Inorganic and Applied Chemistry

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Inorganic and Applied Chemistry

5.5 Buffer

One of the most important applications of the acid-base chemistry is the buffer capacity. A buffer solution is

a solution that counteracts an external action affecting pH. Some of the most well-known buffer solutions are

found in the human body where they help to protect the pH of the blood from external actions and to keep

the blood at constant pH levels. It is essential for the human body to be able to maintain the pH of the blood

at fairly constant levels as certain types of cell only may survive in a close pH window.

A buffer solution may contain a weak acid and its salt or a weak base and its salt. Examples of buffer

solutions is e.g. HF / NaF and NH 3 / NH 4 Cl. Thereby a buffer solution always consists of a weak acid and its

corresponding weak base in amounts close to each other. By choosing the right components and

concentrations it is possible to set up buffer capacity at many different pH levels.

5.5.1 The Buffer equation

Knowing an acids strength exponent pKa it is also possible to calculate pH in a buffer solution and this is the

subject for the following section. We take a starting point in the well-known acid-base reaction:

HA(aq) + H 2 O(l)  H 3 O+(aq) + A-(aq)

The equilibrium expression for the general acid-base reaction may be expressed as:

  

HA

HO A

Ka



3

The equation for Ka is rewritten in order to be able to contain pH. This happens by applying the decimal

logarithm on both sides of the equilibrium sign and do a few simple manipulations. Hereby the buffer

equation arises:

 

^

A

HA

pH pKa log (5- 3)

The buffer equation is also known as the Henderson-Hasselbalch equation. This equation is in principle just

another version of the expression for Ka but it may nevertheless other be easier to apply. Using the buffer

equation one must remember that HA and A- denotes the corresponding acid-base pair and that pKa is the

acid exponent of the acid (HA). In the following example the buffer effect is illustrated in a buffer system

consisting of equal amounts of acetic acid and acetate into which strong base is added.

Acids and bases