# concentrating weak nitric acid



## Geo (Sep 27, 2012)

is it possible to concentrate a weak nitric acid by evaporation? there are a few things i do understand about how the acid behaves. it decomposes when heated. it decomposes in sunlight. im not really worried about loss to NOx as long as i can evaporate to 50%. i saw a youtube video post here that explains how to concentrate nitric acid to white fuming nitric acid but i cant think of anything i would need it for. i have been collecting weak nitric acid by means of collection of NOx from my AR processes. its strong enough to give a good reaction to a piece of copper but nothing like im used to getting when i distill from sulfuric and nitrate. i dont do anything fancy, i just bubble the escaping gas through a flask of water. i do know about h2o2 and an ice bath but i consider this a by-product because rather than trying to make a usable acid, im trying to soak up the fumes from the process.however, i now have a few gallons of this weak nitric acid that i could find a use for if i can figure out how to concentrate it. anyone?


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## butcher (Sep 27, 2012)

Simple answer Yes you can concentrate a weak nitric acid up to azeotropic 70% nitric acid, by careful evaporation, as long as temperature of the solution is controlled below the boiling point of the acid at the concentration of nitric acid to water content of solution

Nitric acid has an azeotope of about 68 % to 70 % this means we can concentrate it to this point with evaporation.

Nitric acid also has a boiling point for each concentration at which point the acid will vapor off of solution.


%HNO3 ---Boiling point C deg
0% --- 100 C (100%water)
20% ---103 C (80% H2O)
30% ---107 C
50% ---116 C
70%---121 C
See note below
90% ---102 C
100% ---86 C

If the nitric acid is below 70% and evaporated below a temperature of the boiling point (for that concentration of HNO3) you can concentrate the acid to azeotope.

Evaporating a weak solution of nitric will vapor off mostly water and very little acid as long as temperature is not too high, as water is removed some acid will be in the fumes but losses will not be high if temperature is kept below the boiling point for the acid at that concentration.

The more concentrated the more acid will vapor off in the fumes of water leaving the solution, refluxing can also help to return the acid to solution and keep from losing the small amount that tries to leave the solution with control of temperature.

If the nitric acid is of greater concentration than 70% you will vapor off acid and lower its concentration to azeotrope70%.

Notice above how the boiling point raises, as acid is concentrated to azeotropic solution for the solution as it is concentrated, and how the concentrated acid has a much lower boiling point than the azeotropic solution.

Now lets look at the boiling temperature in degrees C, and % HNO3 content by weight in liquid and in vapor. This is very helpful when distilling nitric acid.

Boiling temp deg C ---%HNO3 in liquid ---%HNO3 in vapor by weight
100C----0% ------0%
104C----18.5%---1.25% 
107C----31.8%---5.06%
112C----42.5%---13.4%
117C----57.3%---25.6%
119C----67.4%---67% 
120.5C--68.4%---68.4%-azeotropic nitric acid
116C ---76%-----90%
113C----79%-----93%
110C----81%-----98%
96C------90%----99%
88C------94%----99.9%
83.4C----100%---100%


Each concentration of nitric acid also has a density, or specific gravity.

Density @ 15C g/cm3
50% HNO3--1.314
60% HNO3--1.374
70% HNO3--1.419
80% HNO3--1.460
99% HNO3--1.514


The you tube video is wrong or you missunderstood it, you cannot concentrate nitric acid past azeotropic solution of 70%, the nitric would have to be made that concentrated fuming to begin with, when concentrating a dilute acid further evaporation after 70% and you are just evaporating off 70% acid until the evaporating dish is dry.


if you have been collecting NOx from vapors off of aqua regia you also have HCl in solution, so you actually do not have nitric acid but a dilute form of aqua regia.


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## Geo (Sep 27, 2012)

thanks Butcher. thats somewhat what i had in mind, i just didnt want to take something that might be worth something and turn it into something worthless or worse, something harmful.


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## g_axelsson (Sep 27, 2012)

What would happen if you try to partly freeze the weak acid? Would it crystallize ice crystals of pure water and concentrate the acid in the remaining water?

At least salt solutions can be concentrated in this way, never heard anyone doing it for acid though.

/Göran


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## goldenchild (Sep 27, 2012)

g_axelsson said:


> What would happen if you try to partly freeze the weak acid? Would it crystallize ice crystals of pure water and concentrate the acid in the remaining water?
> 
> At least salt solutions can be concentrated in this way, never heard anyone doing it for acid though.
> 
> /Göran



I've found that it takes very little acid to prevent water from freezing. Of course there must be some threshold for the amount of acid water can hold before it no longer freezes but its a very small one.


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## Irons2 (Sep 27, 2012)

g_axelsson said:


> What would happen if you try to partly freeze the weak acid? Would it crystallize ice crystals of pure water and concentrate the acid in the remaining water?
> 
> At least salt solutions can be concentrated in this way, never heard anyone doing it for acid though.
> 
> /Göran



I've concentrated acidic leach solutions during the Winter. The Ice precipitates out as large, flat crystals that are easily filtered.

If your distillate contains HCl, use Silver Nitrate to precipitate out the Chloride, The Silver can be recovered.

No whining about Silver Chloride, Please. Man-up and do it. :mrgreen:


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