2.2 pH and pKa

The strength of an acid depends on its degree of dissociation in water and can be determined from the equilibrium expression (Scheme 1).

Scheme 1

This equation is known as Hendersson-Hasselbach equation and relates a relationship between the concentration of hydrogen ions in solutions and acid strength. Thus, pH is a measurement of the H⁺ concentration in a liquid.  If there is a high H⁺ concentration, the pH indicates that it is a very acidic solution.  If the solution is neutral, there is only a small H⁺ concentration, and the pH reflects that.  If the solution is basic, there is almost no H⁺ concentration, and you can tell that by the pH number. 

As with acids, a similar scale may be derived for bases based on the equilibrium of the protonated form of the base and base itself (Scheme 2).

Scheme 2

However, such an expression creates two separate scales for the determination of strengths of acids and bases. Instead the strength of the conjugate acid is determined then a common scale in terms of pK_a may be used for both acids and bases. Thus, the lower the value of pK_a, stronger is the acid. Similarly, higher is the value of pK_a of base, more basic it is.

2.3 Buffer Solution

A solution of a weak acid and its conjugate base is referred as a buffer solution. A buffer solution has the ability to resist to a small change of pH, because the weak acid can donate a proton to any HO^- added to the solution, and its conjugate base can accept any H⁺, which is added to the solution.

A common example of buffer is a solution of sodium acetate in acetic acid. Human blood is also a buffered solution and maintains a pH close to neutrality.