Reading Volumetric Ware

The parallax problems of linear measurement are compounded with volumetric ware because there are two distinct lines to read. One line is where the liquid makes contact with the walls of the volumetric container, and the other is in the center of the volumetric tube (see Fig. 2.15). 

The distortion of the liquid is caused by the surface tension of that liquid and the walls of the container. This distorted line is called the meniscus. When liquid wets the walls of a container, you read the bottom of the meniscus. When liquid does not wet a containers walls (such as mercury or any liquid in a plastic container), 

It may take a bit of practice to properly see the correct part of the meniscus line for accurate measurement. Fortunately, there are tricks and devices to facilitate the reading. For instance, if the graduation lines on the volumetric ware mostly encircle the tube, it is easy to line up your vision so that you can avoid parallax problems (see Fig. 2.16). 

If the meniscus is difficult to see, you can make it stand out by placing a piece of black paper behind the glass tube, below the liquid line. The liquid picks up the dark color and, like a light pipe, the end of the liquid column will have a dark line. Alternatively, the paper can be folded around the tube and can be held by a paper clip (like a French cuff). This paper cuff can be raised or lowered easily, and it frees both hands. Black paper is inexpensive and easy to use, but if the paper gets wet it will need to be replaced. This problem is not major, but if it occurs midexperiment, it can be an inconvenience. 

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Another concern for accuracy is based on how accurately the user can read calibrations on the volumetric ware. The reproducibility of an individual user will be more consistent than the reading made by a variety of users. Therefore, if there will be a variety of users on any given apparatus, all who are likely to use it should make a series of measurements. This way, the individual errors can be calculated. The ASTM has analyzed the range of errors made by trained personnel, and the reproducibility of these results are shown in Table 2.9. 

Volumetric readings can be made two ways. The easiest and most common is simply reading the volume directly from a piece of volumetric ware. Alternatively, you can weigh a sample and, if you know the molecular weight of the material, you can calculate the volume. Each approach can be affected by barometric pressure, humidity, and temperature. The calculations and tables needed to obtain true volume from observed volume or calculated weight are not difficult to use but should only be used when necessary—that is, when accuracy or precision demand their use. 

There are two different approaches for properly correcting volumetric readings caused by environmental variations because there are two approaches to making volumetric readings those done by reading volume directly from volumetric ware, and those made indirectly by weight. 

The simplest corrections are made when reading directly from volumetric ware. As the volumetric container (and the liquid contained) expands and contracts by temperature variations from 20°C, volumetric corrections are required. These corrections can be found on Table 2.10. 

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