A laboratory desiccator is a round shaped closed vessel made of heavy glass which is a common laboratory glassware item and has multiple uses, such as:
- Storage of standards under dry environment
- Storage of materials for weighing to constant weight
- Prolonged storage of hygroscopic materials
- Determination of loss on drying of manufactured products such as pharmaceuticals
The base of the desiccator supports a perforated plate below which the desiccant drying agent is kept.Commonly used drying agents are anhydrous calcium chloride, drierite (calcium sulphate) and silica. Such desiccants can be regenerated by heating. A small amount of cobalt indicator compound is generally added which turns pink from blue on absorption of moisture. This lets you know when to regenerate the desiccant.
The desiccator is effective only when sealed to prevent contact of contents with outside air. Some desiccators are provided with vacuum fittings for evacuation of inside air for long-term storage of moisture sensitive materials.
The desiccator is often taken for granted and precautions in its handling get often ignored. However, bear in mind that accidents can occur if sufficient care is not exercised in handling and use of desiccators.
The cover of the desiccator is usually sealed to its body with a thin layer of silicone grease. This forms an airtight seal and requires precaution in opening the cover and putting it back.
- Slide cover horizontally with a gentle force until it starts moving. Grasp the knob with one hand and with the other hold the base and apply more force to take it off completely
- Do not get tempted to lift the desiccator by lifting the top knob. Always lift with both hands holding the bottom and the cover.
- To close place lid gently on top and close slowly by applying a mild force till it closes. Rotate lid gently in both directions to ensure an airtight seal.
- Ensure that the rim does not run out of grease but at same time do not use an excessive amount of grease. A thin uniform film is usually sufficient.
- Keep the lid upside down on removal but away from the workbench edge to prevent rollover
- Place a hot substance carefully inside after removal from the hot air oven. Keep the crucible with the lift ajar for a couple of minutes and then close the lid by sliding it to expel hot air so that the lid does not get blown out due to expansion of hot air and also prevent vacuum formation later which can make opening difficult.
Opening a vacuum sealed desiccator
When applying a vacuum take care not to exceed the prescribed limit as it can risk an implosion. Gradually open stop- cock on top to release the vacuum inside at time of opening. Try sliding the lid. If there is difficulty in sliding make use of a thin spatula carefully to create a rift between the lid and the body. On hearing a hissing sound of vacuum release slide the top cover by holding the bottom firmly with the other hand.
A desiccator can be used routinely for storage of moisture sensitive compounds, standards and for applications requiring weighing to constant weight. However, storage of materials that are highly reactive with water such as sodium metal is not recommended as on coming in contact with humid air on opening it can lead to violent reaction.