Buffer Solutions
Buffers are solutions with the ability to resist the addition of strong acids or strong bases, within limits.
They play an important role in chemical processes where it is essential that a fairly constant pH is maintained. In many industrial and physiological processes, specific reactions occur at some optimum pH value. When the pH varies to any extent from the optimum value, undesirable reactions and effects may occur. For example, the pH of your blood lies at about 7.35. If this value drops below 7.0 (acidosis) the results are fatal. Also if it rises above 7.7 (alkalosis) the results are as well fatal. Fortunately our blood contains a buffering system which maintains the acidity at the proper level. If it were not for the protection of the buffering system, we could not eat and adsorb many of the acidic fruit juices and foods in our diet.
A typical lab buffer is CH3COOH and its salt NaCH3COO. Most buffer solutions are made up using a weak acid and its sodium salt! When a strong base such as NaOH is added to the buffer, the acetic acid reacts with and consumes the excess OH- ion. The OH- reacts with the H3O+ ion from the acid in the following reaction:
                H2O + CH3COOH   <------->   H3O+ + CH3COO-

                             H3O+ + OH- <----------> H2O

The OH- reduces the H3O+ ion concentration, which causes a shift to the right, forming additional CH3COO- and H3O+ ions. For practical purposes each mole of OH- added consumes a mole of CH3COOH and produces a mole of CH3COO-.
                   OH- + CH3COOH <------------>   CH3COO- + H3O+
When a strong acid such as HCl is added to the buffer, the hydronium ions react with the CH3COO- ions of the salt and form more undissociated CH3COOH.
                  H3O+ + CH3COO-   <-----------> CH3COOH + H2O
As you would expect, there is a limit to the quantity of H+ or OH- that a buffer can absorb without undergoing a significant change in pH. If a mole of HCl is added to a litre of buffer solution containing 0.5 moles of sodium acetate/acetic acid buffer the H+ completely consumes the buffer and results in a drastic change in pH.
The blood buffer is made up from the dissolved carbon dioxide in the plasma.
              CO2(g) + H2O <------->    H2CO3  <------>  HCO3- + H3O+
When a base is added it reacts with the carbonic acid.
                       OH- + H2CO3 <-------->  HCO3- + H2O
When an acid is added it reacts with the bicarbonate ion.
                     H3O+ + HCO3-    <--------->  H2CO3 + H2O
Because there are both a base-neutralizer and an acid-neutralizer then we have a buffer.
Buffer Components
A buffer has two components.
         HA                      NaA ---> Na+ + A-
    a weak acid   &     a soluble salt of the acid
Therefore any extra H3O+ will be neutralized by the A- in the buffer.
                  H3O+ + A-   <------->  HA + H2O
And any extra OH- that is added will be neutralized by the acid.
                       HA + OH-   <------>  A- + H2O
There are 3 basic types of calculations that can be done with buffer system and these will be covered next.