Laboratory Practices
5940022LabPrac.fm Page 15
Section 2 Laboratory Practices
2.1 Temperature
Most tests in this manual perform most accurately when the sample temperature
is between 20 °C and 25 °C (68 to 77 °F). A note in the individual procedure will
state any special temperature requirements.
2.2 Mixing
Swirling is recommended when mixing samples in a graduated cylinder or a
titration flask. Grip the cylinder (or flask) firmly with the tips of the thumb and
first two fingers (see Figure 1). Hold the cylinder at a 45-degree angle and make a
circling motion from your wrist. This should move the cylinder in an
approximately 12-inch circle, creating enough rotation to complete the mixing in
a few turns.
Swirling is the gentlest method of mixing and offers the least chance for
atmospheric contamination when testing for carbon dioxide and other gases.
Inverting allows for thorough mixing in a capped sample cell or a mixing
cylinder. Hold the cell, or cylinder, in a vertical position with the cap on top.
Invert, so that the cap is on the bottom. Return it to its original position.
(See Figure 1.) This may be repeated as needed.
Both methods are simple, but take a bit of practice to get the best results.
Figure 1 Swirling a Cylinder and Inverting a Sample Cell
2.3 Digestion
Several procedures require sample digestion. Digestion uses chemicals and heat
to break down a substance into components that can be analyzed. This section
describes three different digestion procedures.
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