Sciencemadness Discussion Board - Why did my NH4NO3 synthesis turn brown ?

Why did my NH4NO3 synthesis turn brown ?

Sulaiman - 24-8-2015 at 06:17

for no particular reason I made a small batch of NH4NO3 today:

100 ml 'Jeyes Household Ammonia' (MSDS = 9% to 10% NH3) in a 250 ml pyrex beaker

33ml 68% HNO3 added fairly quickly
then more ammonia added to bring the pH up to 6.8 (c110ml total NH3 solution)

the HNO3 and the NH3 and the mixture of both was water clear.
the mixing was exothermic but not violently so, est. c50C final temperature.

the solution was then heated on a hotplate to remove water,
estimated saturated solution at 100C was c 35 ml.

as the volume decreased not much happened until c35ml remaining,
at which point the solution rapidly turned transparent brown.
Fearing some sort of decomposition I removed the beaker from the heat,
a slushy brown solid formed, which I assumed to be NH4NO3 + H2O + (NO2 ?)

since slight excess HNO3 was used (pH 6.8) I thought that further heating may remove HNO3 and NOx,
so I heated the product some more, not quite to boiling,
the slush melted to a brown liquid which did indeed slowly clear over c10 min.

I now have a 250 ml pyrex beaker with white hard solid stuck in the bottom !

1) was the brown colour from NO2, or something else ?

2) how to get a solid crystalised lump out of a pyrex beaker without scratching the beaker ?
EDIT: re-heated the NH4NO3 to try and get anhydrous NH4NO3 so no need to answer Question2
I may have another go, with slight excess of NH3....

[Edited on 24-8-2015 by Sulaiman]

greenlight - 24-8-2015 at 06:57

Maybe try again with an excess of ammonia.

When I have made it from what I remember I used 62% Nitric acid and slight excess Ammonia, boil down until crystals start to show (about 1/3 volume), chill in freezer and filter.

[Edited on 24-8-2015 by greenlight]

greenlight - 24-8-2015 at 07:53

I just went and tried a quick Ammonium nitrate synth.
Used half of your amount with a slight excess NH3.
Mixed 50ml homebrand cleaning 12% ammonia and 15ml 62% Nitric acid and got an instant precipitation. Did it again with a different brand bottle and the same instant precipitation?
The bottles are very old (4 years+) and when i purchased them I didn't get the instant precipitation and had to boil down. Since then they have been sitting in my bases cupboard.
Must be something to do with the age.
Below is picture with ph strip tests from each. One shows ph 14 and the other ph 12 so definitely neutralized, maybe a little too much excess.

[Edited on 24-8-2015 by greenlight]

Sulaiman - 24-8-2015 at 08:34

instant precipitation !?

I had to boil down to <1/3 volume just to get to a saturated solution.

I just checked the solubility of NH4NO3:
with 12% NH3 + 62% HNO3
there is enough NH4NO3 to precipitate some at 20C !
though most of your product will still be in solution.

Here is the product:

NOT anhydrous.

greenlight - 24-8-2015 at 08:43

When I made it years ago I had to boil it down too, very weird.
I got a thick soup of fine crystals as soon as i added the Nitric acid, it must be something to do with the age of the ammonia because it couldn't be any other compound.

Your product looks nice and white now.

[Edited on 24-8-2015 by greenlight]

Bert - 24-8-2015 at 09:35

Ammonia sold for household cleaning use often contains various OTHER things- soaps, surfactants, perfumes.

Look for a product labeled "clear ammonia", or a product sold as "Janitor's ammonia". Read MSDS from manufacturer as well.

http://m.acehardware.com//product/index.jsp?productId=127232...

Or contact a supplier of blueprint materials, where 24 or 26 Baum strength ammonia water is sold for blueprint copiers.

[Edited on 24-8-2015 by Bert]

Sulaiman - 24-8-2015 at 11:20

The household ammonia that I used was water clear, the non-cloudy type,
the msds only lists NH3 as an ingredient, no detergents etc.
There was no bubbling or frothing during boiling so I guess just NH3.

I love SM ... help, advice, warnings and even instant parallel runs !

P.S. probably part of the reason for today's synthesis was that I just ordered 1l of 33% NH3, http://www.ebay.co.uk/itm/261976894512?_trksid=p2060353.m274...
to replace my household ammonia which is almost finished,
to use up the last bit it was a toss-up between NH4NO3 and NI3 (with some impure/used I2)
not in the mood for problems (suburbia) so NH4NO3 won as it has uses other than just detonation.
I will have another go when the new (pure?) ammonia arrives,
just for comparison.
My plan is to titrate the 33% NH3 against 69% HNO3 which I will do a titer vs. my NaCO3 1M reference.
This will check the NH3 concentration at the same time as the preparation.

Will a stoichiometric mix of HNO3 + NH3 produce NH4NO3 solution + precipitate of pH = 7 ?
(I did an acid:base titration where the stoichiometric combination produced pH>7, not all salts are neutral, which is why I ask)
i.e. is ammonium nitrate a neutral salt?

[Edited on 24-8-2015 by Sulaiman]

greenlight - 24-8-2015 at 19:29

Ah damn, found the MSDS for one bottle but not the other. It says >10% surfacants but no other information. Can't tell at all what impurities are in there now.

Im pretty sure Ammonium nitrate is a slightly acidic salt.

PHILOU Zrealone - 25-8-2015 at 05:15

Quote: Originally posted by greenlight

I just went and tried a quick Ammonium nitrate synth.
Used half of your amount with a slight excess NH3.
Mixed 50ml homebrand cleaning 12% ammonia and 15ml 62% Nitric acid and got an instant precipitation. Did it again with a different brand bottle and the same instant precipitation?
The bottles are very old (4 years+) and when i purchased them I didn't get the instant precipitation and had to boil down. Since then they have been sitting in my bases cupboard.
Must be something to do with the age.
Below is picture with ph strip tests from each. One shows ph 14 and the other ph 12 so definitely neutralized, maybe a little too much excess.

There is no reason it has to do with age...with age NH3 will volatilize through the plastic and eventually form efflorescences of ammonium salts on the bottle (chloride, nitrate, carbonate).
So the concentration will be slightly lower than initially...
NH4NO3 is very hygroscopic and as such remains long in concentrated solutions without precipitating.

Are you sure it was not another amine?
Urea would precipitate like that!
Surfactants may react the same...anionic will precipitate the free fatty acid, cationic will eventually precipitate a salt...non ionic surfactants will remain unaffected by pH variations.

pH 14 is way too basic for ammonia and pH 12 is very far from neutralisation...what is closer to pH 7!

[Edited on 25-8-2015 by PHILOU Zrealone]

greenlight - 25-8-2015 at 05:31

It was definitely 12% ammonia for general cleaning. And it is cloudy ammonia

There was a lot of white precipitate on the inside walls of the bottle as it was half full.
Nitric acid was 62% and quite old as well.
I will take a ph strip reading of the actual ammonia on its own when I get home.
It would have to be one of the surfacants or detergents that's been added in forming an instant precipitate

[Edited on 25-8-2015 by greenlight]