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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 <b>3 </b><b>LARGE
</b>test tubes<br>
(75 mL, with the large openings), marked 1-3. One partner measures
the<br>
Cr(NO3)3 (using a buret) and one measures the EDTA (using a 10-mL
pipet<br>
twice to get a total volume of 20 mL).<br>
<br>
You should have 4 cuvettes, 3 for the 3 solutions and one for the
EDTA<br>
"blank". Mark on the frosted side (if present) numbers 1-3 and a B
for the<br>
EDTA "blank". Make sure a frosted side is pointing toward you when
you<br>
put the cuvette in the machine. The first thing you do before
taking any<br>
absorbance readings for the solutions is put the "blank" in and
calibrate the<br>
machine to "zero". Do this just before mixing the reactant
solutions for<br>
solution 1.<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. If you decide to put the
cuvettes in<br>
so the triangle is set so it points to the side in the direction of
the light beam<br>
write on a side which you will point toward. Just be consistent.<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><sup>+</sup>
and EDTA solutions for solution 1 and start the timer for<br>
that solution. This is time = 0. Then record the Abs and the time
it took to get the<br>
soln in the spectrophotometer from the time of mixing. <br>
<br>
Then mix the Cr<sup class="moz-txt-sup"><span
style="display:inline-block;width:0;height:0;overflow:hidden">^</span>3</sup><sup>+</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 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 4 minutes apart (this
doesn't have to be<br>
"exact"). So every 4th minute (between the readings for solns 3
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>
three solutions over a 60 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 3 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 3. <br>
<br>
</b>Dr. Moga has links to an iPhone and an Android app for this in
her announcement<br>
on Carmen.
However, you'll have to copy and paste them into your browser from<br>
the announcement. Here are the links again,<br>
<br>
Android: Multi Stopwatch & Timer free <br>
<a
href="https://play.google.com/store/apps/details?id=com.ldroid.multistopwatchandtimer"
moz-do-not-send="true">https://play.google.com/store/apps/details?id=com.ldroid.multistopwatchandtimer</a><br>
<br>
iPhone/iPod/iPad: M Stopwatch - A Multiple Stopwatch<br>
<a
href="https://itunes.apple.com/us/app/m-stopwatch-multiple-stopwatch/id912819901?mt=8">https://itunes.apple.com/us/app/m-stopwatch-multiple-stopwatch/id912819901?mt=8</a><br>
<br>
Other good Android apps are:<br>
<a
href="https://play.google.com/store/apps/details?id=com.sjm.apps.watson">https://play.google.com/store/apps/details?id=com.sjm.apps.watson</a><br>
<a moz-do-not-send="true"
href="https://play.google.com/store/apps/details?id=com.egg.multitimer">
https://play.google.com/store/apps/details?id=com.egg.multitimer</a><br>
<br>
If you don't have 3 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 3 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>
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
data in the report.<br>
<br>
The solutions do not need to be spaced out at exactly 4 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 3 solutions (time and Abs columns for each soln, clearly
marked, for a total of <br>
6 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 --> 3, with soln 1 having the lowest abs. and soln 3 the
highest. If not <br>
something may be wrong.<br>
<br>
If you suspect something is wrong speak to the TA, sooner than later
is better. <br>
This way if you've made a mistake there's still time to start over.
You don't<br>
necessarily have to start everything over. If it looks like one
solution has a<br>
problem but the other two appear okay you can just redo the one
"bad" solution.<br>
<br>
Record the data for at least 60 min AND a final Abs value of at
least 0.4. If the<br>
Abs of any soln is less than 0.4 by 60 min continue to record data
for ALL solns<br>
until ALL have Abs values of at least 0.4. If you reach 0.4 for
all the solns before<br>
60 min (not likely, especially for soln 1) you need to continue
recording data until<br>
you hit the 60 min mark. <br>
<br>
There are two water baths, one for the cuvettes and one for boiling
the remaining<br>
solutions in the test tubes after having put some of those solutions
in the cuvettes.<br>
<br>
Make sure you keep the temperature of the water bath for the
cuvettes at a constant<br>
temp between 24-25 C. Choose a temp. in this range and try to hold
your water bath<br>
at that temp. You want water up about 3/4 of the way on the
cuvettes. Make sure<br>
you don't get water in them because then you have to start over.
Put a thermometer<br>
in the beaker with the cuvettes and lean it up against something.
You can take it out<br>
and then put it back periodically to check the temp if you find that
easier. You need<br>
to try to keep the temp constant at a single temp between 24-25 C.
To do this you may<br>
need to 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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