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Author: Subject: How are highly hygroscopic things purified via recrystallization?
aromaticfanatic
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[*] posted on 29-8-2020 at 18:36
How are highly hygroscopic things purified via recrystallization?


So I was looking into some synthesis and I noticed something. Things like sodium hydroxide, NaGHB, CaCl2, etc. that are highly hygroscopic and almost deliquescent are often hard to purify via recrystallization.

NaOH is well known to be hygroscopic but getting the water out of it is challenging. Unlike something like KNO3, putting it in an oven is a very ineffective way of drying it. I assume you can use a vacuum desiccator to dry it but that is pretty advanced for an amateur lab. Because of this I often have to either titrate the NaOH every once in a while or I just add a bit of excess. Recrystallization is almost impossible from what I know. I assume there are some solvents that would work but common ones like ethanol or water don't work to my knowledge. You can evaporate the solvent off but you don't truly recrystallize it. How can you purify NaOH then?

I was looking into GHB synthesis for fun (sadly it's illegal where I reside so I can't make it) and people were having huge issues with purification. People would make the precursors as pure as possible with the most amount of chemicals also being food grade. You can make it with all food grade chemicals. The water is obviously food safe, sodium hydroxide is sometimes food safe (crazy but it's a thing haha, it's just purity standards), you can use baking soda which is food safe, and the nitrite also exists as food safe versions. The solvent for extraction is a different story but ethyl acetate is pretty safe to use. It just hydrolyzes into ethanol which isn't a big deal as far as I know. So people avoid recrystallization by using safe and pure chemicals as well as distilling pure GBL. The problem arises when you try to recrystallize it. Ethanol seems to be the only viable solvent but even then it needs to be pretty anhydrous or else you form a syrup. So I guess you can get around it by using water free ethanol, which is kind of hard/a lot of work to get.

I have CaCl2 and it is highly hygroscopic. I spill some and it turns into a puddle within an hour or so. I never had a need to purify it but I assume you run into similar issues as above for NaOH and NaGHB.

So how in the world can you purify those types of chemicals? I'd love to have some dry and pure NaOH from drain cleaner but I haven't figured out something to purify it sufficiently. GHB is illegal so I don't need to know how to purify, I don't have any reactions that need dry and pure CaCl2 yet but it'd be nice to know how you could purify and dry it without having to buy reagent grade chemicals.
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JJay
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[*] posted on 29-8-2020 at 19:58


Umm... IDK much about GHB, but I think it's a fairly stupid drug - kind of like ethanol in terms of addiction, effects, and ease of overdosing except that an overdose of GHB is a lot lower volume. Just go have a nice martini or something instead if you want to make GHB.

That said, there is a book called Purification of Laboratory Chemicals by Armareggo and Chai that you can read. The latest edition is expensive, but you can probably find it in some library, and I've seen PDF copies of older editions floating around online. It outlines literature procedures on how to purify the vast majority of chemicals that you might come into contact with. The standard method for purifying sodium hydroxide is to recrystallize it from ethanol to eliminate carbonates and then heat it under vacuum to eliminate the ethanol of crystallization. If you really need it super-pure, you should do the entire procedure in a glovebox flooded with inert gas. If you just need to eliminate carbonates, you can just dissolve the sodium hydroxide in anhydrous ethanol, filter, add water, and boil off the ethanol without letting any carbon dioxide get into the apparatus, then titrate. Purifying sodium hydroxide is generally more work than it's worth IMHO, especially considering that it slowly dissolves glass in the presence of water, but legitimate reasons for preparing pure sodium hydroxide do exist.

Anhydrous ethanol is not really that hard to prepare.

A vacuum desiccator is not a super-advanced piece of equipment, but not everyone has one. If you don't have one, you can use a round-bottom flask or a filtration flask and pull a vacuum on it.

I've never needed really pure CaCl2 for anything, but I think you can recrystallize the higher hydrates from water then heat the crystals to drive off most of the water, then dry the product in a crucible. Purification of Laboratory Chemicals might have some ideas.
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aromaticfanatic
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[*] posted on 29-8-2020 at 20:33


Quote: Originally posted by JJay  
Umm... IDK much about GHB, but I think it's a fairly stupid drug - kind of like ethanol in terms of addiction, effects, and ease of overdosing except that an overdose of GHB is a lot lower volume. Just go have a nice martini or something instead if you want to make GHB.

That said, there is a book called Purification of Laboratory Chemicals by Armareggo and Chai that you can read. The latest edition is expensive, but you can probably find it in some library, and I've seen PDF copies of older editions floating around online. It outlines literature procedures on how to purify the vast majority of chemicals that you might come into contact with. The standard method for purifying sodium hydroxide is to recrystallize it from ethanol to eliminate carbonates and then heat it under vacuum to eliminate the ethanol of crystallization. If you really need it super-pure, you should do the entire procedure in a glovebox flooded with inert gas. If you just need to eliminate carbonates, you can just dissolve the sodium hydroxide in anhydrous ethanol, filter, add water, and boil off the ethanol without letting any carbon dioxide get into the apparatus, then titrate. Purifying sodium hydroxide is generally more work than it's worth IMHO, especially considering that it slowly dissolves glass in the presence of water, but legitimate reasons for preparing pure sodium hydroxide do exist.

Anhydrous ethanol is not really that hard to prepare.

A vacuum desiccator is not a super-advanced piece of equipment, but not everyone has one. If you don't have one, you can use a round-bottom flask or a filtration flask and pull a vacuum on it.

I've never needed really pure CaCl2 for anything, but I think you can recrystallize the higher hydrates from water then heat the crystals to drive off most of the water, then dry the product in a crucible. Purification of Laboratory Chemicals might have some ideas.


Yea like I said it's illegal and so I'm not gonna make it, it was just one of the few chemicals I knew of that were very hygroscopic. In terms of effects it is a lot more favorable than alcohol if you dose properly. Better relaxation, euphoria, sleep, no hangover, and nowhere near as harmful to the body as alcohol. Plus you don't get sick if you do it properly. But like I said I don't care for it and just added it to the list since I don't know of many chemicals that are so highly hygroscopic.

Thank you! I'll definitely check that book out.

I figured you might be able to recrystallize sodium hydroxide from an alcohol but I really wasn't too sure and yea most of the time buying the pure stuff is a lot more worth it.

I know it's not hard to make, I just meant it's harder to get your hands on than say, CaCl2, and making it anhydrous is a bit too much work using magnesium. You have to reflux it for multiple hours on end and since you're refluxing highly flammable stuff it's not something I'd just leave alone. So in my opinion it's not worth it.

Yea I knew it wasn't super advanced but for an amateur lab that has relatively basic-medium advanced equipment, it's not something you see often. I have two different condeners, multiple RBF with varying neck numbers, beakers, flasks, test tubes, magnetic stirrer hot plate, etc. and I don't have a vacuum desiccator or vacuum pump. I need to get one soon though I need a vacuum pump and my ghetto vacuum filtration system with an air mattress pump works, but isn't the best and the vacuum is not usable for any other vacuum applications. The flask tip is a really good idea though. I need to get myself a decent vacuum pump and filtration setup. I failed at separating nitrotoluene isomers due to the lack of a proper vacuum filtration system. You need to pull out as much ortho isomer as fast as possible while keeping everything cold or else the para isomer immediately begins to dissolve back into the ortho isomer. It was such a mess, I hated that stuff. I wish I had a vacuum system haha. I have a couple experiments that need vacuum distillations anyway.

Yea I haven't had a use for pure and dry CaCl2 yet either. I just use it for drying and making some jelly alcohol. Neither require pure CaCl2.
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Belowzero
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[*] posted on 29-8-2020 at 23:51


In your example I would simply do the synthesis with whatever chemicals are available and work on the purification of the final product and not so much on its precursors.

And as mentioned the next logical step would be to move to other solvents.

Highly hygroscopic substances are often very soluable too , this feat. can be exploited since a very small amount of water is able to dissolve it leaving the potential contaminations out of solution.
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