Transcript Why Weigh by Difference? The amounts of solid samples weighed in this course are generally small.

Why Weigh by Difference?
The amounts of solid samples weighed in this course are generally
small. I.e., often 500 mg or less.
Analytical balances can weigh objects with a precision of about 0.2 mg.
• For a 500 mg sample, this is 100 x 0.2 / 500 = 0.04 %
We generally strive for better than 1% in precision and accuracy in
exercises
• 1% of 500 mg = 5 mg
• The analytical balance is certainly capable of providing well
beyond this level of precision and accuracy
What else could keep us from achieving an accuracy of 1%?
• The loss of sample in the process of transferring it from one container
to the next.
The loss of a 10 mg* sample by any means would constitute a
• 100 x 10 / 500 = 2% error.
*
For comparison, a drop of water weighs about 50 mg.
• Each time an intermediate container is used, there is a risk of
some of the solid adhering to the container.
• This includes a spatula, watch glass, and especially paper.
• We would like to avoid the use of an intermediate container.
One way to accomplish this is:
• Weigh the final container and leave it on the balance pan
• Transfer solid until the final combined weight of the
container and the solid is the required amount
• Subtract the weight of the container from the final
weight to get the weight of the transferred solid
Problem: The final container (beaker, flask, etc.) almost always
exceeds the capacity of an analytical balance (~100g)!
The top loading balance can accommodate such weight,
but it provides a precision of only about 20 mg
How can we avoid intermediate containers when using the analytical balance?
Weighing by difference!
Weighing by difference will generally work in this
course because substances that need to be weighed
with high precision are almost always distributed in
sample vials which can safely be put on the balance.
• Weigh the sample vial
• Transfer solid until the weight of the sample vial and the solid
is less than the original weight by the required amount
• Subtract the weight of the container and final solid from the
initial weight to get the weight of the transferred solid
While this method may require several transfers, it
will always work and will never cause an excessively
heavy object to be put on the balance pan.
Suppose we wish to transfer a sample of 200 mg
from a vial to a beaker
•
When we are asked to perform such a transfer, it is understood that
the actual weight of the sample can usually be within 20% of the
prescribed weight. For 200 mg this means 200  40 mg.
•
The steps are as follows:
1. Weigh the vial and its contents
2. Transfer a sample estimated to weigh 200 mg or less directly to
the beaker. (The ability to estimate such amounts will improve
with experience.)
3. Reweigh the vial and its contents
If the difference
between initial and final
weights is between 160
and 240 mg
- we have
accomplished our goal.
If the difference
between initial and final
weights is less than
160 mg
- we need to transfer
a (small) additional
amount to the beaker
If the difference
between initial and final
weights is greater than
240 mg
- we must consider
whether to discard the
sample. This will depend
on the rest of the
procedure.
Weigh the Vial and its contents
14.7936 g
Remove Vial from Balance and Tap out Sample into
Container
Reweigh Vial
14.6402 g
14.7936 - 14.6402 = 0.1534 g = 153.4 mg
Result
• In this case, the transferred sample weighs considerably less
than the desired amount
• 153.4 mg is less than 160 mg, which was the lower limit of an
acceptable sample size (Remember, we are trying to weigh
200  40 mg)
• We therefore repeat the process by transferring an additional
amount of material from the vial into the beaker.
• NOTE: We now have a much better basis for the visual
estimation of the weight of a sample
Remove Vial from Balance and Tap out Additional
Sample
Reweigh Vial
14.5783 g
14.7936 - 14.5783 = 0.2153 g = 215.3 mg
Result
• In this case, the weight of the total transferred sample is
within the limits of acceptable size
• 215.3 mg is between 160 mg and 240 mg - the acceptable sample
size
We have succeeded in our original goal!
When several weighings of samples of similar sizes
are required, the subsequent samples are much more
easily estimated by eye.