Sciencemadness Discussion Board

Ridding Apparatus of Water

aga - 26-9-2016 at 11:51

Having seen many references to 'dryness' in write-ups (e.g. Grignards) it seems that there is no standard way to get rid of the majority of atmospheric moisture from glass apparatus.

Some say to heat it all with a blowtorch, others say oven-dry, yet others say to purge with dry gas.

I imagine that purging with warm, dry air would suit most amateurs, as the notion of whacking a blowtorch flame onto precious glassware would be, er, unplatable, and the home oven may not be available for such uses.

This brings us to How the air can be dried.

In the Amateur setting there exist many substances that can be used as drying agents, such as Sand, Dry Rice, Table Salt, NaOH prills, CaCl2, maybe even molecular sieves or highly concentrated suphuric acid.

I suggest a series of simple experiments to establish the efficacy of each available substance that anybody can participate in, the results of which would help to empirically determine the performance of each substance.


As a suggestion, the substance to be tested is prepared according to some documented procedure (document it yourself if none are on hand) such as oven-drying table salt for 1 hour at 200 C, or heating/stirring sand in a beaker for 20 minutes.

Some of this is loaded into a 24/27 extension tube, with cotton-wool used to 'plug' it in place, the whole unit being weighed (s).

1-hour oven dried @ 200C CaCl2 can be used as a reference, some being placed in a second tube, similarly packed with a small amount of cotton wool, then likewise weighed (r).

The two tubes are placed in-line with each other, then a small pump of any kind used to push air through the system for 't' seconds.

(alternatively, each sample could be stuffed into a 'sock' of filter paper, then shoved into each end of a liebig condenser)

The flow-rate 'f' of the pump used can be pre-determined simply by blowing air into an inverted measuring cylinder in a bowl of water for a set time.

By measuring the weight change of each unit, empirical data can be gathered from which a proper comparison can be made of each drying agent.

If the ambient humidity can be measured then the accuracy will be enhanced, although if a series of tests are done in a short time, the relative differences should also provide useful data.


The specifics of how this experiment could/should be done are very much wide open to suggestions, ridicule, hopefully discussion.


The aim is to find out how available substances will work best for amateurs trying to exclude atmospheric moisture from their apparatus.

[Edited on 26-9-2016 by aga]

Texium - 26-9-2016 at 15:05

For Grignards you don't really have to worry about drying the air. You just have to boil your ether or THF for a while prior to adding the alkyl halide, that way the apparatus is filled with solvent vapors. Stick a drying tube at the top of the condenser and then you'll be sure that any air that gets in once the ether cools down is dry. I ran one last weekend when the humidity was 76% and it worked just fine. If you're going for super-ultra-maximum yields then sure you can oven or flame dry the glassware but I feel like that's a bit excessive unless the reagents you're using are incredibly expensive and you need to have a great yield or else waste a lot of money.

Maroboduus - 26-9-2016 at 15:51

I was taught that the surface of glass adsorbs a considerable amount of moisture, and that heating at 200C was needed for scrupulously dry glassware. That and either assembling hot or letting it cool in a desiccator.

I've done some moisture sensitive reactions without this with success, but never anything really finicky Like those Gilman reagents. Maybe with proper procedures you wouldn't need an iodine crystal to get your grignard going, but it sound a lot easier to me to use the iodine.

I do bake my glassware at 200C for dry condition reactions, but I just let it cool in the oven till I can handle it. I don't know if this would help at all if you used a gas oven.

Well, to be honest I'm too cheap to buy a big enough desiccator to fit my condenser in there for it to cool off anyway. Maybe some day when I've taken care of about 25 other priority items for my lab.

EDIT In re post right after mine: Ziqquratu, thanks Hadn't thought of that. I'll go look for some Tupperware that'll fit my condensers etc, I've already got plenty of cake racks.




[Edited on 27-9-2016 by Maroboduus]

[Edited on 27-9-2016 by Maroboduus]

ziqquratu - 26-9-2016 at 16:39

How dry you need your equipment depends on what you're trying to do. Some reactions need to be fairly dry, but are still tolerant of a little water - with Grignards being the classic example where a bit of water won't bother anyone. On the other hand, some reactions genuinely require scrupulous technique in a glove box with water and oxygen scrubbers and recirculated atmosphere and...

For most purposes, I dry my glassware in the oven at 110-130*C for a minimum of 1 h (it's fairly arbitrary, and since I have the luxury of an always-on lab oven, I usually try to keep a selection of glassware in there for use as I need it). I then seal it and allow it to cool under nitrogen; you can also place it in a desiccator (I use silica gel in mine) and assemble it after it has cooled, or use a drying tube to dry the air as it enters (with CaCl2 usually being the desiccant of choice).

Maroboduus, you don't necessarily need a "real" desiccator - you could just buy a sealable plastic container and a heatproof base of some sort (a cake rack would work well).

For extra-dry reactions, flame drying is easy enough - set up your apparatus, allow dry gas to flow through it, and heat with a blowtorch or a heatgun, making sure to keep the torch/gun moving continuously - both to prevent melting the glass, and to ensure even and complete heating. The heat gun is safer, because they generally can't get hot enough to melt borosilicate glass; the torch is fine if you don't stop in one spot too long. I generally flame dry for about 10 minutes to be sure all water has been expelled.

For drying a flow of gas, aga, a lot of the work has been done - we use silica gel, CaCl2, H2SO4, and NaOH fairly routinely, and they all work fairly well. Of course, if you can start with fairly dry gas for your experiments, you're a long way ahead - if you're planning a lot of dry work, a cylinder of welding nitrogen is pretty cheap and will go a long way! But, that said, don't let me discourage you from experimenting! It sounds like you've got a reasonable protocol in mind; perhaps if you could find a way to measure both inlet and outlet humidity, as well as ensuring a consistent flow rate, it might improve the utility of your results.

Ozone - 26-9-2016 at 17:25

There is always the glove box, but...

When I'm really worried about water, especially that adsorbed to glass, I'll pull a vacuum on the apparatus (a Schlenk line is great for this--it also allows for testing how air-tight your apparatus actually is), flame it with a bunsen burner (you can watch the desorbed water move up, sometimes as a ring), purge with inert gas (N2 or Ar) while cooling and thereafter, and either maintain purge gas or put a drying tube/train on there.

Honestly, this rigor can frequently be overcome by the use of a slight excess of reagent. Hell, it has even been found that some sensitive reactions don't go without a tiny catalytic amount of water (any more kills them), so rigorous drying might not always be for the best.

It really depends on the reaction you want to do and what purification strategy you are planning to use.

O3

Dr.Bob - 29-9-2016 at 06:29

Testing drying setups is tough, as everyone starts at a different place, due to variations in temperature, humidity, scale, and solvents. What works in Boston during the winter might not work in Florida in the summer. For most cases, either oven drying or flame drying will work, and as stated, many reactions are just not that sensitive, but some are. For larger scale, often it is not as bad, as the surface area to volume of the glass is lower, and dry inert gases are often available. Just blowing dry N2 through a rbf will remove a lot of moisture, especially if you heat it some. I often use "dry" solvent to rinse the flask to remove most of the remaining water.

aga - 29-9-2016 at 09:14

The intention is/was to test substances on-hand to evaluate their effectiveness in drying air, which is something i'll do in the near future.

CaCl2 will probably come out on top, although i have no idea by what margin over, say, dry rice.

[Edited on 29-9-2016 by aga]

yobbo II - 29-9-2016 at 12:04


If you place glasswware like round bottomed flasks in an oven to dry you must, when removing from oven, suck the contents of the flask out with a vacuum cleaner and a narrow tube as the air inside can be very moist and will condense on the flask when it cools.

Nafion tubing can be used to dry any gas I have read.

PHILOU Zrealone - 27-11-2016 at 07:18

This may help...

Common lab dessicant information page out of Merck Chemical Lab Memento book...

Set by order of efficiency...
CaCl2 is into the third position

Merck-Chemistry-Lab-Memento-p14.JPG - 182kB

Dwarvensilver - 27-11-2016 at 09:34

In school we were taught when prepping for Grignard, rinsing with acetone after lab detergent hot water and air dry with compressed air.

Would only work if air was dry as well? or was this method mostly for cleaning? Been a while lol.

Maroboduus - 27-11-2016 at 10:06

Quote: Originally posted by PHILOU Zrealone  
This may help...

Common lab dessicant information page out of Merck Chemical Lab Memento book...

Set by order of efficiency...
CaCl2 is into the third position



I don't actually speak french, but that looks backward to me.

Phosphorus pentoxide at the bottom?

Also, although I admit I don't speak French, the tittle for the column looks to me like it says something about remaining water in Mg per liter. (after desiccation)

Am I mistaken here?

[Edited on 27-11-2016 by Maroboduus]

EDIT, but I had no idea silica gel was so complete as a desiccant.

[Edited on 27-11-2016 by Maroboduus]

PHILOU Zrealone - 27-11-2016 at 12:16

Quote: Originally posted by Maroboduus  
Quote: Originally posted by PHILOU Zrealone  
This may help...

Common lab dessicant information page out of Merck Chemical Lab Memento book...

Set by order of efficiency...
CaCl2 is into the third position



I don't actually speak french, but that looks backward to me.

Phosphorus pentoxide at the bottom?

Also, although I admit I don't speak French, the tittle for the column looks to me like it says something about remaining water in Mg per liter. (after desiccation)

Am I mistaken here?

[Edited on 27-11-2016 by Maroboduus]

EDIT, but I had no idea silica gel was so complete as a desiccant.

[Edited on 27-11-2016 by Maroboduus]

Lucky you, I speak French :P

Yes P2O5 is a strong dehydratant.
Into the list also silica gel ("Gel de silice")...
I hope you enjoy Gypsum (plaster of Paris as CaSO4 anhydrous = Drierite).
Sadly they don't list Ba(ClO4)2 an old lab dessicant (I own two bottles of it...but clearly I will not use it to dry stuffs but wel to make perchlorates and energetic materials.

They mention the residual moisture into the air above the solid (most likely into a closed vessel like a dessicator) --> mg/L (not Mg/L Mg= Magnesium and transmutation is not into question here ;))

DraconicAcid - 27-11-2016 at 13:03

Quote: Originally posted by Dwarvensilver  
In school we were taught when prepping for Grignard, rinsing with acetone after lab detergent hot water and air dry with compressed air.

Would only work if air was dry as well? or was this method mostly for cleaning? Been a while lol.


Compressed air is a terrible thing to dry glassware with, since it's usually contaminated with water (which condenses when the air is pressurized) or oil (from the pumps). When I was at university, all the undergrad labs had both compressed air and vacuum on tap. The vacuum was perfect for drying glassware, but a significant portion of my students insisted on using the compressed air, making shrieking noises and ruining their experiments. I should have failed them on principle.

DDTea - 27-11-2016 at 13:40

In my current (non-hobbyist) lab, I routinely do air and moisture sensitive reactions with finicky organometallic catalysts. Flame drying with a Bunsen Burner has always worked well for me. After heating, let it cool down in a 110 - 130 C oven, then assemble the apparatus hot. I've not yet melted borosilicate glassware this way.

Oven drying works perfectly fine too. But flaming the glass and watching the condensation migrate out of a flask just feels reassuring.

I'm skeptical of simply purging with hot, dry gas. Then again, it's just not something I ever do. Flaming takes 10 seconds and I've done it hundreds of times.
Quote:
In school we were taught when prepping for Grignard, rinsing with acetone after lab detergent hot water and air dry with compressed air.


Just be careful about removing acetone residues lest they react with your Grignard.

Dwarvensilver - 27-11-2016 at 21:38

Thanks for the tips gentlemen, the learning that goes on here and the knowledge base that is available is bloody amazing.
DDTea do you mean short of totally dry or that acetone molecules have an affinity for glass?

Dwarven

DDTea - 28-11-2016 at 11:23

I mean dry your glassware completely after chasing the water away with acetone. It won't stick to your glass.