Exp 3 (DRL)- recording times, Stopwatch apps, etc.

[robert zellmer]

I often get questions about recording the times for each of the solutions.

Each member of the group (2 per group) should have *3 **LARGE *test tubes
(75 mL, with the large openings), marked 1-3.   One partner measures the
Cr(NO3)3 (using a buret) and one measures the EDTA (using a 10-mL pipet
twice to get a total volume of 20 mL).

You should have 4 cuvettes, 3 for the 3 solutions and one for the EDTA
"blank".  Mark on the frosted side (if present) numbers 1-3 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 before taking any
absorbance readings for the solutions is put the "blank" in and 
calibrate the
machine to "zero".  Do this just before mixing the reactant solutions for
solution 1.

There are some cuvettes which don't have frosted sides.  For these cuvettes
it doesn't matter which side you write on but one side has a little 
triangle at
the top so write on that side to keep things consistent.   Make sure the 
side
you've written on points toward you.  If you decide to put the cuvettes in
so the triangle is set so it points to the side in the direction of the 
light beam
write on a side which you will point toward.  Just be consistent.

Do NOT use both types of cuvettes (some with frosted sides and some 
without).
All the cuvettes used should either have frosted sides or no frosted sides.

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 start the 
timer for
that solution.  This is time = 0.  Then record the Abs and the time it 
took to get the
soln in the spectrophotometer from the time of mixing.

Then 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 then the blank again to recalibrate.

After the initial readings try to space it out so every minute you are 
taking a reading
and the readings for each soln are about 4 minutes apart (this doesn't 
have to be
"exact").   So every 4th minute (between the readings for solns 3 and 1) 
you put the
blank back in and recalibrate.

This will require keeping track of a lot of information, time and 
absorbance for
three solutions over a 60 minute period.

*The easiest thing to do is download an app to your smart phone which 
allows**
**more****than one stopwatch.**  You want an app that allows 3 
stopwatches. **You**
**will use a stopwatch for each solution.* *There are several of these 
for free, but
you'll****probably see messages pop up (trying to sell antivirus 
programs, stuff
about storage,****etc.).  You can always get rid of the app once you're 
through
with exp 3.

*Dr. Moga has links to an iPhone and an Android app for this in her 
announcement
on Carmen. However, you'll have to copy and paste them into your browser 
from
the announcement.  Here are the links again,

Android: Multi Stopwatch & Timer free
https://play.google.com/store/apps/details?id=com.ldroid.multistopwatchandtimer

iPhone/iPod/iPad: M Stopwatch - A Multiple Stopwatch
https://itunes.apple.com/us/app/m-stopwatch-multiple-stopwatch/id912819901?mt=8

Other good Android apps are:
https://play.google.com/store/apps/details?id=com.sjm.apps.watson
https://play.google.com/store/apps/details?id=com.egg.multitimer 
<https://play.google.com/store/apps/details?id=com.egg.multitimer>

If you don't have 3 stopwatches you can simply use a clock (wall or 
phone) or a
single stopwatch and write down the time shown.   This will be more 
difficult in the
end than having 3 stopwatches.   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
             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 data 
in the report.

The solutions do not need to be spaced out at exactly 4 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 3 solutions (time and Abs columns for each soln, clearly marked, 
for a total of
6 columns, as shown in a previous e-mail).  You can record this in one 
partner's
notebook and copy it over to the other partner's notebook before leaving 
lab.

In the beginning, none of your first absorbance values 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.   If your absorbance for 
any given
solution jumps by about 0.3 between successive readings you've likely 
put the
cuvette in wrong (frosted side not pointed toward you).  If you get negative
absorbance readings at any time something is wrong.

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 --> 3, with soln 1 having the lowest abs. and soln 3 the 
highest.  If not
something may be wrong.

If you suspect something is wrong speak to the TA, sooner than later is 
better.
This way if you've made a mistake there's still time to start over. You 
don't
necessarily have to start everything over.  If it looks like one 
solution has a
problem but the other two appear okay you can just redo the one "bad" 
solution.

Record the data for at least 60 min AND a final Abs value of at least 
0.4. If the
Abs of any soln is less than 0.4 by 60 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
60 min (not likely, especially for soln 1) you need to continue 
recording data until
you hit the 60 min mark.

There are two water baths, one for the cuvettes and one for boiling the 
remaining
solutions in the test tubes after having put some of those solutions in 
the cuvettes.

Make sure you keep the temperature of the water bath for the cuvettes at 
a constant
temp between 24-25 C.  Choose a temp. in this range and try to hold your 
water bath
at that temp.  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 at a single temp 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 a
medicine dropper or micropipet.

As you can see, there's a lot going on.  I haven't even covered every 
little thing.
You have to be very well prepared.

Dr. Zellmer
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