水的硬度Hard water

暂时硬度源于石灰岩溶解：CaCO₃ + CO₂ + H₂O ⇌ Ca(HCO₃)₂，加热时平衡左移析出水垢。硬度常换算成 CaCO₃ 的质量浓度表示（mg/L）：小于 60 为软水，60–120 为中等硬水，120–180 为硬水，大于 180 为极硬水；也用德国度（1 °dH = 10 mg/L CaO）。中国饮用水标准规定总硬度不超过 450 mg/L（以 CaCO₃ 计）。

Temporary hardness comes from dissolving limestone, CaCO₃ + CO₂ + H₂O ⇌ Ca(HCO₃)₂; heating shifts the equilibrium left and deposits scale. Hardness is usually expressed as mg/L of CaCO₃: below 60 is soft, 60–120 moderately hard, 120–180 hard and above 180 very hard; German degrees (1 °dH = 10 mg/L CaO) are also used. China's drinking-water standard limits total hardness to 450 mg/L as CaCO₃.

常用 EDTA 络合滴定法：在 pH 10 的氨-氯化铵缓冲液中，以铬黑 T 为指示剂，用 EDTA 标准溶液滴定，溶液由酒红色变为纯蓝色即为终点，测得钙镁总量；在 pH 12–13 下以钙指示剂单独测定钙。这一方法由施瓦岑巴赫在 1940 年代发展。

The usual method is EDTA complexometric titration: at pH 10 in an ammonia–ammonium chloride buffer with Eriochrome Black T, titrate with standard EDTA until the wine-red colour turns pure blue, giving total Ca + Mg; calcium alone is titrated at pH 12–13 with calcon indicator. Schwarzenbach developed the method in the 1940s.

硬水使锅炉和管道结垢、降低传热效率甚至引起爆管，并使肥皂生成不溶的钙镁皂而浪费、去污力下降。软化方法有煮沸、石灰-纯碱法、离子交换（用钠型阳离子交换树脂把 Ca²⁺、Mg²⁺ 换成 Na⁺）和反渗透。适量的钙镁对人体无害，硬水并不等于不卫生。

Hard water scales boilers and pipes, reducing heat transfer and even causing tube failure, and it wastes soap by forming insoluble calcium and magnesium soaps that lower cleaning power. Softening methods include boiling, lime–soda treatment, ion exchange (sodium-form cation resin swapping Ca²⁺ and Mg²⁺ for Na⁺) and reverse osmosis. Moderate calcium and magnesium are harmless to health; hard water is not unhygienic.