络合滴定Complexometric titration

EDTA（H₄Y）是六齿配体，与绝大多数金属离子以 1:1 形成稳定的螯合物，计量关系简单。配合物的稳定性受酸度影响：H⁺ 与 EDTA 结合降低其有效浓度（酸效应），用条件稳定常数 K′ = K/α 描述。准确滴定的条件一般为 lg(c·K′) ≥ 6，因此每种金属离子都有允许滴定的最低 pH（林邦曲线），如 Ca²⁺ 约 pH 7.5，Fe³⁺ 约 pH 1.5。通过控制 pH 和加入掩蔽剂（如三乙醇胺掩蔽 Fe³⁺、Al³⁺，KCN 掩蔽 Cu²⁺、Zn²⁺）可在混合离子中选择性滴定。

EDTA (H₄Y) is a hexadentate ligand that forms stable 1:1 chelates with almost all metal ions, giving simple stoichiometry. Stability depends on acidity, since H⁺ binds EDTA and lowers its effective concentration (the acid effect), described by the conditional constant K′ = K/α. Accurate titration usually requires lg(c·K′) ≥ 6, so each metal has a minimum pH for titration (the Ringbom curve), about 7.5 for Ca²⁺ and 1.5 for Fe³⁺. Controlling pH and adding masking agents, such as triethanolamine for Fe³⁺ and Al³⁺ or KCN for Cu²⁺ and Zn²⁺, allows selective titration in mixtures.

金属指示剂本身是有色配体，与金属离子形成颜色不同且稳定性比 M-EDTA 低的配合物。滴定前溶液呈 M-In 的颜色，终点时 EDTA 夺取金属离子，释放出游离指示剂而变色。常用铬黑 T（pH 10 测 Ca+Mg 总量，酒红→纯蓝）、钙指示剂（pH 12–13 测 Ca）、二甲酚橙（酸性条件测 Zn、Pb、Bi）和 PAN。若金属与指示剂结合太牢，终点不变色，称为指示剂封闭。

A metal indicator is a coloured ligand that forms a differently coloured complex with the metal, less stable than M–EDTA. Before the end point the solution shows the colour of M–In; at the end point EDTA takes the metal and releases free indicator, changing the colour. Common indicators are Eriochrome Black T (Ca + Mg at pH 10, wine red to pure blue), calcon (Ca at pH 12–13), xylenol orange (Zn, Pb and Bi in acid) and PAN. If the metal binds the indicator too strongly the colour never changes, called blocking.

1945 年瑞士化学家格罗尔德·施瓦岑巴赫系统研究了 EDTA 等氨羧络合剂与金属离子的配位反应，建立了络合滴定法，迅速取代了许多繁琐的重量法。广泛用于水的硬度测定、水泥和矿石中钙镁铝铁分析、合金成分分析、电镀液控制和血清钙测定。

In 1945 the Swiss chemist Gerold Schwarzenbach systematically studied complex formation between metal ions and aminopolycarboxylic acids such as EDTA and established complexometric titration, which quickly replaced many laborious gravimetric methods. It is widely used for water hardness, calcium, magnesium, aluminium and iron in cement and ores, alloy analysis, plating-bath control and serum calcium.