TinSandwich
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Problems with recrystallization of KNO3
So I got some potassium nitrate but it wasn't pure (it was technical grade so it probably had some random impurities from the factory and it contained
2% MgO because it's a fertiliser and plants like MgO) si I recrystallized.
The solubility of KNO3 in water goes from about 130 g/l at 0 C° to 2400g/l at 100C°, so I measured the amounts needed, I boiled the water added it
to the beaker full of KNO3 and then heated it until it was disolved, I then filtered it, put it in the fridge and after a bit put it on a paper towel
to dry (I didn't discard any of the leftover water because there really wasn't much) .
But here's the thing: I started with 120g of KNO3 but after recrystallizing I only had 50! I was expecting a loss of maximum 20% and now I'm wondering
where it all went.
Anyways I was wondering if anyone has done this before and has some tips on how to improve my yield
EDIT: I should add that I am using a paper towel as a filter so that might have absorbed a bit of the solution, but still not that much. Also I am new
to this forum: can someone explain why whimsy is blocked? Are there any special requirements to be able to get access to it?
[Edited on 27-2-2016 by TinSandwich]
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aga
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Purification by recrystallisation is essentially a balance between Quantity and Purity.
You will always lose quite a lot of the product when you decant the supernatant liquid containing the impurities + product.
You could try boiling the supernatant liquid to almost-dryness (not actually dry in case there's anything the KNO3 can oxidise) then
dissolve and recrystallise it using the bare minimum of boiling water.
Maximising the yeild in your particular circumstances is mostly a matter of practice and trial-and-error, aka experimentation !
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crystal grower
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Quote: Originally posted by TinSandwich  |
I boiled the water added it to the beaker full of KNO3 and then heated it until it was disolved, I then filtered it, put it in the fridge and after a
bit put it on a paper towel to dry (I didn't discard any of the leftover water because there really wasn't much) .
But here's the thing: I started with 120g of KNO3 but after recrystallizing I only had 50! I was expecting a loss of maximum 20% and
now I'm wondering where it all went.
EDIT: I should add that I am using a paper towel as a filter so that might have absorbed a bit of the solution, but still not that much. Also I am new
to this forum: can someone explain why whimsy is blocked? Are there any special requirements to be able to get access to it?
[Edited on 27-2-2016 by TinSandwich] |
1.) How is it possible that there wasn't much water left if U started with 120g of KNO3?
To dissolve 120g u needed 50ml of water.(100°c)
When it cooled to 0°c there should be still dissolved 65g of KNO3.
2.)there aren't any special requirments, just ask povelrone for a permission and have fun .
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TinSandwich
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| Quote: Originally posted by crystal grower | | Quote: Originally posted by TinSandwich |
I boiled the water added it to the beaker full of KNO3 and then heated it until it was disolved, I then filtered it, put it in the fridge and after a
bit put it on a paper towel to dry (I didn't discard any of the leftover water because there really wasn't much) .
But here's the thing: I started with 120g of KNO3 but after recrystallizing I only had 50! I was expecting a loss of maximum 20% and
now I'm wondering where it all went.
EDIT: I should add that I am using a paper towel as a filter so that might have absorbed a bit of the solution, but still not that much. Also I am new
to this forum: can someone explain why whimsy is blocked? Are there any special requirements to be able to get access to it?
[Edited on 27-2-2016 by TinSandwich] |
1.) How is it possible that there wasn't much water left if U started with 120g of KNO3?
To dissolve 120g u needed 50ml of water.(100°c)
When it cooled to 0°c there should be still dissolved 65g of KNO3.
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There was still some water left from the original 50ml but it was so little that I couldn't really decant it off. Basically after the filtered
solution was cooled it felt like snow: wet, cold and soft. Since all the impurities were left on the filter I decided to just let the water evaporate.
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aga
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Ah.
There needs to be enough solvent (water in this case) to be able to decant off the impurities along with some product.
If you can't pour off the liquid from the crystals then there is no significant purity increase gained.
Likewise, the slower the crystals form the less impurities get trapped, so allowing it to cool slowly gets a purer product = no fridge/freezer
treatment.
Start with more KNO3.
Edit:
If you're short of pot nitrate and are truly new to all this, practice a bit with table salt first.
Mix it with some dirt or something and run the process and see what/how much pure NaCl you get at the end.
[Edited on 27-2-2016 by aga]
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Metacelsus
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I would not recommend using sodium chloride to practice recrystallizations, as its solubility changes relatively little with temperature. (see: http://www.kentchemistry.com/links/Kinetics/SolubilityCurves...)
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aga
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Exactly why it is a good choice.
It's also pretty much available everywhere, and cheap.
[Edited on 27-2-2016 by aga]
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Bezaleel
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| Quote: Originally posted by aga | (...)
Likewise, the slower the crystals form the less impurities get trapped, so allowing it to cool slowly gets a purer product = no fridge/freezer
treatment.
(...) |
Unless you put your beaker in a styrofoam box and put that in a fridge...
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Metacelsus
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I'll agree that it's easy to get, but don't you want a substantial solubility difference between hot and cold? If there isn't one, your recovery will
be poor (not that it matters with NaCl, though).
(However, as you said above, there still must be enough liquid when cold to enable decantation. Is this why you suggested NaCl?)
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TinSandwich
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| Quote: Originally posted by aga | Ah.
There needs to be enough solvent (water in this case) to be able to decant off the impurities along with some product.
gained.
Likewise, the slower the crystals form the less impurities get trapped, so allowing it to cool slowly gets a purer product = no fridge/freezer
treatment.
Start with more KNO3.
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Ok I'll try it with more KNO3, but I can't really agree with the "If you can't pour off the liquid from the crystals then there is no significant
purity increase" part: the impurities in my solution were in suspension and after filtering the end product was extremely pure, it went from a
yellowish color to a transparent solution and then to white crystals
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Sulaiman
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Assuming that the impurity that you want to remove is MgO, possibly Mg(OH)2 in water, both insoluble.
then dissolving in water, filtering and evaporation is enough.
IF the filter can 'catch' all of the insoluble MgO or Mg(OH)2
which I doubt due to tiny particle size, but finer filter = less impurities.
(some very fine colloidal suspensions appear to me to be clear, until I shine a laser through them)
Potassium Nitrate decomposes to nitrite at glowing iron temperatures, so gently heating to dryness should be safe.
EDIT: added the word "gently" above, just in case 
(and always wear appropriate PPE and listen to your mommy)
[Edited on 28-2-2016 by Sulaiman]
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Mabus
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Have you concentrated the leftover water and see how much and what else is in there?
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aga
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| Quote: Originally posted by TinSandwich | | I can't really agree with the "If you can't pour off the liquid from the crystals then there is no significant purity increase" part
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I'm definitely no expert on recrystallisation, i'm just an amateur enthusiast, not an 'expert', if there is such a thing.
Mine is a 25kg bag of 52% KNO3 + 48% K2O agricultural stuff.
I react the mix with HNO3 (also agricultural) to convert most of the 48% K2O to KNO3 then boil it down until i see
some crystals. The point that i stop the boiling is entirely subjective.
When filtered then left to cool, some yellowish crystals form at the bottom and about 60% of the rest of the beaker has yellowish water, which i
decant off from the crystals.
(from this point on measurements are used, as the crystals comprise mostly of the required product)
These are then dissolved in the bare minimum of boiling water and left to cool , a.k.a. 1st recrystallisation.
Again, the liquid above the crystals that form is decanted off, leaving pretty clean white spiky crystals.
The last time i repeated this to get pretty much pure crystals then dried them in an oven for about an hour at 150C to get a dry white powder.
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