Heating is an essential requirement in laboratories for carrying out common operations such as:
- Purification of solvents by distillation
- Extractions using rotary evaporators
- Studies on kinetics of chemical reactions
- Determination of physical parameters such as boiling point, melting point, flashpoint, etc
- Sample digestions
- Drying of cleaned glassware for subsequent use
- Drying of precipitates or determinations of loss on drying
The common heating devices used in labs are bunsen burners, hot air ovens, hot plates, heating mantles, muffle furnaces, hot oil baths and microwave digestion systems. The selection of a device depends on the application in hand. Heating exposes you to burns and boils resulting from contact with hot surfaces, boiling liquids, vapours or flames. Contact hazards generally affect the user only but fires and explosions can cause injuries to others in addition to widespread damage. The risks posed by the common heating devices and precautions to be taken in their handling are discussed briefly in the article.
A Bunsen burner is conventional and convenient means for heating of liquids in test tubes and small beakers. However, a naked flame can lead to fires and explosions if such heating is carried out in close proximity of flammable liquids or explosive materials. Most laboratories do not permit open flame heating due to risks involved so bunsen burners are being edged out by other heating devices.
Hot plates are the next preferred low priced option available after the Bunsen burner. You are advised to examine the hot plates before use. First of all see if the current supply is off or not. Take care as the plate could be hot even if the current supply was switched off recently only. The top plate should be free of spills, cracks or any other damage and the electrical supply cord should not be close to the heating plate. Special care needs to be taken when using old hot plates due to sparks which can result from worn out on-off switches or bimetallic thermostats. Hot plates should be used away from volatile flammable materials
Heating mantles are commonly used for heating round bottom flask for distillations or solvent extractions in rotary evaporation procedures. Before use inspect for any damage to the insulation material and for spillage of water or any other solvent on the mantle. In such case it should be put to use only after proper cleaning and inspection. During operation do not exceed the input voltage as it could lead to damage due to overheating
Hot Air Ovens
Hot air ovens are used for determinations involving loss on drying studies and for drying of cleaned laboratory glassware. A laboratory oven should never be used for warming food items as it can pose serious contamination hazards. Always keep volumetric flasks un-stoppered as otherwise there can be breakages due to expansion of hot air inside. Make use of heat resistant gloves for removal of glassware kept for drying. Take care not to heat volatile organic liquids as they may lead to formation of volatile or toxic vapours inside the oven.
Hot baths provide a heat source that can be maintained at a stable temperature. Sand and salt baths are in use but for most applications oil baths are popular. For heating up to \(200^0 C\) paraffin oil is used but for higher temperatures up to \(300^0 C\) silicone oil needs to be used. The following precautions are necessary for use of oil baths:
- Prevent spillages of volatile hot material into the bath. This can lead to splatter of hot material which can result in burn injuries to those present in the lab
- Do not overfill the oil in the bath. This can result in spillovers due to expansion on heating
- Avoid overheating as it can lead to smoke formation and possibility of oil ignition. Regularly monitor oil temperature. Electrical circuit breaker should be provided to cut off the current supplied to avoid overheating
- Bath should have oil stirring arrangement for uniformity of temperature and to prevent formation of local hotspots on the heating element
- Handle the bath when hot with heat proof gloves
- The bath must be mounted on a stable support and should be removable easily in case of emergencies
Muffle furnaces are capable of reaching temperatures upto \(1800^0 C\) and should be housed in separate rooms away from inflammable or combustible materials. Ordinary glassware should never be put inside and heat resistant crucibles should be removed with suitable tongs and handled wearing heatproof mittens
Microwave ovens offer a faster, safe and convenient means of carrying out time-consuming digestions. The design features are different from household cooking microwave ovens. They have built-in safety features such as prevention of overshoot of temperatures and pressures. However, use of laboratory microwave ovens requires several safety precautions:
- Do not use laboratory microwave ovens for heating or cooking food items to avoid possibilities of contamination
- No metallic containers or objects can be left inside as they can create arcs which can lead to combustion of flammable materials
- Do not exceed recommended temperatures as these can result in rupture of sealed tubes due to high pressure buildup
- Remember to remove metal screw caps, if any, before putting tubes inside
It is essential to know the nature of heating requirements and nature of samples before deciding on applicable heating devices . Always adopt the recommended safety device and observe precautions to prevent accidents.