Laboratory Flask - Types, Procedure, Limitations, Uses

Laboratory flask is a specialized vessel or container commonly used as glassware in scientific laboratory.

It is generally made up of temperature and chemical resistant borosilicate glass. Sometimes it is also made from different plastics depending on the experimental need.

Laboratory flasks are found in different shapes and sizes. Their volume is usually measured in milliliters or liters.

It generally has a wide main body that becomes narrow into one or more tubular necks with an opening at the top. This structure helps to introduce or remove reagents safely.

Laboratory flask is used for holding, mixing, heating, cooling, dissolving and sometimes measuring exact volume of liquids and chemical solutions during experiments.

Purpose of Laboratory flask

The following are the purpose of laboratory flask-

Laboratory flask is used for holding and storing liquids, chemical samples and solutions.
It is used for mixing and dissolving chemicals by swirling and agitating.
It is used for heating, cooling and boiling liquids during laboratory procedures.

It is used to measure precise volume of liquids, mainly in special volumetric flasks.
It is used for setting up and carrying out different chemical reactions.
It is used in separation and purification process like distillation, evaporation and filtration.

It is used for preparing analytical solutions with exact concentration.
It is also used in biological works like cultivation of microorganisms, growing cells and incubation of biological samples.

Materials used to construct Lab Flask

The following are the materials used to construct lab flask-

Borosilicate glass is commonly used for making laboratory flask. Borosilicate 3.3 is mainly composed of silica and boron trioxide.
Quartz glass is also used for making flask where high heat resistance is needed.
Fused silica is used in some special laboratory flasks.
Polycarbonate (PC) is used for making plastic laboratory flask.
Polypropylene (PP) is also used as plastic material for flask.

PETG and PET (Polyethylene Terephthalate) are used for making some types of laboratory flask.
High-Density Polyethylene (HDPE) is used in plastic flask because it can hold different chemical solutions.
Fluoropolymer resins are also used for constructing flask. It includes PTFE (Polytetrafluoroethylene or Teflon), PFA (Perfluoroalkoxy alkane), and FEP (Fluorinated ethylene propylene).

Polyamides are used in some laboratory flask or its parts.
Rubber and elastomers are used in some flask parts. EPDM (ethylene propylene diene monomer) rubber is one example.

Features of Laboratory flask

The following are the features of laboratory flask-

Laboratory flask has a special shape. It generally has a wider main body or base that becomes narrow into one or more tubular necks with opening at the top.

Most laboratory flasks are made up of borosilicate glass. It gives high resistance to thermal shock and harsh chemicals.
Some flasks are also made from plastics like PTFE, polypropylene or polycarbonate. These are used when shatter resistance, flexibility or biological use is needed.
The base of laboratory flask is different according to its work. Flat bottom flask can stand safely on the benchtop.

Round bottom flask has spherical base. It helps in uniform heat distribution during boiling and distillation.
The neck of flask may have tapered ground glass joint. It helps to make tight seal with other laboratory apparatus.
Some flask necks have screw threads or simple rims for fitting rubber stoppers.

Some reaction flasks have more than one neck. These are used for complex chemical reactions.
The wall thickness of flask depends on its purpose. Distillation flask has thinner wall for easy heating.
Filtering flask or Büchner flask has thick and heavy wall. It can withstand negative pressure without imploding.

Some laboratory flasks have volume markings. Volumetric flask has etched graduation line on its long neck for measuring exact liquid volume.
Some flasks have special attachments like side arm tubulation. It is used for connecting vacuum pump or inert gas line.
Some biological flasks have internal baffles at the base. It helps to improve aeration in shaker culture.

Types of Laboratory Flask

The following are the types of laboratory flask-

Erlenmeyer Flask or Conical Flask
It has a flat base and tapered neck. It is used for mixing, swirling and titration without spilling.
Volumetric Flask
It has a long narrow neck with a calibration mark. It is used to measure and prepare solution with highly precise volume.
Round-Bottom Flask
It has a spherical base. It is used for boiling, chemical reactions and distillation because heat is distributed evenly.

Flat-Bottom Flask
It has a flat base and can stand on its own. It is generally used for heating, boiling or storing liquids.
Büchner Flask or Filtering Flask
It is a thick-walled conical flask with sidearm connection. It is used in vacuum filtration and can withstand negative pressure.
Florence Flask or Boiling Flask
It has a round body with a long neck. It is used for continuous boiling and distillation at high temperature.

Distillation Flask
It has a sidearm. It is used to separate liquid mixture based on their different boiling points.
Pear-Shaped Flask
It has a rounded V shaped body. It is used for collecting liquids and removing solid residue after evaporation.
Schlenk Flask
It has a sidearm and stopcock. It is used for reactions which are sensitive to air or moisture.

Reagent Flask
It is used for safe and stable storage of liquid or solid chemicals.
Evaporation or Recovery Flask
It is a round-bottom flask used for heating, evaporation and solvent recovery. It is mostly used with rotary evaporator.
Kjeldahl Flask
It has a bulbous bottom and elongated neck. It is used for digestion of organic matter under high heat for nitrogen analysis.

Iodine Flask
It is similar to Erlenmeyer flask but has a special stopper joint. It is used to prevent volatile substances like iodine from evaporation during titration.
Fernbach Flask
It has an extra-wide conical base. It gives large surface area for maximum aeration in biological cultures.
Claisen Flask
It is a special boiling flask with two stacked stillheads. It is used for complex distillation.

Florentine Flask
It is a spherical flask with a long neck. It is used for extraction of essential oils and uniform heating.
Le Chatelier Flask
It has a bulbous base and long graduated neck. It is used to determine density of fine powders like cement or lime.
Mojonnier Flask
It is a special sedimentation flask. It is used for fat extraction mainly in dairy testing.

Saybolt Flask
It is used as receiving vessel for determining viscosity of petroleum products.
Kohlrausch Volumetric Flask
It is a wide-mouthed flask. It is used for sugar analysis in food chemistry.
Winkler Oxygen Bottle
It is used to measure dissolved oxygen concentration in water samples.

Straus Flask
It is a type of Schlenk flask with double-neck structure. It is used for storing anhydrous solvents.
Combustion Flask
It is made up of borosilicate glass and has platinum sample carrier. It is used for oxygen combustion.
Fleaker Flask
It is a hybrid form between beaker and Erlenmeyer flask. It has cylindrical body and curved neck.

How to use a Laboratory Flask?

The following are the steps to use a laboratory flask-

First check the flask before use. It should be observed for scratches, chips or cracks.
Damaged glassware should not be heated. It may break during heating.

The flask should be heated and cooled slowly. Hot flask should not be placed directly on cold or wet surface because thermal shock may occur.
The flask should be supported properly during use. Coated clamps, ring stands, cork rings or heating mantles are used for this purpose.
Round-bottom flask cannot stand upright by itself. So it should be placed on cork ring or fixed with clamp.

During clamping, the clamp should not be tightened too much. It may break the glass.
If the flask has ground-glass joint, a thin layer of vacuum grease is applied on the upper part of inner joint.
This helps to form air tight seal and prevents glass joints from sticking together.

The joints should be fixed with suitable clips, washers or O-rings.
In Erlenmeyer flask, liquids can be mixed by swirling. Its wide base and narrow neck helps to mix and heat liquid without spilling.
The mouth of Erlenmeyer flask can be closed with rubber bung to prevent evaporation and contamination. But it should not be used for exact volume measurement.

In volumetric flask, solute is first transferred into the flask. Then 20-30% solvent is added and the flask is swirled gently until the solute is dissolved.
More solvent is added until the liquid reaches 1-2 cm below the calibration mark.
The final drops are added by dropper or pipette. The bottom of meniscus should touch the mark at eye level.

Then the flask is closed with stopper and inverted 10 to 20 times to mix the solution completely.
In Büchner flask, the sidearm is connected to vacuum pump or water aspirator by heavy-walled vacuum tubing.
For more safety, the flask may be wrapped with tape or plastic mesh to prevent implosion under negative pressure.

In Schlenk flask, the side-arm stopcock is connected to Schlenk line manifold.
The flask is evacuated and then filled with inert gas like nitrogen or argon. This process is repeated to remove air and moisture before starting reaction.

Uses of Laboratory Flask

The following are the uses of laboratory flask-

Laboratory flask is used for general storage and handling. It is used for confining, collecting and safely transporting liquid samples or solid chemical reagents.

It is used for mixing and swirling of chemicals. Chemicals are agitated, dissolved and precipitated safely. Erlenmeyer flask is commonly used because its conical shape prevents splashing.
It is used for heating and boiling of liquids. Round-bottom flask and Florence flask are used because they distribute heat evenly during boiling and refluxing.
It is used for precise volume measurement. Volumetric flask is used for measuring specific volume of liquid and preparing standard analytical solutions.

It is used in titration. Iodine flask is used in iodometric titration and it prevents vapour loss during the titration.
It is used in distillation and extraction. Distillation flask or retort flask is used for separating liquid mixtures based on their boiling points. Florentine flask is used for extracting essential oils and Mojonnier flask is used for extracting fats.
It is used in vacuum filtration. Thick-walled Büchner flask is connected to vacuum source and used for separating solids from liquids.

It is used for air-sensitive reactions. Schlenk flask is connected to inert gas line and used for reactions that should be protected from air or moisture.
It is used for special property testing. Saybolt flask is used for viscosity, Le Chatelier flask for density, Winkler bottle for dissolved oxygen, Kohlrausch flask for sugar content and Kjeldahl flask for organic nitrogen.
It is used in biological cultivation. Culture flask and wide-based Fernbach flask are used for growing, shaking and aerating bacterial, yeast and mammalian cell cultures.

Advantages of Laboratory Flask

The following are the advantages of laboratory flask-

Laboratory flask helps to prevent spilling of liquids. Its wide base and narrow neck allow swirling of liquids without easy spillage.
It has high stability. The broad and flat base in many flasks helps the flask to stand properly and prevents tipping over.
It helps to reduce evaporation of liquid. The narrow opening decreases the loss of liquid during experiment.

The opening of flask can be closed with stoppers or rubber bungs. This prevents dust and airborne contaminants from entering into the liquid.
Borosilicate glass flask has good thermal and chemical resistance. It can withstand temperature change, thermal shock and harsh chemical reactions.
It does not easily degrade or release impurities into the sample.

Round-bottom flask and Florence flask give uniform heat distribution. This helps to prevent hot spots during boiling and distillation.
Volumetric flask gives accurate measurement of liquid volume. It is used for preparing analytical solutions with exact volume.
Glass flask has clear visibility. It allows observation of chemical reactions, colour changes and liquid level.

Plastic flask is light weight and shatter resistant. It reduces the chance of injury in busy laboratory and field work.
Amber glass flask protects light-sensitive substances from damage by UV light.
Schlenk flask protects reactive chemicals from air and moisture. It is used for handling air-sensitive and moisture-sensitive substances safely.

Limitations of Laboratory Flask

The following are the limitations of laboratory flask-

General-purpose flasks cannot measure exact volume of liquid. Erlenmeyer flask and round-bottom flask have uneven shape, so these are not used for precise or accurate volume measurement.
Some flasks are unstable by their shape. Round-bottom flask and spherical flask cannot stand upright on their own and need clamps, ring stands or cork rings.
Glass laboratory flasks are fragile. They can break or shatter easily if dropped, so careful handling and storage is required.

Vacuum flasks have risk of implosion. Büchner flask or filtering flask may implode under negative pressure if the glass is weakened by scratch, nick or crack.
Plastic flasks have thermal limitation. They cannot tolerate high temperature or direct flame because they may melt or deform. Thick-walled glass flasks like filter flask should not be heated directly on hot plate or open flame.
Plastic flasks may show chemical incompatibility. They can absorb organic solvents, degrade by aggressive chemicals or leach plasticizers into sensitive samples.

Standard thin-walled flasks have pressure and reaction limitation. They are mainly used for collection and storage and cannot safely contain high-energy chemical reactions.
Ground-glass joint flasks may show joint seizing. The joints can freeze or stick together and glass may shatter if pulled apart forcefully.

Precautions of Laboratory Flask

The following are the precautions of laboratory flask-

Flask should be checked before use. Hairline cracks, scratches, chips or star cracks are observed carefully, mainly around the rim and joints. Damaged glassware is removed from use immediately because it can shatter during work.

Broken glass should not be handled with bare hands. If flask breaks, the pieces are collected by dustpan and broom, tongs or forceps. It is disposed in the marked broken glass container.
Sudden temperature change should be avoided. Hot flask should not be placed on cold or wet benchtop. All glassware are cooled slowly to prevent thermal shock and cracking.
Thick-walled glassware should not be heated directly. Filter flask, reagent bottle, jars and volumetric flask are not heated over direct flame or directly on hot plate.

Round-bottom flask is used for boiling liquids. Its shape helps in uniform heat distribution.
Coated clamps and support rings are used to hold and stabilize the flask. Clamps should not be tightened too much because it can crush the glass.
Apparatus is assembled from the bottom up. Clamps are fastened loosely at first. Ground-glass apparatus should not be forced into alignment because it causes stress, breakage and leakage.

Ground-glass joints are lubricated with light coat of vacuum grease or PTFE sleeves. It prevents freezing or seizing of joints. Joints are disassembled and cleaned immediately after use.
Glass tubing and thermometer are inserted into rubber stopper carefully. The hole is lubricated with water or glycerin. The glass is held close to the insertion point with towel and twisted gently into place. It should not be pushed directly or forcefully.
Heavy-walled flask is used for vacuum work. Büchner filter flask is used for vacuum filtration. Standard Erlenmeyer flask should not be used under negative pressure because it can implode.

Sealed flask is treated as pressure-sensitive apparatus. Vented stoppers, mineral oil bubblers or safety valves are used to allow excess gas to escape during reaction or Schlenk line work.
Flask material should be suitable for the chemical used. Glass should not be used for hydrofluoric acid, hot phosphoric acid or strong hot alkalis because they attack and etch glass.
Highly exothermic reactions are handled carefully. Concentrated sulfuric acid and water should not be mixed inside standard glass cylinder or unsuitable flask because the heat produced can break the bottom of vessel.

How to maintain and keep clean a Laboratory Flask?

The following are the ways to maintain and keep clean a laboratory flask-

Eye protection and heavy-duty gloves are worn before washing glassware. The gloves should be slip-resistant and chemically resistant.
Flask is washed quickly after use. This prevents the chemical residues from drying and becoming hard. If washing is not possible, the flask is kept in soapy water for soaking.

Non-abrasive lab grade detergent is used for cleaning glassware. Alconox or other special cleaning products may also be used. Abrasive powder and worn-out brush are not used because they scratch and damage the glass.
Plastic flask is cleaned according to the manufacturer’s instruction. Cleaning agents which attack, discolor or swell the plastic should not be used.
Flask is rinsed properly with tap water. Soap and detergent residues are removed completely, otherwise it may contaminate the next chemical work.

The cleanliness of glass flask is checked by spraying distilled water. In clean flask, water drains as a continuous smooth film and droplets are not left on the inner wall.
Different stains are removed by different solvents. Organic residues are rinsed with acetone, methanol, ethanol or ether. Inorganic residues are removed by soap and water or dilute mineral acid. Oily grease is removed by petroleum ether or hexane. Brown permanganate stain is removed by dilute oxalic acid. Aqua-regia and chromic acid are used only after special training.
Ground-glass joints and stopcocks are opened and cleaned immediately after laboratory work. If they are left with strong bases or old grease, dirt may score the glass and the joints may freeze together.

Sink, soaking bin and dishwasher should not be overloaded with flasks. Rubber sink mat and counter mat are used to decrease impact and breakage.
Flask is dried properly after cleaning. It is kept on towel, lined basket or slip-resistant pad. Large flask may be hanged on pegs. It may also be rinsed with small amount of acetone or dried in laboratory dryer oven.
Clean and unassembled flask is stored safely at the back of bench or hood. It should not be kept near shelf edge. Drawer pads are used to prevent rolling and breaking. New glassware is also washed before first use to remove manufacturing residue and loose particles.

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Laboratory Flask – Types, Procedure, Limitations, Uses