A volumetric flask is used to make up a solution of fixed volume very accurately. The single, rather long, narrow neck is marked (usually by a circumferential scratch or etch on the glass) at a very precise measurement. When the liquid has been added to that point (so the bottom of the meniscus is on the line), it is usually accurate to within a small fraction of a percent and highly precise.
A single flask can usually only be used for a certain measurement, e.g. a 250.00 mL flask can only be used to measure 250 mL. Therefore, volumetric flasks come in standard sizes: 10.00 mL, 25.00 mL, 50.00 mL, 100.0 mL, 250.0 mL, 500.0 mL, etc. A 500-mL volumetric flask measures 500 mL ± 0.2 mL. This is a relative uncertainty of 4 x 10-4 or 400 parts per million.
The use of a volumetric flask almost always involves containing a certain amount of a liquid, either for making solutions or diluting a liquid to the size of the flask. This is done by adding the solvent to the flask until it reaches the neck, at which point it is usually added much more slowly, via pipette or dropper. One must be careful not to add more than the amount the flask is supposed to measure; if one does so, the process may need to be repeated. This can be problematic, especially when working with expensive chemicals.
To make up a solution in a volumetric flask:
Note: Label the flasks with a labeled tag or labeling tape. Never carve or etch the glass surface with sharp objects or chemicals!