Inorganic elements and compounds as drying agents

   It is useful to distinguish between different classes of drying agent as being reagents that react with water either reversibly or irreversibly; the former can be regenerated, usually by heating, while the latter (sometimes classed as dehydrating agents) cannot. Caution is always needed when choosing a drying agent for the following reasons:

• the substance from which water is being removed may react with the drying agent;
• dehydrating agents often react vigorously with water and should not be used to dry very wet solvents, for which a predrying stage is appropriate;
• magnesiumperchlorate, Mg(ClO₄)₂, although an extremely efficient drying agent, is best avoided because of the risk of explosions.

   Many drying or dehydrating agents are compounds of group 1 or 2 metals; concentrated H₂SO₄, molecular sieves and silica gel are also commonly used to absorb water, while phosphorus(V) oxide is a highly effective dehydrating agent.  Agents for drying or predrying solvents Typically, anhydrous salts that absorb water as solvate are suitable for removing water from solvents. Anhydrous MgSO₄, CaCl₂, CaSO₄, Na₂SO₄ andK₂CO₃ are hygroscopic and of these, CaSO₄ and MgSO₄ are particularly efficient and inert drying agents.

  Drying agents that react irreversibly with H₂O Drying agents in this category include Ca and Mg (for alcohols), CaH₂ (for a range of solvents, but not lower alcohols or aldehydes), LiAlH₄ (for hydrocarbons and ethers) and sodium. The latter, generally extruded as wire, is extremely efficient for removing water from hydrocarbons or ethers, but reacts with, for example, alcohols, and is not suitable for drying halogenated solvents.

    Drying agents for use in desiccators and drying tubes Suitable agents for drying samples in desiccators are anhydrous CaCl₂, CaSO₄, KOH and P₂O₅. Gases may be dried by passage through drying tubes packed with a suitable agent, but possible reaction of the gas with the drying agent must be considered. Although P₂O₅ is a common choice for use in desiccators, reaction with water results in the formation of a brown, viscous layer on the surface of the anhydrous powder, thereby curtailing its dehydrating ability.

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