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Author: Subject: Water exchange between different hygroscopic salts
Gargamel
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[*] posted on 25-11-2014 at 05:03
Water exchange between different hygroscopic salts


Hi there

I want to dry hygroscopic salts, ideally using CaCl2.
Among the salts I have in mind to be dryed are
Sodium salicylate
Ammonium nitrate
Strontium chlorate
and several others.

Question:
How great must the difference in critical relative humidity be in practice to effectively dry salts?

For example, AN has a critical relative humidity of ~60%, CaChloride ~30%.
When combined with CaChloride, I would expect the latter to totally dry out the AN until it turns into an unsaturated liquid.

Will this really work in practice, or will there be some kind of equilibrium in the middle?

Question 2.
Concerning salts who create hydrates:
What happens if these are dryed without the influence of heat?
Is it possible to drive off crystal water just putting them in a 10% RH climate?
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jock88
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[*] posted on 25-11-2014 at 06:21


You could use a vacuum too.
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Bezaleel
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[*] posted on 26-11-2014 at 10:11


I cannot answer your question, but I can share a bit of related experience.

A while ago I made PrCl3.7H2O. I used a dessiccator (vacuum), or it would have lasted years (if not decades) to dry to completeness.
I used NaOH as dessiccant. This yielded a solution of NaOH into which all solid NaOH dissolved. And even after all had dissolved, the volume still increased a bit. In my experience NaOH is a considerably stronger dessiccant than CaCl2. When drying a substance from a solution containing a volatile acid, acid vapours will be reacted away by the NaOH in addition, yielding a more neutral dryed salt.

2. It depends much on the nature of the salt, i.e. the strength of the bond of the water molecules. I found that e.g. NH4Fe(SO4)2.12H2O crystals (those beautiful light purple ones :-) ) will loose their crystal water (at least partly) in regular dry air (estimated humidity 30%), so these need to be protected against dry environments. (I'm living in a temperate climate, home humidity is usually 40% -70% here). Other salts will absorb moisture from the air. Yet others are stable.

I really doubt whether you could create dry NaOH by reducing the humidity to a low percentage. But maybe monohydrated CuSO4 could be made. I expect the process to be slow, though. (seek for the term 'dessiccator bag')
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Gargamel
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[*] posted on 30-11-2014 at 04:38


Does vacuum really help if there are two salts involved? Since it would apply to both of them.

Seems like I overlooked sodium hydroxide.
But is has the disadvantage to react to carbonate over time. Is that relevant in practice, how fast does it get useless? Do you use it more than once?
And is the process of forming carbonates accelerated by recycle the hydroxide by heating?


I'm not a chemist by trade, and I find this thing hard to control in practice. I'd like to know what exactly i am working with, and often it's not exactly clear how much water something contains...


Some salts have the unfavorable property that the temperature where you drive out crystal water equals their decomposition temperature.
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[*] posted on 30-11-2014 at 04:53


You won't go far wrong using sodium hydroxide for basic salts, and sulphuric acid for acidic salts. Just put a small beaker of either in a closed container; with your salt to be dried in a separate beaker in the same container.

Correct identification of your salt as acidic or basic is important, or else adding, say, ammonium chloride to a drying chamber with sodium hydroxide in there will cause ammonia gas to be liberated.

The sodium hydroxide will be kept in a closed container, so the amount of CO2 that it can react with is limited. Conversion to carbonate is not a problem.

[Edited on 30-11-2014 by forgottenpassword]
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[*] posted on 30-11-2014 at 06:00


Using vacuum is a good idea, but pumping water vapour is a good way to kill some designs of vacuum pump.

Re.
"Is it possible to drive off crystal water just putting them in a 10% RH climate?"
It should be. Are you somewhere cold?
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[*] posted on 30-11-2014 at 07:42


I successfully used anhydrous calcium chloride to dry some ammonium nitrate recently. It started out as a sort of slush, and dried to a crumbly powder.
I'm not sure if it is actually anhydrous now, but it definitely seemed to pull off most of the water.

[Edited on 11-30-2014 by zts16]




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Gargamel
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[*] posted on 1-12-2014 at 06:59


@zts16:
Thank you for providing that fact.

@unionised:
Central europe. At the moment I've got ~0°C and pretty dry air, my hands and lips start to rupture.
But the 10% RH where meant to be inside the drying box / exsiccator.

@forgottenpassword:
I don't understand why this is important. The salts wont touch each other, and usually do not have any vapor pressure. How can they react with each other?



I have an woulf flask and a fridge compressor, but no really vacuum resistant vessel. I wonder if it is worth the money to spend for a real exsiccator?
Does it make a big difference?

And also, as I wrote, the vakuum will apply to BOTH salts - does it even matter?

Edit:
Or does that apply only to silica gel, which has a different mechanism to grab the water than hygroscopic salts?

[Edited on 1-12-2014 by Gargamel]
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[*] posted on 1-12-2014 at 13:07


Well, if it's very cold outside and you pipe in air from there it will be very dry. If you warm that air up to "normal" room temperature- say 20C, it will have a very low relative humidity.
No drying agents needed, just a fan and a (small) heater.

for example, accodring to this
http://en.wikipedia.org/wiki/Vapour_pressure_of_water the vapour pressure of water at 20C is 2300 Pa
and at -20c according to this
http://www.kayelaby.npl.co.uk/chemistry/3_4/3_4_1.html
it's just 103 Pa
So if you had air at 100% RH and -20 C and warmed it to 20C the RH would fall to roughly 103/2300 *100 %
That's just 4.4% RH which is low enough to dry out most hydrates.
If the outside air is not saturated then the humidity will be even lower when you warm it up.

The biggest advantage to using a vacuum is that the water can move from the salt to the drying agent more quickly because the air is not in the way.

Some salts have more vapour pressure than you expect, but I don't think that will be much of a problem.
Something like ammonium saicylate might be a problem. It's teh sal of a weak acid and a weak base so it will slightly dissociate into ammonia and salicylic acid.
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[*] posted on 1-12-2014 at 18:18


When I made TsOH*H2O, the literature said to use NaOH and CaCl2 to absorb HCl and H2O from the solid, respectively. Adorable. The Calcium chloride did doodley squat and the NaOH crusted over with NaCl. I swapped the calcium chloride to conc. sulfuric acid and the solid very slowly dried. I imagine that the humidity was near zero due to the sulfuric, but the tosic acid is agressively hygroscopic itself.

Ammonium nitrate is easy to dry in a low oven.




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[*] posted on 2-12-2014 at 02:33


Quote: Originally posted by Gargamel  
@forgottenpassword:
I don't understand why this is important. The salts wont touch each other, and usually do not have any vapor pressure. How can they react with each other?

Some, particularly ammonium salts do tend to disproportionate (especially when carried over with moisture; as with moist-salts that need drying!). If there is something for a part of it to react with then it can be a real problem. I can't think of examples off-hand but I will look it up, as I am sure that I have seen this mentioned in laboratory manuals*. Many ammonium salts do smell strongly of ammonia due to this disproportionation.

*Although, come to think of it I may have got the wrong end of the stick. The advice may have been to put BOTH sodium hydroxide and sulphuric acid in separate beakers in the same container. I think that's it, actually, as it would mop-up the ammonia and HCl fumes from any salt that did disproportionate; and the disproportionation is low enough to mean that the losses are minimal. That makes sense to me, so I'll revise my advice to say "put both in". In fact, I seem to recall that the advice was to use both rather than just one, for that very reason. Just using one leaves ammonia or HCl fumes, for example, whereas if you use both then there is no such problem. Worked it out in the end!

[Edited on 2-12-2014 by forgottenpassword]
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[*] posted on 6-12-2014 at 08:50


Thank you for sharing your knowledge.

One more question:
How does silica gel compare to hygroscopic salts?

I cannot find a critical humidity value for it, and I think it's another mechanism behind it.

I just want to estimate if this can be used to dry things that are hygroscopic themselves, or up to what point it it useful, where would an equilibrium approximately be located?
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[*] posted on 6-12-2014 at 13:43


By all means, try it. You can dehydrate silica gel in an oven at 110C. It will not dehydrate the atmosphere as vigorously as the suggested combination of sodium hydroxide and sulphuric acid, though, I doubt; but it may be suitable for your purposes. As mentioned, the combination of sodium hydroxide and sulphuric acid also acts to 'mop-up' any gaseous contaminants that may otherwise build up in your drying chamber. The same cannot be said of silica gel. It depends what you are working with. Silica gel is certainly convenient, and perhaps good-enough for your application. Try it and see what is most suitable for your needs. If you find that you need a more vigorous dehydrating system, then try the sodium hydroxide and sulphuric acid system.
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