Processing kodak motion picture films, module 3 analytical procedures

WEIGHING EQUIPMENT
The internationally accepted unit of mass is embodied in a
platinum-iridium cylinder maintained at the International
Bureau of Weights and Measures at Sevres, France. The
mass of this cylinder is 1 kg exactly, by definition.
Selection
When making analytical reagents or standard laboratory
mixes, it is necessary to weigh the various constituents.
Thus, it is evident that the weighing operation is of
fundamental importance. When weighing a sample to a
specified weight, the tolerance is ± 1 unit in the last decimal
place to the right. For example, 15.0 grams is understood to
mean 15 g ± 0.1 g and 15.000 grams is understood to mean
± 0.001 gram. The type of balance used for weighings
(analytical, torsion) is immaterial provided that the accuracy
of the balance (given in its specifications) is commensurate
with the demand of the weighing operation. Take advantage
(if reagent expiration date permits) of preparing large
volumes of reagents, as this will permit the use of balances
accurate to fewer decimal places and still give a
satisfactorily small percentage error.
Care and Use of the Analytical Balance
1. Weighing Area
If possible, keep the balance in a room separate from
the laboratory. Keep the balance at a reasonably
constant temperature and out of direct sunlight and air
currents. Level the balance and place it upon a solid
support to protect it from vibration.
2. Protection of Knife Edges
To prevent injury to the agate knife edges and planes
when the balance is not in use, raise the beam and the
pan supports. Leave nothing on the pans and keep the
door of the case closed. When weighing, raise the
beam and arrest the pans before placing any object on
the pans. To test for equilibrium lower the beam and
then release the pans. Before removing any object or
weight from the pans, raise the beam to arrest the pans.
3. Protection of Pans
Never weigh chemicals directly on the pans since they
may injure the pans. Never weigh chemicals on paper
in an analytical balance. Use weighing bottles, watch
glasses, or aluminum laboratory dishes as containers
for weighing. The aluminum dishes are used and then
discarded.
Caution
Sodium hydroxide should not be weighed in an
aluminum dish.
4. Temperature
Weigh objects at room temperature. Differences in
temperature will cause air currents which lead to errors
in weighing.
5. Rest Point
Determine the zero rest point at each sitting.
6. Maximum Load
Do not overload the balance. Refer to manufacturer's
literature for maximum capacity.
7. Cleanliness
Keep the balance clean. If any chemical is spilled,
clean it up at once. Do not use liquids for cleaning the
pans. Use a balance brush.
Care and Use of the Weights
Handle Class S weights only with forceps, preferably
bone-tipped. To counterbalance an object, try the large
weights first and then the smaller in systematic order.
Always use the least number of weights possible, for
example, a 3-gram weight in preference to a 1- and a 2-gram
weight. To avoid oscillation, place large weights in the
center of the pan. Use great care to avoid dropping weights.
Always double-check the result of a weighing by adding the
values implied by the empty compartments in the box of
weights and then record immediately in a notebook.
References
1. Standard Specification for Laboratory Glass
Graduated Burets; American Society for Testing and
Materials: ASTM Designation E 287-94, Philadelphia,
PA, March 1994.
2. Standard Specification for Laboratory Glass
Volumetric Flasks; American Society for Testing and
Materials: ASTM Designation E 288-94, Philadelphia,
PA, April 1994.
3. Standard Specification for Glass Volumetric
(Transfer) Pipets; American Society for Testing and
Materials: ASTM Designation E 969-95, Philadelphia,
PA, December 1995.
4. Standard Specification for Glass Measuring Pipets;
American Society for Testing and Materials: ASTM
Designation E 1293-94, Philadelphia, PA, April 1994.
5. Standard Specification for Laboratory Weights and
Precision Mass Standards; American Society for
Testing and Materials: ASTM Designation E 617-91,
Philadelphia, PA, October 1991.
8 Processing KODAK Motion Picture Films, Module 3, Analytical Procedures H24.03