A desiccator is a sealable laboratory container designed to maintain an internal atmosphere of low humidity. Its primary function is to protect moisture-sensitive materials, known as hygroscopic substances, from absorbing water vapor from the surrounding air. The apparatus is also used to cool hot samples, such as crucibles removed from a muffle furnace, preventing them from reabsorbing moisture during the cooling process. This dry, controlled microenvironment ensures sample integrity for accurate weighing or long-term storage.
Selecting and Preparing the Desiccant Agent
The internal dryness is achieved by placing a desiccant, a hygroscopic substance, in the lower chamber beneath a perforated plate. Common choices include silica gel, anhydrous calcium chloride, calcium sulfate (Drierite), and molecular sieves. Silica gel is an amorphous form of silicon dioxide that holds water molecules via adsorption within its porous structure.
To confirm effectiveness, many laboratories use indicator varieties, such as silica gel beads that change color when saturated. These indicators signal that the desiccant requires regeneration or replacement. Before use, the desiccant must be fresh or fully regenerated by heating it in an oven to drive off previously adsorbed water. The prepared desiccant is poured into the lower section, ensuring it remains below the level of the internal sample plate.
Proper Sealing and Sample Loading Procedures
Maintaining the low-humidity environment requires an airtight seal between the lid and the base. This seal is established on the ground glass flange, or rim, by applying a thin, uniform layer of vacuum grease or silicone lubricant. The grease fills microscopic gaps in the glass surface, preventing atmospheric moisture from leaking into the container. Applying too much grease can cause contamination, while too little will compromise the seal.
Samples, often placed in small dishes or weighing bottles, are positioned onto the perforated plate so that they do not contact the desiccant below. When placing a hot sample inside, the lid should be placed slightly ajar for approximately 30 to 60 seconds before being fully seated. This brief period allows the initial rush of warm, expanding air to escape, preventing a partial vacuum that would make the lid extremely difficult to slide off later.
To close the desiccator, the lid must be gently slid horizontally across the base flange, never dropped vertically, which could damage the glass surfaces. For a vacuum desiccator, the stopcock is connected to a vacuum source and opened slowly to evacuate the air, speeding up the drying process. Once the vacuum is established, the stopcock is closed and the source is disconnected. When opening, the stopcock must be opened slowly to gradually vent the pressure before the lid is slid off, as rapid venting can scatter or contaminate samples.
Safety and Maintenance Considerations
Routine maintenance is necessary to ensure the desiccator remains effective. The ground glass flange should be periodically cleaned to remove old grease and then re-lubricated with a fresh, thin layer to maintain the airtight seal. Any visible cracks or chips, especially in glass models, necessitate immediate removal from service due to the risk of implosion, particularly if the apparatus is used under vacuum.
The desiccant must be monitored and regenerated when saturated to maintain peak performance. Regeneration involves heating common desiccants like silica gel in a laboratory oven, often between 135°C and 145°C, to desorb the captured water molecules. For long-term storage, keep the lid slightly offset from the base to prevent the greased surfaces from sticking. Proper handling, which includes carrying the desiccator with both hands supporting the base, prevents damage to the fragile glass components.