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PRESENTER: The balance is arguably the most used piece
of equipment in the chemistry laboratory.
However, only proper technique, care, and maintenance
will allow accurate measurements and will keep the balance
functioning correctly.
A balance is used to determine the mass of a solid or liquid.
You should remember that mass is defined
as a measure of the amount of matter, regardless of location,
and that weight is defined as the force resulting
from gravity, which varies from place to place depending
on the distance from the center of the Earth.
Because weight is directly proportional to mass
on any location on Earth, we will use these terms
interchangeably.
If you decide to do some space travel,
however, take care to remember the differences.
Four major types of balances are commonly
used, the triple beam balance, the top-loading balance,
the analytical balance, and the microanalytical balance.
The difference between these balances
is the amount of precision they afford.
The triple beam balance is precise to 10 milligrams,
the top-loading to 1 milligram, the analytical to 1/10
of a milligram, and the microanalytical to 1/1,000
of a milligram.
Precision comes at a cost, however.
As precision increases, so does the susceptibility
of the balance to environmental conditions
as well as balanced cost.
Therefore, you should choose a balance that is only
as precise as necessary.
The two balances that you will most commonly encounter
include the top-loading balance and the analytical balance.
The top-loading balance is typically
used for rough measurements or for weighing chemicals
that will be used in excess.
The analytical balance is used when precise measurements
are desired.
This video will focus on the use, care, and maintenance
of an analytical balance.
The topics discussed can be extrapolated
to the top-loading balance as well.
To weigh an object on an analytical balance,
turn the balance on if it is not on already
and allow it to warm up for 30 minutes.
Inspect the balance and make sure that it is clean.
If necessary, clean off the balance pan
with a soft painter's brush.
Make sure the doors are closed and press the control bar
to tare the balance.
A reading of 0.0 milligrams with no other display
should appear if the balance is stable.
Gently place the object to be weighed
in the center of the balance pan, close the doors,
and record the weight that is indicated on the display
after the numbers have ceased to fluctuate.
Remove the object from the balance pan
and repeat the process two more times to ensure precision.
Finally, average the three readings to determine
the weight of the object.
When weighing a chemical, care must
be taken to never allow the chemical
to touch the balanced pan.
Instead, weigh the chemical in an appropriate container.
If possible, weigh the chemical directly
into the destination vessel.
This action eliminates the transfer step
and the potential loss of material.
If using the destination vessel is not possible,
choose the smallest weigh boat or piece of paper
that will effectively hold the desired amount of chemical.
The heavier the container, the less accurate
the measurement because just a small weight
difference will exist between relatively large numbers.
In addition, the more surface area, the more
places for chemicals to stick to instead of being transferred
to the destination vessel.
Place the weighing container on the balance pan,
close the doors, and tare the balance
if you are using weigh paper, first,
crease the paper along the diagonal
to facilitate the transfer of chemicals.
Remove the weighing vessel from the balance
and load an estimated amount of chemical
onto it with a spatula.
Placed the loaded weighing vessel back on the balance
pan and close the doors to obtain a reading.
If the weight is too high, take the weigh vessel
off the balance pan and transfer the excess chemical
into an appropriate waste container,
never back into the original reagent bottle.
If the weight is too low, introduce more chemicals
to the weigh container.
Remove the weigh vessel from the balance
pan every time additional chemical is introduced.
Finally, transfer the chemical from the waste paper
or weigh boat into the appropriate reaction vessel.
Reweigh the paper boat to determine
the exact amount of chemical that was transferred.
After finishing with the balance,
use a soft brush to clean up any solids from the balance pan
and wipe up any liquids.
Close the doors to prevent dust and dirt
from entering the balance.
Clean the surrounding area and remove all items
that you brought there.
Leave the balance and area surrounding it
in better shape than when you found it.
The accuracy of your measurements
depends on both good technique and environmental factors.
This video will highlight the most important
of these guidelines.
Keep the balance doors closed at all times
to prevent air currents from causing the balance pan
to bounce slightly.
Never lean or lay a hand on the balance table
to prevent unstable measurements.
Wear gloves to prevent oil from depositing
on the glassware which falsely increases the mass.
Weigh a chemical directly into the reaction vessel.
If that is not possible, use the smallest weigh boat
or weigh paper that will hold your desired
amount of chemical.
Ensure that the outside of the way vessel
is completely dry before placing it on the balance pan,
and be sure to place the object on the center of the balance
pan.
Allow moist chemicals to dry completely
to avoid elevated readings and avoid
breathing on an object which condenses moisture on it
and falsely increases its mass.
Weigh ultra dry objects or ones immediately
removed from a desiccator at identical time intervals
because they absorb moisture and gain weight
the longer they sit in the atmosphere.
Finally, allow hot or cold objects
to reach room temperature before weighing.
A hot object should be cooled in a desiccator
to prevent the pick up of moisture.
Hot or cold objects create a convection current in the air
around the balance pan, which reduces
the air pressure on the balance pan
and results in an unstable reading.
The accuracy of a measurement is only
as good as the instrument that is used.
To keep your balance functioning correctly,
the following precautions should be taken.
Never weigh chemicals directly on the balance pan.
Never add chemicals to the weighing vessel
while it is on the balance pan.
Never pour a chemical directly out of a reagent bottle
and into the weigh vessel.
Use a spatula instead.
Pay attention to the maximum weight
allowed by a particular balance and do not overload it.
When using a balance, be aware of the warning
signs which indicate that the balance is malfunctioning.
After recording the weight of a chemical,
remove it from the balance pan.
The numbers and the display should again
read 0.0 milligrams.
If the displayed value is 0.3 milligrams or greater,
then the 0 level was significantly
shifted during the weighing process
and the chemical must be reweighed.
Check the amount of drift that is occurring.
Drift is characterized by weight readings that do not stabilize
or by unstable readings with no weight applied.
All analytical balances show some degree of fluctuation
on their display and care should be
taken to keep this shift at a minimum.
If you notice a change in the amount of instability
on a particular balance, then it is malfunctioning.
Drift is affected by two environmental factors,
temperature and static electricity.
It can be minimized through their manipulation.
The temperature of the room and the internal temperature
of the balance should be kept constant within 2 degrees
Celsius.
Static electricity can be minimized
by avoiding the use of plastic containers
and by maintaining the humidity above 40%.
If you notice excessive drift, the following situations
may be occurring.
The balance may be exposed to a draft.
Close the doors.
The sample may be moist and undergoing evaporation
on the balance pan.
The sample might be ultra dry and absorbing moisture
on the balance pan.
The sample may not be at ambient temperature, which causes it
to produce convection currents.
The sample may be electrostatic.
If you notice drift while weighing a sample,
the most accurate reading is the first number
that appeared and was maintained for two seconds.
So far, we've discussed the use of a balance
for weighing solids, however, a balance
may also be used to weigh liquid samples.
Generally, it is easier and more accurate to determine
the volume of a liquid by using precision volumetric glassware,
such as a burette, glass pipette, syringe,
or automatic pipette, and then use the liquid's density
to calculate its mass.
However, directly measuring the weight of a liquid
is preferred in some situations, such as when
working with a vicious liquid.
As a final note, when weighing a volatile liquid
or one that evaporates readily, use a weighing bottle
with a lid.
Although using a balance is a relatively simple task,
accurate measurements can only be
ensured through good technique.
Exceptional care must be taken to maintain
a clean environment which helps to keep the balance functioning
properly.
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