Sciencemadness Discussion Board

Potassium Cyanate - Purification

MeSynth - 25-8-2011 at 15:16

Synthesis of potassium cyanate from urea and potassium hydroxide has been discussed in multiple threads but not much has been said about the purification of the resulting cyanate. All info that I have found has been worded poorly without explanation of how the purification actually works. I have searched for data on the solubility of urea, potassium hydroxide, and potassium cyanate but the sources I found had mismatched information. I think the solubility of these compounds should be made clear so that we may decide the best way to purify the potassium cyanate. :)

MeSynth - 25-8-2011 at 15:20

This is one of the methods I have found.

The crude cyanate is ground in the crucible and placed in a beaker. It is then mixed with an equal volume of water. The solution is made alkaline with a few drops of aqueous K2CO3. Once alkaline the mixture is stirred at 50C until only a small amount of solids remain undissolved. The solution is then quickly filtered at the pump to remove the solids. 50 ml of 95% ethanol is added to the filtrate and then cooled by ice. After the solution reaches its coldest temperature and the cyanate has precipitated. The crystals are filtered at the pump and washed with ice cold ethanol.

jon - 25-8-2011 at 17:55

all i did was filter and crash with ethanol there is nothing to this.

Bolt - 25-8-2011 at 19:37

Post in an existing thread! Do not make a new thread if an old one will suffice!

Consult the primary literature if you are unsure of the 'best methods.' Often they will say something like, "The method of John Smith was found to be unsatisfactory."

MeSynth - 25-8-2011 at 23:29

Quote: Originally posted by jon  
all i did was filter and crash with ethanol there is nothing to this.

Was it pure after the crash? Were any tests done?

To be sure that I understand what you mean by crash.

The cyanate (compound being crashed) is taken up in enough water to disolve as much as possible. Insolubles are filtered off. The water is then boiled off until first signs of precipitation is seen (usually as a thin solid layer begining to form). Then a few drops of water (solvent in use) is added to disolve the precipitate. The heat is removed and ice cold ethanol (solvent of choice) is poured in causing the compound to rapidly precipitate. It is then filtered cold and washed with a small amount of ice cold ethanol (solvent of choice) .

Would this purify the KoCN?

spunonrun - 6-9-2011 at 17:33

Solubility in H2O (5°C): ~790g /L
Solubility in H2O (20°C): ~1000 g/L
Solubility in H2O (25°C): ~1200 g/L
Solubility in EtOH: ~50 g/L
Decomp: >132 °C

sodium cyanate:
Solubility in H2O (20°C): ~110 g/L
Solubility in EtOH (0°C): ~0.22 g/100g Solvent
Solubility in Et2O (0°C): insoluble
Melting point: 550°C

sodium hydroxide:
Solubility in H2O (20°C): ~1090 g/L
Solubility in EtOH: ~139 g/L
Melting point: 323°C
One gram dissolves in 0.9 ml water,
0.3 ml boiling water,
7.2 ml abs alcohol,
4.2 ml methanol,
also sol in glycerol


synthesis and purification:
pour 10g urea and 11,5g K2CO3 into crucible
-> heat slowly with burner (H2O, CO2 and NH3 fumes evolving)
-> powder gets dry
-> apply more heat (NH3 fumes evolving)
dont heat to strong!
-> a clear molten mass shoud be in your crucible
-> let cool and crush to powder

->solve powder in min. amount of water
->add CH3COOH until no more CO2 evolves
->add two 2x EtOH (by volume)
->put in freezer
->filter out NaOCN
->rinse with EtOH (CH3COONa is removed)

Please report back when youve done.

[Edited on 7-9-2011 by spunonrun]

overload - 9-10-2011 at 18:57

I decided to go with potassium bicarbonate.

overload - 14-10-2011 at 17:01

Urea and potassium bicarbonate were reacted in a porcalin crucible with a 3 mmol excess of urea.

111_1182.JPG - 74kB

Once the chemicals liquified there was some bubbling which died down into a light fizz which got less and less until it finally stopped which signals the end of the reaction.

111_1185.JPG - 69kB

The cyanate cakes were disolved into water with heating until no more would disolve.

111_1187.JPG - 65kB

111_1189.JPG - 62kB

The remaining solids were filtered out and the filtrate was crashed with ice cold alcohol and the precipitates were filtered.

111_1190.JPG - 72kB

I'm certain that there are still impurities due to the filtrate forming crystals before I could crash with the alcohol. The alcohol was ice cold which may have caused other impurities to precipitate too. When I get more 95% alcohol I will recrystalize from a supersaturated solution with warm hot alcohol by adding it very slowly and then allowing time for the crystals to form. Then wash the filtered crystals with ice cold alcohol. I also plan to do this in a deep sink with arm length gloves to prevent exposure to the cyanate.

111_1192.JPG - 57kB

[credit | pipernigrum]

[Edited on 15-10-2011 by overload]

Eli25 - 4-7-2015 at 06:11

I react Urea with Potassium Hidroxide ....the product is black!!!! I solve it in water and filter the black substance. then evaporate the water ....a fairly white powder remains....

If it were Potassium Cyanate it had to melt above 315 C....but it doesn't!!!!!!!!!!!!!

Has anyone have any idea?

Eli25 - 4-7-2015 at 06:15

how much Ethanol should I add? The solution itself should be warm?

Magpie - 4-7-2015 at 10:54

from Inorganic Syntheses, vol II, p. 88:

"Fifty grams of finely powdered crude potassium cyanate is stirred mechanically with 50 ml of water at 50° until most of the solid has dissolved. Glacial acetic acid (10 - 20 drops) is added until the solution is neutral to phenolphthalein. The warm solution is then quickly filtered into a 500 ml flask. 5 ml of water at 50° is used to rinse out the flask. The filtrate is treated with 250 ml of alcohol. After thorough mixing, the flask containing the crystalline slush is placed in a refrigerator overnight. The crystals are then removed with a Buchner funnel and washed with alcohol. The crystals must be dried thoroughly over barium oxide or a similar desiccant to prevent partial hydrolysis and the formation of ammonia on storage. Yield is 40g (80 percent)."

For assay see my procedure in the Prepublication section of this forum.

[Edited on 4-7-2015 by Magpie]

aga - 4-7-2015 at 12:27

Quote: Originally posted by Eli25  
I react Urea with Potassium Hidroxide ....the product is black!!!! I solve it in water and filter the black substance. then evaporate the water ....a fairly white powder remains

How much Urea. What purity ?
How much KOH ? What Purity ?
How did you 'react' it ? In a solvent or by hitting it with a hammer ?
How much water ?
How white is the powder ?
How much powder ?
Is it dry powder ?

Please try to get the idea.

Chemical reactions can take many pathways depending on the relative concentrations of the reagents, any impurities, the reaction conditions, the solvent(s) used etc etc etc.

Simply saying "I react Urea with Potassium Hidroxide ....the product is black" is no use at all.

If you can be more specific, people may be able to help more.