# Safe drying agents for drying machine

When I was milling NaOH for some purpose, I noticed that the powder is incredibly hygroscopic. Because I have problems with drying certain chemicals safely (without extensive heating), I thought this might be a safer solution:

But I'm not so sure about that safety. Can NaOH exit into the air and pollute the compound in the second beaker? If it can, which compounds are safe and strongly hygroscopic?

Could I use this technique to increase sulfuric acid concentration? What about acetic acid?

• I think downvotes should be followed by a comments, especially on a Beta site, where guidelines are not so obvious and need to be discussed. – Tomáš Zato - Reinstate Monica Oct 12 '14 at 21:22

The first part of your schematic basically describes a dessicant dryer or deliquescent dryer. These are common industrial air drying processes that produce very low dew point properties. Generally air (or other inert gas) is passed through a column of desiccant, under increased pressure.

Desiccant dryers use common desiccants that include activated alumina, molecular sieves and silica gel. In all cases, there is a maximum amount of water that can be adsorbed/absorbed before the desiccant needs to be regenerated; typically regeneration is done by heating the desiccant column, although other methods, such as purging can be used. Industrial desiccant dryers have the heater element installed as part of the construction, although any bench chemist can probably tell you about heating red silica gel beads from desiccators in the oven until they turn blue.

Deliquescent dryers are similar, but involve a highly hygroscopic agent that slowly dissolves, or deliquesces, as it absorbs more water. Usually the agent cannot be regenerated, and the waste liquid is drained and discarded. Fresh drying agent then needs to be added. This is most likely the scenario for NaOH.

So, this would need to be a major consideration for your proposed system; how to safely and effectively dry your NaOH. This is a very caustic material to have to try to continually regenerate or replace.

Yes, I would be concerned that you would transfer NaOH from the first chamber. To check this, you could easily test the gas outlet with some litmus.

Can you use dry air to remove excess water from other materials (such as acids)? In theory, this may be possible and will depend on the relative hygroscopicity of the drying agent and the material to be dried; you need to be sure that the air you are supplying your wet material with is dry enough to receive water and not simply supply more. Purge drying (passing dry air over a desiccant) is another form of regenerating industrial desiccants, but is not as effective as heating. Sulfuric acid is itself used as a desiccating agent as it is very hygroscopic. I don't know the relative hygroscopicity of NaOH and H2SO4/CH3COOH, but there is no way I would ever consider passing air from one container directly into the other chamber. This is a very hazardous setup, and is fraught with much danger. Heating an acid solution in a suitably ventilated environment under controlled conditions is still, to my knowledge, the most effective way to increase the concentration of the solution.

You should also consider the energy efficiencies of trying to 'dry' a sulfuric acid solution by passing dry air over or through the solution versus heating it. The amount of time and materials required to produce sufficient dry air to effectively concentrate a sulfuric acid solution is likely to be far in excess than the time/material investment required to simply heating it.

• I heard rumours over the internet, that heating vinegar produces toxic gases and actually does not produce more concentrated acid. Since there were many such rumours, I was being careful. What danger is this setup creating? – Tomáš Zato - Reinstate Monica Oct 12 '14 at 23:26
• Sorry - I have replaced vinegar with sulfuric acid in that last paragraph. Probably need to use a fractionating column to purify vinegar, as acetic acid azeotropes with water. Heating vinegar will release acetic acid vapours which are known to be toxic (and will kill birds if you read the internet;-)). – long Oct 12 '14 at 23:55
• Your setup has the potential to be very dangerous if you have large quantities of concentrated acid and base in close proximity, in a closed system, with a shared environment. In the (perhaps unlikely) event of things going wrong, this is a very messy and nasty mixture to have to deal with. – long Oct 13 '14 at 0:00
• @long Acetic acid is purified by freezing, that's why the purified acid is called glacial acetic acid. – LDC3 Oct 13 '14 at 0:07
• @LDC3 I don't think freezing works. I have frozen a vinegar bottle and I filtrated the ice chunks from the rest. The both final samples smell the same, taste the same (aarrgh!) and the ice even regained the original orange color after melting. – Tomáš Zato - Reinstate Monica Oct 13 '14 at 14:50

Sodium hydroxide (potassium hydroxide is even better) is indeed used as a drying agent in the chemical laboratory, as are many other things: sulfuric acid, phosphorus pentoxide, molecular sieves are but a few.

The important consideration is compatibility with the substance to be dried. You wouldn't even think of, say drying a stream of chlorine gas with potassium hydroxide, or a propylene stream with sulfuric acid.

You want gypsum sold here to labs as "Drierite" in the anhydrous $\ce{CaSO4}$ form.

Drierite, or anhydrous calcium sulfate, is readily used in labs for drying gases, etc. It's fairly safe (I wouldn't eat it, but it's used as plaster), hygroscopic, and easily re-usable. Generally you mix the plain white form with a little bit of indicator, and then bake at 210 °C to drive off the water before re-use.

Common drying chemicals for air dryiers I'm aware of are (anhydrous) copper sulfate and calcium chloride, the second awailable as granules and dirt cheap. Sodium hydroxide reacts with carbon dyoxide, so it's a questionable choice. However in case one need to dry an inert solvent, like benzene or diethyly ester, sodium hydroxide is a valid choice for the first run (with sodium metal for the second run).

At the extreeme corner phosphorous pentoxide and sodium metal and metal hydrides lie. They are effective, but dangerous and should not be used as first-stage dryers, as they may react too lively. In addition, some solvents/gases may react with them, so something more inert must be used. This includes silica, molecular sieves and so on.

In case of drying gases it is common to bubble the gas throw layer of concentrated sulfuric acid, using two safeguard vials before and after the vial with sulfuric acid.

Can NaOH exit into the air and pollute the compound in the second beaker?

If you leave the NaOH as pellets, it is not likely to transfer. The powder might.

Could I use this technique to increase sulfuric acid concentration? What about acetic acid?

No and no. Glacial acetic acid is already 100%. If you could remove water from sulfuric acid, you would get fuming sulfuric acid. The excess $\ce {SO3}$ would dissipate quickly.

which compounds are safe and strongly hygroscopic?

The more hygroscopic the chemical, the more hazardous it is.

If you really need to dry a solvent, it would be better to follow the information here.

• I was not talking about glacial acetic acid, but a 7% vinegar. And I was talking about 35% sulphuric acid (I don't need to deydrate that in fact, since I have a 98% one). Why a compound that is hygroscopic is also hazardous? – Tomáš Zato - Reinstate Monica Oct 12 '14 at 21:25
• @TomášZato I think you need to look at the 98% sulfuric acid again, 35% sulfuric acid is 18M, as strong as it comes. Phosphorous pentoxide is a good drying agent, but becomes a strong acid and very sticky when wet. Sodium metal will burn when it comes into contact with your skin. $\ce {LiAlH4}$ will react with water in the air, releasing a lot of heat. There are other hazards. – LDC3 Oct 12 '14 at 21:52
• I disagree that hygroscopic chemicals need to be hazardous. Some are, but drierite is fairly safe. Molecule sieves are also quite safe. – Geoff Hutchison Oct 12 '14 at 21:53
• @GeoffHutchison Yes, those are quite safe, but TomášZato was asking about more hydroscopic ones. – LDC3 Oct 12 '14 at 21:55
• @LDC3: Sieves and alumina are the about the best drying agents out there. The alumina/copper drying/deoxygenation systems invented by Brookhart have all but replaced circulating stills. – Abel Friedman Oct 12 '14 at 22:15