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madcease
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Dry HCL Generation
Wondering on the ratio that whilst gassing using the drip method onto sulfuric if one was to obtain 30g dry gas and the HCL being used is 300g/ltr
will 100ml give me 30g back or is it .25X?
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pip
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correct me if i'm wrong but your trying to drip hcl solution on sulfuric acid to get dry hcl? you should be dripping sulfuric acid onto table salt to
get dry hcl.
oh and i'm sure you need this for gassing an organic compound but this is still inorganic chemistry and belongs in that forum.
for 30 grams dry hcl you need a little less than a mole so take the time to look up how much a mole of salt and a mole of sulfuric acid weighs and use
that much.
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madcease
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Yes table salt is one way on this is the other way.
Im not using it for anything which is why its in this forum its for knowledge
Does 300grams per litre HCL acid make 300grams Dry HCL GAS?????
If not what is the ratio
thnx
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gsd
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Yes it does. As long as you use enough excess of H2SO4.
gsd
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madcease
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thats what i thought thanks for straight reply.
If doing in 1Ltr RBF do you think using 250ml of H2SO4 would be enough.
I have heard that you have to keep changing the H2SO4 and it starts to draw back from the wash bottle. Once changed with new h2SO4 its back to normal.
Maybe to much water in the reciever?
Can one just fill it up 500ml and will still generate?
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Nicodem
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Thread Moved
30-7-2009 at 10:02 |
gsd
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It is my guesstimate that if the strength H2SO4 falls below about 70 %, then part of HCl will remain in that diluted H2SO4. So if you want to liberate
300 gms of HCl gas from 1 Kg of Hydrochloric Acid then I think you will need about 1.6 kg (900 ml) of 98 % H2SO4.
Now do you maths on volume of the RBF available with you 
gsd
P.S. : Please note that it is a very bad laboratory practice to add any chemical which contains appreciable quantity of water into
H2SO4. Due to very large density difference and extremely high heat of dilution, instances of bumping, splashing and even glassware breakage are not
uncommon. You should always do it OTHER WAY ROUND. i.e. add H2SO4 to Hydrochloric Acid.
gsd
[Edited on 31-7-2009 by gsd]
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woelen
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The HCl you make from 30% hydrochloric acid and concentrated H2SO4 is gaseous, but not really dry. You should bubble the HCl through a wash bottle
with concentrated sulphuric acid.
Besides that, I think that making HCl from hydrochloric acid and sulphuric acid is not the most economic if you want to squeeze out the last molecule
of HCl from the sulphuric acid. You'll need a truly large excess amount of H2SO4 to get that last little amount of HCl. It is cheaper to take fresh
hydrochloric acid and add H2SO4 to that. The net effect is that you use more hydrochloric acid, but much less H2SO4. Given the fact that H2SO4 is much
more expensive, then it is clear that you should minimize on usage H2SO4.
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entropy51
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Dry HCl can be generated by dropping conc HCl onto anhydrous CaCl2:
http://jchemed.chem.wisc.edu/Journal/Issues/1995/Dec/abs1139...
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woelen
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That is an interesting one. I never heard about that and I'll certainly try it! I am wondering how this could work. Isn't HCl much more attracted to
water than CaCl2? I know that CaCl2 is a good drying agent, but I'm really surprised to read that it can take the water away from concentrated
hydrochloric acid, leaving gaseous HCl behind.
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bfesser
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Refer to page 280 of Brauer:
http://www.sciencemadness.org/library/books/brauer_ocr.pdf
Attachment: JCE1995p1139.pdf (868kB)
This file has been downloaded 1468 times
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woelen
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I tried the method of CaCl2 with HCl 30%. I took around 1 gram of CaCl2 and added appr. 1.5 ml of acid (30% HCl). When the acid is poured on the
CaCl2, only faint fizzling occurs. Hardly any HCl escapes.
Then I started heating the mix. The CaCl2 quickly dissolves and then production of HCl starts and it goes fairly vigorously (and producing quite some
foam, which is not pleasant at all). The mix must be gently heated (60 C or so) in order to drive off the HCl. I collected some of the gas in a 100 ml
erlenmeyer with narrow opening (through a tube). Then I stoppered the erlenmeyer and put it in a tub, filled with water, upside down and then removed
the stopper. At least 90% of the volume of gas dissolved in the water, so indeed, the gas produced in this way is HCl.
This method is a nice addition to the set of available methods, especially for those people, who do not have access to concentrated sulphuric acid. I
do not think that its yield is as good as for the HCl/H2SO4 method, but it certainly is useful. I also think that the process can be optimized to
increase its efficiency. I just tested the feasibility of the process and I'm sure that a carefully controlled setup can improve the yield.
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chloric1
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Quote: Originally posted by woelen  | I tried the method of CaCl2 with HCl 30%. I took around 1 gram of CaCl2 and added appr. 1.5 ml of acid (30% HCl). When the acid is poured on the
CaCl2, only faint fizzling occurs. Hardly any HCl escapes.
Then I started heating the mix. The CaCl2 quickly dissolves and then production of HCl starts and it goes fairly vigorously (and producing quite some
foam, which is not pleasant at all). The mix must be gently heated (60 C or so) in order to drive off the HCl. I collected some of the gas in a 100 ml
erlenmeyer with narrow opening (through a tube). Then I stoppered the erlenmeyer and put it in a tub, filled with water, upside down and then removed
the stopper. At least 90% of the volume of gas dissolved in the water, so indeed, the gas produced in this way is HCl.
This method is a nice addition to the set of available methods, especially for those people, who do not have access to concentrated sulphuric acid. I
do not think that its yield is as good as for the HCl/H2SO4 method, but it certainly is useful. I also think that the process can be optimized to
increase its efficiency. I just tested the feasibility of the process and I'm sure that a carefully controlled setup can improve the yield.
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Woelen, don't forget to mention that the drying agent is easier to regenerate than the H2SO4. There might be minimal hydrolysis of calcium chloride
but it is generally dehydrated at 200 C IRC.
Would like to mention that I was reading about zinc chloride and I remember that heating even the hydrated salt yields dry HCl. Now, as far as yields
are concerned I am not sure but in any case you just redissolve the oxychloride in more HCl and chrystalize and repeat. Might work for the bromide as
well. I doubt the iodide would be practical given the endothermic nature of HI.
Fellow molecular manipulator
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madcease
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Just curious about this suckback from the wash bottle which has HCL dripped on H2SO4 and comes out sweet but starts going up the wash bottle. Noticed
that when the H2SO4 was full of water after long gassing periods but not at the start
Maybe im thinking in the wash bottle the is an excess of H2SO4 could this make pressure for suck back or push thru. Its more of it building in the
wash bottle and coming out the tube which should b dry gas but its not
How much H2SO4 should be submergeged with the tip of the wash bottle. Its in about 2cm right now and im thinking maybe up it to about 50mm.
Let me no if anyone has experience with this method and error
Tnx
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peppovsky
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I have another (sort of) approach to this issue.
I've connected a separation funnel containing conc HCl to a büchner flask (using a rubber stopper with a two holes in it - one for the stem of the
sep funnel and one for air-intake), with dry CaCl2 in it. The flask is connected via a PVC pipe to another (smaller) büchner flask, from the top
(using a rubber stopper with only one hole for the PVC pipe to fit in perfectly - this should be super-tight!), and the PVC tubing it inserted to the
bottom of this second-flask. This flask also contains a magnetic stirrer bar, and is connected to a vacuum source (aspirator in my case).
The stirrer is turned on, some HCl is dropped on the CaCl2, and after closing the sep funnel's stopcock, the vacuum is turned on. This makes the dry
HCl flow from the first flask to the bottom of the second flask, containing the solvent. A sketch is attached to explain the process.
The system was tried to be successful, though not a lot of HCl gas is produced. This could be due to bad HCl that was used (chep hardware store type,
had a strong yellow tint to it), or the flask not being heated.
I can imagine that since there's an air-intake occurring in the first flask, it somewhat "undries" the HCl gas (due to air humidity) , so one should
consider packing the PVC pipe between the flasks with CaCl2, to dry everything that flows through it. Furthermore, one should also consider heating
the dry-HCl-generating flask to about 70C in order to produce more HCl gas. If anyone is interested, I can provide actual pictures of the system
set-up.
[Edited on 5-12-2009 by peppovsky]
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bbartlog
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Y, this setup as shown seems to have some weaknesses (as you already described). First of all you're effectively vacuum distilling the HCl without a
heat source, so of course it's going to get chilled and produce gas at a rather low rate (determined by ambient heat flow into the flask). Second, you
end up putting your target solution under vacuum when it seems like you could as easily just deliver HCl under positive pressure by heating it; of
course this may be an advantage depending on what you're doing.
Depending on the volumes involved and your supply of CaCl2 it seems like you could effectively end up passing the HCl gas through CaCl2 just by
filling the flask 2/3 full of the stuff, rather than setting up a separate tube. Then whatever gas comes off of the HCl layer in the bottom has to
pass through the layer of granules above.
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peppovsky
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I just really like the idea of forcing the HCl through my solution by "sucking" it with the other flask. Oh well. Are you suggesting I should heat the
flask generating the dry HCl (while stoppered properly), and just letting the PVC tube go straight to the bottom of a beaker (with stirring)? How do I
overcome the problem of undissolved/unreacted HCl gas escaping the beaker?..
EDIT: I guess the last issue can be overcome by bubbling the dry HCl through an erlenmeyer flask that is stoppered correctly (the PVC going through
the rubber stopper).. am I right?
[Edited on 5-12-2009 by peppovsky]
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aonomus
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I would think the best way would be to finely disperse the gas. I found at the pet store plastic air 'stones'. They are white plastic cylinders that
are more or less sintered grains of either PE or teflon, they have no reactivity to conc. HCl from me using it to bubble air into a conc HCl solution
with copper metal to make CuCl2.
I suppose the tricky part would be to get it to fit into the glass tube, but once you get the arrangement built it should have lots of use, both to
put gasses into solutions, or to scrub byproducts as a reaction.
For generating dry HCl via CaCl2, I would think that maybe using positive pressure would be better. Since the HCl needs time to dissolve in whatever
solvent (eg: MeOH), you could just leave the recieving flask sealed and use a little air pressure to force HCl(aq) into the first flask, pushing a
little HCl into the recieving flask. Once the atmosphere is mostly HCl, as the solvent dissolves HCl, it should also pull conc HCl from the dropping
funnel. Using a vacuum would pull some HCl out once its partial pressure in the air above the solution dropped low enough.
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peppovsky
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I see. The question is how do I "push" HCl from the generating flask to the receiving flask? (except, of course, heating it)
Let's be more specific. I have chloroform in the receiving flask with caffeine (freebase) dissolved in it*. I want to bubble dry HCl through the
chloroform solution in order to get caffeine.HCl, which should crystallize out of the solution. In this case - does the dry HCl gas should dissolve in
the chloroform first? or is bubbling HCl through it at slow rate would suffice?
*It's part of a beginner project I'm doing - extracting caffeine from tea leaves, and getting the purest material I can have. I wish to learn as much
as can from this project - refluxing, filtering, separating, washing and extracting was done. My next learning step is to get caffeine.HCl without the
need to evaporate water (If and when I get it I'll recrystallize it from a solvent of choice, and do a melting-point test).
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User
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Well one simple way is just letting the HCl gass push itself through the system by plain pressure build up, same as blowing trough a straw in a glass
of water.
Alternative would be to use a pressurized air stream to transport the HCl.
The first seems the most easy.
Or am I missing something here?
You could use a air dispersion stone (ones used for aquariums) these things make absorption much more efficient.
Down side would be that this could \\\'contaminate\\\' your setup especially when crystals come falling right out.
Also it is quite common to use a long receiver such as a measuring cylinder.
The gas has to travel further before reaching the surface.
Keep your receiver stone cold, this makes gas dissolving more efficient.
[Edited on 6-12-2009 by User]
What a fine day for chemistry this is.
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Vogelzang
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If you've got a pipette like the one in the picture, you can use a propane torch or bunsen burner to heat it and create two bends and use it in a
gassing apparatus like in the picture. The large part of the pipette keeps liquid being sucked back from going all the way back to the flask.
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peppovsky
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ok, thanks for all the help. I've tested my system with the receiving flask in an ice bath, and the gas-generating flask on a hot-plate. works much
better (although currently I'm bubbling the HCl gas to water, organic solvents will come next). I ran out of CaCl2 though, so some shopping will have
to be made before re-experimenting 
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peppovsky
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I just realized there's an important issue that I'm not quite certain about.
Does the HCl gas has to dissolve in the solution before reacting with whatever in it, or can it react with whatever in it without dissolving in the
solution first?..
[Edited on 22-12-2009 by peppovsky]
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havarti_gouda
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This idea came to me on one of those long nights when babysitting a 16 hour reaction. After I cleaned all of my glassware, instead of watching re-runs
of Who's the Boss?, I thought, "why not prepare an anhydrous, slightly acidic solution so I don't have to continually construct and re-construct a dry
gas generator over and over?" So, I bubbled dry HCl into a few (pre-dried) solvents of choice (dcm, acetone, ether, etc.) and then stored in air tight
containers. Makes crystallization much easier for the time-conscious. But how long will my solutions last? There shouldn't be any sort of
decomposition or increase or reduction in acidity?
[Edited on 30-1-2010 by havarti_gouda]
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entropy51
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16 hour reactions and half kilo of methylamine? Sounds kind of industrial scale.
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bbartlog
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Will depend on what you've bubbled your HCl into. Seems like it couldn't do much to DCM, but some other things might be subject to slow chlorination.
On the other hand, how much HCl can you actually dissolve in DCM anyway? It has no -OH so it seems like it would hardly stay in solution...
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