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The first chart shows mixtures of different SRMs,
where we have sulfur on the x-axis
and the uncertainty on the y-axis.
We wish to make about a 1% sulfur standard,
using two bituminous coals, 2693 and 2692b.
The mixture of those coals at 1%
will give us an uncertainty of about .01% sulfur absolute.
Going to the second chart, we see that, to make that blend,
we would need to use 24% 2693 and 76% 2692b.
First, going to weigh out some 2693 onto weighing paper,
and let it equilibrate with the room atmosphere.
We do this so that we will not have
changing weights as we're doing our weighings.
So I've tared a piece of weighing paper.
I'm now going to weigh out about 1 or 2 grams of 2693,
after thoroughly mixing it, using a glass scoop.
So, I've gone into the bottle, filled the scoop
with about 1 gram, which I then put on the weighing paper.
Take a second portion, put it on the weighing paper.
Close the bottle.
Then, gently spread out the coal so that it can equilibrate
with the room atmosphere.
Place the weighing paper and the coal back
on the toploader.
And we will monitor that weight for a short period of time,
until it stabilizes and is no longer
picking up moisture from the atmosphere.
So record that weight.
Now, mixing the second component, 2692b.
Again, we're going to take about 2 grams
and place it on weighing paper.
We record the weight
and note the time that the weighing was taken.
We're now going to weigh
the first component for the sulfur standard.
We'll place the sulfur crucible on the five-place balance.
We will record its empty weight in the notebook.
Taking a clean spatula, we're going to remove 60 milligrams,
approximately, from the sample on the weighing paper
while it's on the toploader and add that
to the sulfur crucible that is on the five-place balance.
And now, we will record that weight,
and the difference between this weight
and the initial weight of the crucible will give us
the exact weight of how much 2693 we added to the crucible.
And because the sulfur crucible is already
on the five-place balance, we will remove
about 190 milligrams of sample and place it
into the sulfur crucible to make the total about 250 milligrams.
Okay, we've now zeroed the balance because we want
to remove 190 milligrams of sample and place it
in the crucible that is on the five-place balance.
Taking about 190 milligrams and adding it
to the 60 milligrams, approximately,
that's on the balance, for a total of 250 milligrams.
And the sulfur concentration
of these two combined components will be about 1%.
Samples of each SRM were taken, for moisture determinations,
at the same time as the sulfur samples were weighed.
We get the exact weights of the amount added
to the sulfur crucible by difference.
After the moisture determinations
have been completed and the exact dry weights
of the two components have been determined, we can then go
to our web site or a computer program and determine
what the exact concentration of the standard is.