Exp 17 - recording times, Stopwatch apps, etc.

[robert zellmer]

I often get questions 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" (you recalibrate about every 5 min, in
between the readings for solutions 4 and 1).

There are some cuvettes which don't have frosted sides.  For these cuvettes
it doesn't matter which side you write on but make sure the side you've 
written
on points toward you.

Do NOT use both types of cuvettes (some with frosted sides and some 
without).
All the cuvettes you use 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 that is 
time = 0 (start
the stopwatch).  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
and the readings for each soln are about 5 minutes apart (this doesn't 
have to be
"exact").  So every 5th minute (between the readings for solns 4 and 1) 
you put the
blank back in and recalibrate.

This will require keeping track of a lot of information, time and 
absorbance for
four solutions over a 75 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 4 
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 17. ***Get one of these apps as it will make recording the time 
for each
soln. way easier.**

If you don't have 4 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 4 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
        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.

The solutions do not need to be spaced out at exactly 5 minute 
intervals, it's just
easier to keep track of things if you do.  It is NOT an error if you 
don't take readings
at exactly 5 min intervals as long as you record the actual time of the 
Abs reading.

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, as shown in a previous e-mail).

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 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 --> 4, with soln 1 having the lowest abs. and soln 4 the 
highest.  If not
something may be wrong.

If you suspect something is 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. Choose a
temp. in this range and try to hold your water bath at that temp. In a 
small beaker,
put the 4 cuvettes sitting on the rubber stopper and add water so it 
covers about
3/4 of the way up 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 so you don't accidentally knock it over. 
Alternatively, 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 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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