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Chapter 2 Basic Tools of Analytical Chemistry 29

Second, when filling a pipet or volumetric flask, set the liquid’s level exactly
at the calibration mark. The liquid’s top surface is curved into a meniscus, the meniscus
bottom of which should be exactly even with the glassware’s calibration mark The curved surface of a liquid contained
(Figure 2.6). The meniscus should be adjusted with the calibration mark at eye in a tube.
level to avoid parallax errors. If your eye level is above the calibration mark the
pipet or volumetric flask will be overfilled. The pipet or volumetric flask will be
underfilled if your eye level is below the calibration mark.
Finally, before using a pipet or volumetric flask you should rinse it with several
small portions of the solution whose volume is being measured. This ensures that
any residual liquid remaining in the pipet or volumetric flask is removed.

2D. 3 Equipment for Drying Samples

Many materials need to be dried prior to their analysis to remove residual moisture.
Depending on the material, heating to a temperature of 110–140 °C is usually suffi-
cient. Other materials need to be heated to much higher temperatures to initiate
thermal decomposition. Both processes can be accomplished using a laboratory
oven capable of providing the required temperature.
Commercial laboratory ovens (Figure 2.7) are used when the maximum de- Figure 2.7
sired temperature is 160–325 °C (depending on the model). Some ovens include the Conventional laboratory oven used for
ability to circulate heated air, allowing for a more efficient removal of moisture and drying materials.
shorter drying times. Other ovens provide a tight seal for the door, allowing the
oven to be evacuated. In some situations a conventional laboratory oven can be re-
placed with a microwave oven. Higher temperatures, up to 1700° C, can be achieved
using a muffle furnace (Figure 2.8).
After drying or decomposing a sample, it should be cooled to room tempera-
ture in a desiccator to avoid the readsorption of moisture. A desiccator (Figure 2.9)
is a closed container that isolates the sample from the atmosphere. A drying agent,
called a desiccant, is placed in the bottom of the container. Typical desiccants in-
clude calcium chloride and silica gel. A perforated plate sits above the desiccant,
providing a shelf for storing samples. Some desiccators are equipped with stopcocks
that allow them to be evacuated.

Figure 2.8
Example of a muffle furnace used for
heating samples to maximum temperatures
of 1100–1700 °C.
Courtesy of Fisher Scientific.

desiccator
A closed container containing a
desiccant; used to store samples in a
moisture-free environment.

desiccant
(a) (b) A drying agent.
Figure 2.9
(a) Desiccator. (b) Desiccator with stopcock for evacuating the desiccator.
Photos courtesy of Fisher Scientific.