Bezaleel
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Metathesis reactions for caesium thiocyanate, CsSCN
Dear all, I'm trying to find a good method to create CsSCN through metathesis reactions of CsCl. The old literature suggests mixing molar equivalents
of NH4SCN and Cs2CO3 (2:1) in strong solution, and heating to decompose the dissolved (NH4)2CO3 into H2O, CO2 and NH3. This seems easy and viable, but
then I first need to create Cs2CO3 from CsCl.
The method for obtaining the Cs2CO3 that comes to mind, is metathesis between K2CO3 and CsCl, exploiting the lower molar solubility of KCl, as
compared to the solubilities of both CsCl, Cs2CO3, and K2CO3. (At 20 °C, molar solubilities per litre are: CsCl 11.1, KCl 4.6, K2CO3 8.0.) I guess
that both starting materials should be calcined before weighing, to get an accurate molar ratio.
Supposed I would indeed use this method, a problem is the exceptional solubility of the Cs2CO3. How do I separate the KCl crystals from the solution,
without loosing too much of the Cs2CO3 solution?
Any suggestions? Thanks!
[Edited on 3-8-2016 by Bezaleel]
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Praxichys
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You could try using oxalic acid. Add oxalic acid to a solution of CsCl and steam distill until the distillate is no longer acidic (indicating the
removal of substantially all the HCl). Evaporate to dryness with mild heating (the oxalate salt of caesium is highly soluble) and then heat when dry
to decompose the oxalate into the carbonate. Use excess oxalic acid to ensure total removal of all chloride since the heating process will decompose
any excess oxalic acid anyway.
Alternatively you may be able to use KOH and try and precipitate KCl, which would leave mostly CsOH in solution. Bubbling CO2 through this followed by
evaporation would yield a crude product. Definitely not as pure as the first method, and CsOH attacks glass rapidly.
[Edited on 3-8-2016 by Praxichys]
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Metacelsus
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If you were to make CsOH, you could just react that with ammonium thiocyanate instead of adding carbon dioxide first.
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Bezaleel
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Thanks, these are good suggestions. The oxalate hadn't come to my mind yet. I don't have that much oxalic acid, so I'll look into different methods
first.
Maybe it's yet easier to work from Cs2SO4, which can be easiliy made from CsCl and H2SO4. Calcining will evaporate any excess H2SO4. Metathesis with
KSCN (which I also have) should precipitate the K2SO4 easily when comparing solubilities again. This is closely similar to the oxalic acid method, but
saves my oxalic acid.
The use of KOH is a good suggestion as well. Glassware can't be used in that case; would a nickel crucible do, or would it be attacked too? It's a
99.9% Ni crucible, I have. And indeed, direct combination of the CsOH solution with NH4SCN is the easier route. When preparing CsOH there will be
carbonate in it anyway, as I can't work under a protected atmosphere.
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Bezaleel
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Odd behaviour of evaporation of acid Cs2SO4 solution
I had some time for the next step in this synthesis. I decided to make Cs2SO4 and try metathesis with KSCN to obtain CsSCN.
50g of CsCl were dissolved in about 50ml of distilled water. A notable temperature drop of the liquid occurred (from 22°C down to 9°C). The solution
was poured into a 250ml volumetric flask, and 16.0ml of 98% H2SO4 (a 3.5% excess) were added dropwise. The idea behind the use of a volumetric flask
is that aerosoles may condensate in its long neck.
The solution was heated until boiling and large clouds of mainly HCl vapour left the flask. A few days later, the solution was transferred to a wide
neck erlenmeyer flask and heating was continued. All water was tried to evaporate while the flask was constantly shaken.
At one point the following happened. A hard foam suddenly formed and no liquid was felt moving while shaking the flask. The foam was destroyed with a
glass rod, and coalesced, after which a liquid was formed again.
Heating was continued until boiling was no longer observed. A few specks of a white substance were on the walls of the erlenmeyer, but the rest was a
clear non-boiling liquid! Since nothing happed while heating was continued for about 2 more minutes, I decided to see what would happen when the flask
was left to cool down. It took as much as a few minutes before a fonfant like mass began to be formed at the sides of the liquid. This quickly became
more vicous and finally all became an off-white solid.
The melting point of Cs2SO4 is 1005°C, so apparently the excess H2SO4 has a considerable influence on the crystallisation, which I ecpected would
take place.
A few small lumps of the fondant like Cs2SO4 were dissolved in a bit of water. The pH was below 0.
My idea was to calcine the Cs2SO4 to remove the excess H2SO4. For Pr2(SO4)3 this worked as expected, but now I'm left with either a thick lump of
solid which is strongly attached to the glass of the flask, or I retain a liquid.
* Has anyone seen such behaviour before?
* I'm a afraid that if I would heat the liquid that first formed to above 337°C (the BP of H2SO4) that the very hot liquid might sputter, and I
surely want to keep everything in the flask and avoid contact with it. Any thoughts?
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softbeard
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Wouldn't you be forming caesium bisulfate, CsHSO4.xH2O instead of caesium sulfate directly from CsCl if you've used excess
sulfuric acid?
I suspect you'll have to go to a very high temperature to drive off the excess H2SO4 as SO3 and steam; not just
'boiling off' excess H2SO4 like you expect or want.
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j_sum1
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Not to backtrack too much but I thought Praxichys' oxalate suggestion was the most viable.
It should not be too difficult to get a hold of some more oxalic acid. The dihydrate is readily available. And if not, it can be made from sucrose
and nitric acid.
It seems to me that oxalates are easily overlooked but have some very useful properties and in this case are ideally suited to what you want.
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morsagh
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Just electrolysis of CsCl with salt bridge.
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