THE MEASUREMENT
OF WEIGHT
I BASIC PRINCIPLES

Weight is the force of gravity on an object.

Balances measure this force.
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WEIGHT versus MASS


Mass: amount of matter in an object; units
are kilograms.
Mass doesn’t change when object is moved
to new location.


Astronaut is “weightless” in space but mass is the
same.
In the lab, we weigh objects.
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WEIGHING AN OBJECT

We compare pull of gravity on sample with
pull of gravity on standard(s) of established
mass.
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BASIC BALANCE
When the beam is
exactly balanced,
gravity is pulling equally
on sample and standard
they are the same
weight.
Hence, “balances”.
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OLDEST SCIENTIFIC
INSTRUMENTS


Mechanical balances have one or more
beams; objects are placed on a pan attached
to a beam.
Have been used for hundreds of years.
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MECHANICAL BALANCES

Still used for some purposes – balancing
centrifuge tubes.
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ELECTRONIC BALANCES

Still measure pull of gravity on objects but do
not have beams.

Use an electromagnetic force rather than
weights to counterbalance the sample.
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ELECTRONIC BALANCES

Produce an electrical signal when a sample is
placed on the weighing pan, the magnitude of which
is related to the sample’s weight.

To convert electrical signal to a weight value,
balance compares the electrical signal from the
sample to the signal from a standard(s) of known
weight.
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ELECTRONIC BALANCES

Electronic balances make the comparison
between sample and standard sequentially:


Calibrate with a standard at one time
Later the sample is weighed
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II CHARACTERISTICS AND
TYPES OF BALANCES

Range is the span from the lightest to the
heaviest weight the balance can measure.

Capacity is the heaviest sample balance can
weigh.
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SENSITIVITY AND
READABILITY

Sensitivity: smallest weight that will cause a
change in the response of the balance.

Sensitivity determines the number of places to the right
of the decimal point that the balance can read accurately
and reproducibly.
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
Extremely sensitive balances weigh accurately
to the nearest 0.1 microgram (or 0.0000001 g).

Less sensitive balance might read to the
nearest 0.1 gram. Manufacturers express the
sensitivity of their balances by their readability.
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ANALYTICAL BALANCES

Analytical balances optimize sensitivity and
can weigh samples to at least the nearest
tenth of a milligram (0.0001 g).

Are both mechanical and electronic balances
of all types.
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RANGE, CAPACITY,
SENSITIVITY



Range, capacity and sensitivity are
interrelated.
Don’t use analytical balance to weigh
samples in the kilogram range and vice
versa.
Choose best balance – not simplest to
operate.
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III PROPER OPERATION;
AVOIDING ERRORS

Accuracy and precision of modern balances is
primarily affected by:




User technique and lab conditions
Maintenance
Design and construction of balance
Accuracy and precision of instruments is excellent
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OPERATING AN ELECTRONIC
ANALYTICAL BALANCE
1. Level balance.
2. Adjust the balance to zero with pan clean and empty and
chamber doors closed.
3. Tare the weighing container or weigh the empty vessel.
4. Place sample on weighing pan; read the value for the
measurement.
5. Remove sample, clean.
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WAYS TO GET SYSTEMATIC
ERROR:






Don’t level the balance
Don’t adjust to zero
Allow vibration, drafts, jostling
Don’t close balance doors
Touch samples and their containers
Allow temperature to fluctuate
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




Ignore static charge
Ignore loss or gain of moisture
Place overload on weighing pan
Select wrong weighing vessel
Make a mess and don’t clean up
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QUESTION
A technician weighs a cell preparation on an
analytical balance and observes that initially
the weight is 0.0067 g. A few minutes later
the weight is 0.0061 g. What might be
happening here?
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ANSWER
Assuming the technician avoided temperature
effects and drafts, sample probably lost
moisture or other volatile component.
Analytical balances are very sensitive, so see
these effects.
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MAINTENANCE AND
CALIBRATION



Calibration brings balance readings into
accordance with internationally accepted
standards.
Calibration must be periodically checked in
the laboratory of the user.
Practice in lab today.
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CALIBRATION




Level balance
Set to zero
Place standard on balance, often 100 g
Adjust to upper weight
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HOW OFTEN TO CALIBRATE?





It depends
Time, use and abuse affect response
Check whenever balance is moved
In microgram range, check when weather
changes
Mechanical balances – when service person
comes
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STANDARDS



Calibrate with standards; metal objects
whose masses are known (within limits of
uncertainty).
Traceability comes from standards.
Accuracy of any weight determination is
limited by the accuracy of the standards used
for comparison.
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STANDARDS

Handle standard weights with tongs because
they are damaged by skin oils and cleaners.

Standard weights need to be periodically
recertified since change over time due to
scratches, wear, and corrosion.
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QUALITY PROGRAM




Calibrate periodically.
Check precision and linearity periodically.
Consistently check and record weights of
standards – tests accuracy.
Follow SOPS.
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LINEARITY



Linearity error occurs when a balance is properly
calibrated at zero and full-scale (the top of its range)
but the values obtained for weights in the middle of
the scale are not exactly correct.
If a balance has linearity error, have it repaired
professionally.
Practice in lab today.
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QUESTION

A technician is verifying the performance of a
laboratory balance and weighs a 10 g standard six
times.
a. What feature of the balance is she checking?
b. What is the standard deviation for the balance?
(Remember the units.)
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c. The balance specifications require that its
precision be better than or equal to 0.001 g. Does it
meet these specifications? If not, what should be
done?
WEIGHTS
10.002 g
9.998 g
10.001 g
10.002 g
9.999 g
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10.002 g
ANSWER
The SD is 0.002 g, which does not meet the
specification.
There may be a problem with the balance
although it is possible that the operator is not
using good weighing practices.
The balance should be taken out of use and the
cause of the imprecision should be
investigated.
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IV MASS versus WEIGHT

Value read from a balance is the weight of an
object, not its mass.

May seem surprising. After all, the object is
directly compared to a standard whose mass
is known.
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

Standard has a mass of 1 gram and the
sample exactly balances the standard.
Therefore, it seems logical that the sample
has a mass of 1 gram. Not so.
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BOUYANCY EFFECT

Major force measured in weighing is force of
gravity


But there is slight buoyant force by air that makes
any object appear “lighter” than it really is.
If the same object is weighed in air and in a
vacuum, the object will be slightly heavier in
the vacuum.
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BUOYANCY EFFECT

Principle of buoyancy: any object will
experience a loss in weight equal to the
weight of the medium it displaces.


ships float
helium balloons rise
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
Balances are calibrated with metal mass
standards. Metal has a relatively high density
compared to aqueous solutions and other
materials.
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
A 1 g metal mass standard displaces less air than 1
g mass of water


The metal standard is buoyed less by the air.
Difference in buoyancy of mass standard and
sample explains why the standard and sample have
the same weight but different masses.
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
If a balance were calibrated with a mass standard
whose density were identical to that of the sample,
then the weight of the sample would equal its mass.

Or, if the standard and sample were weighed in a
vacuum, then weight and mass would be equal.

But, neither is the case.
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
Call this buoyancy error.



Weight readings for aqueous solutions have a buoyancy
error of roughly 1 part in 1000.
This buoyancy error is small enough as to be of little
concern in most applications.
Distinction between mass and weight is generally
ignored except when very high accuracy
measurements must be made.
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
If necessary, there are equations to correct
for the buoyancy effect that take into
consideration the air density at particular
atmospheric conditions and the density of the
object being weighed.
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QUESTION

1. Suppose you need to prepare a solution
with a concentration of 15 mg/mL of a
particular enzyme. You try to weigh out 15
mg of the enzyme on the analytical balance,
but find that it is extremely difficult to get
exactly 15 mg. Suggest a strategy to get the
correct concentration even if you cannot
weigh exactly 15 mg.
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ANSWER
Weigh out as close to 15 mg as possible and then
calculate the amount of buffer required to dilute
the enzyme to a concentration of 15 mg/mL.
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QUESTION

Suppose a 100 g standard mass is
accidentally dropped on the floor. As a result,
its mass is slightly less than 100 g. If the
standard is used to calibrate a balance, what
will happen to subsequent readings from that
balance? What type of error is this?
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ANSWER
All subsequent readings with that balance will
be a little too high. Using this standard
causes a systematic error.
Dropping it was a gross error.
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