Measuring the Volume of Liquids
Some chemistry glassware, called volumetric glassware, is inscribed with markings to make measuring the volume of liquids easier. The pieces of volumetric glassware found in the chemistry laboratory are beakers, Erlenmeyer flasks, graduated cylinders, pipets, burets and volumetric flasks.
To Deliver and To Contain
Volumetric glassware can be divided into two categories: those designed to contain a specified amount of liquid and those designed to deliver a specified amount of liquid. Glassware designed to contain, like graduated cylinders and volumetric flasks, are usually marked with a TC. When liquid is poured from a piece of glassware a small amount remains behind, clinging to the sides of the vessel. A 100 ml volumetric flask is designed to hold exactly 100 ml, but if the liquid is poured out it will actually deliver a little less than 100 ml. Glassware designed to deliver, like pipets and burets, are marked with a TD. These pieces of glassware account for the small amount of liquid that remains behind. A 100 ml pipet contains a little more than 100 ml of liquid, but when the liquid is drained from a pipet, exactly 100 ml is delivered.
When water is placed in a glass or plastic container the surface takes on a curved shape. This curve is known as a meniscus. Volumetric glassware is calibrated such that reading the bottom of the meniscus, when it is viewed at eye level, will give accurate results. Viewing the meniscus at any other angle will give inaccurate results.
The accuracy of the markings on volumetric glassware varies greatly. The markings on beakers and flasks are usually about plus or minus 5% of the volume of the container. As such, they should be used only when a rough estimate of volume is required. The tolerance on graduated cylinders is about 1%. Volumetric flasks, burets and pipets are the most accurate with tolerances of less than 0.2%. To achieve these accuracies the person using the device needs to use the proper technique and the measurements need to made at the temperature for which the glassware was calibrated (usually 20 degrees C).
Read about proper pipet technique
|. Read about proper buret technique.