From: b_duke@darwin.ntu.edu.au (Brian Duke)
Newsgroups: sci.chem
Subject: I2 + I-/I3- equilibrium
Date: 4 Sep 92 10:15:06 +0900

Some time ago I posted a question to sci.chem about replacing carbon 
tetrachloride in the partition experiment to determine the I2 + I-/I3- 
equilibrium. This produced no traffic in sci.chem, but several e-mail 
messages to me. The responses are below, with my original message in 
the first one, and a summary of our recent experience at the end.
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In article <1992Jul21.104001.3491@darwin.ntu.edu.au> you write:
>Dear Chem netters.
>
>There is a classic experimnet using the partition coefficient of iodine
>between water and carbon tetrachloride to determine the I2 + I-/I3-
>equilibrium constant.
>
>Carbon tetrachloride is now a "not advised" chlorocarbon. What is the 
>best alternative solvent?
>
>I will post the results and maybe get my students to try out different 
>solvents this coming semester (starts next week here "down under").

In many simple experiments involving chlorinated hydrocarbons
you can probably use methylene chloride (dichloromethane) as a substitute
for either chloroform or carbon tetrachloride.  Since the partition coef. 
experiment only requires a nonionizing solvent that doesn't mix with water
but will dissolve iodine handily, methylene chloride ought to work fine.  
(Its solvent properties are like those of CHCl3 and CCl4.)  CH2Cl2, unlike
chloroform and carbon tet, is not thought to be carcinogenic in humans.  It's
still somewhat nasty stuff -- it'll cause all the noncarcinogenic problems
that CCl4 does (skin irritation, liver damage, all that fun stuff), so
the usual organic-solvent precautions apply.

One caution:  since its boiling point is low, around 40 C, CH2Cl2 can (and
will) sometimes spurt out of a loosely stoppered flask if you handle it (the 
flask, that is ;-) a lot with your hands.  Personal experience here.

Good luck.
-- 
David Nash                  | Gradual Student, Chemistry
                            | University of Illinois (Urbana)
(nash@aries.scs.uiuc.edu)   | This .sig is made of 100% recycled electrons.
(nash@c.scs.uiuc.edu)       | No binary trees were killed to make it.

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I'm not sure about 'The Best' but two alternatives would be Toluene
and Trichloroethylene.

-- 
	 L.Piacenza - Chemistry Department - University of Transkei
	 Internet: sppp@hippo.ru.ac.za  Tel. 27-471-3022384
	 Internet: chpp@unitrix.utr.ac.za
-----------------------------------------------------------------------
We started using dichloromethane at Red Deer College in
Red Deer, Alberta where I taught for a couple years, though
it had flaws too:  most notably, it was more volatile so more
pressure built up in the sep. funnels.  It did work, though.

JOhn Olson
john@segovia.mit.edu
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I taught general chemistry lab here at Yale last year, and we did a similar
experiment. The solvent used was dodecane (C12H26). 

Michelle Lamb
Graduate Teaching Assistant, Yale University
michelle@rani.chem.yale.edu
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I haven't learned yet not to be afraid of these things, so I'd probably use
CCl4 anyway:-)

Honestly, I think the most common substitute for carbon tet is tetrachloro-
ethylene. I believe that is what the dry cleaners now use. I would guess that
the partition constant stays the same, but you could check that by doing it
yourself and fitting the partition coefficient to last years' data.

Hope this helps.

Kevin
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We use hexane in our general chemistry laboratory, as suggested by M. N.
Ackermann in the Journal of Chemical Education (1978, page 795).  The value
of the equilibrium constant for I2(in hexane) + I-(aq) --> I3-(aq) comes out
to be about 20 L/mol.

Another possibility is trichlorotrifluoroethane, a liquid freon which we
have been able to obtain cheaply as a commercial dry-cleaning solvent.

Howard DeVoe
Department of Chemistry and Biochemistry
University of Maryland, College Park, Maryland (USA)
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That's it, folks, for the comments I received. I've since had my students
on the problem, and I offer the following comments:-

1.	Toluene is not on. After leaving the mixtures from one week to the 
	next (both those with I2 only and those with I2/I-, a horrible
	crud developed. We suspect this may be oxidation of the CH3- to
	give benzoic acid, but we have not had time to check this out.
	This reaction may be a function of the high temperature in our
	labs. Even with air con it is about 25C.

2.	Hexane and Heptane work fine, and higher straight chain hydrocarbons
	such as dodecane mentioned above (we had none in stock) would also
	be fine as the I2 solubility increases with chain length. However
	there is a problem with these solvents. They are lighter than water.
	It is necessary to increase the amount of the organic layer, to 
	allow easy pipetting of the sample from the layer. With CCl4 the 
	organic layer is at the bottom. Tipping the flask slightly allows 
	easy pippetting of the sample even from a layer with a volume only
	slightly more than the pipette volume.

3.	Dichloromethane is too volatile here in the tropics. It may be OK
	in cooler climes.

4.	Methylchloroform is good and it gives a lower organic layer. This
	along with dichloromethane are the recommended replacements for
	CHCl3 and CCl4 here.

Thanks for all your help.
-- 
                  Brian Salter-Duke (Brian Duke)
School of Chemistry and Earth Sciences, Northern Territory University
        GPO Box 40146, Casuarina, NT 0811, Australia.
Phone 089-466702   FAX 089-410460    E-mail B_DUKE@DARWIN.NTU.EDU.AU