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Author: Subject: Redistilling HCl?
aonomus
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[*] posted on 28-9-2010 at 13:33
Redistilling HCl?


So I've been using some store bought 38% HCl (muriatic acid) for a series of experiments, however recently I used some Na2CO3 to neutralize a drop or 2 of spilled acid, which immediately turned into a thick brown precipitate (I'm assuming iron carbonate for now).

Regardless of the impurity, I'm thinking about redistilling it in glass to clean it up, however I'm wondering how best to do this, since HCl vapors aren't easy to condense, and in general I want to avoid losing HCl.

The method I was thinking of using was setting up a distillation system, extending the drip tube in the receiver down below the surface of a volume of water, and cooling the water to absorb as much HCl as I can. The only hard part would be assaying the HCl and adjusting to 38% without a pH meter or pH indicator.

Anyone have any better ways to do this?
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bahamuth
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[*] posted on 28-9-2010 at 14:52


If you extend the drip tube below the surface of the water you WILL have suckback!
To avoid this attach a hose to the drip tube connected to a funnel which you have inverted in a beaker or similar, so that the funnel when it fills from the suckback will aspirate air, before it fills the inverted funnel, breaking the water column and thus preventing the suckback to enter your system.
But this will waste your acid anyhow so I suggest to distill your acid into a flask with as much magnetic stirring you can get, to dissolve the HCl vapors in clean water. Either way you will have losses, plus HCl gas is really nasty shit.

Or if you have the possibility to have positive pressure on your system, driving the HCl gas out into water, preferably through a glass frit bubbler...




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not_important
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[*] posted on 28-9-2010 at 15:04


Alternative if you mostly use HCl of 20% or less. diluting it before use or adding to water+whatever, is to dilute to 20% and distill the constant boiling acid. This doesn't fume, meaning a lot less corrosion around the acid storage area.

No matter what strength acid you distill you should guard against carrying over spray or mist from the still book, as this bring contaminates into the distilled acid.

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mr.crow
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[*] posted on 28-9-2010 at 15:10


You could dilute the hardware store acid (31% IIRC) to 20.2% and distill that. I'm sure your condenser(s) could handle it. It would be a known concentration and still useful.

To get 38% you would have to use the gas :( Sounds... unpleasant




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[*] posted on 28-9-2010 at 15:29


Well, there is a really easy method described by DR. Robert Thompson ( of illustrated guide to home chemistry fame) here:

http://www.youtube.com/watch?v=jv1Ms6Subg4

This work for you?
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[*] posted on 29-9-2010 at 00:36


I have experience with distilling 30% acid and I must say this was not pleasant at all. First a lot of gas is produced and this gas does not condense, so I had clouds of HCl all over the place. Next, I decided to put a little water in the receiver for dissolving the HCl and this worked somewhat better, but the received became hot, soooo hot. The reaction between gaseous HCl and water is very exothermic. You need good cooling.

Finally I ended up with acid of 25% concentration and lost some HCl in the process. If most of your experiments can do with 20% acid, then I would suggest diluting it down to just over 20% and perform the distillation with that.




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bahamuth
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[*] posted on 29-9-2010 at 06:22


A couple of days ago I made p-toluenesulfonic acid according to Vogel, which calls for recrystallizing from water saturating it with HCl gas, inside the house, on my stove, almost chocked when I was vacuum filtering and washing the crystals with sat. HCl....



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wink.gif posted on 29-9-2010 at 07:17


Cooks gas stuff in closets, misting the air with a NaOH solution to keep the fumes down. You babies need to stop complaining.



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[*] posted on 29-9-2010 at 08:23
Passing gas


Running the numbers on 38% HCl, I get 10 Molar.
10 moles of gas at RTP is ~250 litres or 66 gallons,
underscoring HCl's unusual solubility in water.

For preparative amounts, boiling
HCl out of solution generates a lot of gas.

Careful, there.
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peach
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[*] posted on 29-9-2010 at 09:33


You could always use the HCl generator route, dehydrating the original HCl by dripping sulphuric on, to drive the HCl(g) out for redissolving. That would avoid the hotplate and boiling.

In terms of assaying a pH, you're going to struggle without a pH meter or ANY form of pH indicator :P

Universal indicator ain't THAT expensive, and there are tons, tons more. My chemistry teacher used to have a thing for phenolphthalein (I can never remember how to spell that). I'm not sure why, if it was for the clear end point, to get used to working at things other than 7 for the end point (which made the resulting calculations fun of coarse), safety or more simply, he liked the neon pink flasks all around the room.

Then there's red cabbage indicator of coarse. Even if your cabbage is manky, the dyes still respond in specific ways to the pH. And, you've got a soxhlet, I knowZ so.

Hydrochloric will naturally settle it's own concentration around 38% anyway (once 'overgassed'), as the water saturates with HCl(g) around that point under standard conditions.

It will also change it's pH whenever you open the lid, as it'll fume a little and pick up moisture. I think my supplier bottle started out as 27-28%, probably to avoid some of the fuming problems, cost and changeability. But it does still fume a bit when opened. A little and a bit depend on your ideas of bad fuming. Mine looks like this;



If you're using it for precise pH control, you can't trust what's written on the bottle and you can't trust the additions even it's super expensive Sigma grade bottles. You'll need to check pH's and / or titrate as you go to account for variations in the acid and in the thing being changed.

You may want to avoid the digital meters when working at extreme pH's, as it tends to upset the bulb. You can always dilute down and make things easier to measure.

I also knowZ you have quite an accurate balance. You could potentially titrate the sample by making a stock from some pure base solid weighed on the balance (do the water on there too), then salt out the sample and weigh the resulting solid.

If you know how much base is in one sample, and what salt you'll be getting out from the acid, you can see the change in mass in the total dried result and work out how much of the base must have converted to that particular salt to create that change.

[Edited on 29-9-2010 by peach]




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dann2
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[*] posted on 29-9-2010 at 10:59


Hello,

Some discussion here:
http://www.sciencemadness.org/talk/viewthread.php?tid=9386

I made the 38% stuff from 12% + conc. Sulphuric acid + NaCl
The HCl gas dissolves into the 30% (and increasing) acid (HCl and water).
User inverted funnell as the device to introduce the gas to the 'acid'.

Dann2
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peach
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[*] posted on 29-9-2010 at 12:10


Quote: Originally posted by sswenson  
Well, there is a really easy method described by DR. Robert Thompson ( of illustrated guide to home chemistry fame) here:

http://www.youtube.com/watch?v=jv1Ms6Subg4

This work for you?


Nice link! Subscribbled.

The salt generator is an idea too, better than messing with your other bottle and sulphuric.

You obviously don't need to scrub the water traces from the generator's stream, but you will want some activated carbon in the way to stop sulphuric droplets being carried over.

When I next do HCl from salt, I'm going to experiment with using plastic containers, because running it in glass is upsetting when the cake sticks in there; you have to start poking around with metal tools. If you try quenching it, it will burst some flasks as well.

So, I was thinking of using ..PE containers and screw on tubing adaptors. Stick some tubing from the top of the salt bottle to the top of the inverted sulphuric to equalize the pressure, then a medical / tubing clamp to control the acid flow.

98% sulphuric comes in PE containers anyway. It's just the HCl(g) that needs looking at. If that can be done safely and produce in a controllable manner, it'll cost next to nothing (be free), emptying the cake will be as hard as squishing the bottle with your hand and if all goes tits up, they can have a rinse out and go in the domestic waste.

It'd also be easy to scale it up using 5l PE jerry bottles for doing a bottle of HCl in a go.

Hydrochloric isn't expensive, but making it yourself means you can be 99% sure you know what's in the end result. Two kilos of salt costs about 70p, 1 litre of concentrated sulphuric is about £7. Bake the salt first to get all the water out. Redistilling the sulphuric is annoying, takes ages and is fairly dangerous, so I'd just live with the few % water in there. What's 50p's worth of sulphuric versus 6 hours redistilling the stuff, and the risk.

If you use a suitable setup (NOT a repressurized propane tank), you can heat the salt cake to drive it through to the second stage as well, for maximum death stench.

[Edited on 29-9-2010 by peach]




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