Sciencemadness Discussion Board

Is bubbling air through a solution an effective way to rid residual gas?

JefferyH - 5-7-2014 at 15:16

I have a reaction that is using NO2 gas to perform a nitration. I need a way to remove the excess NO2 gas that may be left in solution from the solution. Boiling is not an option.

Would bubbling a stream of air through the stirring solution be an effective way of pushing the excess gas upwards out of the solution? I would assume so, I just have no experience in doing so. I am doing this under a fume hood course.

Brain&Force - 5-7-2014 at 15:39

It depends on the gas. Nitrogen dioxide is really easy to recycle by bubbling it through some hydrogen peroxide - you get some nitric acid in return. It's weak acid, but still useful.

Other gases can be removed by heating the solution into which it is bubbled in a well-ventilated area.

HgDinis25 - 5-7-2014 at 17:06

BrainForce, Nitric Acid is a strong acid. You probably meant Nitrous Acid.

elementcollector1 - 5-7-2014 at 17:25

Quote: Originally posted by HgDinis25  
BrainForce, Nitric Acid is a strong acid. You probably meant Nitrous Acid.

Nope - 'weak' as in 'dilute'.

Brain&Force - 5-7-2014 at 17:42

Quote: Originally posted by elementcollector1  
Quote: Originally posted by HgDinis25  
BrainForce, Nitric Acid is a strong acid. You probably meant Nitrous Acid.

Nope - 'weak' as in 'dilute'.


This is what I meant - I always confuse people on this point. The hydrogen peroxide will oxidize the nitrogen dioxide to nitric acid most efficiently:

2NO2 + H2O2 + H2O → 2HNO3


Water can be used, but this leads to the formation of nitrous acid (though slightly more nitric acid, as well as a small amount of nitrogen monoxide, will be formed):

2NO2 + H2O → HNO3 + HNO2
3NO2 + H2O → 2HNO3 + NO

Metacelsus - 5-7-2014 at 18:03

You could put it under vacuum.

JefferyH - 5-7-2014 at 18:06

Good replies. But how does this help me get my the NO2 out of the solution? If I need to keep the water layer and continue working with it, adding H2O2 would just make aqeuous nitric acid, would it not? Neutralizing that will just contaminate my water layer with a nitrate salt, and extracting that salt would be no easy task without interfering with my reactants.

One of the reactants for instance is an aromatic with multiple polarizing water-solubizing groups on the aromatic ring, yet this compound is soluble in both polar and non-polar solvents, to an extent.

Is there no procedure for me to rid the NO2 without heating? Would heating even work if I could do it? I don't need to recycle the NO2, I am just assuming it as waste at this point. The quantities I'm working with a very small here.

HgDinis25 - 5-7-2014 at 19:16

You would be better of bubbling an inert gas into the solution, like argon or nitrogen. Oxygen in the air may cause undesired unexpected side-reactions. To complement, you could use both a vacuum and, at the same time, be bubbling the inert gas into the solution. This would most likely work.

Brain&Force, this is pretty straight forward: the terms Weak and Strong about an acid are related to their ionization and the respective equilibrium constant (aka acidity constant). If you don't desire to confuse people, I would sugest refering to a weak acid solution as a weak acid solution.

JefferyH - 5-7-2014 at 20:39

Would a vacuum work if it was the only thing I used? These gasses have a much lower boiling point than any of the solvents/water, so I see the possibility there. If I put the apparatus under a vacuum would all the gas just get pulled out, behaving like air would in the system? Or is bubbling air through the solution a requirement?

I would probably have an activated charcoal trap set up in the middle of that to stop any corrosive gas from getting to the vacuum.

If it is this easy to pull gases out of aqueous or otherwise solutions, could this method be also used to pull gas out of an aqeous solution it is contained in and then through a flask were it would immediately react? Like if I wanted to pull a hydrogen halide molecule from its respective aqeuous solution and into another solution?

HgDinis25 - 6-7-2014 at 04:22

Vacuum reduces the parcial pressure of the gases immediately above the solution, therefore shifting the aqueous-gas equilibrium to the gas phase, however I'm not sure how fast it would go without a gas inlet bubbling through the solution. Also remember that, when only using vacuum, if you reach something like 0,5 psi, water may also leave your reaction vessel.

Yes, the vacuum method can be used to drive, for instance, HCl from it's aquous solution, to a certain degree I believe. You should assume, though, that it won't leave the vessel completrly dry.

AJKOER - 6-7-2014 at 20:37

Try cooling with stirring as the vacuum method may impact your volatile compunds.

If that doesn' t work, add more of the compound that you trying to nitrate with stirring and let the NO2 bubbles(?) dissolve slowly via nitration.

Ascaridole - 7-7-2014 at 13:57

Vacuum works fine, we degas water for HPLC this way, you do however need vigorous stirring however while pulling vacuum for things to go at a reasonable rate. On the same not if your worried about liquid leaving your reaction just put a condenser between the vacuum take off and your solution and cool the crap out of it.

If you do go with the bubbling method inert gas is highly recommended as air is full of wonderful CO2 and other trace gases that can screw with you. Helium is the best but super expensive. Also use of a gas dispersion tube is necessary to achieve efficient use of sparging (bubbling inert gas through a solution).

ziqquratu - 7-7-2014 at 15:35

The three common methods for degassing a liquid are to sparge with an inert gas, sonicate under weak vacuum or freeze-pump-thaw.

Sparging (bubbling) with an inert gas - usually nitrogen or argon - is effective enough and particularly good when you're dealing with large volumes. It does basically replace the dissolved gases with the inert gas, which can be problematic for some applications (e.g. avoiding bubbles in polymers), but it's easy and may very well be best for your application, as dissolved nitrogen is probably not an issue to your application! The trick is to get the smallest gas bubbles you can reasonably manage - a tube with a fine frit at the bottom works well, or some glass wool can be better than nothing (although "nothing" on a reasonably fine tube can also be OK if you sparge a bit longer). Avoid reactive gases (including oxygen and carbon dioxide in air), though, unless you're certain they will have no effect on your system.

Sonication under vacuum forces the gases out and they are not replaced. It's also quick and probably more effective than sparging, but does require vacuum (although an aspirator should be more than enough) and takes longer than sparging. Also, being under vacuum, it can be less useful for low-boiling solvents.

Freeze-pump-thaw is the best method for total degassing, but also the most tedious. You place your solvent under inert gas, freeze in liquid nitrogen, apply high vacuum to the flask, seal the evacuated flask and thaw the solvent. The gas enters the headspace as the solvent melts. Once it's fully melted, you freeze, evacuate, seal and thaw again... and again, and again... until no more bubbles evolve. Once you're done, fill the flask with inert gas and use as required (quickly if you don't want the inert gas to dissolve!). Because the solvent is frozen when the flask is open to the pump, the method can be used for any solvent, regardless of boiling point. You do need to be sure, though, that your vessel doesn't have any leaks, or it will allow air to get in when the solvent is melting, meaning your time is wasted.

Chemosynthesis - 14-7-2014 at 02:04

How NO2-free do you need it?

I've used inert sparging under stirring for 30min to an hour, if I remember the timing right, on some HPLC solvents, and similar to Ascaridole mentions above, have seen vacuum used for similar purposes.

I think sparging would probably be acceptable to you if it's relevant to your polymerization thread, as I have seen the technique used in polymer-nanoparticle bulk-heterojunction synthesis before.

AJKOER - 14-7-2014 at 09:03

Try salting-out also by adding NaCl, or CO2(?), assuming that it remains unreactive. Here is a reference http://www.freedrinkingwater.com/water_quality/water-science... . To quote:

"When an ionic salt like NaCl is added to water, the ions from the salt introduced will attract the water molecules in an effort to "solvate" the ions. This has the tendency to decrease the weak affinity of non-polar oxygen molecules to water and drive the dissolved oxygen out of the polar water. In general, the solubility of a gas in a solvent is affected significantly by the presence of other solutes in the solution.

The maximum amount of dissolved oxygen a body of water can hold (saturated solution) depends on several factors. Dissolved oxygen solubility is affected by water temperature, atmospheric pressure and salinity. Cold water can dissolve more oxygen than warm water.... Solubility of dissolved oxygen also decreases as salinity increases.

Dissolved oxygen refers to the amount of oxygen contained in water. Oxygen has limited solubility in water usually ranging from 6-14 mg/L....

The solubility of gases in water usually decreased by the addition of other solutes, particularly electrolytes. Aerated drinks have carbon dioxide dissolved in it under pressure. Therefore, when salt is added to an aerated drink, the dissolved carbon dioxide is "salted out". The drink fizzes as many small bubbles of carbon dioxide are released from the drink. The extent of this "salting out effect" varies considerably with different salts, but with a given salt the relative decrease in solubility is nearly the same for different gases including dissolved oxygen."

Now in the case of NO2 in purely water, the addition of MgCl2 or NaCl or CaCl2 (in declining order of preference), one may increase the nitrating activity level (by decreasing the activity coefficient of water in the ionic solution), and thereby increase the consumption of NO2 without raising the temperature.

[Edited on 14-7-2014 by AJKOER]