Exp 17 - recording times, etc.

[Robert Zellmer]

I got a question this morning about recording the times for each of the 
solutions.

You should have 5 cuvettes, 4 for the 4 solutions and one for the EDTA.  
Mark
on the frosted side (if present) numbers 1-4 and a B for the EDTA "blank".
Make sure a frosted side is pointing toward you when you put the cuvette in
the machine.  The first thing you do is put  the "blank" in and 
calibrate the
machine to "zero".

Technically, the moment of mixing is time = 0 for each solution. You are 
not
taking times relative to solution 1.

Thus, you mix the Cr^3+ and EDTA solutions for solution 1 and that is 
time = 0.
Then you record the abs and the time it took to get the soln in the 
spectrophotometer
from the time of mixing.

Then you mix the Cr^3+ and EDTA solutions for solution 2.  This is time 
= 0 for
solution 2.  Try to make it so you take the reading for soln 2 about a 
minute after
soln 1.  This is not critical but just makes it easier in the long run 
so the readings
are spaced about a minute apart for each soln (including the "blank").

And so forth for soln 3 and 4 and then the blank again to recalibrate.

After the initial readings try to space it out so every minute you are 
taking a reading.
So every 5th minute (between the readings for solns 4 and 1) you put the 
blank back
in and recalibrate.

The easiest thing to do is simply use the wall clock or a clock on your 
phone and
write down the time on the clock.  Later you will use this to get your 
*ELAPSED *times,
which is what you ultimately need.  I often see people use a single 
stopwatch on
their phones and write the following:

     1 min for soln 1
     2 min for soln 2
     3 min for soln 3
     4 min for soln 4
     etc.

This is not necessarily wrong in what you are doing in the lab but it 
will be wrong in
the data analysis.  you need to use the *ELAPSED *times so each solution 
should be
starting at time zero and you will have to correct for this in the 
report sheets.

If you use your phone and can have 4 clocks or stopwatches that would 
work but will
quickly become difficult to keep track of and you are likely to make 
mistakes if you
are not careful.  Just use one clock or stopwatch (I find the clock 
easier) and write
down the time of each reading and get the elapsed times later when you 
do the
calculations.

The solutions do not need to be spaced out at exactly 5 minute 
intervals, it's such
easier to keep track of things if you do.

Remember, have a SINGLE data table on ONE page for the time and Abs 
readings
for all 4 solutions (time and Abs columns for each soln, clearly marked, 
for a total of
8 columns).

In the beginning, none of your first readings should be over 0.100. If 
so, something is
probably wrong.  If it's around 0.3 you put the cuvette in wrong with 
the frosted side in
the path of the light beam.  For the first few readings things may look 
strange in terms
of the order of the absorbance values.  This is just due to not having 
each solution
evenly spaced in time because of "fumbling around" at the beginning 
until you get
used to things.  After about 15-20 minutes your absorbance values should 
be in the
order of  solns 1 --> 4, with soln 1 having the lowest abs. and soln 4 
the highest.  If not
something may be wrong.  Speak to the TA.

Record the data for at least 75 min AND a final Abs value of 0.4. If the 
Abs of any
soln is less than 0.4 by 75 min continue to record data for ALL solns 
until ALL have
Abs values of at least 0.4.  If you reach 0.4 for all the solns before 
75 min (not
likely, especially for soln 1) you need to continue recording data until 
you hit the
75 min mark.

Make sure you keep the water bath for the cuvettes around 24-25 C. You 
want water
up about 3/4 of the way on the cuvettes.  Make sure you don't get water 
in them
because then you have to start over.  Put a thermometer in the beaker 
with the
cuvettes and lean it up against something.  You can take it out and then 
put it back
periodically to check the temp if you find that easier.  You need to try 
to keep the
temp constant and between 24-25 C.  To do this you may need to 
periodically remove
some water from the water bath and add cool or warm water (you can use a 
little
of the water from the boiling water bath for this) using an eye dropper 
or micropipet.

Dr. Zellmer

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