Nemo_Tenetur
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Problems with magnesium nitrate hexahydrate dehydratation
Hi!
Last year I got a bag with magnesium nitrate hexahydrate, fertilizer grade, brand name is "Krista-MAG" from Yara in Germany. The bag was sealed and
bought directly from an established supplier/wholesaler, therefore adulteration is very unlikely.
Yesterday I decided to open the bag and to dry a kilogram to get (almost, it´s difficult to remove the water completely without decomposition)
anhydrous magnesium nitrate as desiccant.
The bag contained granules/prills of a few millimeter diameter, no crystals, free-flowing, completely white, somewhat shiny. I poured about one
kilogram in a stainless steel bowl (carefully cleaned) and heated it several hours above my closed fire place .
Soon after heating, a weak hissing noise was detectable and the prills start to melt. After about an half hour, a clear yellow liquid was formed. A
very gentle boil was visible and the dehydratation proceeded.
As far, as good. But after about an hour, the yellow color changed to amber and finally brown and a flocculent precipitate was formed.
Finally, a considerable (see attached jpeg) amount of dark brown crap was formed. I have no idea how I could remove this crap from the hot molten
magnesium nitrate. As soon as I started to mechanically separate the crap (for example, with a stainless steel spoon), it starts to solidify at least
partially. Under these circumstances, a filtration with a buchner funnel will not work , nor will it be possible with a stainless steel sieve
(immediately clogged and solidified).
Surely, filtration of 200 to 300 degree centigrade hot molten salt is technically possible, but under amateur conditions (typical sciencemadness
setup) it´s very difficult and no feasible way IMHO.
So I think the crap must be removed prior to heating/dehydratation. I have no idea what kind of additive YARA used. The material safety data sheet
stated unter chapter three number 3.1 "substance with a single compound" (sic!)
99% magnesium nitrate
Yara behaves in a sneaky way as they add the following statement:
"Enthält keine weiteren Inhaltsstoffe, die nach gegenwärtigem Kenntnisstand des Lieferanten
eingestuft sind und zur Einstufung des Stoffes beitragen und die dadurch in diesem Abschnitt
genannt werden müssten"
That means about "there are no other scheduled compounds which are important for the classification and therefore necessary to get explicitly
listed here".
I took some liberties for translation, english isn´t my mother language, I think you understand what I mean.
So my questions are:
1.) What kind of substance is added ? My best guess is some organic anti-cake or thin organic layer to protect the hygroscopic prills. I don´t think
cab-o-sil or any other anorganic matter. The discoloration and charring indicates organic matter IMHO.
2.) How could I remove the additive coating-crap in an affordable and practical way? For example, extraction / washing with a cheap and available
solvent like acetone or isopropanol?
Comments are welcome, thank you for your ideas to solve the problem.
Attachment: Krista_MAG_6.0_20250305.pdf (555kB)
This file has been downloaded 27 times
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SplendidAcylation
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I would go out on a limb and say that your mistake is heating/melting magnesium nitrate in a stainless steel bowl...
The nitrate ion is a powerful oxidizer, I would imagine that it would oxidize most metals, except the most noble ones.
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Deathunter88
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To add to that, only a very small amount of organic impurities is needed to turn a large volume of solution brown/black. I would say that your brown
magnesium nitrate is good enough for use as a dessicant.
I have dehydrated USP pharmaceutical grade magnesium sulfate heptahydrate several times and each time the result was a solid that ranged from yellow
to dark brown, despite the purity being over 99.5%.
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Nemo_Tenetur
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Thank you for replies and suggestions. Yes, hot nitrate solutions are aggressive and could dissolve a fraction of the stainless steel bowl. On the
other hand, I´ve used this bowl to evaporate and dry concentrated ammonium nitrate solutions several times , sometimes up to the melting point and
I´ve never observed such a discoloration and flocculent precipitate.
I´ve also checked the weight of the bowl and it´s still the same. Maybe a milligram quantity is lost, not enough to detect it with my scale.
I´d like to use another bowl (not stainless steel) to check if the discoloration comes from the metal, but I have no idea what material I could use
instead. The solidified salt cake is hard like a rock and I required many hammer blows to get it out. Therefore porcelain or glass isn´t a good idea.
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imidazole
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If I remember correctly, Magnesium Nitrate actually doesn't release water on heating, It instead decomposes to oxy-hydroxides, and such, like with
Magnesium Chloride. To properly create the anhydrous form, you'd need some way to shift it back to the salt, like heating under NO2 and dehydrating
agents.
You might be fine using the hydrate, and if you crush the stuff to get past the coating, probably will work well enough for whatever you're planning.
If you still want to remove the organic coating, read on:
The sciencemadness wiki had some mention of anticaking agents for KNO3, as Calcium Stearate, which is soluble in pyridine and hot hydrocarbons. if
it's anything similar, it could be possible to (carefully!) dissolve it in hot petroleum ether, though then you'd have your oxidizing salt covered in
fuel, and that could end very badly. :/
If you're still interested in removing the organic stuff, there is an annoying, although possible way to remove it:
Given the fact you aren't really able to dehydrate past the hydrate, you might be interested in re-crystallization and filtration, and then trying to
slowly remove the excess water in a dessicator or with careful heating.
I'm a new member so let me know if there's anything I can do to improve my responses
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Nemo_Tenetur
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Hi imidazole,
thank you for your reply. As far as I know, the dehydratation of magnesium nitrate hexahydrate is at least partially possible. I didn´t smell any
nitrous gases (NO x) which should be evolved if a certain degree of decomposition (more than traces) happens.
Remember that I dried about one kilogram inside my room, not outdoor.
Extraction with low boiling hydrocarbon could be an option because it´s very unlikely that magnesium nitrate hexahydrate is soluble in this solvent.
And if I wait with heating/dehydratation until the last traces of hydrocarbon are gone (outside my room, of course) then no explosion risk is
involved. Pyridine is no option for me as the smell of this stuff is so nasty and makes me really crazy, I could vomit if I merely think about it.
I think your response is well-written and understandable for me as a non-native english speaker. Please keep in mind that this is an international
forum with members from countries around the world. Some abbreviations are well established and understandable for most of us non-native english
speakers (for example, IMHO and LOL), whereas slang terms are often difficult for us.
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Texium
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How much weight did your magnesium nitrate lose after heating? Wikipedia doesn’t list a melting point for anhydrous magnesium nitrate, only that it
decomposes at 330 °C. I think it’s more likely that you only partially dehydrated it to the dihydrate.
If all you need it for is a desiccant, get magnesium sulfate instead. It’s much easier to handle and works great.
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Fulmen
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Texium is right, you cannot dehydrate magnesium nitrate fully, at least not by heating. I think you can get the dihydrate, but that's it. But it
should still function as a desiccant, it's even been used in the mfg of 100% nitric acid. But personally I would choose a desiccant that doesn't melt
during drying.
We're not banging rocks together here. We know how to put a man back together.
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Nemo_Tenetur
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Calculations are disappointing
You´re both right. I´ve just calculated the weight loss, the water content of the hexahydrate is about 42% (148,3 / 256,4).
I´ve used about 1070 gram hexahydrate ( far away from exact measurement, my lab scale just go up to 200 gram, the kichen scale up to 5 kilogram).
I got about 920 gram yield (weight loss about 14%, this indicates a tetrahydrate).
I´ve decided to try magnesium nitrate as it is recommended and patented as a dehydrating agent for nitric acid, it´s cheap (about 2 Euro per
kilogram) and well-available as fertilizer.
I´ve also used sulphuric acid as desiccant. It works fine, but it´s more expensive and the workup of the spent acid is no fun and also dangerous.
The magnesium nitrate route looks theoretically more appealing, but the dehydratation of the hexanitrate is also no fun. Apart from the discoloration
I simply don´t know what kind of vessel or pan I should use, if the standard stainless steel can´t resist.
Surely, there are exotic alloys (hastelloy or better) or noble metals which can resist such conditions and are used in the industry, but the
availability and pricing are out of my scope :-(
Porcelain or glass is brittle, you can´t get the salt cake out without destruction.
Maybe I should go back to the old-fashioned route potassium nitrate and sulphuric acid and simply discard the potassium hydrogen sulphate.
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chempyre235
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I've heard that copper II sulfate is a bit easier to use. The pentahydrate is blue, while the anhydrous form is white, which makes it a good indicator
as well. According to Wikipedia, the temp required to get the anhydrous form is 200°C. The salt is readily available in the US, as a defoliant to kill algae in ponds,
or as a root killer for plumbing. It can be purchased OTC for about $20/lb, but is somewhat cheaper online.
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Fulmen
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Ah, you're after nitric acid. I looked at the same approach some 30 years ago, so my memory is a bit fuzzy. But IIRC the biggest downside is that you
will need a lot more magnesium nitrate compared to sulfuric acid. So for a given volume sulfuric is better.
Have you tried vacuum? You'll need a still for the nitric acid so vacuum drying should be possible.
We're not banging rocks together here. We know how to put a man back together.
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Osmiridium
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Have you tried to dissolve some of the fertilizer in water? Is there any residue or insoluble part? Fertilizer prills can be covered with something
wax/parrafin like to avoid caking since this would be quite undesired when distributing it with machines and that's its actual intended use.
[Edited on 25-3-2025 by Osmiridium]
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Nemo_Tenetur
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@chempyre235: Copper sulphate could work, but with some limitations. According Wikipedia, Copper sulphate pentahydrate loses two moles water at 63
degree centigrade, well below the boiling point of anhydrous fuming nitric acid. That means, much more anhydrous Copper sulphate is necessary.
@Osmiridium: No, but I´ll try to wash the prills with an aliphatic hydrocarbon and see if it leaves a waxy residue. At present time my complete lab
space is filled with copper and nickel salts
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Precipitates
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Quote: Originally posted by Nemo_Tenetur  | | @chempyre235: Copper sulphate could work, but with some limitations. According Wikipedia, Copper sulphate pentahydrate loses two moles water at 63
degree centigrade, well below the boiling point of anhydrous fuming nitric acid. That means, much more anhydrous Copper sulphate is necessary.
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Vacuum distill
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woelen
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I have a bag of fertilizer NaNO3 (called chili salpeter in NL). These also are white prills, somewhat shiny, free flowing.
If I add these to water, then I get a turbid solution, with a lot of white scum on it. The scum feels greasy. So, I'm quite sure that this is due to
some layer of organic insoluble stuff, which is not harmful, when going into the soil.
I also tried to clean my NaNO3. I took a lot of the somewhat turbid solution, with the scum on it. I filtered it through double filter paper. This
gave me a (nearly) clear and colorless solution. Next, I boiled down the solution in a little pan with teflon coating on a hot plate, attempting to
get pure dry powder. At a certain point, I have a white slurry, but on further heating, the slurry turned yellow/ochre and became smelly. At that
point I stopped the process and quit. the pan was not damaged, the grey teflon layer still is intact. The sodium nitrate is brown. it burns fine, when
mixed with sulfur and charcoal. Its solutions are yellow though.
My experience is very similar to your experience with the magnesium nitrate. I quit working with this fertilizer grade sodium nitrate. Too much hassle
for getting pure NaNO3 from fertilizer, I could buy very pure KNO3 lateron for just 25 euros for a bag of 2.5 kg and I am happy with that.
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Dr.Bob
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I think some fertilizers may be coated with something to prevent them from absorbing water easily. Maybe a laquer, wax, varnish, polymer, who knows.
But without that, some compounds will self dissolve in their own water of hydration.
If you want to purify a nitrate and it is already a hydrate, for some, you could go ahead and dissolve it in hot water and then filter and
recrystallize it to get rid of impurities. Melting a nitrate seems like a poor way to purify it, as some can decompose when hot. And doing it over
a fire seems even more dangerous. But drying in a vacuum oven is the best way I have found to dry salts, and grinding them in a mortar and pestle
first helps to dry faster and get break up coatings.
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Precipitates
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I had the same problem, but simple recrystallisation and processing gave a "pure" free-flowing white powder.
Those were back in the days when I lived in the UK, and people could just go down to their local gardening shop and buy nitrate of soda cheaply by the
kilo.
In terms of magnesium nitrate, you should be able to recrystallise. Losses will be substantial as the solubility curve isn't great, but IMHO it's
worth it to get rid of any rubbish that also has a good chance of reacting with your nitric acid.
Edited to relate back to magnesium nitrate.
[Edited on 27-3-2025 by Precipitates]
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