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Chapter 2 Basic Tools of Analytical Chemistry 27

(a)

Figure 2.4
Common examples of glassware
used to measure volume:
(a) beaker; (b) graduated cylinder;
(c) volumetric flask; (d) pipet;
(e) dropping pipet.
Photos courtesy of Fisher Scientific. (b) (c)

ume contained by the volumetric flask is usually within 0.03–0.2% of the stated
value. Volumetric flasks containing less than 100 mL generally measure volumes to
the hundredth of a milliliter, whereas larger volumetric flasks measure volumes to
the tenth of a milliliter. For example, a 10-mL volumetric flask contains 10.00 mL,
but a 250-mL volumetric flask holds 250.0 mL (this is important when keeping
track of significant figures).
Because a volumetric flask contains a solution, it is useful in preparing solu-
tions with exact concentrations. The reagent is transferred to the volumetric flask,
and enough solvent is added to dissolve the reagent. After the reagent is dissolved,
additional solvent is added in several portions, mixing the solution after each addi-
tion. The final adjustment of volume to the flask’s calibration mark is made using a
dropping pipet. To complete the mixing process, the volumetric flask should be in- (d) (e)
verted at least ten times.
A pipet is used to deliver a specified volume of solution. Several different pipet
styles of pipets are available (Figure 2.5). Transfer pipets provide the most accurate Glassware designed to deliver a specific
means for delivering a known volume of solution; their volume error is similar to volume of solution when filled to its
calibration mark.
that from an equivalent volumetric flask. A 250-mL transfer pipet, for instance,
will deliver 250.0 mL. To fill a transfer pipet, suction from a rubber bulb is used to
pull the liquid up past the calibration mark (never use your mouth to suck a solu-
tion into a pipet). After replacing the bulb with your finger, the liquid’s level is ad-
justed to the calibration mark, and the outside of the pipet is wiped dry. The
pipet’s contents are allowed to drain into the receiving container with the tip of the
pipet touching the container walls. A small portion of the liquid remains in the
pipet’s tip and should not be blown out. Measuring pipets are used to deliver vari-
able volumes, but with less accuracy than transfer pipets. With some measuring