Sciencemadness Discussion Board

Ammonium chromate, what the fuck has happend there?

pinkamena - 12-11-2016 at 11:45

Okay, I was just neutralising a concentrated chromic acid solution with ammonia to get ammonium chromate, and added about 100% excess ammonia. The next day, ammonium chromate crystallised out, which is not really unusual, but a few hours later when I looked at it, and there were super fluffy orange crystals only on one side of the beaker all the way to the top. The crystals really were like a mushroom or something like that, I have absolutely no explanation for that. Has anyone an idea of how that could happen, the solution was alkaline, and there were no chloride ions present, so there is no chance that some volatile CrO2Cl2 has formed, and there were only crystals in the beaker, nowhere else, so I don't think that there was any volatile Cr 6+ compund escaping.

Rhodanide - 14-11-2016 at 05:56

Quote: Originally posted by pinkamena  
Okay, I was just neutralising a concentrated chromic acid solution with ammonia to get ammonium chromate, and added about 100% excess ammonia. The next day, ammonium chromate crystallised out, which is not really unusual, but a few hours later when I looked at it, and there were super fluffy orange crystals only on one side of the beaker all the way to the top. The crystals really were like a mushroom or something like that, I have absolutely no explanation for that. Has anyone an idea of how that could happen, the solution was alkaline, and there were no chloride ions present, so there is no chance that some volatile CrO2Cl2 has formed, and there were only crystals in the beaker, nowhere else, so I don't think that there was any volatile Cr 6+ compund escaping.


You should change the title.
Besides that, I have no idea.
Edit: Actually, By neutralizing the Chromic acid, you probably created some heat, causing turbidity, and maybe subsequently moving the crystals around? (Other users feel free to correct me, for I could be incorrect!)
Edit2: Add some pics if you can! (That is, if you still have the formation intact!)


[Edited on 14-11-2016 by Tetra]

[Edited on 14-11-2016 by Tetra]

phlogiston - 14-11-2016 at 07:51

Sometimes you can have a crust of crystals form on the sides of the container which then wicks up the saturated solution by capilary action. As the solvent evaporates on the surface of the mass, it deposits more salt. Odd shaped crusts can sometimes form in that way.

I don't know if that is what happened in your case, but your description sounds like it might have.

[Edited on 14-11-2016 by phlogiston]

wg48 - 14-11-2016 at 09:20

Yes the crystals form above the solution surface due to capillary action up its walls. Its not necessarily even perhaps due to how clean the sides of the container are or if one side of the container is at a different temperature.

Below is pic of some waste solution that evaporated in a jar. It was only about 1/2 to 2/3 full.



waste.jpg - 15kB

and a pic of the bottom of the jar



wast24.jpg - 27kB

[Edited on 14-11-2016 by wg48]

woelen - 15-11-2016 at 00:10

Indeed crystals, formed near the surface, sticking to the glass, suck up additional liquid, which evaporates and leaves more solid. This can grow so large that it even grows out of the beaker, onto the table around it. This type of crystals is less pure than those, formed in the liquid. Any non-volatile impurities, present in the liquid, will also be in the crystals above the liquid. If in your case you started with pure chromic acid and pure/clean ammonia, then the fluffy crystals above the surface of the liquid will be pure as well.

You did not make ammonium chromate in this way, but you made ammonium dichromate. The former is golden yellow, the latter is bright orange. Ammonium chromate is not stable, it loses ammonia on standing in contact with air, leaving ammonium dichromate behind. Making pure ammonium chromate is very difficult. I never managed to obtain the pure compound, it always is strongly contaminated with ammonium dichromate.

Rhodanide - 15-11-2016 at 10:55

Quote: Originally posted by woelen  
Indeed crystals, formed near the surface, sticking to the glass, suck up additional liquid, which evaporates and leaves more solid. This can grow so large that it even grows out of the beaker, onto the table around it. This type of crystals is less pure than those, formed in the liquid. Any non-volatile impurities, present in the liquid, will also be in the crystals above the liquid. If in your case you started with pure chromic acid and pure/clean ammonia, then the fluffy crystals above the surface of the liquid will be pure as well.

You did not make ammonium chromate in this way, but you made ammonium dichromate. The former is golden yellow, the latter is bright orange. Ammonium chromate is not stable, it loses ammonia on standing in contact with air, leaving ammonium dichromate behind. Making pure ammonium chromate is very difficult. I never managed to obtain the pure compound, it always is strongly contaminated with ammonium dichromate.


Indeed, I have had that happen with Potassium Ferricyanide over the course of a year.

mayko - 15-11-2016 at 11:46

I had an outgrowth of this sort, trying to dry copper chloride solution with sodium hydroxide. The lye deliquesced and developed a feathery tree-like growth which eventually invaded the copper chloride container. There's a picture in this post:

https://www.sciencemadness.org/whisper/viewthread.php?tid=26...