定义Definition
弱酸是在水中只部分电离的酸，溶液中存在电离平衡 HA ⇌ H⁺ + A⁻，大部分仍以分子形式存在。酸的强弱用酸解离常数 Ka（或 pKa）衡量，如乙酸 Ka = 1.74 × 10⁻⁵ mol dm⁻³（pKa 4.76）。
A weak acid ionises only partly in water: HA ⇌ H⁺ + A⁻, with most of it remaining as molecules. Acid strength is measured by the acid dissociation constant Ka (or pKa); ethanoic acid has Ka = 1.74 × 10⁻⁵ mol dm⁻³ (pKa 4.76).
pH 计算Calculating pH
设电离很少，[H⁺] ≈ √(Ka × c)。例：0.100 mol dm⁻³ 乙酸，[H⁺] = √(1.74 × 10⁻⁵ × 0.100) = 1.32 × 10⁻³ mol dm⁻³，pH = 2.88，而同浓度盐酸 pH = 1.00。常见弱酸包括羧酸、碳酸、氢氟酸、次氯酸、H₂S 和 NH₄⁺。
Assuming little ionisation, [H⁺] ≈ √(Ka × c). For 0.100 mol dm⁻³ ethanoic acid, [H⁺] = √(1.74 × 10⁻⁵ × 0.100) = 1.32 × 10⁻³ mol dm⁻³, pH 2.88, whereas hydrochloric acid of the same concentration has pH 1.00. Common weak acids include carboxylic acids, carbonic acid, hydrofluoric acid, chloric(I) acid, H₂S and NH₄⁺.
与强酸的区别Distinguishing from strong acids
同浓度下弱酸的 pH 较高、导电性较弱，与镁或碳酸盐反应较慢；但中和等量碱所需的体积相同，因为平衡会随着 H⁺ 被消耗而不断右移。弱酸与其盐组成的溶液是缓冲溶液。
At the same concentration a weak acid has a higher pH, conducts less well and reacts more slowly with magnesium or carbonates, but it needs the same volume of alkali to neutralise, because the equilibrium keeps shifting right as H⁺ is used up. A weak acid with its salt forms a buffer.
年表Timeline
- 1887
阿伦尼乌斯提出电解质部分电离的观点
Arrhenius proposes that electrolytes ionise partly
- 1888
奥斯特瓦尔德提出稀释定律，定量描述弱电解质电离
Ostwald states the dilution law for weak electrolytes
相关公式Formulas
例题Worked example
0.10 mol/L 乙酸（Ka = 1.8×10⁻⁵）：[H⁺] = 1.34×10⁻³ mol/L，pH = 2.87，电离度 1.3%。
0.10 M acetic acid: pH = 2.87, 1.3 % ionised.