In which of the following pieces of glassware is the meniscus of the liquid inside unimportant to its primary function? (A) Pipet, (B) buret, (C) beaker, (D) graduated cylinder, or (E) volumetric flask.
A meniscus is the curve on the surface or at the top of a liquid which is inside a container. The container or vessel, as well as the composition of the liquid, can influence the shape of the meniscus. A meniscus can appear concave or dipping down in the middle — and this is usually found in most aqueous solutions — or convex or dipping down at the sides, at the edges, near the vessel. And this is found in, for example, liquid mercury. It is the cohesive forces or cohesion within the liquid which determines its surface tension and the adhesion or adhesive forces between the liquid and the sides of the container, which together affect the meniscus shape.
The curve is more evident or more pronounced in very narrow vessels and is quite hard to see and looks very flat in vessels that have a very wide mouth. When reading a meniscus, the eye must be level with the bottom of the curve in a concave liquid or the top of the curve in a convex liquid to avoid the error of parallax. So in a very narrow vessel, it is easier to read the volume value from the meniscus because of the distinct curve. Thus, some types of glassware are much more suitable to taking accurate volume measurements because of their narrow nature.
A meniscus is easier to read in pipets, burets, graduated cylinders, and volumetric flasks because of the pronounced curve. Beakers are wide mouthed. And it is harder to read a meniscus accurately in a beaker because the meniscus appears flatter. Thus, beakers are less accurate when measuring volumes than the other four pieces of glassware mentioned. But there is another factor which determines the accuracy of a reading one can take with a particular piece of glassware. And that factor is temperature. Temperature can directly influence both the volume of a liquid and the capacity of a vessel by making the liquid and the glass slightly expand or contract.
This expansion or contraction is not noticeable to the human eye but can affect the accuracy of a volume reading when accuracy is needed. So glassware in the lab used to measure liquid volumes are all calibrated. This means that the volume markings printed onto the sides of the vessels are printed in a specific place for use of that glassware at a specific temperature. If we look carefully on these pieces of glassware, we will notice a temperature value printed on the side. This is the calibration temperature and is usually 20 degrees Celsius. When liquids are measured at this temperature in the lab, the volume readings for pipets, burets, graduated cylinders, and volumetric flasks are highly accurate.
These four pieces of glassware are calibrated to within one percent accuracy for the specific temperature printed on the side. Beakers, however, are not calibrated with such precision. And they’re usually given a reading of about 10 percent accuracy for volume measurements at a specific temperature. We must remember that the low accuracy when using a beaker is also due to its wide neck and the difficulty in reading the meniscus.
To summarize, pipets, burets, graduated cylinders, and volumetric flasks are used for measuring or transferring accurate volumes of liquids. While beakers are usually used to measure, transfer, or stir, because of the wide neck, approximate volumes of liquids or mixtures. Finally, the piece of glassware where the meniscus of the liquid inside is unimportant to its primary function is beaker.