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cnidocyte
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[*] posted on 23-1-2011 at 02:04
CaCl2 + H2SO4


I searched for threads on this but the closest thing I found was a thread on NaCl + H2SO4. I was thinking about reacting CaCl2 with H2SO4 to make some CaSO4 and noticed that theoretically this should be an easy way to produce HCl. If I'm not mistaken, if I add a mole of CaCl2 to a 1M H2SO4 solution and filter out the CaSO4 precipitate I should be left with a 2M solution of HCl.
CaCl2 + H2SO4 --> CaSO4 + 2HCl
Is that what will happen or is there something I'm not taking into consideration?
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hissingnoise
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[*] posted on 23-1-2011 at 04:10


The reaction of NaCl with H2SO4 for HCl production is preferred because NaHSO4 is soluble!
CaSO4, being insoluble complicates matters!
You could try dissolving CaCl2 in water and then adding a roughly stoichiometric quantity of H2SO4 to it.
You'll get impure, dilute HCl this way . . .

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[*] posted on 23-1-2011 at 09:03


Well if all he wants is a fairly dilute aqueous solution of HCl, his proposal will work fine, and in fact the insolubility of CaSO4 is probably a plus since he can filter it out. The discussions on this forum that I recall involve methods for producing gaseous, preferably dry HCl (because aqueous HCl is an article of commerce, most places). Different ball of wax and for that I think NaCl would definitely be preferred.
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[*] posted on 23-1-2011 at 09:15


Why is that preferred? With NaSO4 being insoluble, it precipitates out allowing it to filtered out. Assuming all CaCl2 reacted, the NaSO4 obtained should be pure. The HCl solution on the other hand will contain some NaSO4 since its slightly soluble. Also the solubility of CaCl2 is 745g/L which is about 6.8M. Mixing 6.8M CaCl2 solution with conc. H2SO4, you could get a HCl solution of around 6.5M couldn't you.
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[*] posted on 23-1-2011 at 09:30


Use the standard method for best results!
Dry NaCl and ~98% H2SO4 and lead HCl to water with the inverted funnel trick . . .

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[*] posted on 23-1-2011 at 10:28


This is complicated by the fact that CaSO4 is slightly soluble, more so in strong acids as it can form CaHSO4 (bisulfate); however it is much less soluble that NaSO4 or its hydrates - I think that cnidocyte mistyped back up there.

Adding _solid_ CaCl2 to H2SO4 would also complicate things, because it gets coated with CaSO4 which slows down further reaction.

As discussed elsewhere, adding strong aqueous HCl to CaCl2 is a decent way to produce HCl gas, possibly better because only hydrochloric acid is consumed - the CaCl2 can be recovered.
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[*] posted on 23-1-2011 at 13:16


Hm, Maybe I'm missing something here, but how exacly CaHSO4 can exist? The charge of Ca is +2 and H is +1, but the SO4 anion has a -2 charge, so the charges are not equal (maybe you meant NaHSO4, sodium bisulfate).
Also, it should be Na2SO4, if I'm not mistaken (maybe you meant CaSO4, calsium sulfate) ?
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[*] posted on 23-1-2011 at 13:31


I'm completely lost. I can't see how NaCl + H2SO4 could be a better method of producing HCl. Na2SO4 is pretty soluble in water. Yeah using solid CaCl2 wouldn't be as effective as reacting a saturated CaCl2 solution with concentrated H2SO4. As CaSO4 precipitates out, more CaCl2 can be dissolved in the solution and since HCl has such high solubility, I'd say you can concentrate the HCl a fair bit.
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[*] posted on 23-1-2011 at 13:55


Mixell: HSO<sub>4</sub> has a charge of -1. Ca(HSO<sub>4</sub>;)<sub>2</sub>.

The reason for using dry salts and conc. sulfuric acid is because it produces HCl gas, meaning it can be bubbled into water as hissingnoise and others have mentioned, free of sulfate or cationic impurities.




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[*] posted on 24-1-2011 at 05:39


Quote:
I'm completely lost. I can't see how NaCl + H2SO4 could be a better method of producing HCl.


To summarize:
If your goal is to produce moderately dry HCl *gas*, then what you want is a salt that will react fairly completely with concentrated H2SO4. In this context, the relative solubility of NaHSO4 is an asset because it allows new NaCl to be exposed to attack by H2SO4. CaCl2 is likely to become covered with CaSO4 before it finishes reacting, though I have to think that it could still be used (fine powder, heating, agitation and so on).
If your goal is to produce an aqueous *solution* of HCl, especially a dilute one as you initially indicated, then an insoluble salt as a product is obviously an advantage.
Just make sure you don't try to produce an aqueous solution of 70% HCl or something.
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[*] posted on 24-1-2011 at 06:44


Quote: Originally posted by bbartlog  
If your goal is to produce an aqueous *solution* of HCl, especially a dilute one as you initially indicated, then an insoluble salt as a product is obviously an advantage.
Just make sure you don't try to produce an aqueous solution of 70% HCl or something.
Obviously? What's so obvious about it? You can't produce pure (or strong) HCl by this method.

Obviously you've never tried to filter CaSO4!

Now if you filtered it and then distilled the filtrate properly you could obtain relatively pure HCl at 20% concentration, but you're starting to lose some of the simplicity of the proposed method.

[Edited on 24-1-2011 by entropy51]
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[*] posted on 24-1-2011 at 06:55


Here, let me help you all out a bit with some real experiments using NaCl and CaCl2 for HCl generators. There's a bit more on the page after that one.

Quote: Originally posted by bbartlog  
If your goal is to produce moderately dry HCl *gas*, then what you want is a salt that will react fairly completely with concentrated H2SO4. In this context, the relative solubility of NaHSO4 is an asset because it allows new NaCl to be exposed to attack by H2SO4. CaCl2 is likely to become covered with CaSO4 before it finishes reacting, though I have to think that it could still be used (fine powder, heating, agitation and so on).
If your goal is to produce an aqueous *solution* of HCl, especially a dilute one as you initially indicated, then an insoluble salt as a product is obviously an advantage.
Just make sure you don't try to produce an aqueous solution of 70% HCl or something.


Bbart, there’s no need to put asterisks around the words gas and solution. HCl gas really exists and it’s called hydrogen chloride, similarly HCl solutions are very real too and are called hydrochloric acid. The maximum concentration that can be achieved is about 37 w%, known as concentrated hydrochloric acid.

The reasons why hydrogen chloride dissolves so enthusiastically in water are multiple:

• reacts with water and dissociates: HCl(aq) + H2O(l) === > Cl-(aq) + H3O+(aq). Solvation energy is released (ΔH < 0), the solution heats up during formation.
• The transition non-solution (state 1) === > solution (state 2) ALWAYS leads to increased entropy (is more probable): ΔS = S2 – S1 > 0
• Thus ΔG = ΔH – TΔS < 0


[Edited on 24-1-2011 by blogfast25]
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[*] posted on 24-1-2011 at 07:19


Quote: Originally posted by entropy51  
Obviously you've never tried to filter CaSO4!


Tell me about your troubles?

I’ve frequently made impure NH4Cl by combining a saturated solution of garden grade (NH4)2SO4 with a saturated solution of CaCl2: (NH4)2SO4(aq) + CaCl2(aq) === > 2 NH4Cl(aq) + CaSO4(s) and never had any filtering problems, quite the contrary: it filters like a dream!



[Edited on 24-1-2011 by blogfast25]
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[*] posted on 24-1-2011 at 07:26


Quote: Originally posted by blogfast25  
and never had any filtering problems, quite the contrary: it filters like a dream!
Really! My experience has been that it filters slowly, if at all. Were you using filter paper, fritted glass, or what?
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[*] posted on 24-1-2011 at 09:19


Coffee filter paper. The fact that the filtrate was hot and quite concentrated in NH4Cl may have helped (I must have done this three or four times by now). It runs through quite fast and completely clear.

I also disagree somewhat where you wrote:

”You can't produce pure (or strong) HCl by this method.”

Hmmm… much of the world’s hydrochloric acid sloshing around is still produced by heating conc. H2SO4 and NaCl in cast iron pans, hence the Fe contamination that’s still so common in ‘hardware store muriatic acid’.

In my simple experiments (linked to above) I could have made 37 w% HCl too, just not very much of it! I chose to capture the HCl gas in a volume of water that was suitable for direct titration and thus direct yield determination, that’s all…
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[*] posted on 24-1-2011 at 09:32


Quote: Originally posted by blogfast25  
I also disagree somewhat where you wrote:

”You can't produce pure (or strong) HCl by this method.”

Hmmm… much of the world’s hydrochloric acid sloshing around is still produced by heating conc. H2SO4 and NaCl in cast iron pans, hence the Fe contamination that’s still so common in ‘hardware store muriatic acid’.

I was referring to H2SO4 + CaCl2 followed by filtration. I know that H2SO4 + NaCl still works and use it myself. I routinely make 34% HCl that way.

I rarely buy the hardware store muriatic acid anymore because I don't use a lot of it and I don't like to have it sitting around corroding everthing near it.

[Edited on 24-1-2011 by entropy51]
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[*] posted on 24-1-2011 at 11:54


Quote: Originally posted by madscientist  

The reason for using dry salts and conc. sulfuric acid is because it produces HCl gas, meaning it can be bubbled into water as hissingnoise and others have mentioned, free of sulfate or cationic impurities.

What apparatus do you need for bubbling?
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[*] posted on 24-1-2011 at 12:08


Quote: Originally posted by blogfast25  

Bbart, there’s no need to put asterisks around the words gas and solution. HCl gas really exists and it’s called hydrogen chloride, similarly HCl solutions are very real too and are called hydrochloric acid.
[Edited on 24-1-2011 by blogfast25]


I'm sure he's using the asterisks to show he's talking about gas production, as opposed to using dilute acids and wet salts to have the gas captured in the original solution and present as hydrochloric, not to mock us. :)

If you're trying to produce the sulphate and don't want the hydrogen chloride around, bubbling it into water is one method. Add some base to encourage it to stay put.

Calcium Chloride will yield twice as much of the gas per unit of salt volume - which is useful if you don't have big flasks to hand as you shouldn't really be opening the generator mid work to refill it - it will stink, ruin the finish on every metallic fitting in the room, moisture will start running in and the gas stream is then contaminated with atmosphere; some of which may or may not be problem depending on what you're doing.

The three most common methods I know of to hydrogen chloride are table salt / calcium chloride + sulphuric and hydrochloric onto calcium chloride. Which you choose and how you do it depends largely on what you're doing with the resulting hydrogen chloride.

If the goal is making hydrochloric, that rules out the latter of the three, since you need hydrochloric to run it - but it can be used to purify shitty hydrochloric.

If it's making hydrochloric very easily with bits you'll readily find on the shelf, it's going to be sulphuric onto table salt, as you can pick up kilos of it at the cheapo stores like Aldi and Lidl for about 30p a bag and sulphuric is easier to find than surfactant free hydrochloric.

If you want clean hydrochloric, I'd go with bubbling the resulting gas into some distilled water.

The benefit of bubbling is that it will separate all the rest of the rubbish in your impure reagents out of the product; with a very high ratio of separation for next to no effort.

If you want a very well known gas stream, for anhydrous work and such, you'll want the hydrochloric onto calcium and to blow it through carbon or some more anhydrous salt; this way you know the acid in the generator is the same species as the gas you're producing and it doesn't have mist coming over with it.

You have to be careful assuming gas generators are truly anhydrous and pure if they're just a flask with a funnel and some tubing. I have watched reactions fail from what appears to be damp and / or acid coming over - probably as a result of the mechanical fizzing going on picking it up.

Calcium chloride generators using sulphuric do work when the salt is present as granules, but grinding it up will encourage it to go quicker. Going quickly is usually not the objective with gas generators. If anything, I find I want them running slower to be sure that the gas has a chance to be absorbed where it's going; not bubbling through it and out the exhaust.

You can bake NaCl generators to drive more hydrogen chloride out, but it's more mess to think about for what amounts to very little extra money. I'd also recommend you're careful when it comes to cleaning out calcium / sulphuric generators, as I've found big lumps of it that have been difficult to remove, and actually seen a flask pop when trying to rinse one out.

Because of this, I started looking into using plastics instead. From what I remember, HDPE isn't stunningly resistant to sulphuric according to the tables, yet I have bought sulphuric, many times, in HDPE bottles. Neither is it excellent against hydrogen chloride. But when a HDPE bottle costs less than a pound, and a boro flask costs tens of pounds, I don't really see it as a big problem if the plastic one goes in the bin after a while.

Theoretically, you should have 98% sulphuric and freshly baked salt to ensure you get the absolute maximum out of the generator, and none of it is left behind dissolved in the moisture present. In practice, the amount of effort and risk involved in redistilling sulphuric is no where near the cost of loosing a small amount of hydrogen chloride.

Incidentally, the cost for a lecture bottle of dry hydrogen chloride is over £100, and the Monel regulators / manifolds are £500->£2k for corrosive gases.

Generating the same amount using these methods costs about £5->10 from memory. So super duper efficiency isn't really essential when comparing it with the alternative.

You can buy dirty grade hydrogen chloride at about £140 for 5 kilos, but the damages you'll pay when 5kg of it goes wrong will swamp that with the first letter to the lawyer.

You also can't easily rent cylinders of the reactive gases as they can be used as chemical weapons.

To bubble gas a length of tubing will work. As hydrogen chloride has such a high solubility in water, if you're running the generator slowly, you may see few or no bubbles, as the gas dissolves into solution before escaping as a bubble.

[Edited on 24-1-2011 by peach]




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[*] posted on 24-1-2011 at 13:06


Quote: Originally posted by peach  
To bubble gas a length of tubing will work. As hydrogen chloride has such a high solubility in water, if you're running the generator slowly, you may see few or no bubbles, as the gas dissolves into solution before escaping as a bubble.

[Edited on 24-1-2011 by peach]


Yeah, that’s what I saw, at moderate rates of HCl generation: at the end of the submerged tube there was like on bubble protruding into the water but never quite leaving the tube. Swirls of denser HCl solution can then be seen to sink from the permanent bubble to the bottom of the water. That’s why the inverted funnel method also works fine.

It’s very similar with ammonia (NH3) form ammonium salt + NaOH: that too is really keen to dissolve in water.
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[*] posted on 24-1-2011 at 13:08


entropy51: I see, my bad.
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[*] posted on 24-1-2011 at 14:28


Quote: Originally posted by blogfast25  
...
Hmmm… much of the world’s hydrochloric acid sloshing around is still produced by heating conc. H2SO4 and NaCl in cast iron pans, hence the Fe contamination that’s still so common in ‘hardware store muriatic acid’....


I'd like to see references for that. From what I've read the main sources are byproduct HCl from chlorination of organics (which contains iron from attack on the pipes and so on), from burning H2 and Cl2 from electrolytic production of NaOH, from controlled incineration of chlorine-containing organic waste. Production using H2SO4 adds the cost of H2SO4 and the waste sodium sulfate production cost.

http://www.hydrochloric-acid.co.uk/hydrochloric-acid-manufac...

http://www.solvaychlorinatedinorganics.com/docroot/chlo_inor...

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[*] posted on 24-1-2011 at 15:42


Acid gas release from insulation on electrical wires is enough of a concern carbolite build a furnace specifically for checking it.

THREAD DERAIL SNIPPERS APPLIED -> X <- HERE

[Edited on 1-25-2011 by Polverone]




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[*] posted on 24-1-2011 at 16:09


As far as making clean hydrochloric cheaply and easily goes, it's drain cleaning sulphuric onto table salt bubbled into distilled water.

Using a gas to water method (with a state change involved) avoids the possibility of insoluble products and soluble contaminants in the reagents getting through filter papers - you are effectively 'distilling' the acid.

Use CaCl2 if you can find it cheaply, since you'll get more gas per flask of salt you use. Bearing in mind CaCl2 will only give you twice as much, it can't cost much more than double a bag of NaCl or it's questionable whether or not it's worth it.

Job done!


[Edited on 25-1-2011 by peach]




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[*] posted on 25-1-2011 at 01:01


Quote: Originally posted by peach  

To bubble gas a length of tubing will work. As hydrogen chloride has such a high solubility in water, if you're running the generator slowly, you may see few or no bubbles, as the gas dissolves into solution before escaping as a bubble.

As in a boiling flask (containing the HCl source) and a stopper with glass tubing through it and the other end of the glass tubing submerged in water in another flask? HCl would melt rubber stoppers though wouldn't it.
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[*] posted on 25-1-2011 at 05:48


It does attack natural rubber yes - making it go dry and crumbly.

But it doesn't do it instantly, and rubber stopper's don't cost a whole lot. You could try a silicone stopper instead, or painting silicone caulk over the whetted surface of the natural rubber. You could even bed PTFE tape over the silicone before it cures.

More of a problem with your proposed method is that adding all that concentrated sulphuric to your dry salt will cause the reaction to get going straight away at full pace, meaning you may end up loosing some of the gas.

For generating gases, you really want some control over how quickly and to what extent the two components meet - which means a Kipp's apparatus or flask and funnel setup.

In my scheming with regards to generating large quantities on the cheap, I envisaged having one HDPE bottle with the solid in and another, above it, with the liquid in. I'd connect the top bottle to the bottom one with two bits of tubing - one going to the top of that bottle above, to equalise the pressure between the two, and the other draining the liquid out into the lower bottle. Then use a clamp to control the rate the liquid drains into the lower bottle.

This is basically a flask and funnel deal, but hopefully one that's significantly cheaper and harder to break. If it works with HDPE, you can get 5l jerry cans that'd allow you to produce a lot per go.

I had a pack of HDPE bottles turn up in the post last week and would be interested to see how it works, so I could give it a go.

I could have done with some slightly bigger ones really, since these 250ml ones have fairly small caps and the cap will be the best place to attach things. I could also do with some hose barbs, which will then get stuck in place with a blob of silicone caulk.

{edit}Begins wondering if there are barbs easily to hand. If I can find something suitable, I'll give it a try and let you know.

[Edited on 25-1-2011 by peach]




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