Desiccants are useful for keeping small closed spaces dry, such as tool storage containers. Desiccants absorb water vapour, and then need drying out to work again, usually with heat. Some desiccants change colour when saturated.
- Normally contains a wet/dry indicator
- Old silica gel uses pink/blue indicator colours
- New silica gel uses brown/yellow-brown indicator colours
- Non-indicator silica gel is occasionally seen.
- Easy to dry for re-use
- Hydrated lime (bagged lime) is safe and absorbent
- Quicklime (anhydrous lime) absorbs more water, but is much more reactive, and can be a fire risk if it gets wet
- Reactivation not an option (requires over 1000C)
- No indicator
- No guarantee that bought product will absorb, though it normally does
- Very cheap
Also known as table salt, this is sometimes used as a cheap and safe desiccant due to its hygroscopic properties. More effective desiccants are available, but few are safe for humans to ingest, making this a good choice where infant access is likely.
Plaster of Paris
- Also known as Gypsum, Anhydrous Calcium Sulphate, & 'Drierite' (a US trade name)
- Anhydrous MgSO4 is an effective desiccant
- Epsom salts is hydrous MgSO4
- aka Montmorillonite clay
- A strong desiccant
- Can cause injury, handling precautions wise
- Used caustic desiccant is still good for clearing drains etc
Desiccants must be dried when saturated so they can absorb water again. There is considerable variation in what this entails.
Silica gel is dried in an oven at 120-150C. Non-indicating silica may be dried at higher temps, though there seems little reason to do so.
Sticking to 120C is wisest to minimise risk of package damage.
Lime requires over 1000C to dry it, so reactivation is not a realistic proposal.
Adding an Indicator
Its possible to add a saturation indicator to desiccants. A relatively easy to use indicator is copper chloride, CuCl2. This may be dissolved in water and the solution added to silica gel.
CuCl2 is made by adding hydrochloric acid to copper. Ensure excess copper remains after the reaction.
- Yellow brown when dry
- Green when the desiccant is saturated
Copper chloride is toxic if ingested, and should be fully cleaned off all surfaces to avoid the possibility of food contamination later. Copper is an essential nutrient, but only in miniscule amounts; excess intake causes serious problems.
- Sulphuric Acid
- Activated alumina
- Calcium hydride
- Copper(II) sulfate
- Lithium hydride
- Phosphorus pentoxide
- Potassium carbonate
- Sodium chlorate
- Sodium hydroxide
- Sodium sulfate