緩衝液

A buffer solution contains a weak acid and its conjugate base, or a weak base and its conjugate acid. Buffers resist pH changes when small amounts of strong acid or base are added, which is critical for chemical and biochemical processes. For example, human blood maintains pH near 7.4 using a carbonic acid-bicarbonate buffer system.

When acid is added, the conjugate base component neutralizes hydronium ions. When base is added, the weak acid component neutralizes hydroxide ions. This conversion of strong acids and bases into weak conjugate pairs causes minimal pH change. The buffer works as long as the conjugate acid-base pair concentration exceeds the added strong acid or base.

Conjugate acid-base pairs, such as acetic acid and acetate, do not react with each other because they are in equilibrium. However, when a weak acid like acetic acid is mixed with a weak base like ammonia, they will react to form a salt. This distinction is fundamental to buffer formation and stability.

In an unbuffered solution, adding strong acid causes a sharp pH drop due to increased hydronium ion concentration. In a buffered solution, the acetate component reacts with the added acid, producing acetic acid and chloride ions, resulting in minimal pH change. This demonstrates the protective effect of buffers.

Common buffers include acetic acid with sodium acetate, and ammonia with ammonium chloride. The acetic acid-acetate buffer consists of a weak acid and its salt, while the ammonia-ammonium chloride buffer consists of a weak base and its salt. Both effectively resist pH changes in their respective pH ranges.

A buffer can prevent drastic pH changes only when the concentration of the conjugate acid-base pair is higher than the concentration of added strong acid or base. Once this buffering capacity is exceeded, the buffer loses its protective effect and pH changes become significant.

When strong base like sodium hydroxide is added to a buffer, the weak acid component reacts with it, producing sodium acetate and water. This reaction shifts the acid ionization equilibrium to the right, restoring hydronium ion concentration and maintaining relatively stable pH despite the base addition.