66 Buffer Solutions
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Preparing a Buffer Solution with a Specific pH
A buffer is a solution of weak acid and conjugate base or weak base and conjugate acid used to resist pH change with added solute.
LEARNING OBJECTIVES
Describe the properties of a buffer solution.
KEY TAKEAWAYS
Key Points
- Buffer solutions are resistant to pH change because of the presence of an equilibrium between the acid ([latex]\text{HA}[/latex]) and its conjugate base ([latex]\text{A-}[/latex]).
- When some strong acid is added to a buffer, the equilibrium is shifted to the left, and the hydrogen ion concentration increases by less than expected for the amount of strong acid added.
- Buffer solutions are necessary in biology for keeping the correct pH for proteins to work.
- Buffers can be prepared in multiple ways by creating a solution of an acid and its conjugate base.
Key Terms
- aqueous: Consisting mostly of water.
- equilibrium: The state of a reaction in which the rates of the forward (reactant to product) and reverse (product to reactant) reactions are the same.
- pKa: A quantitative measure of the strength of an acid in solution; a weak acid has a pKa value in the approximate range −2 to 12 in water and a strong acid has a [latex]\text{pKa}[/latex] value of less than about −2.
Buffers
A buffer is an aqueous solution containing a weak acid and its conjugate base or a weak base and its conjugate acid. A buffer’s pH changes very little when a small amount of strong acid or base is added to it. It is used to prevent any change in the pH of a solution, regardless of solute. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. For example, blood in the human body is a buffer solution.
Buffer solutions are resistant to pH change because of the presence of an equilibrium between the acid ([latex]\text{HA}[/latex]) and its conjugate base ([latex]\text{A}^-[/latex]). The balanced equation for this reaction is:
[latex]\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-[/latex]
When some strong acid (more [latex]\text{H}^+[/latex]) is added to an equilibrium mixture of the weak acid and its conjugate base, the added acid reacts with the conjugate base to create more HA. However, because of the equilibrium, some of the newly created HA turns back into the conjugate base. This causes the hydrogen ion ([latex]\text{H}^+[/latex]) concentration to increase by less than the amount expected for the quantity of strong acid added. Similarly, if a strong base is added to the mixture, the hydrogen ion concentration decreases by less than the amount expected for the quantity of base added. This is because some of the newly created HA turns back into the conjugate base due to the equilibrium.
Buffer solutions are necessary in a wide range of applications. In biology, they are necessary for keeping the correct pH for proteins to work; if the pH moves outside of a narrow range, the proteins stop working and ca