Processing kodak motion picture films, module 3 analytical procedures

4. Graduated Cylinders and Tip-up Pipets
In many cases the volume of a solution to be used in an
analytical method need be only an approximation of
the specified volume. For example, the method may
prescribe 20 mL of a reagent whereas only slightly
more than 15 mL would suffice. In these cases,
graduated cylinders or “tip-up” pipets are used.
Graduated cylinders are calibrated to delivery (TD) or
to contain (TC). See Table 1 for tolerances. “Tip-up”
pipets require less time for operation and are.
therefore, preferable. Portable “tip-up” pipets, shown
in Figure 6, are available in different sizes. They have
the accuracy of a graduated cylinder. They may be
purchased with either standard taper, ground glass,
male joints, or for use with rubber stoppers. It is
suggested that Erlenmeyer flasks of 250-, 500-, or
l000-mL capacity be selected with the ground glass
joint to match. The restraining wires as shown in
Figure 6 need not be used.
Figure 6 Tip-up Pipet
F002_0910AC
5. Burets
Burets are graduated to deliver variable known
volumes of liquids. Methods are generally developed
to employ 30 to 50 mL of solution as measured from a
buret. For such purposes, a 50-mL capacity buret is
used. In those cases in which 10 to 20 mL of solution
are measured from a buret, a 15 or 25 mL buret is used.
Class A burets are required. See Table 1 for tolerances.
Analysts should understand and apply the following
specific instructions for the use of burets.
a. Cleanliness
Use a clean buret. The buret does not have to be
dry before rinsing it with the solution to be used.
If the buret is not perfectly clean, drops of the
solution will adhere to the wall and the buret will
deliver less than the indicated volume.
Furthermore, the contaminants may affect the
results.
b. Perfect Tip
Use a buret with a good tip. A buret with a
broken tip may deliver a volume other than the
rated volume when the tip is touched against the
wall of the receiving vessel. A buret with a
broken or chipped tip sometimes can be firepolished
and salvaged.
c. Stopcock Seal
Use a buret which will hold a constant reading
for at least 5 minutes. If the stopcock seal is
defective, the solution will leak and thus lower
the buret reading. Teflon stopcocks do not
require grease, and are preferred.
d. Rinsing
Rinse the entire inner surface of the buret two or
three times with portions of the solution to be
used.
e. Lowering Meniscus to the Zero Mark
Fill the buret well above the zero mark. With the
buret zero mark at eye level, lower the meniscus
to the zero mark. Allow a minute or two for
drainage, then make the initial reading, or
readjust the buret precisely to the zero mark.
During the waiting period, check for leaks and
make certain that air bubbles are expelled either
at the top or from the tip. After the meniscus has
been adjusted, remove the final drop by touching
the tip with the wall of a waste-solution beaker
which is kept under the buret except during
titration. One-second contact is adequate.
f. Position
The buret should be clamped in a vertical
position during the readings and while the
solution is being titrated.
Processing KODAK Motion Picture Films, Module 3, Analytical Procedures H24.03 5