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Volumetric Flask – Instructions for Use
|Volumetric flasks are used for making up solutions. Below are a set of simple instructions for use.
The flask should be scrupulously cleaned and dried. Any grease or dirt on the flask will potentially contaminate the sample and cause the meniscus to form poorly. This will give rise to a small error in the reading of the volume.
The solvent is generally water, but can sometimes be other solvents. Be aware that different solvents form a different shaped meniscus in the neck of the flask which may affect the volume.
The solute may be a solid or a liquid. It will usually dissolve completely to give a clear solution. It may form a suspension rather than a true solution in which case it will be cloudy or opalescent.
The operation should be carried out as close to 20°C as possible as the flasks are calibrated for use at that temperature.
A known quantity of solute is weighed or pipetted into the flask. If any solute adheres to the neck it should be rinsed into the flask with the solvent. It is good practice to avoid this if possible. More solvent is cautiously added until the bulb is around 3/4 full. The flask is then swirled but not shaken to dissolve the solid or disperse the liquid completely. If there is still some undissolved solid a sonic bath can be used to hasten the dissolution. Care should be taken not to splash any of the solution above the volume mark.
Once the solution is completely homogeneous further solvent is carefully added until the bottom of the meniscus just touches the top of the graduation line. A black card held just below the line will shade the meniscus. See diagram. Notice that the the eye must be level with the meniscus so that the ring around the neck appears as a line, not an oval. This is to avoid parralax error.
The stopper is then placed in the top of the flask and the flask is inverted several times to homogenise the solution. The solution is then ready for use.
A closeup of a flask neck with a shading black card showing the correct level of meniscus