16.7 Buffers
Learning Objectives
By the end of this section, you will be able to:
- Define buffer and describe how it reacts with an acid or a base.
Weak acids are relatively common, even in the foods we eat. But we occasionally come across a strong acid or base, such as stomach acid, that has a strongly acidic pH of 1–2. By definition, strong acids and bases can produce a relatively large amount of hydrogen or hydroxide ions and, as a consequence, have marked chemical activity. In addition, very small amounts of strong acids and bases can change the pH of a solution very quickly. If 1 mL of stomach acid [which we will approximate as 0.05 M HCl(aq)] is added to the bloodstream, and if no correcting mechanism is present, the pH of the blood would go from about 7.4 to about 4.9 — a pH that is not conducive to life. Fortunately, the body has a mechanism for minimizing such dramatic pH changes. This mechanism involves a buffer, a solution that resists dramatic changes in pH.
Watch Buffers, the Acid Rain Slayer: Crash Course Chemistry #31 (11min 40s).
Video Source: Crash Course. (2013, September 16). Buffers, the acid rain slayer: Crash course Chemistry #31 [Video]. YouTube.
Buffers resist dramatic changes in pH by being composed of certain pairs of solutes: either a weak acid plus a salt derived from that weak acid, or a weak base plus a salt of that weak base. For example, a buffer can be composed of dissolved acetic acid (HC2H3O2, a weak acid) and sodium acetate (NaC2H3O2, a salt derived from that acid). Another example of a buffer is a solution containing ammonia (NH3, a weak base) and ammonium chloride (NH4Cl, a salt derived from that base).
Characteristics of a Good Buffer
Good buffering systems have the following characteristics:
- The solution contains a weak acid and its conjugate base OR a weak base and its conjugate acid
- The buffer resists changes in pH by reacting with added acid or base, so these ions do not accumulate.
- Any added acid reacts with the conjugate base to resist pH changes
- Any added base reacts with the conjugate acid to resist pH changes
Buffers cannot be made from a strong acid (or strong base) and its conjugate since these solutions ionize completely in water. Also take note, water is not a buffer.
Source: “Characteristics of a Good Buffer” by Jackie MacDonald, CC BY-NC-4.0
How Buffers Work
Let’s consider an acetic acid – sodium acetate buffer to demonstrate how buffers work. If a strong base — a source of OH−(aq) ions — is added to the buffer solution, those hydroxide ions will react with the acetic acid in an acid-base reaction:
HC2H3O2(aq) + OH−(aq) → H2O(l) + C2H3O2–(aq)
Rather than changing the pH dramatically by making the solution basic, the added hydroxide ions react to make water, and the pH does not change much.
If a strong acid—a source of H+ ions—is added to the buffer solution, the H+ ions will react with the anion from the salt. Because HC2H3O2 is a weak acid, it is not ionized much. This means that if