Volumetric flask
A volumetric flask (vol flask for short) is a piece of laboratory glassware used to measure a very precise and accurate amount of a liquid, and is used for such when the amount is too big for a pipette or burette. It is shaped like a Florence flask with a flatter bottom so as to not tip over. The neck is marked (usually by a scratch or etch on the glass) at a very precise measurement. When the liquid has been added to that point (so 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 250.00mL flask can only be used to measure 250mL. The volume mark is usually made by machine, so it can be more assuredly accurate than a hand-made mark.
Use
The use of a volumetric flask almost always involves either measuring a certain amount of a liquid, 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. Be careful you don't add more then the amount the flask is supposed to measure. If you do it usually means you have to do it all over again. When the flask has been completely filled it is then stoppered and inverted several times if this is a dilution.
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