双语化学辞典Chem Lexicon
反应 · 普通化学 · 溶液反应 General chemistry · Solution reactions

沉淀反应Precipitation (chemistry)

拼音Pinyin chén diàn fǎn yìng　别名Also 沉淀； precipitation reaction

定义Definition

沉淀反应是溶液中的离子结合成难溶固体（沉淀）从溶液中析出的反应。当离子积超过溶度积（Q > Ksp）时，沉淀生成。

Precipitation is the formation of an insoluble solid (precipitate) from ions in solution; it occurs when the ion product exceeds the solubility product (Q > Ksp).

过程Process

过饱和 → 成核 → 晶体生长。过饱和度大时成核快，得到细小颗粒甚至胶体；陈化（奥斯特瓦尔德熟化）使小晶粒溶解、大晶粒长大，便于过滤。均相沉淀法（如尿素水解缓慢释放 OH⁻）可得到大而纯的晶体。

Supersaturation → nucleation → growth. High supersaturation gives many small particles or even colloids; digestion (Ostwald ripening) dissolves small crystals in favour of large ones, easing filtration. Homogeneous precipitation (e.g. slow OH⁻ release from urea) gives large pure crystals.

应用Applications

离子检验（Ag⁺ + Cl⁻ → 白色 AgCl；Ba²⁺ + SO₄²⁻ → 白色 BaSO₄）；重量分析；分步沉淀分离金属离子（如用 H₂S 控制 pH）；水处理（石灰软化、明矾絮凝除磷）；纳米材料与催化剂前驱体的共沉淀制备。

Ion tests (white AgCl, white BaSO₄); gravimetric analysis; selective precipitation of metal ions (e.g. pH-controlled sulfides); water treatment (lime softening, alum flocculation, phosphate removal); co-precipitation of nanomaterials and catalyst precursors.

影响因素Influencing factors

同离子效应降低溶解度；盐效应、酸效应（弱酸根沉淀溶于酸）、配位效应（AgCl 溶于氨水）会增大溶解度。共沉淀和后沉淀会带入杂质。

The common-ion effect lowers solubility; salt, acid (weak-acid anions dissolve in acid) and complexation effects (AgCl dissolves in ammonia) raise it. Co-precipitation and post-precipitation introduce impurities.

反应Reactions

m Mⁿ+(aq) + n Xᵐ−(aq) ⟶ MₘXₙ(s)

离子积 Q > Ksp

Ion product Q > Ksp

  1. 混合使溶液过饱和。

    Mixing creates supersaturation.

  2. 成核。

    Nucleation.

  3. 晶体生长与陈化。

    Growth and ageing.

Pb2+ + 2 I− ⟶ PbI2↓黄色Pb2+ + 2 I− ⟶ PbI2↓(yellow)

“金雨”实验。

The ‘golden rain’ demonstration.

年表Timeline

  • 1889

    能斯特提出溶度积概念

    Nernst introduces the solubility product

  • 1900

    奥斯特瓦尔德描述熟化现象

    Ostwald describes ripening

相关公式Formulas

溶度积 Solubility product→
\[\mathrm M_m\mathrm X_n(s) \rightleftharpoons m\mathrm M^{n+} + n\mathrm X^{m-}:\quad K_{sp} = [\mathrm M^{n+}]^m[\mathrm X^{m-}]^n,\qquad s = \left(\frac{K_{sp}}{m^mn^n}\right)^{1/(m+n)}\]
例题Worked example

AgCl：Ksp = 1.8×10⁻¹⁰，s = 1.3×10⁻⁵ mol/L；Ag₂CrO₄：Ksp = 1.1×10⁻¹²，s = (Ksp/4)^{1/3} = 6.5×10⁻⁵ mol/L——Ksp 更小，溶解度反而更大。

AgCl s = 1.3×10⁻⁵ M; Ag₂CrO₄ (smaller Ksp) s = 6.5×10⁻⁵ M — more soluble.

重量分析换算因数 Gravimetric factor→
\[w_\text{待测} = \frac{m_\text{称量形} \times F}{m_\text{试样}}\times 100\%,\qquad F = \frac{a\,M_\text{待测}}{b\,M_\text{称量形}}\]\[w_\text{analyte} = \frac{m_\text{weighed form} \times F}{m_\text{sample}}\times 100\%,\qquad F = \frac{a\,M_\text{analyte}}{b\,M_\text{weighed form}}\]
例题Worked example

由 Fe₂O₃ 求 Fe₃O₄：F = 2M(Fe₃O₄)/(3M(Fe₂O₃)) = 2×231.5/(3×159.7) = 0.9666。

Fe₃O₄ from Fe₂O₃: F = 0.9666.