Page 42 - Modern Analytical Chemistry

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Chapter 2 Basic Tools of Analytical Chemistry 25

The weight percent C 10 H 20 N 2 S 4 in the sample, therefore, is
.
0 32295 g C 10 HN S
20
2 4
´ 100 = 7 001% w/w C 10 HN S
.
20
2 4
g
0.4613 sample
2D Basic Equipment and Instrumentation
Measurements are made using appropriate equipment or instruments. The array of
equipment and instrumentation used in analytical chemistry is impressive, ranging
from the simple and inexpensive, to the complex and costly. With two exceptions,
we will postpone the discussion of equipment and instrumentation to those chap-
ters where they are used. The instrumentation used to measure mass and much of
the equipment used to measure volume are important to all analytical techniques
and are therefore discussed in this section.

2D.1 Instrumentation for Measuring Mass
An object’s mass is measured using a balance. The most common type of balance balance
is an electronic balance in which the balance pan is placed over an electromagnet An apparatus used to measure mass.
(Figure 2.2). The sample to be weighed is placed on the sample pan, displacing the
pan downward by a force equal to the product of the sample’s mass and the acceler-
ation due to gravity. The balance detects this downward movement and generates a
counterbalancing force using an electromagnet. The current needed to produce this
force is proportional to the object’s mass. A typical electronic balance has a capacity
of 100–200 g and can measure mass to the nearest ±0.01 to ±1 mg.
Another type of balance is the single-pan, unequal arm balance (Figure 2.3). In
this mechanical balance the balance pan and a set of removable standard weights on
one side of a beam are balanced against a fixed counterweight on the beam’s other
side. The beam itself is balanced on a fulcrum consisting of a sharp knife edge. Adding
a sample to the balance pan tilts the beam away from its balance point. Selected stan-
dard weights are then removed until the beam is brought back into balance. The com-
bined mass of the removed weights equals the sample’s mass. The capacities and mea-
surement limits of these balances are comparable to an electronic balance.

Detector Light source

Balance
pan

N
S S
Figure 2.2
(a) Photo of a typical electronic balance.
Control (b) Schematic diagram of electronic balance;
circuitry adding a sample moves the balance pan
down, allowing more light to reach the
detector. The control circuitry directs the
electromagnetic servomotor to generate an
opposing force, raising the sample up until
Electromagnetic
the original intensity of light at the detector
servomotor
is restored.
(a) (b) Photo courtesy of Fisher Scientific.