[javhert]

Hi. Here in my lab we are testing microwave digestions of samples using HNO3 (10 mL concentrated acid for 0,5 g sample). Problem is, the high temperature and pressure conditions generate pressurized nitrogen dioxide gas which comes up as a stream of red fumes when opening the digestion vessels.

I decided I can't trust our fume hood to get rid of the bulk of it since the velocity at which it escapes disperses it faster than the hood is pulling it out and even if it did, fumes of visibly red, corrosive and toxic gas coming out of the air duct could get us into trouble. So I'm looking for the best way to supress and consume the nitrogen dioxide before it reaches the air extractor.

Info is ambiguous about how to treat this gas. Scrubbers or gas washers seem to be the most affordable solution. Some say NO2 is quite soluble in water (I guess in respect to NO) and some say it's not. Some speak of scrubbing it with oxidizing solutions of sodium hypoclorite or hydrogen peroxide while some others talk about using a reducing agent like sodium sulfide or ammonia.

I've created a pair of gas washers using long glass tubes with a fritted end and 19 L carboys, so bubbles of gas would travel across 40 cm of liquid. Which solution would give a good absorption and reaction rate to make sure most of it doesn't make it to the outside?

[Corribus]

There's really nothing you can do about this except open your tubes in a fume hood rated for acid work. Which you should be using anyway if you're preparing solutions of acid digestion.

If fumes are reaching your face rather than being sucked into the hood duct, then there is something wrong with the flow of your hood or you are not using the hood properly (e.g., sash height, working too close to the front of the hood, large objects in the hood that obstruct flow, etc.).

The cooler the vessels are the less volatile the acids will be. You can try refrigerating them before opening them.

We do microwave-assisted nitric acid digestions all the time and have no problem with acid fumes when opening the vessels. If you have a rated fume hood and are using it properly, you shouldn't need to take extra precautions. (However your microwave exhaust should ideally be scrubbed before letting it into the hood.)

[javhert]

Updating, I've found the hard way that the gas washers I concocted weren't good for the purpose. Even when attempting to do this in a regular sized glove chamber, extracting all the NO₂ at an acceptable rate for the gas washers to absorb it proved to be take way too much time (I could have seen it coming using a complete mix tank dilution model but I digress) so I'm looking for a less improvised way to deal with it.

I've researched about fume hoods and so far I can safely discount using a ductless hood with a carbon filter because gas filter suppliers explicitly say there is no good solid filtering media for nitrogen dioxide. Some fume hood brands offer wet scrubbers mounted above the fume hood in order to wash down "water soluble gases" before they exit through the exhaust duct. Seems like a good option although again, it all depends on whether nitrogen dioxide is considered a water soluble gas or not and information doesn't seem to be clear at that (given the equilibrium with N2O4, a quick look to the Henry constants isn't enough to tell). Anyone here knows by experience if fume hood wet scrubbers are good enough or rated to deal with nitrogen dioxide?

At most we would be processing 24 samples which equate to 240 mL of 70 % nitric acid or 171,696 g of potential NO₂ (provided all HNO₃ would be transformed).