The pH of water is 7. If we add 0.1 mole of strong acid (HCl for instance) to 1.000 L of water, the pH value drops to pH 1. Upon adding 0.1 mole of a strong base (NaOH for instance) to 1.000 L of water, the pH increases up to pH = 13. These are major changes.
If we add 1.0 mole of the weak acid HOAc and 1.0 mole of the conjugate weak base OAc- (present in NaOAc, that dissociates in OAc- and Na+, the latter a weaker acid than water) to 1.000 L of water, we obtain a buffer solution. The pH of this buffer solution can be calculated as follows:
HOAc(aq) H+(aq) + OAc-(aq)
Notice that [HOAc] is 1.0 mol/L, since HOAc is a weak acid, so only a few molecules dissociate: [HOAc] = coHOAc. Also notice that [OAc-] = 1.0 mol/L. The added salt NaOAc will dissociate completely and only a few of the OAc--ions will accept a proton, the OAc--ion being a weak base. The Na+-ion is a weaker base than water and can be neglected. So: [OAc-] = coOAc-.
If we add 0.1 mole of a strong acid (HCl for instance) to 1.000 L of this buffer solution, the pH value will decrease, but only a little bit. The protons from the strong acid HCl will react with the buffer base OAc-:
H+(aq) + OAc-(aq) HOAc(aq)
1.000 L
H+(aq)
+
OAc-(aq)
HOAc(aq)
Before
0.1 mole
1.0 mole
1.0 mole
D
-0.1 mole
-0.1 mole
+0.1 mole
After
0 mole
0.9 mole
1.1 mole
After this reaction, there is still a buffer solution, but with different concentrations:
The pH has decreased, but only from 4.74 to 4.66.
If we add 0.1 mole of a strong base (NaOH for instance) to 1.000 L of this buffer solution, the pH value will increase, but only a little bit. The strong base NaOH with the buffer acid HOAc:
OH-(aq) + HOAc(aq) OAc-(aq) + H2O
1.000 L
OH-(aq)
+
HOAc(aq)
OAc-(aq)
Before
0.1 mole
1.0 mole
1.0 mole
D
-0.1 mole
-0.1 mole
+0.1 mole
After
0 mole
0.9 mole
1.1 mole
After this reaction, there is still a buffer solution, but with different concentrations:
The pH has increased, but only from 4.74 to 4.83.
In general we can define a buffer as a solution of a weak acid A and the conjugate weak base B in water. Both A and B can be ions! The pH of the buffer is given by:
A(aq) H+(aq) + B(aq)
Summary
Buffer Solutions
= solution of a weak acid A and the conjugate weak base B in water
Small amounts of acid or base can be added without causing a significant change in pH.
H+(aq) + OAc-(aq)
HOAc(aq)
