沉淀滴定Precipitation titration

莫尔法：以 K₂CrO₄ 为指示剂，在中性或弱碱性（pH 6.5–10.5）溶液中用 AgNO₃ 滴定 Cl⁻，AgCl 溶解度小于 Ag₂CrO₄，先析出 AgCl，过量 Ag⁺ 与 CrO₄²⁻ 生成砖红色 Ag₂CrO₄ 指示终点；福尔哈德法：以铁铵矾为指示剂，在酸性溶液中用 NH₄SCN 滴定 Ag⁺（生成红色 [Fe(SCN)]²⁺ 指示终点），或先加过量 AgNO₃ 再返滴定测卤素离子；法扬斯法：用荧光黄等吸附指示剂，化学计量点后沉淀表面吸附过量 Ag⁺ 带正电，吸附指示剂阴离子而变色（黄绿→粉红）。

Mohr method: with K₂CrO₄ indicator in neutral or weakly alkaline solution (pH 6.5–10.5), AgNO₃ titrates Cl⁻; AgCl, less soluble than Ag₂CrO₄, precipitates first, and the first excess Ag⁺ forms brick-red Ag₂CrO₄. Volhard method: with ferric alum indicator in acid, NH₄SCN titrates Ag⁺ until red [Fe(SCN)]²⁺ appears, or excess AgNO₃ is added and back-titrated to determine halides. Fajans method: with adsorption indicators such as fluorescein, the precipitate surface, positively charged by excess Ag⁺ after the equivalence point, adsorbs the indicator anion and changes colour from yellow-green to pink.

1832 年盖-吕萨克用氯化钠滴定银，用于精确测定银币中的含银量（“混浊消失法”）。1856 年莫尔提出以铬酸钾为指示剂；1874 年福尔哈德提出硫氰酸盐返滴定法；1924 年法扬斯提出吸附指示剂。

In 1832 Gay-Lussac titrated silver with sodium chloride to assay silver coins precisely, watching for the turbidity to clear. Mohr introduced the chromate indicator in 1856, Volhard the thiocyanate back-titration in 1874 and Fajans adsorption indicators in 1924.

用于测定食盐、海水、食品和水中的氯化物，以及电镀液和感光材料中的银。莫尔法不能在酸性溶液（CrO₄²⁻ 转化为 Cr₂O₇²⁻）或氨性溶液（生成银氨配离子）中进行，也不宜测 I⁻ 和 SCN⁻（强烈吸附）。福尔哈德法测 Cl⁻ 时需防止 AgCl 转化为溶解度更小的 AgSCN，可加硝基苯覆盖沉淀或滤去 AgCl。

Precipitation titration determines chloride in salt, seawater, food and water, and silver in plating baths and photographic materials. The Mohr method cannot be used in acid, where CrO₄²⁻ becomes Cr₂O₇²⁻, or in ammonia, where silver forms ammine complexes, and suits I⁻ and SCN⁻ poorly because they adsorb strongly. In the Volhard determination of Cl⁻, AgCl must be prevented from converting to the less soluble AgSCN, by coating it with nitrobenzene or filtering it off.