When I dissolve ammonium nitrate in 98% sulfuric acid, the solution turns bright red. The product still produces the lovely white choking fumes of
nearly anhydrous HNO3, but I am worried that the replacement reaction that creates the (NH4)HSO4 and HNO3 would be exothermic enough to partially
decompose the HNO3 and produce dissolved NO2. Glyceryl trinitrate created from these materials is not the colorless oil I was told to expect from
nitration of pure glycerol (i.e. free from acrolein compounds), instead having a brownish tint to it, even after bicarbonate wash and removal of
occluded water via CaCl2 solution.
Do solutions of ammonium bisulfate in mixed acid always have this reddish color and impart a brown tint to nitrated compounds, or am I adding the
prills too quickly and putting myself in danger?
Would addition of urea help here, or would it simply become urea sulfate and not interact with any dissolved NO2?
[Edited on 21-12-2011 by killswitch]AndersHoveland - 21-12-2011 at 11:04
There must be impurities, because as much as I know there should not be any bright red color in the reaction of concentrated H2SO4 and NH4NO3.
Nitrogen dioxide, however, can give the solution a redish-brown color.
I have also read that the use of ammonium nitrate, rather than sodium or potassium nitrate, in nitrations can sometimes be problematic. NH4NO3 has the
advantage that it is much more soluble. But reaction with concentrated sulfuric acid can also cause some decomposition.
When ammonium nitrate is heated with concentrated sulfuric acid, about 85% of its nitrogen is given off as nitrous oxide. Most of the remainder as
nitrogen gas. Typically, ammonium nitrate is only dehydrated when a large excess (50 times as much) of sulfuric acid is used and the temperature is
raised to 150degC. http://pubs.acs.org/doi/abs/10.1021/ja01681a501Luftwaffe - 21-12-2011 at 14:09
I too have encountered this red nitrating solution with ammonium nitrate from instant cold packs that I later found to be quite impure. It's
deffinatly not nitrogen oxides making the colour, it's much too bloody looking and when I used purified AN the nitrating solution is clearish brown.
While the solution looks sinister my nitration yielded a white product. I am still at a loss with what exactly is the contaminant as it turned red in
a nitrating solution and brown when boiled at a little over 100C and has been a mystery to this day in all the threads I have read/started on
purifying AN in instant cold packs.AndersHoveland - 21-12-2011 at 14:23
This is just a complete stab in the dark and probably wrong, but...
If calcium chloride is the impurity, could it potentially be possible that it is somehow reacting in the nitration mixture to transiently form ClO2[+]
ions? Chloryl ions impart a bright red coloration. This might be less far-fetched then it first seems. I know that NaCl can be oxidized to chlorine by
mixed acid nitration. And chlorine has an equilibrium in solution.
Cl2 + H2O <=> HCl + HOCl
I will spare you the details of further equilibrium, but needless to say the HOCl could effectively be further oxidized.
Mixtures of chlorate and concentrated sulfuric acid have been observed to turn red, although such mixtures more often explode spraying corrossive acid
everywhere! "Chemistry, inorganic and organic", Charles Loudon Bloxam, p166
While compounds containing the chloryl ion or group can only be isolated using obscure regents in extreme reactions, chloryl ions can more accessibly
form in equilbrium in concentrated sulfuric acid solutions, imparting their distinctive red color.
But if the red coloration persists in your solution even after the nitration acids have been diluted, then the color is not due to ClO2+ because
chloryl ions cannot exist in the presence of water.
[Edited on 21-12-2011 by AndersHoveland]Luftwaffe - 21-12-2011 at 15:28
Very convincing argument as the colour does go away when diluted. Maybe I should consider myself lucky that I didn't detonate my RBF. O.O
This would make an interesting denaturant and is odd that they would pick something that could blow you up.
The red colour persists but is a bit more swashed out as product precipitates out throughout the nitration. Thanks Anders for the find!AndersHoveland - 21-12-2011 at 15:33
I suspect a portion of ammonium chloride is added to the NH4NO3 in many cold packs.
Wikipedia says:
Quote:
Instant cold packs use an endothermic reaction to cool down quickly. Common types include solid ammonium nitrate , or
ammonium chloride dissolving in water
Some cold packs may also contain some urea, in addition to the ammonium chloride.
I do not think there is anything particularly to worry about in terms of explosion hazard from chloride in a mixed acid nitration bath. Of
course, nitration in the presence of any organic compound can potentially be hazardous (though I would not worry about an explosion from urea
or benzene). I hope this goes without saying, but ALWAYS wear splash-resistant safety googles when performing a nitration!
[Edited on 21-12-2011 by AndersHoveland]Luftwaffe - 21-12-2011 at 15:54
I guess a good way to find out for sure is to test for chlorides, I'll try that when I mess with cold packs again.Sedit - 21-12-2011 at 19:23
Can we see a picture by any chance?Luftwaffe - 21-12-2011 at 21:01
I don't have a picture, but it looks like liquid bromine without any fuming. The liquid is opaque.
EDIT: The nitrating bath looks like this guys solution for making nitric acid, which is essentially the same.
