often better to make a proper allowance for this small zero error than
to disturb the balance by an attempt at correction. Unless a student
thoroughly understands the construction of a balance he should never
attempt to make adjustments, but should apply to the instructor in
charge.
The object to be weighed should be placed on the left-hand balance pan
and the weights upon the right-hand pan. Every substance which could
attack the metal of the balance pan should be weighed upon a
watch-glass, and all objects must be dry and cold. A warm body gives
rise to air currents which vitiate the accuracy of the weighing.
The weights should be applied in the order in which they occur in the
weight-box (not at haphazard), beginning with the largest weight which
is apparently required. After a weight has been placed upon the pan the
beam should be lowered upon its knife-edges, and, if necessary, the
pan-arrests depressed. The movement of the pointer will then indicate
whether the weight applied is too great or too small. When the weight
has been ascertained, by the successive addition of small weights, to
the nearest 5 or 10 milligrams, the weighing is completed by the use of
the rider. The correct weight is that which causes the pointer to swing
an equal number of divisions to the right and left of the zero-point,
when the pointer traverses not less than five divisions on either side.
The balance case should always be closed during the final weighing,
while the rider is being used, to protect the pans from the effect of air
currents.
Before the final determination of an exact weight the beam should
always be lifted from the knife-edges and again lowered into place, as it
frequently happens that the scale pans are, in spite of the pan-arrests,
slightly twisted by the impact of the weights, the beam being thereby
virtually lengthened or shortened. Lifting the beam restores the proper
alignment.
The beam should never be set in motion by lowering it forcibly upon
the knife-edges, nor by touching the pans, but rather by lifting the rider
(unless the balance be provided with some of the newer devices for the
purpose), and the swing should be arrested only when the needle
approaches zero on the scale, otherwise the knife-edges become dull.
For the same reason the beam should never be left upon its knife-edges,
nor should weights be removed from or placed on the pans without
supporting the beam, except in the case of the small fractional weights.
When the process of weighing has been completed, the weight should
be recorded in the notebook by first noting the vacant spaces in the
weight-box, and then checking the weight by again noting the weights
as they are removed from the pan. This practice will often detect and
avoid errors. It is obvious that the weights should always be returned to
their proper places in the box, and be handled only with pincers.
It should be borne in mind that if the mechanism of a balance is
deranged or if any substance is spilled upon the pans or in the balance
case, the damage should be reported at once. In many instances serious
harm can be averted by prompt action when delay might ruin the
balance.
Samples for analysis are commonly weighed in small tubes with cork
stoppers. Since the stoppers are likely to change in weight from the
varying amounts of moisture absorbed from the atmosphere, it is
necessary to confirm the recorded weight of a tube which has been
unused for some time before weighing out a new portion of substance
from it.
WEIGHTS
The sets of weights commonly used in analytical chemistry range from
20 grams to 5 milligrams. The weights from 20 grams to 1 gram are
usually of brass, lacquered or gold plated. The fractional weights are of
German silver, gold, platinum or aluminium. The rider is of platinum or
aluminium wire.
The sets of weights purchased from reputable dealers are usually
sufficiently accurate for analytical work. It is not necessary that such a
set should be strictly exact in comparison with the absolute standard of
weight, provided they are relatively correct among themselves, and
provided the same set of weights is used in all weighings made during a
given analysis. The analyst should assure himself that the weights in a
set previously unfamiliar to him are relatively correct by a few simple
tests. For example, he should make sure that in his set two weights of
the same denomination (i.e., two 10-gram weights, or the two
100-milligram weights) are actually equal and interchangeable, or that
the 500-milligram weight is equal to the sum of the 200, 100, 100, 50,
20, 20 and 10-milligram weights combined, and so on. If discrepancies
of more than
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