The reason buffers work is equilibrium, and Le Chatelier’s Principle. If you add some acid (or base) the
equilibrium shifts to keep the concentration ratios constant and hence maintain the pH.
Let’s look at it in terms of chemical equilibrium: HA(aq)
H+(aq) + A-(aq)
+
If OH- is added, it removes H (aq) by forming water. Equilibrium shifts to the _________to replace the H+(aq),
thereby maintaining the pH. Remember, there is a lot of HA around because it’s a weak acid.
 If H+ is added, it shifts the equilibrium to the left by reacting with the A-(aq) to form HA. The A- comes from the
conjugate base that we added as the salt (see table below).
Buffer Solutions (Overview)
Definition
A solution which resists a change in pH when a ________ amount of acid or base is added
Composition
“Type” 1 A mixture of a weak Bronsted ___________ (HA(aq)) and its conjugate base (A-(aq))
The concentration of the conjugate base must be increased. This is accomplished by adding a
soluble, ______________ salt containing the conjugate. (e.g. NaA(aq))
OR
“Type” 2  A mixture of a weak Bronsted base and its conjugate acid.
Preparation
Method I  mix a weak acid or base and its conjugate. (i.e. salt of the conjugate)
OR
Method II  partially neutralize a weak acid (or base) with a strong base (or acid)
Composition
Why a mixture? Since the acid is weak, there is very little of the conjugate acid around to react with any added
acid. The source of the “extra” conjugate base is the salt or the products of a partial neutralization.
Preparation
Method I  the first method of preparation is easy – mix the weak acid/base and the salt. Usually it’s in equal
proportions, but it doesn’t need to be.
Method II  the most common method of preparation is “partial neutralization”: react two parts weak acid with
one part strong base. This means that there will be one part of excess weak acid left over and one part salt
(conjugate) produced.
1
That is:
Before reaction:
HA(aq) + NaOH(aq)  Na+A-(aq) + H2O(l)
2
1
0
0
(excess) (limiting)
After reaction:
*After the reaction, you have one part weak acid and one part conjugate salt….that’s a buffer.
*You could do the same with a weak base and a strong acid.
*The capacity of a buffer is determined by the _______________ of its acid-base conjugate pair.
The Henderson - Hasselbalch Equation
Derived:
HA(aq)
H+(aq) + A-(aq)
B(aq) + H2O(l)
BH+(aq) + OH-(aq)
Shortcut:
If [salt] = [acid], then the value of the fraction becomes 1, and the log of 1 is _____, so pH = ______.
How can this be applied?
1. When preparing a ____________: When we are _______ way to the ________________ point,
which is the partial neutralization that we used when preparing a buffer. (i.e. when the pH of the
reaction mixture equals the pKa of the acid, a buffer solution results.)
2. To determine the ________ of an acid:This is also an experimental method for determining the
Ka of an unknown weak acid. You carry out the titration to find the equivalence point. Then you
use your graph (i.e. titration curve) to determine the pH at 1/2V eq. This is the value of the pKa.
2