Sciencemadness Discussion Board

Precipitation of the "wrong" salt - solubility of NH4NO3 ?

Daddy - 13-3-2009 at 07:44

Hello everyone,

I am posting this under "Beginnings" since I am a beginner (some details about me mentioned in the thread "Introducing myself" under "Miscellaneous").

I read that if you mix in solution two different salts, containing four different ions, that of the four possible combinations, the salt which is least soluble will precipitate. (Sounds logical...) - so for example, if you mix NaNO3 and KCl, KNO3 will precipitate.

I tried this with ammonium nitrate and potassium chloride, where potassium nitrate should precipitate (at least below 20ºC, and I had 15ºC in my solution). After some time I had a white precipitate and a clear solution above it. But I found that the precipitate was ammonium nitrate again - the *most* soluble salt of these, according to my sources.

The tests I did were these: I dissolved the precipitate in water, which cooled the water, a property of ammonium nitrate (but not of KNO3, as far as I know). I also heated the decanted clear solution with lime, which should have produced ammonia gas if any ammonium ions were in the solution; but it did not.

As a matter of fact, the ammonium nitrate never dissolved completely; it rendered always a turbid, milky liquid. It cooled the water, though; so *some* dissolution must have taken place. I had bought the ammonium nitrate as fertilizer; on the bag it said "stabilized", does this mean it has been treated or mixed with something which affects its solubility? Or how else do I have to interprete my results? I am confused about it and would appreciate help.

hissingnoise - 13-3-2009 at 08:07

Fertiliser grade AN is usually in the form of coated prills.
The turbidity is caused by the insoluble coating which is a type of fine clay.
It will settle to the bottom over time time leaving a clear AN solution.
Carefully decant or filter the solution to give relatively pure AN on evaporation.
The clay, AFAIK, is a mostly insoluble carbonate and this will mess-up your metathetical precipitations.

Ebao-lu - 13-3-2009 at 08:45

Not all ammonium salt solutions (better to say, very few) give an ammonia gas while boiling. It is only the case with weak acid salts. Still sometimes a slight NH3 smell is present, but usually it is caused by bases in the solution. If your initial AN fertilizer solution smells when boiled, that is probably because of some admixtures, maybe the same precepitate if it is CaCO3. If it does not dissolve entirely(AN), that means it is bacause of admixtures, as AN is very good soluble in water, and the solution is transparent. Try to decant it the solution before use as it was suggested (and you can then boil it to eliminate rest base untill NH3 odour dissapears(if it was present initially.)

Ebao-lu - 13-3-2009 at 14:43

Quote:
I read that if you mix in solution two different salts, containing four different ions, that of the four possible combinations, the salt which is least soluble will precipitate. (Sounds logical...) - so for example, if you mix NaNO3 and KCl, KNO3 will precipitate.
It is also needed that salt concentration in resulting solution should be more then its solubility for precepitation. You can use tables and curves of solubility to estimate wether salt would precepitate or not (like this one http://en.wikipedia.org/wiki/Solubility_table).
KNO3 is indeed least soluble at 15C, but the diffrernce between its solubility and NH4Cl is not enough substantial to be sure that precepitate is only KNO3. Depending on the concentrations and volumes of solutions you used, you may get no precepitate(too dilute solutions), KNO3(rather narrow range of concentrations and volumes), or KNO3 with some NH4Cl (if too concentrated NH4NO3 used). It is quite difficult to predict what you'll get.
So better, if you use more relevant differences in solubilities. You can cool your KNO3 up to 0 - its solubility would further decrease.

not_important - 13-3-2009 at 15:55

Quote:
Originally posted by Daddy
had bought the ammonium nitrate as fertilizer; on the bag it said "stabilized", does this mean it has been treated or mixed with something which affects its solubility?


As hissingnoise said, an anticaking coating, plus fertilizer grade ammonium nitrate nowadays often contains some calcium nitrate.

Do as he suggested, for the easy way. Slowly evaporate the solution, but not to dryness, to crystallise out the NH4NO3.

Check the label for calcium, or make a few ml of clear solution and add a bit of a solution of sodium carbonate or oxalate, a white precipitate indicates the presence of calcium.

As you are using KCl as the potassium source, the calcium won't interfere as its chloride is quite soluble, but it will make calculating the proper amount to use difficult and the crystallised ammonium nitrate won't dry.

[Edited on 14-3-2009 by not_important]

Daddy - 20-3-2009 at 07:22

Ok, thank you very much all of you. So I will try to purify my ammonium nitrate and then try again.
Quote:

Not all ammonium salt solutions (better to say, very few) give an ammonia gas while boiling. It is only the case with weak acid salts. Still sometimes a slight NH3 smell is present, but usually it is caused by bases in the solution. If your initial AN fertilizer solution smells when boiled, that is probably because of some admixtures, maybe the same precepitate if it is CaCO3.


Ebao-lu: I did not just boil the solution, I added lime to it and then heated it. This did indeed give off quite a lot of ammonia when I did it with the original fertilizer; but none at all with the solution that remained after the experiment with KCl. - But when I have time I will try again after purifying the AN as suggested.

hissingnoise - 20-3-2009 at 07:39

Quote:
Originally posted by Daddy
I did not just boil the solution, I added lime to it and then heated it.

That is another metathetical reaction giving calcium nitrate and ammonia (gas).
If you need NH4OH at any time you can use it.
Ammonia can be led into water using an inverted funnel but its low density means the solution must be actively stirred. . .

Daddy - 20-3-2009 at 08:15

Yes, I did this ... I led the ammonia gas into a flask with water through a tube which reached down to the bottom of the flask, so the gas bubbled upwards through the water and it dissolved well that way. I only had to control the reaction speed carefully so there was not too much and not too less ammonia produced at a time.

(Edit: I have no automatic stirrer, so I cannot do it with the inverted funnel.)

But what confuses me, about the experiment I described at the beginnning: if ammonium nitrate is so soluble, why did that solution not produce any ammonia when heated with lime? Must I then not assume that all the ammonium ions are in the precipitate?

[Edited on 20-3-2009 by Daddy]

Ebao-lu - 20-3-2009 at 14:26

Indeed strange. Then it should be, that fertilizer's residue is the substance causing NH3 evaluation, not lime i suppose. Can you check, if the fertilizer solution also produces NH3 when boiled(without lime) ? If not, then it means that the reaction between NH4+ and CaCO3 requires high concentrations of NH4+, and the concentration in the filtrate is not sufficient. You can try several dilutions of AN boiling with lime to see how the rate depends on concentration?

And if the fertilizer solution produces NH3 when boiled, try to get rid of the residue and then boil it with CaCO3 to see wether it reacts with AN or not(at appropriate rate)

Daddy - 26-3-2009 at 07:20

OK, I am back ... I purified the amonnium nitrate as suggested and did the experiment again (dissolving it together with KCl and letting it precipitate). This time, when I mixed the clear solution with lime, it gave off a distinct ammonia smell when agitating, I did not even have to heat it. So this time the ammonium ions were obviously in the solution.

I did not yet perform any tests with the precipitate to check if it is actually KNO3. (At this moment I am still waiting for the water to evaporate from it; I am just letting it dry in the sun.) But since KCl would not precipitate in these conditions, I think it is safe to assume it is KNO3.

So it seems that it were really the impurities in the ammonium nitrate which prevented the KNO3 from precipitating the first time.

PS: Lime is Ca(OH)2; CaCO3 would be limestone.
OH- + NH4+ = NH3 + H2O, this should work with any hydroxide and ammonium salt.