# Reusing NO2 from nitric acid



## kjavanb123 (Apr 18, 2009)

I am just curios, according to Hokes book, nitric acid and let's say copper, would produce NO2 as the brown fume, can this gas be somehow turn into HNO3? is it possible chemicaly to reuse this to produce nitric acid?


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## Noxx (Apr 18, 2009)

Yes you can.
There are a few patents on this around.


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## kjavanb123 (Apr 18, 2009)

Thnx

what would be a good searching phrase for this? How to make nitric acid??


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## Noxx (Apr 18, 2009)

I don't know... Most of the patents are on how to neutralize it.

You should be a simple scrubbing column where NO2 could bubble with a lot of surface.

I'm not sure if you could reach a high HNO3 concentration...


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## Palladium (Apr 18, 2009)

If you use H2O2 in the nitric you can cut down considerably on the fumes.


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## Palladium (Apr 18, 2009)

:arrow: http://goldrefiningforum.com/phpBB3/download/file.php?id=1235


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## butcher (Apr 18, 2009)

distill off NO2 bubble into water to make nitric acid, if less than 68% concentration can boil off water untill it reach's 68% further boiling will boil off your 68% acid, if stronger acid needed for some unknown reason, you would need to add 2 times the amount of sulfuric acid and distill off somewhere greater than 80% maybe stronger up to fuming acids.if doing this check on boiling points at concentration, my memory says 103 deg. for 20%, 120 deg for 60%, 83 deg for concentrated 96%, but memory is not good look it up to be sure.

I have been getting back alot of my acids, better to use them to dissolve base metals with than to neutralize them, of course not sure if I am spending more on electrical heat than would cost me to buy them???? OH well, if I realized what I spend getting this gold would probably kick myself.


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## semi-lucid (Apr 18, 2009)

butcher

For the past week or so, I've been working on a design for a rig to regenerate some the nitric from the NO2. But after recent discussion and reading, I've all but abandoned the idea.

Have you done it successfully?

Edit. I'm talking about recovering the brown fumes that occur while dissolving metal.

Thanks.
John


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## butcher (Apr 19, 2009)

yes you can do it this is one process to make nitic acid. bubble them into water.


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## qst42know (Apr 19, 2009)

This works to retain and to get the most of your acid. Its not collected for other uses but gets returned to the reaction to keep working. You can use less acid to get the job done.

http://goldrefiningforum.com/phpBB3/viewtopic.php?t=3269&start=0&postdays=0&postorder=asc&highlight=keeping+genie

It works much better than my cheezy lab would indicate. :lol: I need to retake the photos while using a "safe glass" reaction vessel.


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## 4metals (Apr 19, 2009)

The conversion of the nitric fumes back into nitric acid is possible in a closed system which has been purged with pure oxygen. The system schematic is below, it is a relatively simple system to operate for digesting silver in 50% nitric acid. While in theory only pure oxygen comes out the top of the packed tower (which is 14 feet tall) we still have the discharge exhausted into our ductwork. The reaction tank is stainless steel and the lid is weighted and gasketed to provide a leak-proof seal, forcing the NOx up into the packed tower. The packed tower also has a cooling coil inside so the flow is provided by the oxygen input rate which we judge by the bubble rate coming out the vent at the top of the tower which is submerged about 12” into a water bath. 

The reaction is started by adding distilled water to the tank under the packed column deep enough to cover the drain pipe to assure the gas doesn’t bubble out the bottom. This liquid is pumped through the column where it reacts with the NOx and the O2 to make nitric acid, which ends up in the reservoir with the water. We change out the acid in the reservoir when it is about 50% nitric (measured with a hydrometer) and we re-use this acid for the silver dissolve. This setup is probably overkill for all but large silver digestions but when I was experimenting with this technology I had a scaled down rig made up with standard organic lab glassware using a 5 liter vessel which, leaving room for a rise in the reaction, digested 40 ounces of silver. There was no red fume coming out of the top of the glass setup either. That setup would actually make a nice small rig for inquarting 10 ounces of karat with 30 ounces of silver and producing 99% pure gold as a by product. The main contaminant is silver, so if you’re a jeweler cleaning up scrap it can be re-alloyed as is, or cleaned up by melting with some creative fluxing. Then you can drop the silver with salt, reduce it with sugar and use it on the next batch.


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## semi-lucid (Apr 20, 2009)

Metals

Thanks for the post. The first question that comes to mind is about the materials used. I was under the impression that I needed to restrict my materials to glass only, with the necessary exception of a couple of PTFE gaskets.

The pump is of particular interest. Is it a PTFE lined pump? Centrifugal?

The column is packed. With what exactly?

O2 is key? N2 interferes/ buffers the reaction?

Thanks.
semi


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## semi-lucid (Apr 20, 2009)

The other thing is cooling. I had already incorporated cooling into my design, With the possibility of taking the temperature all the way down to 0 deg. C if there was an advantage to the reduced temp. 

I've read that nitric is more stable at 0 C, and I thought it might help in the regeneration process.

semi


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## Platdigger (Apr 20, 2009)

You can use a paristaltic pump.
Randy


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## Harold_V (Apr 20, 2009)

300 series stainless is very acceptable for use with nitric acid. It is packaged in such containers for distribution. You could make a recovery system from that kind of material with excellent results. 

Harold


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## semi-lucid (Apr 20, 2009)

Harold_V said:


> 300 series stainless is very acceptable for use with nitric acid. It is packaged in such containers for distribution. You could make a recovery system from that kind of material with excellent results.
> 
> Harold



Does that also apply to AR?

John


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## Harold_V (Apr 20, 2009)

No, stainless will readily dissolve in AR.

Harold


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## 4metals (Apr 20, 2009)

The unit I described is for digesting silver which has been inquarted with karat scrap. The system uses only nitric acid and therefore stainless steel is acceptable. the pump is also stainless steel. You could use a peristaltic pump but the hose which comes in contact with the solution wears out periodically and has to be replaced. The reason I don't use a peristaltic pump is that periodically always seems to happen at the worst possible moment.

If you were to build the identical system with materials resistant to aqua regia and pump in chlorine gas instead of oxygen you will recover aqua regia in the tank. The process is more difficult to control because the aqua regia reaction proceeds much more rapidly than the silver digestion I do causing a vacuum which inhibits flow. The simple bubbler I use on top of the tower for nitric dissolves will not work and it has to be replaced with vacuum relief valve and the chlorine flow controlled while monitoring a manometer.

The difficulty in using the system with chlorine, plus the materials needed for an aqua regia resistant setup, make this system a challenge. I don't know what you need as a final product from your refining, maybe you have to use aqua regia, but if you use the setup for nitric and inquart everything with silver you will come out with high purity gold. The gold can be re-boiled in nitric and rinsed and dried to yield a product that may serve your purposes. Read some of Harold's posts, he knows the benefits of inquarting and speaks of it often.


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## 4metals (Apr 20, 2009)

Semi 
Whoops, I forgot to tell you about the tower packing. The only function of the tower packing is to provide surface area in the column while allowing airflow through the column. The reaction between the O2 and the NOx happens on these surfaces. There are products made for this purpose, they're called tower packing and they're expensive. you can also use bio-balls used in aquariums to provide a similar function for bacteria to colonize. They're expensive as well. Now for the cheap alternatives. Ladies plastic hair rollers, cut up pieces of CPVC pipe. (short half to 3/4 inch slices of inch and a half to two inch pipe) piled up in the column. The diameter and length of the packed area of the column will determine the rate at which you can add metal to be dissolved. Our 18" diameter 14 foot tall tower handles 1350 ounce silver/karat inquarts without a problem or any red fumes leaking out.


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## semi-lucid (Apr 20, 2009)

I don't want to be a pain with all these questions, but I have a couple more.

The other day, when I asked about using metals in the acid lab, I got responses indicating the fumes would attack all common metals, including 400 and 300 series stainless, as well as being rough on plastics. That and a few other things I read steered me towards glass only.

I had expected to be able to collect the fumes from AR digestion, as well as from nitric digestion. Both processes release the same NO2 fumes do they not?

From Wiki: Dissolving gold in AR

Au (s) + 3 NO3- (aq) + 6 H+ (aq) → Au3+ (aq) + 3 *NO2* (g) + 3 H2O (l) 
and
Au3+ (aq) + 4 Cl- (aq) → AuCl4- (aq).

I don't see the difference between the nitric fumes and the AR fumes, and I'd rather not produce AR in the re-generator. I'm I reading this Wrong?



> and pump in chlorine gas instead of oxygen



Oh no :!:  O2 gas is scary enough for me, and I've used up many bottles of it in my day. I was once in an accident where I sustained severe second degree burns to my face and ears. Fortunately I was wearing a cap and safety glasses, so my eyes were not injured. There was a line across my nose where the glasses sat, that went from intact skin to raw meat. It wasn't an explosion involving purified O2, but people reading this should appreciate the fact that O2 can be dangerous. I've been around Chlorine gas to. Pretty raunchy stuff.

On the packing.



> There are products made for this purpose, they're called tower packing and they're expensive. you can also use bio-balls used in aquariums to provide a similar function for bacteria to colonize.



I was thinking of the aquarium ball's when I read your post. Someone, I think GSP, mentioned using plastic flowers in a neutralizing scrubber. 

The other question is *cooling*. Do you apply just enough cooling to maintain room temperature, or do you go below room temp? Quoting Wiki: *"anhydrous nitric acid should be stored below 0 °C to avoid decomposition"* If reduced temps can avoid decomposition, it makes me wonder if reduced temps can promote composition.

Would reduced temps result in N2O4?

http://en.wikipedia.org/wiki/Dinitrogen_tetroxide

Would N2O4 be corrosive to organic plastics? Makes me like glass. 

Thanks again.
John


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## 4metals (Apr 20, 2009)

It's better to be a pain asking questions than to be in pain from a reaction thats gone bad! I'll answer anything I can.

Glass is good for the reaction except for the fact that it can break, glass lined steel reactors are the best but incredibly expensive. I've heard of refineries using titanium vessels but I have no experience with them, I know that the titanium has to be passivated first but that's all I've heard of it. For the fumes a vinyl ester fiberglass holds up well, I've installed them over 20 years ago and they're still holding up to aqua regia which is caustic scrubbed. The fumes coming off an aqua regia reaction have some chlorides in them and will condense as a dilute aqua regia, more nitric than hydrochloric. When I say they will condense it is when they are passed through a condenser to cool them and collect the condensate. Usually when doing stone removal I like to condense the fumes to make the nitric perform double duty. If working in a condensed reactor I like to use the aqua regia made up 5:1 (so 1 part nitric) because the recycling of the condensate saves on acid. So to sum it up, nitric dissolves do not produce the same fumes, they may look the same because of the dominant player is the nitric, but aqua regia passes off chlorides which will condense if cooled.

The cooling in the packed tower is just so the tower stays cooled to help in the condensing of the fumes, it needs to be cooler than the reaction producing the fumes, the cooler it is the more efficient it condenses the fume. 

Plastic flowers, that's right up there with plastic hair curlers! The thing you want to avoid with packing is voids or depressions where either unreacted fumes or falling solutions will collect, we want a film on the packing caused by surface tension of the liquids, no puddles. So if flowers work, go for it! 

Anhydrous nitric acid is pure acid and the best you'll get is 50% by condensing it from fumes. Don't get too carried away with cooling, cold running tap water in a teflon coated coil will work fine.


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## Lou (Apr 20, 2009)

Anhydrous nitric acid has no purpose in refining and is a serious hazard--it is stored cold because it is constantly decomposing. It is a radically different animal than the regular 16M 67-70% that everyone here is familiar with and will visibly attack and destroy plastics which are otherwise inert at lower concentrations. 

As for column packing, one can find small marbles. Those are better than using saddles or raschig rings because there is less risk of holdup and accidental overpressure.


Glass reaction vessels are just fine if one is careful. I use jacketed glass reactors that have a 5" flange at the top and use overhead stirring and recirculating heaters. 5L, 12L and 50L but that would depend upon how much material. 

Glass lined reaction kettles are indeed expensive, and will rust all to hell, even if they're epoxy painted. Pfaulder makes these kettles. Every now and then you'll see one up on ebay for 500 or so. Which is a deal, because a 50 gal glass-lined reactor costs about 20 times that new (or more!).


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## goldsilverpro (Apr 20, 2009)

Lou said:


> Glass lined reaction kettles are indeed expensive, and will rust all to hell, even if they're epoxy painted. Pfaulder makes these kettles. Every now and then you'll see one up on ebay for 500 or so. Which is a deal, because a 50 gal glass-lined reactor costs about 20 times that new (or more!).



Several places I worked had glass lined Pfaudler kettles. They were all jacketed and steam was used to heat them. Instead of a reactor top, they were all covered with a round sheet of fiberglass with a fiberglass fume vent coming up out of it. The lid was hinged for easy access and cleaning. As far as I'm concerned, they are the ultimate dissolving machine. Far better than those huge awkward round bottom glass flasks sitting on a heating mantle.


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## 4metals (Apr 20, 2009)

No doubt that glass lined phaudler vessels are top of the line, but a 72 liter glass reactor with a mantle on a tipping base is cheaper and tough to beat. You never have to pick up the vessel, just invert it to empty and hose it clean. Condensers and mixers can all be added to the 4" conical head with a simple clamping device.


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## Lou (Apr 20, 2009)

Much like my setup, but I do not using a heating mantle...mine is jacketed.

Glas-col heating mantles are still around 2k for that size. Do you use overhead stirring?


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## 4metals (Apr 20, 2009)

Funny you should ask about stirring, I use these reactors for stone removal, once years ago I did have an expensive gasketed stir motor so the reaction stayed sealed and boy did it look scientific! Running the stirrer caused the diamonds to swirl around and score the glass which needless to say resulted in the acid draining out of the wrong end of the reactor. After replacing the mantle (you're right they are expensive) and the 72 liter flask (again pricey) the stirrer earned a corner of distinction from which it was never removed. Glass does have its drawbacks, although my glassblower doesn't see it that way!


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## Harold_V (Apr 21, 2009)

4metals said:


> The gold can be re-boiled in nitric and rinsed and dried to yield a product that may serve your purposes.


I'm very outspoken about not using nitric for the wash process. From experience, I know that the chlorides are not easily eliminated from precipitated gold, so the average guy is likely to encounter some re-dissolving of the recovered gold. My process recommends HCl for the wash, which appears to do an adequate job without any of the risks or complexities of using nitric. It in no way implies that nitric is not suited, just that it presents problems that may or may not be easily overcome by those with limited equipment. 

Harold


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## oldtimmer (Apr 21, 2009)

There is one for sale on ebay: 370153970905..


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## goldsilverpro (Apr 21, 2009)

The one on Ebay doesn't look like a kettle. I think it's columns. It's not what I was thinking of.


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## 4metals (Apr 21, 2009)

The e-bay posting is only parts of a complete column, I think that most would be surprised at the amount of money you would have to spend to complete the column; clamps, caps, and elbows cost big bucks when purchased new and most times, when you buy parts at a "great" price, the only way to get the missing pieces is to buy new. In the long run you'll spend more than you intended to. It is also critical when buying used glass lined equipment to closely inspect the glass to assure there are no small cracks which will allow the acids we use to attack the steel. Valves are the hardest things to inspect when buying used.


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## 4metals (Apr 21, 2009)

4metals said:


> The gold can be re-boiled in nitric and rinsed and dried to yield a product that may serve your purposes.





Harold_V said:


> [I'm very outspoken about not using nitric for the wash process. From experience, I know that the chlorides are not easily eliminated from precipitated gold, so the average guy is likely to encounter some re-dissolving of the recovered gold. My process recommends HCl for the wash, which appears to do an adequate job without any of the risks or complexities of using nitric. It in no way implies that nitric is not suited, just that it presents problems that may or may not be easily overcome by those with limited equipment.



Harold
For aqua regia refining I completely agree with your advice against using nitric acid to remove silver chlorides. The process I was talking about does not use aqua regia, only nitric as a parting acid. The resultant gold sponge, which has never dissolved, is slightly contaminated with silver but it is metallic silver, not the chloride. For this reason nitric will clean it up nicely. What this process does is essentially the same as the parting acids do in a huge fire assay. This process will not produce .9995 gold but may approach .999. We have produced gold for sale directly to Johnson Matthey Canada this way because their acceptance criterion was only .995.


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## Harold_V (Apr 22, 2009)

4 metals said:


> Harold
> For aqua regia refining I completely agree with your advice against using nitric acid to remove silver chlorides. The process I was talking about does not use aqua regia, only nitric as a parting acid.


Thanks for the clarification, 4metals. I failed to consider that you were referencing nitric processed material, so it would make perfect sense in that situation, and I'd certainly endorse the process. 

The HCl wash I referenced isn't intended to remove silver chloride, as you likely may understand. It's intended to help remove drag down of other contaminants, and appears to be very effective. The contaminants that are removed are generally very visible in the way of discolored solutions. 

Because silver chloride isn't removed by this process, I used a wash in ammonium hydroxide to remove any traces of silver chloride. That seemed to be effective, for in the end, my quality was very acceptable. 

Harold


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## semi-lucid (Apr 26, 2009)

Metals

I've spent quite a bit of time studying column's, and working on a CAD model. 
I'm wondering if the 14 feet number is the overall height, water tank + column. Or if it is the column alone. I'm trying to calculate the volume of the packed area, so I can look at it on a percentage basis. Do you think a volume of 33% to 50% would be overkill?

Better to uber-kill it, than to under-kill it.

John


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## 4metals (Apr 26, 2009)

Semi,

The tower I referenced was 14' tall by itself. Now despite how cool it sounds to do all of the chemical and mathematical calculations what it comes down to is this;

How high is your ceiling?
What size column can you get at the right price?

Refineries and tall ceilings are nice, as a rule of thumb with tower packing the fume should be redistributed every time it moves up by twice the diameter of the column. To redistribute the fume you need a short unpacked section of column and more packing support. Packing support is the grid the packing sits on to keep it from falling to the bottom. So in a 18" diameter column I would redistribute the fume every 36 inches. I would leave a 12" void before the next set of packing supports. In the void I like to add spray nozzles so whatever chemistry you are recirculating can enter the column. In our 14 foot column there are 3 sections of packing plus the necessary voids where the solutions are sprayed and the fume redistributes. If you are building a scrubber to scrub and neutralize NOx the air will be pulled through the scrubber with a blower, because of this a tightly woven packing, as compared to the relative density of your primary packing, needs to be above the top spray nozzles to act as a mist eliminator. This keeps large drops of your chemistry from spraying out of the column.

So to summarize, overkill never hurts when scrubbing, if you design to produce 1000 ounces a day, it never fails you'll need to process 1500! So I would use the fattest, tallest scrubber I could find at a good price, and still fit in the doors and under the ceiling.


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## semi-lucid (Apr 26, 2009)

Yes. You don't study column's very long before you start looking at the ceiling. :? 

J


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## dick b (Apr 26, 2009)

Its quite easy to break a 15' High column into three short 5' columns with the discharge of the first column being piped down to the bottom of the second column with the water sprays in the falling section of pipe, etc.
That way you can achieve any length of column necessary to scrub the fumes plus you can change the chemistry in each set of sprays to scrub for different fumes and or neutralize them.


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## 4metals (Apr 26, 2009)

Absolutely true, all you need is a bigger exhaust blower to make up for the resistance. 

Good point!


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## semi-lucid (Apr 26, 2009)

I was thinking about that also. You could have a separate water reservoir for each column, and route the fumes to the column with the strongest acid first. Then when you changed out the acid and installed fresh water, change the order of the series. Why keep it simple when you can make it complicated. :roll: 

John


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## 4metals (Apr 26, 2009)

I'm a huge believer in KISS technology.

KEEP IT SIMPLE STUPID!


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## semi-lucid (Apr 26, 2009)

When I first started thinking about this, I planned to bubble the fumes through a series of reservoirs, and as the concentration got high enough in the first reservoir, I would remove it, and move the second reservoir to the first position, and the third reservoir to the second position. Then add a new reservoir with fresh water to the third position.

That line of thinking naturally led me to thinking about a series of column's, because I wonder if the fresh water would be more effective at reacting with the fumes then a 50% acid concentration. With a series, you would always have a low acid concentration at the end of the scrub.

Not sure if it's worth the complication though. I have heard a quote attributed to Albert Einstein; Keep it as simple as possible, but no simpler.

John


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## semi-lucid (Apr 26, 2009)

That makes me want to pose the question.

As the acid concentration gets higher, at some point it should reach a maximum, and lose its effectiveness at scrubbing the fumes. Wouldn't this be true?

If so, is the fall off in effectiveness sudden or gradual?

John


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## Anonymous (Apr 26, 2009)

would hydrogen peroxide be a better liquid to pump the NO2 into? You would get nitric for every peroxide and no left of NO? just a thaught

jim


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## semi-lucid (Apr 26, 2009)

Jim

I'm not sure what concentration of H2O2 your speaking of, but my guess would be it would be cheaper to use distilled water and O2 gas.

John


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## 4metals (Apr 26, 2009)

The reaction in the scrubber where the N2O and O2 react forming HNO3 is probably most efficient when the water is pure H2O but that's where the height of your column comes in. The more possible reaction sites available the more chances for complete conversion. I'd guess that as the nitric acid concentration increases in the reservoir, the point in the scrubber where there is no more N2O available to react is higher up the column. It's easy to monitor the nitric concentration with a hydrometer and when the concentration is high enough, just siphon off the acid / water mixture and add deionized water. It's critical to use chloride free water. I would bet if you 'push' the column capacity to scrub until you see red fume out the bubbler, adding water would help, as would increasing the O2 flow.


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## semi-lucid (Apr 26, 2009)

Metals.

Thanks. That all makes sense to me. I really appreciate your help.

John


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