Monday, March 09, 2009

How to make dry HCl gas


Dry HCl gas is essential for certain reaction types and can come in handy for making various saturated HCl solutions. The easy solution to this problem is to have an HCl cylinder handy. However, I’m sure that some of you have experienced (or have heard of) the horrors of the corroded gas regulator on an HCl cylinder. Regulators on HCl cylinders have a very bad habit off snapping off! One of my good friends had a very close call with a big HCl cylinder. Luckily he was standing right next to the door so the only thing that needed to be replaced was that particular lab. If you must have an HCl cylinder standing around I’d recommend a lecture bottle (see photo). These are (in theory) less likely to go off since they have a relatively short life time and if they go off they are likely to kill fewer chemists. Nonetheless, this particular HCl lecture bottle in my current lab has decided to corrode/fuse. A tech-guy has been by three times over the last 6 months trying to get the regulator off. I just wish he would take the damn thing with him and not store it in the hood next to me. A safer, relatively simple, cheap and convenient way to get hold of some dry HCl gas is to make it yourself. The standard approach that I’m guessing most chemists still use is to add conc. sulfuric acid to sodium chloride or conc. HCl. However, avoiding the use of conc. sulfuric is desirable because it’s nasty and you’ll have to clean up afterwards. So in the interest of safety I would recommend the addition of conc. HCl to calcium chlorid. It’s cheap, the HCl gas that you generate is completely dry, it’s relatively easy to clean things up and the reaction is very easy to control. Here’s the original reference:
A Convenient Way to Generate Hydrogen Chloride in the Freshman Lab, Francisco J. Arnáiz, Journal of Chemical Education, 199572 (16), 1139.
On the photo you can see one of my recent set-ups. Here I am adding conc. HCl to calcium chloride with a pressure equalising addition funnel and bubbling the HCl directly into my reaction flask. Please note the use of a Pasteur pipette for bubbling the gas into the reaction flask. Do not get tempted to use a metal needle. Also I often add a wash bottle between the reaction flask and the gas source in case of unexpected suck backs. D!

34 comments:

petedskier said...

Excellent option. I was accustomed to using lecture bottles of HCl with a needle valve. This was my experience. Using the lecture bottle with the needle valve was very reliable and if I had a bottle that couldn't be opened, I just replaced it with a new one. However, I found that I would regularly remove the needle valve, cap the bottle, and then rinse and purge the needle valve with solvent and dry it under vacuum. If you are that fastidious, then reusing the lecture bottle and valve will be like new. You should also hide this valve because unless you can convince everyone else to treat it in the same manner, it will be frozen tight when the borrower returns it.

Chemgeek said...

Another alternative I've used for some reactions is HCl in ether. That avoids the whole gas cylinder issue. It may not be the best option, but it is an option for anhydrous ether of known concentration.

Ψ*Ψ said...

Yeah, a good friend of mine got a lungful of HCl (and evacuated the building and closed down a semi-major street) when his boss snapped off a regulator that way...
(he's OK, though)

Will said...

Very nice setup. Haven't needed to use dry HCl gas before, but I'll keep this in mind if the need ever arises.

Daniel Sejer said...

I just discovered that the paper has been html'd by Rhodium: http://designer-drugs.com/pte/12.162.180.114/dcd/chemistry/equipment/calcium.hcl.generator.html, D!

RB Woodweird said...

I also like the F&F method using dry alcohols and acetyl chloride. An online experimental using this method is here:

http://www.dfrc.ars.usda.gov/DFRCWebPDFs/1998-Ralph-JAFC-46-2531.pdf

Syn-chemist said...

I would say a simple and best method (no need to dry..).. good. Thanx, for the info..

Daniel Sejer said...

An experimental (and named) procedure that involves adding an acid chloride to a protic solvent! The method works equally well with thionyl chloride and sulfuryl chloride. However, you still have to dry your solvents and it obviously only works for protic solvents. D!

milkshake said...

SOCl2+methanol (SOCl2 added at -78C) is suitable for large-scale esterifications but the intermediate MeOSOCl takes some effort to pump out from the product in the end. I like TMS-Cl + methanol for small-scale work - its less corrosive, does not have to be cooled with dry ice and TMSOMe and TMSOTMS are low-boiling.

For anh HCl work I like 4M HCl in dioxane - it is best stored in teflon-cap bottles, Sure-Seal caps rust like crazy.

I like the HCl+H2SO4 method in the lab - it works quite well and the stream of gas is easily contollable.

At my first company job I gassed the whole lab with a giant tank of HCl gas - I put a spiffy new Monel-alloy "corrosion-resistant" regulator on (cost about $800!!) opened the main ventil on the tank and it immediately became clear that the regulator was defective - the lab filled with corrosive fog in an eye-blink. I shut my eyes, blind closed the tank and run out from that lab screaming. A patrolling police car was just passing by our company, and they stopped me and demanded an explanation. It was hard to tell the cops - please i will show you my ID but first let me wash that acidic mist off my hair and face!

By the way, the Monel Alloy corrosion-resistant regulator was impressively corroded all over from the accident.

Daniel Sejer said...

Milkshake, you don't mention AcCl in alcoholic solvent. I also find this and excellent way to generate HCl in situ and for example methyl acetate is easily removed. D!

CARBON CHEMISTRY said...

good idea. I'll keep this in mind if the need in future experiments............

Daniel Sejer said...
This comment has been removed by the author.
Synthetic Nature said...

good job... reminds of my trials a few months ago to generate dry CO2 gas with dropping aqueous HCl to sodium bicarbonate. at first sight the reaction worked well but after about 15 minutes the carbonate was consumed... so i filled another portion in and the whole mixture blowed into my fume hood. luckily i installed a security bottle but this was the last time i tried generating CO2 this way... at the moment im using dry ice on a water bath, drying it on CaCl2 before let it bubble into my reaction vessel...

maybe i come back to your HCl generator...

gigio said...

the issue number should be 12 rather than 16. it surely is a nice and safe method!

Anonymous said...

Is it safe to use a regular plastic tubing?

Thanks.

Lelijkebaas said...

I've just made some ethanolic HCl using the CaCl2/conc HCl method, regular plastic tubing is fine.

I had serious suck-back problems of the EtOH into the pasteur pipet + tubing of the HCl generator. I assume this is either b/c the HCl (aq) + CaCl2 -> HCl (g) + hydrated CaCl2 reaction is quite endothermic, or the HCl takes much longer to get released than it is taken up by the EtOH.

Stubborn as I am, I didn't put a washbottle in between ;-(. So: be sure to use one when you perform this method. I think next time I'd rather use NaCl-H2SO4.

Daniel Sejer said...

Lelijkebaas, Two things: Firstly, if you require HCl in a protic solvent it is far easier to add for example acetyl chloride directly to the solvent. Today I added acetyl chloride to methanol to generate HCl in methanol. Very simpel and straightforward.
Secondly, NaCl-H2SO4 method is just as prone to suck back as the method I have suggested in this post. However, I must say that you are the first suck-back case I have heard off with this method. You must have been exceptionally unfortunate to have this happen. D!

Lelijkebaas said...

Yeah, always been an unlucky guy ;-).

Tried the same again today using NaCl-H2SO4. Worked perfectly. It seems as if I got a more steady HCl flow. I wouldn't let some inexperienced student do it that way, though.

I know the trick of the AcCl/EtOH. But I like building stuff :-). Perhaps I'll try it when I'm in a lazy mood...

yeomanf said...

Would it be possible to add dehydrated MgSO4 instead of Calcium chloride?

Daniel Sejer said...

Yeomanf, I don't know. I have never seen it done so that could be an indication that it doesn't work very well. But in principle I guess that anhydrous MgSO4 should be powerful enough to dehydrate aq. HCl. D!

Seb said...

MgSO4 works as well, but expect to use a bit more HCl as it will partially convert to MgCl2 which can form a hexahydrate. Probably not an issue though as most HCl generators are setup to create excess HCl...

uday said...

You can use Hcl for preparation of Dioxane Hcl . added H2So4 in to Con Hcl purge in to Dioxane you will get Dioxane HCl

peach said...

How often do you, and the others, use brass fittings for handling the reactive gases?

Since I don't have a registration or VAT code, I can't order them at home and have to generate them.

I may soon to be acquiring those things however, could opens up the possibility of renting the speciality gases.

Having generated them so many times in glass, I am well accustomed to the corrosion problems, and the cylinder in the photo is a sight I've seen before. I have also been renting gas cylinder, alone, from the age of about 16.

I called the suppliers last year to enquire about the training courses on the speciality gases, and renting them, and had a quarter of an hour phone chat with one of the chemists.

He was fairly insistent on the use of Monel. And I know Matheson have inert purge ports on their manifolds for clearing them out after use.

With that, and my own experience in mind, I was quite surprised to see the brass on the cylinder in the photo.

How many of you still use brass? And I'm assuming if you do, you're purging it on a routine basis.

Of coarse, Monel regulators are in no way cheap!

Armil@gas south atlanta said...
This comment has been removed by a blog administrator.
Anonymous said...

Thanks for the nice pic of the setup i used it yesterday and it worked great. Do you have one for generating NH3?

Daniel Sejer said...

I have never tried generating ammmonia myself. I believe the way to go is treating solid ammonium chloride with an appropriate base. It's described here: http://www.emporia.edu/scimath/KansasScienceTeacher/KST_Vol6/documents/TheAmmoniaFountainRevisited.pdf
However, there may be a better way of doing this that I'm not familiar with. D!

Nogari said...

Thanks for this useful post. I used this setup today as I was doing a Pinner reaction and it worked very well,.. had no problems with suckback and thought you can regulate gasflow quite easily.

Anonymous said...

where is the wash bottle you said in your photo?thanks i am trying to generate dry HCl gas in lab these days ,thanks

Daniel Sejer said...

@Anonymous: You should add the wash botlle between the two reaction flasks so that the reaction mix can't be sucked back into the flask where you are generating HCl. D!

yogesh said...

pl show me how will we could keep wash bottle for escaping HCl

Abhishek Kognole said...

I get mixture of HCl gas and water vapor from reaction. And I want to remove the water vapor to get dry HCl gas. how to do this?

mamid said...

@Abhishek Kognole: Use some concd. H2SO4 in a wash bottle right after the generator.
chemtips has a fresh post on this subject (some useful links, too):
http://chemtips.wordpress.com/2012/12/18/snap-crackle-pop-anhydrous-hcl-generation/

Anonymous said...

I'd like to know the following:

How much CaCl2 and HCl should be required to saturate 1 L methanol with HCl gas?

How long should it take to reach saturation?

What is the saturation concentration of HCl gas in Methanol?

I imagine the concentration can be verified via titration, but what is the best method for doing so?

I am using a glass Pasture pipette to bubble in the HCl gas, but it seems a diffuser of some sort would work better to get more of the gas into solution. Are there any recommendations or suggestions for this purpose? Or should I just draw out the pipette to a finer tip to get smaller bubbles?

Whizbang Togglesprocket said...

As a makeshift diffuser you might try white sand in the bottom of the flask and bury the end of the pipette under the sand.