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Author: Subject: Sodium cyanate from hydroxide or carbonate?
MeshPL
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[*] posted on 6-10-2017 at 12:13
Sodium cyanate from hydroxide or carbonate?


So we have 2 simple reactions for making sodium cyanate:
NaOH + CO(NH2)2 ---> NaOCN + H2O + NH3
Na2CO3 + 2CO(NH2)2 ---> 2NaOCN + H2O + CO2 + 2NH3

So far I used the first one due to better reactivity of sodium hydroxide vs. usually hydrated sodium carbonate or sodium hydrogen carbonate which could be decomposed in situ to carbonate. But is my method any good? Is carbonate superior to hydroxide?

So far I just mixed a stochiometric amount of hydroxide and urea with additional excess of urea in a mortar, than dumped it into porcelain dish (yeah, alkalis are bad for ceramics, but so far it worked out ok) and heated it on a gas burner till it melted and resoldified then colected the product. Heating usually was not long and 10 minutes was often enough.
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Melgar
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[*] posted on 6-10-2017 at 20:01


If I remember right, the potassium salt works better for this reaction, due to its lower interaction with glass and ceramic, and certain solubility characteristics. If the smell of ammonia is still lingering in the area, then it probably occurred as it should. I remember that you need to heat to 400C, and hold it there for a while, since any potential impurities would be unstable at this temperature. Also, you should use a slight excess of urea, because urea can burn off whereas carbonate salts won't. Carbonate salts seem to be preferred, at least for the potassium salts, but both reactions will occur. There may be issues with recrystallizing in order to purify them though.



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MeshPL
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[*] posted on 6-10-2017 at 21:57


Ok, thanks! I will use potassium salts next time, seems to be a good idea. I will try carbonate too, though I am not too concerned about purity, because if the next step is reducion with Mg or C or whatever else, purity of the product (cyanide) is going to be abysmal anyway.
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[*] posted on 6-10-2017 at 23:17


For sodium cyanate, hydroxide is better, as it doesn´t requires as high temperatures.
You can apparently even use metallic sodium and react that with urea.
But if you use the carbonate, you have to reach around 500°C when melting both reagents together.

So I would second Melgars opinion on using potassium carbonate instead, that only requires around ~350°C.

I did this synthesis myself years ago, tried both Na2CO3 and K2CO3 with urea, and I´ve never really succeeded with sodium cyanate due to the temperatures, the urea does likely decompose before it is able to react properly with the sodium carbonate.
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Melgar
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[*] posted on 7-10-2017 at 05:09


Oh, I remember another reason the potassium salt is preferred now: potassium cyanate has a lower melting point, by quite a lot, and so the reaction can take place below the decomposition point of urea derivatives. However if your goal is cyanide, then sodium cyanide has a lower melting point than KCN. There's a thread on that here, although you may have read it already:

http://www.sciencemadness.org/talk/viewthread.php?tid=809




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[*] posted on 7-10-2017 at 23:37


That kinda became confusing, but from what I understand, K2CO3 works fine. So does NaOH and KOH but Na2CO3 is no good. I may try them out a bit, determining whether to use sodium or potassium cyanate may take some tinkering, as I am using reduction with Mg but may also try reduction with carbon... I will see. Should I get any nice results, I will share with you. So far I just know that I got SOME cyanide from reducing NaOCN with Mg.
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