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I'm sending this due to a question from a student concerning the
pre-lab.<br>
<br>
For exp 3 (DRL) there is a pre-lab. You will be plotting data for a
Beer's Law<br>
graph, as explained in the manual. You are following the absorbance
of the CrEDTA-<br>
product during lab and you need to determine how the Abs is related
to the conc.<br>
of the product, [CrEDTA-]. The eqn that relates them, given on page
20, is<br>
A = m[CrEDTA-] (where m = e*b, m is the extinction coef., e is
molar absorptivity<br>
and b is the path length of light through the solution, the
cuvette). The pre-lab gives<br>
you 3 data points for [CrEDTA-] and Absorbance. There is a fourth
data point you<br>
are suppose to use. As described in the manual you are suppose to
add the (0,0) data<br>
point. You do this because when [CrEDTA-] = 0 the Absorbance has to
be zero. The<br>
manual states your line should go through the origin. You are
suppose to force it<br>
through the origin. When doing this graph for the report and using
Excel and you add<br>
a trend line there is an option which will force the line through
the origin and give<br>
an eqn similar to y = 189 * x (note, the intercept is zero). You
are suppose to do<br>
this.<br>
<br>
However, for the pre-lab the program does not give an option for
doing this and does<br>
not do this. You enter the data given and the (0,0) point. You
then click on "Plot Data".<br>
You then need to calculate the slope. You can do this by taking
points from the line<br>
on the graph. You can use Excel to plot the data given, which would
be easier. However,<br>
do <font color="#ff0000"><b>NOT</b></font> force the line through
the origin for the pre-lab.<br>
<br>
If you calculate the slope by hand from the graph you'll need to
take points from the graph.<br>
It's a little hard to read the labels on the x-axis. The numbers
plotted are 0.0000, 0.0004,<br>
0.0008, 0.0012, 0.0016, etc. (out to the 4th decimal place).<br>
<br>
Dr. Moga sent an e-mail about this exp earlier today. I will be
sending a couple myself.<br>
<br>
I hope this e-mail and the others will help. I will also be
discussing this exp further in<br>
lecture on Monday.<br>
<br>
Dr. Zellmer<br>
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