Introduction
A laboratory desiccator is a sealed vessel that keeps its contents in a dry, low-humidity atmosphere. A drying agent sits in the base, a perforated plate supports your sample above it, and the airtight lid shuts out moisture, so hygroscopic reagents, dried solids and cooled samples stay dry.
In South African laboratories the desiccator is a familiar workhorse, from minerals and assay work to environmental and water testing, SANAS-accredited quality control and university chemistry. It stores moisture-sensitive materials, cools crucibles and samples after ignition or drying without letting them pick up water, and protects standards and reagents. It is central to accurate gravimetric work, where even a little reabsorbed moisture changes a weight.
This guide sets out what a desiccator is, what it is used for, how it works, the main types, the desiccants that go inside, and how to choose and use one.
What is a desiccator?
A desiccator is a thick-walled vessel, most often borosilicate glass or polycarbonate, closed by a lid that seals onto a ground flange. A removable perforated plate divides it: the drying agent, or desiccant, sits underneath, and your sample rests on the plate above, clear of the desiccant.
The sealed air inside is dried by the desiccant, which absorbs water vapour and holds the humidity low. A vacuum desiccator adds a stopcock on the lid so the chamber can be evacuated with a vacuum pump for faster, deeper drying. Tap a part below to see how the pieces fit.
Tap a part to learn what it does.
What a desiccator is used for
The common thread is keeping things dry in a controlled, sealed space. Typical uses in South African labs include:
- Storing hygroscopic reagents and chemicals
- Cooling crucibles and samples after ignition or drying
- Protecting moisture-sensitive standards and samples
- Slow drying of solids at room temperature
- Sample preparation ahead of accurate weighing
- Keeping electronics, optics or wafers dry in storage
In gravimetric analysis the desiccator is essential: a heated sample is cooled inside it so it does not reabsorb moisture before weighing, which would throw off the result.
How it works
A desiccator works by sealing a small volume of air and letting a desiccant strip the moisture from it. Step through the setup and watch the humidity fall.
STEP 1 OF 6
Types of desiccator
A few styles handle most needs. Tap one to see where it fits.
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Desiccants compared
The drying agent in the base does the real work. Switch between the common options.
Bars are general guidance for comparison, not measured performance figures.
Find the right setup
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How to use a desiccator
Setting one up and running it well comes down to six steps:
- Add fresh desiccant. Put a dry drying agent in the base. Indicating silica gel shows its colour when it still has capacity and when it is spent.
- Fit the plate. Set the perforated plate on top of the desiccant bed so air can circulate underneath.
- Place your sample. Rest the sample or dish on the plate, clear of the desiccant, so nothing sits in the drying agent.
- Seal the lid. Lightly grease the ground flange, or check the O-ring, and slide the lid on with a twisting motion for an airtight seal.
- Evacuate if vacuum type. On a vacuum desiccator, connect a vacuum pump through the stopcock, draw the vacuum, then close the valve.
- Open with care. Release any vacuum slowly, then slide the lid sideways rather than lifting it straight up.
Common mistakes to avoid
A few habits account for most lost seals and spoiled samples. Watch out for:
- Using spent desiccant, so the chamber never really dries
- Forgetting to grease or seat the lid, breaking the seal
- Lifting the lid instead of sliding it, which can chip the flange
- Putting very hot items straight in, risking thermal shock or condensation
- Releasing a vacuum too quickly and scattering the sample
- Letting the sample sit directly in the desiccant bed
Each one either loses the dry atmosphere or spoils the sample, so build them into your routine.
Standard vs vacuum desiccator
The two look similar but get to dryness differently. Compare them below.
Frequently asked questions
What is a laboratory desiccator used for?
To keep contents in a dry, sealed atmosphere. It is most often used to store hygroscopic reagents and moisture-sensitive samples, and to cool heated crucibles and samples after drying or ignition without reabsorbing water.
How does a desiccator work?
A desiccant in the base takes up water vapour from the sealed air, lowering the humidity inside. Your sample rests on a perforated plate above the desiccant and stays dry. A vacuum desiccator also lowers the pressure to dry faster.
What desiccant should I use?
Indicating silica gel is the common all-rounder and shows when it is spent. Molecular sieves hold a lower humidity for sensitive work, Drierite is a tough reusable option, and calcium chloride is high-capacity but messy. Most can be regenerated by heating.
What is the difference between a standard and vacuum desiccator?
A standard desiccator holds a dry atmosphere at ambient pressure using the desiccant alone. A vacuum desiccator adds a stopcock so the chamber can be evacuated with a vacuum pump, reaching lower humidity and drying faster.
How do I regenerate indicating silica gel?
Heat it in an oven until the indicator returns to its dry colour, then let it cool in a sealed container before use. This drives off the absorbed moisture so the gel can be used again.
Why cool samples in a desiccator?
A hot sample cooling in open air picks up moisture, which adds weight and skews gravimetric results. Cooling it in a desiccator keeps it dry so the weight you record is accurate.
How do I open and close a desiccator properly?
Slide the lid sideways across the greased flange rather than lifting it straight up, which keeps the seal and protects the ground glass. On a vacuum desiccator, release the vacuum slowly through the stopcock first.
Final thoughts
The desiccator is a simple vessel doing an important job: holding a dry, stable atmosphere so samples, reagents and standards keep their integrity. Understanding how the desiccant and seal work together helps you choose the right type, pick a suitable drying agent, and get reliable, repeatable results. For any South African lab handling moisture-sensitive work, it remains a bench essential.
Keep it dry
Browse glass, plastic and cabinet desiccators, plus desiccants, at LabFriend ZA.
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