16.2
缓冲液
包含大量弱共轭酸碱对的溶液称为缓冲溶液或缓冲液。 添加少量强酸或强碱时，缓冲溶液可抵抗 pH 值的变化。 乙酸和乙酸钠的溶液是由弱酸及其盐组成的缓冲液的一个示例： CH3COOH (AQ) + CH3COONA (AQ)。 由弱碱及其盐组成的缓冲液的一个示例是氨和氯化铵的溶液： NH3 (AQ) +…
向溶液中加入少量的酸或碱，可导致 pH 值显著降低或升高。然而，许多化学和生物化学过程 需要一个稳定的 pH 值才能发挥作用。缓冲液可以防止溶液的 pH 值发生剧烈变化，在没有超过它们缓冲能力的情况下。缓冲液含有弱酸及其共轭碱 或弱碱及其共轭酸。例如，人类血液将其 pH 值保持在 7.4 附近，通过一种缓冲液，由碳酸（一种弱酸）和碳酸氢根离子（它的共轭碱）组成。共轭酸碱对会形成缓冲液，是因为它们不会中和它们的共轭酸或碱。例如，醋酸和醋酸盐不能反应。然而，如果把醋酸（一种弱酸）和氨（一种弱碱）加在一起，它们就会反应生成一种盐—醋酸铵。在缓冲液中，弱酸通过与产生的氢氧化物离子 反应来中和任何添加的碱，而其共轭碱通过与任何水合氢离子 反应来中和任何添加的酸。类似的机制也适用于弱碱 及其共轭酸的情况下。两个烧杯，X 和 Y，含有相同体积的 不同溶液，每个溶液的 pH 值为 7.2。烧杯 X 中的溶液没有进行缓冲。相反，烧杯 Y 中的溶液 含有乙酸-醋酸盐缓冲液。如果向烧杯 X 中添加盐酸，溶液的 pH 值会由于 水合氢离子浓度的增加 而突然下降。相反，烧杯 Y 中的溶液 烧杯 Y 中的溶液显示出几乎恒定的 pH 值，因为其中一种缓冲组分，醋酸盐，与盐酸反应，生成 氯离子和醋酸。同样地，如果向烧杯 X 中添加氢氧化钠，溶液的 pH 值会由于 氢氧化物离子浓度的增加 而突然升高。另一方面，当向烧杯 Y 中添加 氢氧化钠时，烧杯 Y 中的溶液的 pH 值 变化很小，因为其中一种缓冲组分，醋酸，与氢氧化钠反应，生成乙酸钠 和水分子。只要共轭酸碱对在溶液中的浓度高于 加入的强酸或强碱浓度，缓冲液就可以 防止溶液的 pH 值发生剧烈变化。
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Q1: What is a buffer solution and why is it important?
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.
Q2: How does a buffer prevent drastic pH changes?
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.
Q3: Why don't conjugate acid-base pairs neutralize each other?
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.
Q4: What happens when you add strong acid to a buffered versus unbuffered solution?
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.
Q5: What are common examples of buffer solutions?
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.
Q6: What limits a buffer's ability to resist pH change?
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.
Q7: How does adding strong base to a buffer solution work?
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.