CHEM 240: Introduction to
Bioanalytical Chemistry

 J. D. Cronk
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7. header

Lecture 7. Acid-base chemistry: Buffers.

Wednesday 31 January 2007

Calculations of pH in buffered aqueous systems: The Henderson-Hasselbalch equation. Preparation of buffers.

Reading: Harris - Ch.9, p.175-181. Problems: Ch.9 - 10, 12, 13, 15, 19, 20.

 
7. Summary

Lecture 7 summary

In today's lecture, we look carefully at the characteristics and action of buffers. We will cover 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. We also consider practical aspects of buffer preparation and use, including the concept of buffer capacity.

An online resource on buffers in biological applications is available and highly recommended: Calbiochem booklet on Buffers.

The Henderson-Hasselbalch equation

One of the most useful equations in acid-base chemistry is the Henderson-Hasselbalch equation.
The derivation of the Henderson-Hasselbalch equation for a weak acid of the form BH+ is shown at right. One always starts with acid dissociation equation, which for BH+ produces the neutral base B and H+. The definition of Ka follows by applying the usual rule for constructing an expression for the equilibrium constant to the acid dissociation equation (line 2). Next apply the definition of pKa = –log(Ka) (line 2, 2nd eqn.). The property of the log function log(ab) = log a + log b is used to separate log[H+] from the log([B]/[BH+]) term (line 3), then the definition of pH is applied (line 4). This is essentially the Henderson-Hasselbalch equation, which we write in whatever form is most convenient for the problem at hand. Rearranging this equation (line 5), we have the form that is presented to us in the text (see Harris, p.176).   Derivation of the Henderson-Hasselbalch equation

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.

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