Volumetric flask

Volumetric flasks of various sizes.

A volumetric flask is a piece of laboratory glassware used in analytical chemistry for the preparation of solutions. It is made of glass or plastic and consists of a flat bottomed bulb with a long neck, usually fitted with a stopper. The stopper is normally made in a chemically resistant plastic such as polypropylene rather than glass. The neck has a single ring graduation mark and a label. The label should show the nominal volume, tolerance, calibration temperature, class, relevant manufacturing standard and the manufacturer’s logo. The glass or plastic is generally clear but may be amber colored for handling light sensitive compounds such as silver nitrate or vitamin A.

Use

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Volumetric flasks are used for making up solutions to a known volume. A typical procedure to make up a 1M solution of sodium chloride would be to weigh out 58.4g (1Mol) of sodium chloride into a 1000ml volumetric flask and add approximately 800ml of distilled water. Swirl the flask gently until all the solid is dissolved, then add more distilled water until the bottom of the meniscus touches the graduation line. The meniscus can be made clearer by holding a black card behind the neck just below the line of sight^[1]. The stopper is then placed in the neck and the whole flask inverted repeatedly to homogenize the solution^[2]. Care must be taken when making up concentrated solutions because 500ml of a concentrated solution added to 500ml of distilled water do not necessarily make 1000ml and thus the volume may change when the solution is homogenized. A well known example of this found when mixing water ethanol mixtures. In this case it is better to make up near the mark, then homogenize, then make up to the mark. Volumetric flasks have also been used as decanters, for brandy, malt whisky, or other alcoholic beverages. They are less decorative than the fancy crystal decanters made by Baccarat and similar vendors, but they have stoppers that seal well enough to prevent evaporative losses.

Standards

Volumetric flasks generally come in two different standards. The higher standard flasks (Class A, United States Pharmacopeia or equivalent depending on the country) are made with a more accurately placed graduation mark, and have a unique serial number for traceability. Where this is not required, a lower standard (Class B or equivalent) is used for qualitative or educational work.

References