Desiccants and silica gel: how drying agents work and keeping vials dry

A desiccant is a material that takes up water vapour from the air around it. The small "do not eat" sachets in shoe boxes and electronics packaging are the most familiar example. In the lab, desiccants keep moisture-sensitive materials dry, including freeze-dried peptides. This guide explains how the common types work, what the colour change in indicating silica gel means, and how to use and regenerate them.

Terms are defined in the glossary.

How desiccants take up water

Desiccants work in one of two ways.

• Adsorption. Water molecules stick to a surface. Silica gel and molecular sieves work this way. The material stays dry to the touch and keeps its shape, even when full.
• Absorption or reaction. Water is taken into the material or reacts with it. Calcium chloride takes up so much water that it dissolves into a liquid, which is why household damp traps collect a pool of brine. Calcium sulfate and phosphorus pentoxide react with water chemically.

Adsorbing desiccants are the usual choice for storing samples. They don't turn liquid, and most can be dried and reused.

How much water a desiccant can hold depends on the humidity around it and the temperature. Each type has its own pattern:

• Silica gel holds the most water when the air is moderately to very humid. At low humidity it takes up less.
• Molecular sieves hold a little less water overall, but they keep working at very low humidity, where silica gel has little effect. They are the choice when a space needs to be very dry.
• Clay desiccants (bentonite) are cheap and work well at room temperature and normal humidity. They are common in shipping packaging.

Silica gel

Silica gel is made by setting a solution of sodium silicate with acid, then washing and drying the gel. The result is hard, glassy beads or granules full of tiny pores. That internal surface is very large for the weight, which is why a small sachet can hold a useful amount of water.

Silica gel is chemically inert and is not the same as crystalline silica dust. The "do not eat" warning on sachets is mainly about choking and the risk from indicating dyes in some types.

Indicating silica gel

Indicating gel contains a dye that changes colour as the gel fills with water. It tells you when to replace or dry it.

• Orange to green or colourless. The common modern type. It uses an organic indicator.
• Blue to pink. The older type, which uses cobalt(II) chloride. Cobalt chloride is classified as a carcinogen under EU and GB chemical rules, so this type has largely been replaced. Handle it with gloves and dispose of it as chemical waste.

The colour change is a guide, not a measurement. For a sealed container, a humidity indicator card or a small digital hygrometer gives a clearer reading.

Regenerating silica gel

Saturated silica gel can be dried and reused. Spread it in a thin layer on a tray and heat it in an oven, typically at around 110–120 °C for a few hours, until the indicator returns to its dry colour. Follow the maker's instructions, as some indicator dyes break down at higher temperatures. Let it cool in a sealed container, or it will start taking up water again from the room air while it cools.

Molecular sieves need much higher temperatures to regenerate, usually in the range of 200–300 °C, and often under vacuum or a flow of dry gas.

Molecular sieves

Molecular sieves are zeolites: crystalline aluminosilicates with a network of pores all the same size. The type number gives the pore size in ångströms (Å):

Because the pores are so uniform, sieves pick out water by size. That makes 3A sieves useful for drying solvents without taking up the solvent itself (Williams & Lawton, 2010).

Desiccators

A desiccator is a sealed container with desiccant in its base and a perforated plate above it for samples. Glass desiccators have a greased ground-glass rim to seal the lid. Plastic cabinet desiccators are common for storing larger numbers of items.

A vacuum desiccator has a tap so air can be pumped out. Removing the air speeds drying. Vent it slowly when opening, so the rush of air doesn't disturb fine powders.

For a fridge or freezer, a simple sealed box with a desiccant sachet does the same job on a small scale.

Using desiccants with freeze-dried samples

Freeze-dried (lyophilised) peptides are hygroscopic: they take up water from the air quickly. See hygroscopic powders for why this matters and how to handle vials. Desiccants help in a few ways:

• Storage. Keep sealed vials in a closed container with a desiccant sachet, especially in a freezer that frosts up. See how to store lyophilised peptides.
• Warming up. Let a cold vial reach room temperature inside a closed desiccator or sealed box before opening it. This stops water condensing on the cold glass and powder.
• Between uses. Return opened and resealed vials to the dry container straight away.

A few practical points:

• Keep fresh desiccant sealed until you need it. It starts taking up water from the room as soon as it's exposed.
• Don't let desiccant touch the sample. Use sachets or a separate tray, and never put loose beads in with powder.
• Check it regularly. A saturated desiccant does nothing, and if it warms up it can release water back into the container.
• Size it to the space. A large box opened often needs more desiccant, and more frequent changes, than a small one that stays shut.
• Don't rely on it alone. A desiccant can only keep up with a well-sealed container. A loose lid lets in more moisture than a sachet can take up.

Related guides

For how water content is measured on a COA, see Karl Fischer titration and water content. For how the dry cake is made, see what lyophilisation is. Our vials are shipped sealed, and each product page lists the recommended storage condition for that compound. See Quality.