exp 3 (DRL) pre-lab

[robert zellmer]

I'm sending this due to a question from a student concerning the pre-lab.

For exp 3 (DRL) there is a pre-lab.  You will be plotting data for a 
Beer's Law
graph, as explained in the manual.  You are following the absorbance of 
the CrEDTA-
product during lab and you need to determine how the Abs is related to 
the conc.
of the product, [CrEDTA-].  The eqn that relates them, given on page 20, is
A = m[CrEDTA-]  (where m = e*b, m is the extinction coef., e is molar 
absorptivity
and b is the path length of light through the solution, the cuvette).   
The pre-lab gives
you 3 data points for [CrEDTA-] and Absorbance.  There is a fourth data 
point you
are suppose to use.  As described in the manual you are suppose to add 
the (0,0) data
point.  You do this because when [CrEDTA-] = 0 the Absorbance has to be 
zero.  The
manual  states your line should go through the origin.  You are suppose 
to force it
through the origin.  When doing this graph for the report and using 
Excel and you add
a trend line there is an option which will force the line through the 
origin and give
an eqn similar to y = 189 * x  (note, the intercept is zero).  You are 
suppose to do
this.

However, for the pre-lab the program does not give an option for doing 
this and does
not do this.  You enter the data given and the (0,0) point.  You then 
click on "Plot Data".
You then need to calculate the slope.  You can do this by taking points 
from the line
on the graph.  You can use Excel to plot the data given, which would be 
easier.  However,
do *NOT* force the line through the origin for the pre-lab.

If you calculate the slope by hand from the graph you'll need to take 
points from the graph.
It's a little hard to read the labels on the x-axis.  The numbers 
plotted are 0.0000, 0.0004,
0.0008, 0.0012, 0.0016, etc. (out to the 4th decimal place).

Dr. Moga sent an e-mail about this exp earlier today.  I will be sending 
a couple myself.

I hope this e-mail and the others will help.  I will also be discussing 
this exp further in
lecture on Monday.

Dr. Zellmer
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