Download The Analytical Balance

Chemistry 119: Experiment 1
The Analytical Balance
Operation of the Single-Pan Analytical Balance
Receive instruction from your teaching assistant concerning the proper
operation of the Sartorius BP 210S analytical balances that you will be using in the
C119 laboratory.
Learn what is meant by direct weighing and weighing by difference. With
few exceptions, weighing of samples of chemicals must be performed by placing those
materials in a vessel and obtaining the combined weight of the vessel and sample. The
weight of the sample is obtained by subtracting the weight of the clean vessel. In the
Sartorius balances, a “taring” mechanism exists by which the balance is adjusted to
read zero mass when the clean vessel is on the balance pan. Then the mass of sample
added to the pan can be read directly. (The analytical balance determines the mass of
the object, i.e., the result is independent of the local gravitational field. The weight
(gravitational force on the object) does depend on local conditions. For example, your
weight on earth’s moon would be much less than on the earth itself. Nevertheless,
scientists tend to be fuzzy about this distinction, i.e., we say “weight” when we really
mean “mass”).
For accurate weighing of corrosive materials, approximate amounts should be
weighed first on a triple-beam or other auxiliary balance to protect the analytical
balance from spillage and corrosive vapors. The procedure for weighing silver nitrate
or iodine, for instance, is as follows. Weigh an empty vessel on an analytical balance
(using a cover if the material is volatile). Record the weight. Take the vessel to a
rough balance and weigh into it the approximate amount of material to be taken.
Accurately reweigh the vessel plus material on the analytical balance. The vessel must
not be handled directly between analytical weightings; use tongs, finger cots, or other
protection.
Note that accurate weighing of materials, the density of which differs greatly
from that of the stainless steel weights (which have a density of 7.8 g/mL) that are used
to calibrate the balance, will require a correction for air buoyancy. This correction can
be as large as 10 mg, as is demonstrated in the pipette calibration which follows in
Experiment 2.
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Rules for Using an Analytical Balance
1. On the front of the balance are seven keys or push
switches. Of these, only the key labeled “ | / Ó ” and the two
“TARE” keys at the extreme left and right are needed for
operation of the balance in this course. The “ | / Ó ” key is a
Standby control. If the balance display shows a small “O” in
the lower left corner, the balance has been set to standby and
needs to be activated by pressing the
“ | / Ó ” key. Any deviations from the above should be
reported immediately to the laboratory instructor.
2. With the balance activated and nothing on the balance pan,
the reading should be close to 0.0000 g. To restore the
readout to exactly 0.0000 g, press one of the two TARE keys.
The readability of the balance is 0.1 mg. You should always
record your weights to the nearest 0.1 mg.
3. You are now ready to weigh an object. Open the balance
door and gently place the object on the center of the balance
pan. Close the balance door while weighing an object to
prevent air currents from disturbing the reading. Read the
weight from the balance display. You may need to wait 5-10
seconds for the reading to stabilize. When finished, the
operator should close the balance door to prevent dust and
dirt from entering the balance. The balance may be left in the
activated condition for use by the next student.
4. Only glass, ceramic, metal (nonreactive), or plastic objects
and containers should be placed in direct contact with the
balance pan to prevent corrosion.
5. Handle dried objects with tongs (or finger pads)—not with
bare hands—to prevent grease and moisture from being
added.
6. To be weighed accurately, all objects must be at room
temperature. A warm object sets up convection currents inside
the balance case, which will make an object appear lighter
than it really is. Also, warm air inside a container is less dense
than the air it displaces and this also leads to a negative
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determinate error. The error can exceed 20 mg for a 50 g
container!
7. Objects that possess a static charge may give erroneous
weight readings. Waiting a few minutes may be sufficient for
the charge to dissipate, or the object may be wiped with a
faintly damp chamois cloth.
8. Volatile materials require special precautions, such as
weighing in sealed glass containers, such as an ampoule. Even
when weighed in conventional containers, the containers
should be tightly covered to prevent loss and corrosion to the
balance.
9. To prevent reading errors, record the weight in your
notebook and then consciously read the number again and
place a check next the recorded number to signify that you
have verified the reading. An inversion of numbers, for
example, will lead to an error in the analysis just as surely as
any other mistake in procedure.
10. When weighing chemicals (dry solids) that you place in a
container (vial or beaker), you should use the TARE feature to
“zero out” the mass of the container. Then the weight you
read is the actual mass of the chemical that has been added to
the container. Under no circumstances should the chemical be
added to the container while it is on the balance pan. Remove
the container from the balance case, add the chemical, then
return the container to the balance case and determine its
weight. The reason for this stricture is to avoid spilling
chemicals inside the balance case.
11. The balance and case should be kept scrupulously clean.
A camel's hair brush is useful for the removal of dust or spilled
material.
12. Under no circumstances should a balance be adjusted
without prior consent from the instructor. Any balance that is
not level (bubble indicator) or that cannot be zeroed should be
reported immediately.
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Laboratory Exercise: Direct Weighing of Pennies
In this experiment, we will practice the use of the analytical balance by
weighing an inert material, such as a penny. We will try to determine the average
weight of a penny from several weighings. Some experience with simple statistics will
also be included.
Apparatus:
210-g capacity Sartorius BP 210S analytical balance
2, 100-mL beakers
10 new pennies
drying oven (110 oC)
tongs or forceps
Chemicals:
95% ethanol, 10 mL per student
Procedure:
1.
Obtain a clean, dry 100-mL beaker. Weigh this to ±0.1 mg after zeroing the
balance carefully.
2.
Add 10 clean pennies to the beaker, and add just enough 95% ethanol to
cover the pennies with liquid. Swirl and pour off the liquid into another clean
beaker.
3.
Place the beaker in a 100 oC oven for 5 minutes (no more!), then let it cool to
room temperature. Weigh each of the 10 pennies (in the 100-mL beaker),
taking care not to touch them with your fingers.
4.
To obtain some idea of the reproducibility of weighing a given object,
determine the mass of one of the pennies five times.
5.
Weigh the beaker and all 10 pennies together.
Calculations:
1.
Calculate the mean and standard deviation for weighing the single penny
five times. Compare the reproducibility and readability of the analytical
balance.
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2.
Calculate the weights of each penny and the weight of all ten pennies. Then
calculate the mean, median, standard deviation and confidence interval at 90 and 95%
confidence for the ten penny weights. Calculate the
relative standard deviation by
the following formula:
RSD (%) = s 100
x
3.
Calculate the fraction of pennies having weights within 1 and 2 standard
deviations from the mean.
4.
Add the 10 penny weights, and compare with the measured total weight. Do
these agree? If not, explain why there is a discrepancy on your report sheet.
Report:
1.
Answer all questions given above.
2.
Report the estimate of the mean weight of a your group of pennies, with
standard deviation, relative standard deviation and 90 and 95% confidence
intervals.
3.
Report the sum of weights of the ten pennies and the weight of all ten pennies
taken together.
4.
Comment on any discrepancy between the measured total weight and sum of the
individual weights.
5.
Assume that your mean penny weight and standard deviation are the same as the
mean weight and standard deviation (σ) of the population of all existing new
pennies
and that this population of weights is normally distributed (by the way, there is no basis for the
truth of such an assumption). What are the
chances that you will find a “rich” penny,
one that exceeds the mean weight by 10%?
6.
Pennies are constructed principally of zinc (with a thin copper coating). Assuming
that the average density of a penny is identical to that of pure zinc, calculate the mean
volume of a penny. Compare this to the volume of the same mass of pure
copper. The
periodic table of the elements should give you an idea of why these
two volumes are very
similar. Explain.
This experiment has been adapted from a laboratory manual authored by Professor S. D.
Brown. Last revision: 8/22/97.
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