BIOL 1406

PreLab 3.2

What are acids and bases?

Although several definitions for acids and bases exist, in this class we will define an acid as a proton donor, and a base as a proton acceptor. Remember, a proton is just another name for a hydrogen ion (H⁺). When an acid donates a proton to a base, the acid becomes a conjugate base (since it is now a proton acceptor) and the base becomes a conjugate acid (since it is now a proton donor):
 

General Example

Specific Example

Because acids are proton donors, they act to increase [H⁺] when dissolved in water. For example, when hydrochloric acid (HCl) is dissolved in water, it dissociates to form H⁺ and Cl^- . The release of H⁺ ions increases [H⁺] in the solution. Also, remember that in any aqueous solution as [H⁺] increases [OH^-] will decrease.

On the other hand, because bases are proton acceptors they act to decrease the [H⁺] when dissolved in water. Bases can decrease the [H⁺] in two ways:

1. They can dissociate to form OH^-. For example, sodium hydroxide (NaOH) is a strong base that dissociates to form Na+ and OH^-. As [OH^-] increases, [H⁺] decreases, because some of the H⁺ ions that were present in the water before the NaOH was added will combine with the extra OH^- to form water.
2. They can combine directly with H⁺ ions, thereby removing “free” H⁺ ions from the solution. As the number of H⁺ ions declines, more water molecules dissociate, increasing the number of OH^- ions. For example (see figure above), ammonia (NH₃) combines with H⁺ ions in water to form ammonium ions (NH₄+), thereby decreasing the concentration of free H⁺ ions.

In sum, when an acid is added to an aqueous solution, [H⁺] increases and [OH^-] decreases. When a base is added to an aqueous solution, [OH^-] increases and [H⁺] decreases.

Acids and bases can vary in the extent to which they alter [H⁺] and [OH^-]. Strong acids, such as hydrochloric acid (HCl) and sulfuric acid (H₂SO₄), almost completely dissociate in water. Therefore, they produce a relatively large increase in [H⁺] and a correspondingly large decrease in [OH^-]. On the other hand, weak acids, such as acetic acid and citric acid, have a much lower level of dissociation. Therefore, they produce a relatively small increase in [H⁺] and a correspondingly small decrease in [OH^-]. Similarly, strong bases, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH), produce a large increase in [OH^-] when dissolved in water, while weak bases such as ammonia (NH₃) and Trizma® base produce a much smaller increase in [OH^-]. Most biomolecules have properties of weak acids and/or bases.

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