Page 43 - Modern Analytical Chemistry

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26 Modern Analytical Chemistry

Fulcrum
Balance beam
Fulcrum
Counterweight

Removable weights

Figure 2.3
Schematic diagram of single-arm mechanical
balance. Balance pan

The mass of a sample is determined by difference. If the material being weighed
is not moisture-sensitive, a clean and dry container is placed on the balance. The
mass of this container is called the tare. Most balances allow the tare to be automat-
ically adjusted to read a mass of zero. The sample is then transferred to the con-
tainer, the new mass is measured and the sample’s mass determined by subtracting
the tare. Samples that absorb moisture from the air are weighed differently. The
sample is placed in a covered weighing bottle and their combined mass is deter-
mined. A portion of the sample is removed, and the weighing bottle and remaining
sample are reweighed. The difference between the two masses gives the mass of the
transferred sample.
Several important precautions help to minimize errors in measuring an object’s
mass. Balances should be placed on heavy surfaces to minimize the effect of vibra-
tions in the surrounding environment and should be maintained in a level position.
Analytical balances are sensitive enough that they can measure the mass of a finger-
print. For this reason, materials placed on a balance should normally be handled
using tongs or laboratory tissues. Volatile liquid samples should be weighed in a
covered container to avoid the loss of sample by evaporation. Air currents can sig-
nificantly affect a sample’s mass. To avoid air currents, the balance’s glass doors
should be closed, or the balance’s wind shield should be in place. A sample that is
cooler or warmer than the surrounding air will create convective air currents that
adversely affect the measurement of its mass. Finally, samples dried in an oven
should be stored in a desiccator to prevent them from reabsorbing moisture from
the atmosphere.

2D.2 Equipment for Measuring Volume
Analytical chemists use a variety of glassware to measure volume, several examples
of which are shown in Figure 2.4. The type of glassware used depends on how exact
the volume needs to be. Beakers, dropping pipets, and graduated cylinders are used
to measure volumes approximately, typically with errors of several percent.
volumetric flask Pipets and volumetric flasks provide a more accurate means for measuring vol-
Glassware designed to contain a specific
volume of solution when filled to its ume. When filled to its calibration mark, a volumetric flask is designed to contain a
calibration mark. specified volume of solution at a stated temperature, usually 20 °C. The actual vol-