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Author: Subject: Silver and Copper Cyanide from Potassium Ferrocyanide
Pok
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[*] posted on 18-5-2020 at 18:29


It seems that the addition of NaOH is not needed, when the concentrations are not too high and when Na2SO3 is used as a reducing agent. The original source is a patent. Attachment: Patent DE000000055152A.pdf (194kB)
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It says "In order to avoid further exposure of the copper cyanide to the 6 molecules of sulphuric acid formed, either work in a highly diluted solution or neutralize most of the free sulphuric acid formed by adding alkali, unless you prefer to use a sulphite from the outset."

I used about a 10 % solution of copper sulphate, heated it nearly to the boiling point and added a solution of about 10 % sodium sulfite/ferrocyanide very slowly with stirring. According to the patent a slight excess of copper sulfate is required to avoid the formation of a iron-rich precipitate. Maybe that's the brown stuff? So I used a 10 % excess of copper and about 10-15 minutes after the whole mixture was added, the precipitate has turned almost pure white. (I made a few q&d tests before including NaOH, Cu(OH)2 and so on and the products turned brown very fast and contained noticable amouns of iron.)

I washed the precipitate with pure water and pressed it between filter paper. My sample is drying now and looks similar to your last picture, but rather with a gray tint than pink. Some small pieces are already dry and look off-white. I'm still afraid it could be contaminated with iron. I will check that later.

1.jpg - 449kB 2.jpg - 258kB 3.jpg - 338kB
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Boffis
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[*] posted on 19-5-2020 at 05:29


There you go, your question answered!

Is the presence of iron a big issue? I suspect that it is present either as a ferric oxide hydrate or as a copper cyanoferrate complex, niether of which should be a problem in organic chemistry.
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[*] posted on 19-5-2020 at 13:30


Nice preparation!

May I asked what did you do with filtrate? I know that it probably containted only very small amount of free cyanide ions but still there were present in it. Did you add bleach in it to oxidised cyanide in to cyanate?

Btw CuCN can be also used for hydrocyanation of alkynes to make unsaturated nitriles. See: http://organicreactions.org/index.php?title=Hydrocyanation_o...




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[*] posted on 19-5-2020 at 13:35


I prefer pure compounds. ;) My goal is to make KCN from it and iron impurities could be a problem.

Unfortunately, my dry product is not as white as shown in the second picture. Better washing could have helped. Or maybe the colour only comes from tiny Fe impurities. I will test for iron soon. My yield is only about 90 %. Maybe it still was too acidic and some HCN escaped. But a single addition of some NaOH just after adding all the K4Fe(CN)6 should be enough I think. I will make further tests. :)

I didn't throw away the solution yet. It still contains some copper. I will probably add bleach and a precipitation agent for the copper.

[Edited on 19-5-2020 by Pok]
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[*] posted on 19-5-2020 at 21:44


How did you convert CuCN in to KCN? I firstly think about double displacement between CuCN and KI because corresponding Ag salt is less soluble than AgCN. But I found solubility product of CuCN and CuI and CuCN is much less soluble than CuI.

Another way which comes to my mind is reaction between CuCN and Mg or Zn to make magnesium or zinc cyanide and copper. Than heating with potassium carbonate to form KCN and insoluble basic carbonate.

[Edited on 20-5-2020 by Bedlasky]




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[*] posted on 20-5-2020 at 06:56


Does anyone have a reference to an example of the use of cuprous cyanide to prepare a nitrile by halogen displacement either aliphatic or aromatic? I have found a few using alkali cyanides but I was wondering if Cu-1 cyanide can be used as a substitute? anomalous in his orginal post indicates that it work in the case of some aromatic cyanations of haloaromatic compounds, presumably reactive ones like nitrobromobenzenes etc.

I have a few details for Sandmeyer type diazo reactions to give aromatic nitriles but its not the same.

[Edited on 20-5-2020 by Boffis]
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[*] posted on 20-5-2020 at 08:47


That looks great.
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[*] posted on 23-5-2020 at 14:30


Quote: Originally posted by Bedlasky  
How did you convert CuCN in to KCN? I firstly think about double displacement between CuCN and KI because corresponding Ag salt is less soluble than AgCN. But I found solubility product of CuCN and CuI and CuCN is much less soluble than CuI.

Another way which comes to my mind is reaction between CuCN and Mg or Zn to make magnesium or zinc cyanide and copper.

I planned to convert the CuCN (which was a bit brown after drying)
0.jpg - 181kB

to Copper(I) oxide with KOH. Unfortunately, it turned out that a complex copper potassium cyanide is formed, which doesn't get further attacked by KOH. After mixing a small amount of CuCN with a solution of KOH in excess, everything seemed to work. Yellow hydrated Cu2O formed with a colourless solution above (not visible in the following picture because the whole mixture was stirred)
1.jpg - 67kB

After heating the precipitate became brown (water-free Cu2O) and crystals appeared during cooling, which seem to be not very soluble in water.
2.2.jpg - 215kB

There are many different complexes of CuCN and KCN and this one doesn't seem to be very usefull. After letting it sit for 1-2 days under the colourless liquit it became a mixture of a black compound (probably CuO) and a green compound (some basic copper salt).
3.jpg - 269kB

The liquid above didn't decolorize a copper tetrammine solution.

The CuCN seems to be quite pure. Only very small amounts of iron seem to be present. I also tried to convert it to Mg(CN)2. 250 mg of CuCN were suspended in about 2-3 ml water and 150 mg (or so) Mg powder was added. The mixture became quite warm and some gas was produced. After a few hours a brown solid (Cu + remaining Mg) and a clear solution was visible. Again, this solution didn't decolorize a copper tetrammine solution.

I don't see an easy way to convert CuCN to KCN. Silver cyanide should offer a much better way.

[Edited on 23-5-2020 by Pok]
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[*] posted on 23-5-2020 at 16:04


Has anyone considered that the precipitate is cuprous ferrocyanide when white (and the basis for Williams' "bullshit" method for quantifying ferrocyanide) and cupric ferrocyanide when red-brown?



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[*] posted on 23-5-2020 at 20:49


Quote: Originally posted by Pok  

The CuCN seems to be quite pure. Only very small amounts of iron seem to be present. I also tried to convert it to Mg(CN)2. 250 mg of CuCN were suspended in about 2-3 ml water and 150 mg (or so) Mg powder was added. The mixture became quite warm and some gas was produced. After a few hours a brown solid (Cu + remaining Mg) and a clear solution was visible. Again, this solution didn't decolorize a copper tetrammine solution.


Dissolve in your cyanide solution huge excess of sodium thiosulfate. Than add some ferric salt. Upon acidification with dilute acid you should obtain red solution (with some sulfur from thiosulfate decomposition). Cyanides reacts with thiosulfate or sulfur to form thiocyanate which form with ferric ions red complex. Be sure that acid which you use doesn't contain iron (test it with thiocyanate, salicylic acid or other iron sensitive reagent). From my expirience vinegar is good choice - commercial HCl or battery acid contain lots of iron (especially battery acid).

Another good test is mixing your solution with Fe(II)+Fe(III) solution. If cyanide is present, there is a formation of prussian blue.

Quote: Originally posted by S.C. Wack  
Has anyone considered that the precipitate is cuprous ferrocyanide when white (and the basis for Williams' "bullshit" method for quantifying ferrocyanide) and cupric ferrocyanide when red-brown?


I didn't find any mention about Cu(I) ferrocyanide. But I was thinking about this few days ago. I made few calculations for silver cyanide formation and in theory it shouldn't work, because silver ferrocyanide is more insoluble than silver cyanide. But reaction takes place in hot acidic solution. At these conditions HCN should form which can react with Cu(I) or Ag(I) to form CuCN or AgCN.

[Edited on 24-5-2020 by Bedlasky]

1.png - 16kB




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[*] posted on 24-5-2020 at 11:13


Regarding the solubility: AgCN is less soluble than AgCl. But you can make HCN from AgCN by letting it react with HCl!

Probably some complexes are formed which make this reaction possible.

@S.C. Wack: it's not cuprous ferrocyanide. The compound doesn't contain considerable amounts of iron!

I also tried to make HCN from CuCN by letting it react with HCl (formation of sparingly soluble CuCl). But this didn't work. The CuCN dissolved in the concentrated HCl. This is typical for CuCN.

[Edited on 24-5-2020 by Pok]
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[*] posted on 24-5-2020 at 12:15


Was there any distillation of that?

Has anyone compared their cyanide from ferrocyanide to cyanide from cyanide?

[Edited on 24-5-2020 by S.C. Wack]




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[*] posted on 30-5-2020 at 13:34


Quote:

Has anyone compared their cyanide from ferrocyanide to cyanide from cyanide?

I did. CuCN from ferrocyanide is slightly darker than CuCN from KCN, but all tests I made gave the same results (but Fe contamination).
Besides, white Cu4Fe[(CN)6] is described in legendary "Chemistry of cyanogen compounds" by Williams.
I still dare to claim, that the simplest and cheapest method utilizes CuCl solution prepared in situ from CuSO4, NaCl and Cu.
Anyway, obtained CuCN is pretty useless, however its preparation from FeCy6 is interesting and surprising.
At least one question is explained: influence of Cu(I) on acidolysis of K4Fe[(CN)6].




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