Volumetric flask

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A volumetric flask. The target volume mark is faintly visible as a thin line encircling the neck of the flask.

A volumetric flask (vol flask for short) is a type of laboratory flask (piece of laboratory glassware) used to contain or measure a very precise and accurate amount of a liquid. It is shaped like a Florence flask with a flatter bottom so as to not tip over. 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. However, 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. The volume mark is usually made by machine, so it can be more assuredly accurate than a hand-made mark.

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 (or the liquid to be measured) to the flask until it reaches the neck, at which point it is usually added much more slowly, via pipette or dropper depending on the size of the flask. 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. If a dilution is called for, the completely-filled flask is then stoppered and inverted several times.

Volumetric flask capped with a stopper at the top

Volumetric flasks are often marked TC meaning "To Contain", often with a certain temperature at which the volume measurement is most accurate. TC means that when filled to the circumferential mark on the neck, the flask will contain the designated amount of liquid, as opposed to TD (To Deliver) on other glassware meaning a vessel will deliver a certain amount of liquid.

Volumetric flasks typically come with a stopper or cap for capping the opening at the top of the neck. Such stoppers can be made of glass or plastic. When a glass stopper is used (see image at left), the opening at top of the neck has an outer (or female) tapered (conical) ground glass joint and the glass stopper has a matching tapered inner (or male) ground glass joint surface, but often only of stopper quality. Very small volumetric flasks (capacity of a milliliter or less) need a base to stand on so they will not tip over easily.