Sciencemadness Discussion Board - Getting the Most Out of Solution

Getting the Most Out of Solution

MrHomeScientist - 5-1-2019 at 08:58

This is probably a simple problem, but I'm not completely sure how to do the math on it.

My goal is to make 100g NH₄Cl. To do this, I am using the reaction:
KCl + NH₄NO₃ --> KNO₃ + NH₄Cl

I dissolved 139.25g KCl and 149.53g NH₄NO₃ in 536mL of distilled water. I chose this amount of water specifically to get the maximum amount of KNO₃ out at my fridge's temperature of 3C. I just pulled the dish out of the fridge and the crystals are drying on the filter paper now. So, let's say I have X grams of KNO₃ that has been removed from solution.

The next step is to boil down the water to a lower level, put it back in the fridge, and gather more KNO₃, and repeat this to squeeze as much nitrate as possible out of the solution. Finally, I'll allow the rest of the liquid to evaporate and collect my pure NH₄Cl.

My question is: how much should I reduce the water volume to pull out more KNO₃, while leaving everything else still in solution? I've removed X grams of potassium and nitrate ions, but there's still potential for KCl, NH₄Cl, and KNO₃ to crystallize. I made a chart of the various solubilities that I'll attach here.

Maybe I'm overthinking this? I think the potential for KCl crystallizing is what's throwing me off.

Attachment: ammonium nitrate.xlsx (15kB)
This file has been downloaded 364 times

fusso - 5-1-2019 at 09:12

I see the solubility as mass per volume. Shouldn't molarity be used instead?

happyfooddance - 5-1-2019 at 09:16

Try it exactly as you said. I have made kilos of ammonium chloride exactly this way. Some NH4Cl will crystallize with the KNO3 if you boiled it down too much, or cool too intensely/quickly, but 2 things:

1) it is very easy to distingush KNO3 (long straight needle-like), KCl (square pyramidal), and NH4Cl (fractal, snowflakes) salts by eye, and even determine relative concentrations by taste.

2) it is fairly easy and fast to get a clean separation, filtering or decanting is easy.

Edit: it sometimes helps to collect fractions by temp, for example, after decanting, putting solution in the freezer to collect more crystals. Sometimes if it gets too cold the whole thing will turn to a mass of ammonium chloride crystals, so you have to watch it.
So you are right about your concerns but I also think you might be overthinking it.

But because you are aware of these things I think this will be really easy for you.

[Edited on 1-5-2019 by happyfooddance]

morganbw - 5-1-2019 at 10:44

Quote: Originally posted by fusso

I see the solubility as mass per volume. Shouldn't molarity be used instead?

I love using mols, makes life simple. To use molarity, I have always had to convert to grams. That is something my scale can understand, it does not weigh in mols per se but needs a transformation into grams.

When someone states grams instead of mols, pls be aware that the grams may have been determined from the mol weights.

r0749547 - 5-1-2019 at 12:54

try to desolve it in an ammonia/NaOH solution once you have pulled most of the KNO3 out.
it will saturate the sollution with ammonium making the NH4Cl drop out of the solution first.
while the KCl, NH4NO3 and KNO3 will stay in the solution.

and if you want us to do the math, the volume of the solution would be nice

if my calculations are right... I kind of made a lot of roundings...
you should be able to boil the solution down to about 250ml.
but to be safe I would go to 300ml-400ml.

[Edited on 5-1-2019 by r0749547]

MrHomeScientist - 13-1-2019 at 14:39

I was able to remove 50.8g of KNO₃ after cooling. I reduced the solution volume to 350mL, and allowed that to cool. I thought this would keep everything in solution, but a small amount (maybe 10g) of square crystals formed. Not sure if this is KCl or NH₄Cl.
After these had formed, I filtered them off. To my surprise, I returned later to find more crystals had grown, but this time it was a mixture of needles, squares, and little "snowballs." Now I'm even less sure what's happening! I filtered those off as well, and I'll put the remaining liquid back in the fridge and see what happens.

MrHomeScientist - 14-1-2019 at 17:50

Well it's definitely a different crystal habit now. Much more snowy, definitely not the pure needles of KNO₃ from the first batch. A little frustrating because there should still be over 100g of nitrate in there! This isn't a great way to make ammonium chloride, it seems. I have a ton of ammonium nitrate so I was hoping I could use what I had.