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I often get questions about recording the times for each of the
solutions. <br>
<br>
Each member of the group (2 per group) should have 4 <b>LARGE </b>test
tubes<br>
(75 mL, with the large openings), marked 1-4. One partner measures
the<br>
Cr(NO3)3 (using a buret) and one measures the EDTA (using a 10-mL
pipet<br>
to get a total volume of 20 mL).<br>
<br>
You should have 5 cuvettes, 4 for the 4 solutions and one for the
EDTA.<br>
Mark on the frosted side (if present) numbers 1-4 and a B for the
EDTA<br>
"blank". Make sure a frosted side is pointing toward you when you
put<br>
the cuvette in the machine. The first thing you do is put the
"blank" in and<br>
calibrate the machine to "zero". <br>
<br>
There are some cuvettes which don't have frosted sides. For these
cuvettes<br>
it doesn't matter which side you write on but one side has a little
triangle at<br>
the top so write on that side to keep things consistent. Make sure
the side<br>
you've written on points toward you. <br>
<br>
Do NOT use both types of cuvettes (some with frosted sides and some
without).<br>
All the cuvettes used should either have frosted sides or no frosted
sides.<br>
<br>
Technically, the moment of mixing is time = 0 for each solution. You
are not <br>
taking times relative to solution 1.<br>
<br>
Thus, you mix the Cr<sup class="moz-txt-sup"><span
style="display:inline-block;width:0;height:0;overflow:hidden">^</span>3</sup>+
and EDTA solutions for solution 1 and that is time = 0. <br>
Then you record the abs and the time it took to get the soln in the
spectrophotometer <br>
from the time of mixing. <br>
<br>
Then you mix the Cr<sup class="moz-txt-sup"><span
style="display:inline-block;width:0;height:0;overflow:hidden">^</span>3</sup>+
and EDTA solutions for solution 2. This is time = 0 for <br>
solution 2. Try to make it so you take the reading for soln 2 about
a minute after <br>
soln 1. This is not critical but just makes it easier in the long
run so the readings <br>
are spaced about a minute apart for each soln (including the
"blank"). <br>
<br>
And so forth for soln 3 and 4 and then the blank again to
recalibrate. <br>
<br>
After the initial readings try to space it out so every minute you
are taking a reading<br>
and the readings for each soln are about 5 minutes apart (this
doesn't have to be<br>
"exact"). So every 5th minute (between the readings for solns 4
and 1) you put the<br>
blank back in and recalibrate. <br>
<br>
This will require keeping track of a lot of information, time and
absorbance for<br>
four solutions over a 75 minute period.<br>
<br>
<b>The easiest thing to do is download an app to your smart phone
which allows</b><b><br>
</b><b>more</b><b> </b><b>than one stopwatch.</b><b> You want an
app that allows 4 stopwatches. </b><b>You</b><b><br>
</b><b>will use a stopwatch for each solution.</b> <b>There are
several of these for free, but<br>
you'll</b><b> </b><b>probably see messages pop up (trying to sell
antivirus programs, stuff<br>
about storage,</b><b> </b><b>etc.). You can always get rid of
the app once you're through<br>
with exp 17. </b><br>
<br>
If you don't have 4 stopwatches you can simply use a clock (wall or
phone) or a<br>
single stopwatch and write down the time shown. This will be more
difficult in the<br>
end than having 4 stopwatches. Later you will use this to get your
<b>ELAPSED </b>times, <br>
which is what you ultimately need. I often see people use a single
stopwatch on their<br>
phones and write the following: <br>
<br>
1 min for soln 1 <br>
2 min for soln 2 <br>
3 min for soln 3 <br>
4 min for soln 4 <br>
etc. <br>
<br>
This is not necessarily wrong in what you are doing in the lab but
it will be wrong in <br>
the data analysis. You need to use the <b>ELAPSED </b>times so
each solution should be<br>
starting at time zero and you will have to correct for this in the
report sheets. <br>
<br>
The solutions do not need to be spaced out at exactly 5 minute
intervals, it's such <br>
easier to keep track of things if you do. <br>
<br>
Remember, have a SINGLE data table on ONE page for the time and Abs
readings <br>
for all 4 solutions (time and Abs columns for each soln, clearly
marked, for a total of <br>
8 columns, as shown in a previous e-mail). You can record this in
one partner's<br>
notebook and copy it over to the other partner's notebook before
leaving lab.<br>
<br>
In the beginning, none of your first absorbance values should be
over 0.100. If so,<br>
something is probably wrong. If it's around 0.3 you put the
cuvette in wrong with<br>
the frosted side in the path of the light beam. If your
absorbance for any given<br>
solution jumps by about 0.3 between successive readings you've
likely put the<br>
cuvette in wrong (frosted side not pointed toward you). If you get
negative<br>
absorbance readings at any time something is wrong. <br>
<br>
For the first few readings things may look strange in terms of the
order of the<br>
absorbance values. This is just due to not having each solution
evenly spaced in<br>
time because of "fumbling around" at the beginning until you get
used to things.<br>
After about 15-20 minutes your absorbance values should be in the
order of <br>
solns 1 --> 4, with soln 1 having the lowest abs. and soln 4 the
highest. If not <br>
something may be wrong.<br>
<br>
If you suspect something is wrong speak to the TA. <br>
<br>
Record the data for at least 75 min AND a final Abs value of 0.4. If
the Abs of any <br>
soln is less than 0.4 by 75 min continue to record data for ALL
solns until ALL have <br>
Abs values of at least 0.4. If you reach 0.4 for all the solns
before 75 min (not <br>
likely, especially for soln 1) you need to continue recording data
until you hit the <br>
75 min mark. <br>
<br>
Make sure you keep the water bath for the cuvettes around 24-25 C.
Choose a<br>
temp. in this range and try to hold your water bath at that temp.
You want water <br>
up about 3/4 of the way on the cuvettes. Make sure you don't get
water in them <br>
because then you have to start over. Put a thermometer in the
beaker with the <br>
cuvettes and lean it up against something. You can take it out and
then put it back <br>
periodically to check the temp if you find that easier. You need to
try to keep the <br>
temp constant at a single temp between 24-25 C. To do this you may
need to <br>
periodically remove some water from the water bath and add cool or
warm water<br>
(you can use a little of the water from the boiling water bath for
this) using a<br>
medicine dropper or micropipet. <br>
<br>
As you can see, there's a lot going on. I haven't even covered
every little thing.<br>
You have to be very well prepared. <br>
<br>
Dr. Zellmer
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