Melgar
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Electrochemical cell: Chloroform and NaOH from salt, water, and isopropanol
I wanted to get some experience designing an electrochemical cell for a well-understood reaction, so I could focus more on how to properly design the
cell rather than try and guess what the reactants are doing. I also wanted really cheap materials, so I could just toss the reactants out and start
over if something went wrong, and an organic component to the reaction, so that I could better understand organic electrochemical redox reactions. I
also wanted to produce something mildly interesting, and do something that hadn't already been done a million times before; basically my goal was to
add some novel approach to a well-understood reaction. So I chose the well-documented haloform reaction.
Because of acetone's volatility, and because I want to minimize formation of chloroacetone (a potent tear gas), I'm proposing to use isopropanol
instead, which would be oxidized by chlorine/hypochlorite to acetone, acetone being more reactive than isopropanol with hypochlorite. This would
ensure that the amount of acetone present would always be low, thus minimizing the concentration of chloroacetone. Isopropanol also has a higher
boiling point than acetone, which would minimize its evaporation, and is also a bit cheaper. Additionally, with isopropanol, there's no need to worry
about acetone peroxides forming or anything like that. There would be about 20% more chlorine necessary to perform the reaction, but that seems to be
the only downside.
Acetate, formed as a product of the haloform reaction, could react with radicals at the anode to form either methanol, ethane (via Kolbe
electrolysis), or chloromethane. It's been my past experience with organic electrochemistry that it's never clean; there's always a mixture of many
different compounds, probably due to the continuous radical formation at the electrodes. It will be interesting to see what will form from just these
simple reactants, and if they match my predictions.
I'd planned to use graphite electrodes, and have a thermoelectric cooler extracting heat so that not much chloroform evaporates. The chloroform would be collected in a glass separation funnel
attached to the bottom of the cell, and removed periodically. I hadn't thought much about the CO2 that would be formed from oxidizing acetate ions,
but I realize now that the main end product would probably be sodium carbonate, not the hydroxide. Still, I'm interested in learning how pH changes
as this reaction moves forward, and whether the pH would more closely approximate NaHCO3, Na2CO3 or NaOH.
After this, I'd like to try making a chloralkali cell using galinstan instead of mercury. Of course, since I don't want to immediately have a leaking
sodium galliate solution, I'd have to react the sodium with something besides water, likely an alcohol, in order to produce the akoxide, which is
probably more useful to me than the hydroxide anyway. I'd be interested in this reaction mainly to document how to substitute mercury with something
non-toxic in a famous reaction. I think that would be an interesting contribution, no? However, I want to design and build a cell that doesn't need
anything expensive to operate first, and neither gallium nor indium come cheap. Thus, I'm thinking the haloform reaction would be a good one to start
with.
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WGTR
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Sounds fairly well thought out. I'd suggest using a divided cell, to try controlling the pH around the anode. If it goes too high, then the produced
oxygen will go all piranha-like on your graphite anode. I've tried this experiment making bromoform, and that is one problem I had since I didn't use
a divided cell. I did manage to produce some bromoform this way, heavy drops would fall off the anode to the bottom of the cell. A lot of salt was
wasted, however, since the way I controlled pH was by replacing the solution. If you want to see the product as its formed, then try adding small
amounts of acetone at a time, not all at once. I don't recall any tear gas forming, other than the usual halogen pungency.
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Melgar
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Hmm, I have a platinized titanium electrode that's for electroplating. Do you think there would be any danger of that degrading under those
conditions? I wouldn't expect it to, but at the same time, it wasn't exactly cheap, and I don't want to risk damaging it.
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WGTR
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I don't specifically know, although as a general rule I keep chloride away from platinum.
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