Handling of volumetric flasks
What are volumetric flasks?
Volumetric flasks are precise volumetric instruments used to prepare solutions with an exact defined volume. Volumetric flasks consist of a body (pear-shaped) with a long, narrow neck bearing the ring mark for the respective nominal volume. Small-volume volumetric flasks, which are increasingly used in modern analytical methods, can tip over more easily due to their small base and comparatively unfavorable center of gravity. Trapezoidal volumetric flasks provide an alternative. Their lower center of gravity and significantly larger base make them considerably more stable than conventional volumetric flasks of a comparable volume. Volumetric flasks can typically be closed with a ground joint stopper. Amber volumetric flasks provide UV protection while clear volumetric flasks do not.
Volumetric flasks are calibrated to “In” (TC = to contain), meaning that when correctly filled to the ring mark, the volumetric flask contains exactly the specified volume. They are available in accuracy classes A and B, sometimes with a larger neck and a larger error limit together with the alternative markings “AW” or “BW”. Volumetric flasks are used in particular to fill samples to be analyzed and to prepare defined solutions as standard solutions for titration or as a dilution series.
How to prepare a standard solution with a volumetric flask
1. Insert the precisely weighed amount of substance, or rinse in a standard solution concentrate.
2. Fill the flask with distilled water to about half. Swirl the flask to dissolve and mix the contents thoroughly.
3. Top up the flask with distilled water to just below the ring mark.
4. Top up the remaining volume, using a wash bottle or pipette, until the meniscus is exactly at the ring mark. Important: meniscus must be read at eye level. The wall of the flask must not be wetted above the mark.
5. Close the flask and shake upside down to mix contents.
