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madcease
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[*] posted on 30-7-2009 at 06:58
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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[*] posted on 30-7-2009 at 07:36


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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[*] posted on 30-7-2009 at 07:50


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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[*] posted on 30-7-2009 at 08:39



Yes it does. As long as you use enough excess of H2SO4.

gsd
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madcease
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[*] posted on 30-7-2009 at 08:47


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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gsd
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[*] posted on 30-7-2009 at 20:45



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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[*] posted on 30-7-2009 at 22:45


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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[*] posted on 31-7-2009 at 05:41


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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[*] posted on 31-7-2009 at 06:50


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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[*] posted on 31-7-2009 at 06:58


Refer to page 280 of Brauer:
http://www.sciencemadness.org/library/books/brauer_ocr.pdf

Attachment: JCE1995p1139.pdf (868kB)
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[*] posted on 31-7-2009 at 10:25


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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[*] posted on 31-7-2009 at 12:00


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.


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.




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madcease
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[*] posted on 2-12-2009 at 19:57


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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[*] posted on 5-12-2009 at 05:17


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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[*] posted on 5-12-2009 at 11:15


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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[*] posted on 5-12-2009 at 12:04


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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[*] posted on 5-12-2009 at 18:46


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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[*] posted on 6-12-2009 at 02:00


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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[*] posted on 6-12-2009 at 02:58


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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[*] posted on 20-12-2009 at 14:29


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.

gasser.jpg - 45kB
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[*] posted on 22-12-2009 at 02:11


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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[*] posted on 22-12-2009 at 09:15


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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[*] posted on 29-1-2010 at 16:47


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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[*] posted on 29-1-2010 at 16:59


16 hour reactions and half kilo of methylamine? Sounds kind of industrial scale.:)
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[*] posted on 29-1-2010 at 18:24


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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