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Buffers

Buffers

A buffered solution is resistant to large pH changes upon the addition of small amounts of H+ or OH–. The mixture of a weak acid and its conjugate base acts as a buffer. Examples of calculations used in preparation of buffers, and those used to predict changes in pH of a buffered system occurring upon addition of specified amounts of strong acids or bases can be found in the references given below, and many other sources. They also consider practical aspects of buffer preparation and use, including the concept of buffer capacity.

 
As we saw in the discussion of acids and bases according to the Brønsted-Lowry definition, the acid dissociation equation can be written for any Bronsted acid, and from it the acid dissociation constant Ka is defined. An equation that is indispensible for treatment of buffers, the Henderson Hasselbalch equation (lower equation) is readily derived from the definition of the acid dissociation constant by taking the logarithim of both sides and applying the definition of pH and pKa and the properties of the log function.
Equartions: Acid dissociation equation, definition of pKa, and Henderson-Hasselbalch equation   This form of the Henderson-Hasselbalch equation can be expresses pH in terms of pKa and the log ratio of conjugate base to acid. These equations are very useful for buffer calculations and in determining the proportions of conjugate pairs that exist at a given pH.

It is easy to see, for instance, that at a pH equal to the pKa of a weak acid HA, the population of unionized acid molecules is exactly balanced by the number of molecules of the conjugate base. At a pH above that of the pKa, the anionic form will predominate, and at low pH, the unionized acid species is favored.

In biochemistry, especially in discussion of amino acid and protein chemistry, frequent use is made of the concept of pKa of an ionizable group.

Practical aspects of buffers

  • Buffer preparation
  • Actual pH vs. calculated pH
  • The pKa depends on
    • Concentration
    • Temperature
    • Ionic strength

Buffer capacity

The capacity of a buffer depends on the total (formal) concentration of the buffer species ( [acid] + [conjugate base] ), and for a given total concentration, buffer capacity will be greatest when [acid] = [conjugate base] − that is, when pH = pKa for that buffer's acidic form.

If one knows in advance that the buffer will need to resist an increase in pH due to the addition of base, and that protection against acid is not needed, one can prepare a buffer with an initial excess of the acidic species. This would provide an extra measure of buffer capacity in this special situation. A similar idea holds if the buffer must protect against addition of acid and not base: A buffer with an initial excess of the basic species may be called for.
 

References

  1. Calbiochem booklet on Buffers. (An online resource on buffers in biological applications is available and highly recommended.)
  2. Segel IH. (1976). Biochemical Calculations: How to Solve Mathematical Problems in General Biochemistry, 2nd ed. Ch.1, Acid-Base Chemistry.
  3. Harris DC. (2009). Exploring Chemical Analysis, 4th ed.
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