than the ionization constant of the weak base, the solution will be acidic. If the constant
is greater for the weak base, the solution will be basic. If the constants are equal, the
resulting solution will be neutral.
Section VI. BUFFERS
Buffer systems are commonly used in the laboratory to help maintain a constant
pH in a reaction mixture. A buffer solution acts to resist a change in pH. A buffer is
composed of either a weak acid and its salt or a weak base and its salt. Buffers made
up of a weak acid and it's salt are called acidic buffers and function from pH 0 to 7.
Basic buffers consist of a weak base and its salt and function from pH 7 to 14.
9-21. MECHANISM OF ACTION
a. Addition of a Strong Acid. A common buffer used in the laboratory is the
acetic acid/acetate buffer. As acid is added [H ] into the solution, it will react with the
acetate anion from the salt (proton acceptor) forming acetic acid. Since acid is added, a
solution will be controlled by the ionization constant of the acetic acid formed and only a
slight change in [H ] takes place.
HC2H3O2/NaC2H3O2 (Acetic acid/sodium acetate)
Buffer system with HCl added:
H + Cl- + Na + C2H3O2- <====> HC2H3O2 + Cl- + Na
b. Addition of a Strong Base. As base is added to this buffer, acetic acid
reacts with the base to form salt (acetate ions) and water. Once again a shift in the
equilibrium of the solution occurs but with a minimal change in pH.
Na + OH- + HC2H3O2 <====> Na+ + C2H3O2 + H2O
(1) A buffer's ability to resist changes in pH is limited to the concentration of
either the weak acid of base and its salt. The addition of an excessive amount of either
acid or base will "exhaust" the buffering capacity of the buffer.
(2) The closer the pH of the buffer to the pKa or pKb of the weak acid or
base respectively the greater the buffering capacity.