干燥剂Desiccant

干燥剂不能与被干燥物反应或溶解于其中：酸性干燥剂（浓硫酸、P₂O₅）不能干燥碱性物质（NH₃、胺）；碱性干燥剂（CaO、NaOH、KOH）不能干燥酸性物质（HCl、CO₂、羧酸）；无水 CaCl₂ 会与醇、胺、酚形成配合物，不能用于干燥这些物质。还要考虑吸水容量（能吸多少水）和干燥效能（干燥后残留水的多少）。

A desiccant must not react with or dissolve in what it dries: acidic desiccants (concentrated H₂SO₄, P₂O₅) cannot dry bases such as NH₃ and amines; basic ones (CaO, NaOH, KOH) cannot dry acidic substances such as HCl, CO₂ or carboxylic acids; and anhydrous CaCl₂ complexes alcohols, amines and phenols, so it cannot dry them. Consider both capacity (how much water it takes up) and efficiency (how little water it leaves).

无水 MgSO₄：中性、快速、容量大，适用面广；无水 Na₂SO₄：中性、温和但较慢；无水 CaCl₂：便宜，适于烃和卤代烃；3Å/4Å 分子筛：效能极高，用于溶剂深度干燥；变色硅胶：含 CoCl₂ 指示剂，吸水后由蓝变粉红，用于干燥器；P₂O₅：效能最强之一；金属钠和二苯甲酮：回流干燥醚类溶剂并以蓝紫色指示无水无氧；CaH₂：干燥 DMF、二氯甲烷、胺。

Anhydrous MgSO₄ is neutral, fast and high-capacity, suitable for most uses; anhydrous Na₂SO₄ is neutral and mild but slower; anhydrous CaCl₂ is cheap and suits hydrocarbons and alkyl halides; 3 Å and 4 Å molecular sieves are extremely efficient for drying solvents thoroughly; indicating silica gel contains CoCl₂ and turns from blue to pink when wet, for desiccators; P₂O₅ is among the most powerful; sodium with benzophenone dries ethers at reflux, a deep blue-violet colour showing they are dry and oxygen-free; CaH₂ dries DMF, dichloromethane and amines.

食品和药品包装中的小袋干燥剂多为硅胶或生石灰（CaO + H₂O → Ca(OH)₂，遇水放热，不能食用）；电子产品和精密仪器用分子筛防潮；工业上用三甘醇脱除天然气中的水分。

Sachets in food and medicine packs usually contain silica gel or quicklime (CaO + H₂O → Ca(OH)₂, which heats up with water and must not be eaten); molecular sieves protect electronics and instruments from moisture; and industry dries natural gas with triethylene glycol.