Description About Volumetric Flasks

Volumetric glassware is a staple of scientific laboratories. Scientists and researchers use this type of glassware to accurately contain or deliver a calibrated amount of liquid. Volumetric glassware includes burets, graduated cylinders, pipets and flasks.

Description

A volumetric flask consists of a round bulb with a long neck. A graduation line circumvents the neck of the flask. A volumetric flask is traditionally made out of clear glass. However, flasks can be made out of other materials to meet the needs of light-sensitive work or work involving glass dissolving chemicals. Volumetric flasks have either a plastic or ground-glass stopper or cap.

Calibration and Acccuracy

Based on a series of standards and tolerances, the international standards organization, ASTM, defines volumetric flasks as either Class A, for the highest level of accuracy, or Class B. Volumetric glassware is calibrated to a specific temperature. Depending on the class of glassware and the experimental conditions, a volumetric flask is accurate down to plus or minus 0.1 percent. By comparison, glassware that is not calibrated is accurate to plus or minus 5 percent.

TD Versus TC

Volumetric glassware is either marked TD, for “To Deliver,” or TC, for “To Contain.” TD signifies that the piece of glassware will deliver the volume of material specified. A volumetric pipet is marked TD because it is designed to deliver the amount of solution specified to another container. In contrast, a volumetric flask is marked TC because it contains the specified volume of solution when filled to the graduation line.

Meniscus

The key to filling a piece of volumetric glassware lies in the curved surface at the top of the liquid inside the glassware. This curved surface is called a meniscus. Volumetric glassware is properly filled when the bottom of the meniscus barely touches the top of the graduation mark on the glassware. To read the meniscus correctly, the line of sight of the person filling the volumetric glassware must be parallel to the graduation line.

Using a Volumetric Flask

To make a solution using a volumetric flask, measure out the material to be diluted and dissolve it into the required solvent in a beaker. Do not use the entire amount of solvent expected while the material is dissolving. The final amount of solvent will be added in the volumetric flask. Once the material is dissolved into solution, pour the solution into the volumetric flask. Being careful not to leave any solvent drops on the sides of the flask’s neck, use a dropper to add solvent until the meniscus indicates the solution has reached the amount indicated on the flask. Stopper the flask and invert it several times to completely mix the solution. The volumetric now contains the specified amount of the solution.

References

• ChemLab: Volumetric Flasks
• The Lab Depot: Volumetric Glassware Definitions and Examples
• ASTM: E694-99 (2005) Standard Specification for Laboratory Glass Volumetric Apparatus