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Corrosive Joeseph
National Hazard
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This page is a personal favorite of mine regarding general chemical aquisition............
http://makezine.com/setting-up-a-home-science-lab3/
Being well adjusted to a sick society is no measure of one's mental health
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macckone
Dispenser of practical lab wisdom
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+1 to gdflp, filtering gypsum with concentrated nitric acid is just not safe or even
doable. The only way this works is to let the gypsum settle (days or weeks) and
then decant off the nitric acid that isn't bound in the gypsum.
Also upsilon's attempt to make sodium hydroxide in a salt bridge cell
with 2L of salt solution and a .5 A supply is not really going to work well.
There is probably about 600g or so of salt. To reach 2% which is really
about the maximum for a salt bridge cell would mean making 40g or so
of sodium hydroxide. That is around 1 moles. At 26 amp-hours per mole,
that is going to be about 22 days at 10% efficiency and .5 amps. You can't
run to completion or get high efficiency with a salt bridge.
You can use a diaphram cell and get 10% or so solution but the efficiency is still
pretty low. Electrical efficiency of a homemade diaphram cell is likely to be
around 20% maximum. But you can get higher efficiency and solution concentration.
The key with a diaphram cell is ensuring there is no sodium hydroxide flow to
the chlorine side of the cell. This requires keeping the solution on the chlorine
side above the level of the sodium hydroxide side. In a long running cell
this requires constantly draining and replacing the solution.
In either case you have to purify the sodium hydroxide solution
and then dry it. Drying sodium hydroxide is not easy.
[Edited on 25-9-2015 by macckone]
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Upsilon
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Isn't most NaOH produced industrially via electrolysis though? What do they do that lets them run it to completion? Or do they not and have a method
for removing the NaCl contaminant?
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woelen
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In the industrial processes, the electrolysis does not run to completion. The dissolved NaOH/NaCl mix (so called caustic brine) is evaporated in
multiple stages, causing NaCl (and also some NaOH) to crystallize. The remaining caustic brine contains less NaCl and after several stages of
evaporation, NaOH-purity can be better than 99%. At this stage, however, probably 90% of all produced NaOH is "waste", contaminated with a lot of
NaCl. The "waste" product of the evaporation (NaCl/NaOH-mix) is fed back into the electrolysis cell so that the NaOH is not lost (subsequent
electrolyses start off with some NaOH in it already).
In reality, this is not a discrete process as sketched above, but a continuous process of electrolysis, taking away caustic brine, evaporation and
partial feedback of caustic brine to the cathode electrolyte cells, together with addition of fresh brine to the anode electrolyte cells. The energy
needed for all these feedbacks mostly comes from the waste-heat of the electrolysis process itself. These feedbacks of "waste"-flows, "waste"-heat,
and the staged evaporation of brine to get progressively more pure NaOH are marvels of engineering ingeniousity and certainly are not easily built at
home. This is exactly why these industrial processes for making basic chemicals like NaOH are so hard to duplicate in the lab. Many of these processes
only work on an industrial scale. The ingenious engineering makes these processes highly efficient, in terms of used energy and in terms of net yield
(ratio of useful products to waste which must be dumped).
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annaandherdad
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Thanks for that explanation, woelen. I had always wondered about this myself (how they purify the NaOH). I suppose if you want really pure NaOH
electrolysis is not a good method of production?
Any other SF Bay chemists?
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macckone
Dispenser of practical lab wisdom
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The three industrial methods are: mercury cell (being phased out), diaphram (requires purification), and membrane (most commercial operations use
this).
The mercury cell and membrane can produce very pure sodium hydroxide.
Neither method actually runs to completion but the sodium chloride is completely
isolated from the sodium hydroxide. Membrane can only produce around 10%
solution but it is very pure. The mercury cell method can go to 50% but
it is being phased out due to the toxic waste it produces. The main drawback
for the home chemist is a membrane cost $300/sq ft. At $100-$200 per pound
for mercury the mercury cell is also quite expensive for the home chemist.
The diaphram method with a good diaphram is much less efficient and is
used in a few places for food grade sodium hydroxide (used in making pretzels).
But the sodium hydroxide contains substantial salt and requires extensive
purification. This is the method that is most useful for a home chemist
but you can't use the product without purification and it is not easy to
purify.
Purifying sodium hydroxide from a diaphram cell is not easy.
Woelen went into the difficulty. Industrially they rewash the salt
to dissolve the sodium hydroxide preferentially. And unlike a home
cell they can get 10% sodium hydroxide solution with high concentration
of salt. While a home cell is likely to only get up to 2% unless you
spend a lot of time on engineering. Plus you have to come up with a
way to constantly remove the mix while adding pure salt.
With any of these methods you need to purify the salt before you
use it for electrolysis if you want a pure product. As table salt is
often a mixture with silicates and other things that are safe to consume
but will contaminate your end product.
Another note, getting anhydrous sodium hydroxide adds another
level of complexity as you need to heat the sodium hydroxide well
above melting with a constant stream of absolutely dry carbon dioxide
free gas. Industrially this is air that has the carbon dioxide removed
with the conveniently available lye solution and then dried using a variety
of methods including condensing units and beds of drying agents.
The air is recylced as are the drying agents, the water from the
condensing units is very high purity and is fed back into the
various cells.
Quite frankly doing all of this is difficult in a home environment.
Although if you are doing it for the learning experience it can be
quite challenging and even fun.
It is easier to purify drain cleaner or just buy a lye based one.
In fact you might want to try purifying a drain cleaner first
as the process is a challenge and will prepare you for
cleaning up the crude lye from electrolysis.
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Upsilon
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Yeah, thanks for the info. It seems like large-scale industry always has the upper hand over the lab in terms of flexibility. I suppose that's to be
expected though since they have profits at stake. So I guess household electrolysis is only useful for producing gases?
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macckone
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I would say household electrolysis can be useful.
But you need to know the limitations.
I think it is more for fun and technical ability than
generating a useful product.
But that is mostly why a lot of us do chemistry.
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woelen
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For some chemicals, household electrolysis can be useful. I myself made KClO3, KBrO3, Br2, and KIO3 in this way. I wrote webpages about these
processes:
http://woelen.homescience.net/science/chem/exps/miniature_ch...
http://woelen.homescience.net/science/chem/exps/KBrO3_synth/...
http://woelen.homescience.net/science/chem/exps/OTC_bromine/...
For all experiments you need some chromate or dichromate as well. For this, in the meantime I also found a very easy way to make it, no need to
isolate.
Use potassium chrome alum, or chromium sulfate and dissolve in as little as possible amount of water (e.g. 5 ml).
Add bleach (5% household) while gently heating and stirring without perfumes and detergents until you get a clear yellow solution (first the solution
is turbid and then the precipitate redissolves again and the liquid becomes yellow).
The liquid, thus obtained, can be used in all electrolysis processes, described in the links, given above. For each 100 ml of liquid, add 2 ml or so
of the yellow solution.
Chrome alum or chromium sulfate is easy to obtain, e.g. on eBay and it is non-toxic:
http://www.ebay.nl/itm/500g-Chromium-Potassium-Sulphate-high...
You can also buy potassium dichromate or sodium dichromate, ready for use. They are sold by many sellers on eBay (even more so than for chromium(III)
salts), but they are toxic and inhalation of dust must be avoided at any cost.
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ave369
Eastern European Lady of Mad Science
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Quote: Originally posted by macckone  | buy a glass vacuum
distillation set and a water aspirator vacuum pump.
You can get one of these for under $200 |
Buy yourself a retort, man. Mine costed me about $5. It's a veritable nitric acid factory. And also useful for other things.
Smells like ammonia....
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Upsilon
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I just tried my own version of PTFE-coating a rubber stopper. I used Teflon tape and superglue with a #2 stopper. I started with 4 vertical wrappings
to completely cover the bottom of the stopper. I then wound it horizontally around the sides of the stopper. All while applying superglue to the
stopper to hold it down. I think it turned out pretty good, you can't see any exposed rubber or even any seams unless you look really closely.
It was tedious as hell and I managed to trash my fingers with superglue. There's also some residual superglue on the outside of the Teflon tape so
I'll need to wash it with acid before using it. I'm not using this for distillation or anything; a stopper with a hole in it would make covering it
with Teflon infinitely more complicated. This was mostly just for being able to store acids that attack rubber (like conc. sulfuric) in regular beer
bottles.
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