blogfast25
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Synth of KSbCl6
Previous attempts at synthesising NH4SbCl6 failed, I believe largely due to the high solubility of the salt, which is perhaps not surprising as it is
the ammonium salt of a very strong acid (HSbCl6 – replace Cl with F and you’ve got a veritable ‘superacid’!).
So I had a go at the potassium salt (potassium hexachloroantimonate). Here’s what I did:
5.1 g of Sb (advertised 99.2 %), a coarse powder.
90 ml 22 % HCl
25 ml 38 % HNO3
All combined in a 250 ml beaker and gently heated till reaction started. It’s very vigorous when it gets going and I had to stir hard to avoid NOx
foam from causing a spill. The solution process takes about 5 mins and a completely clear solution was obtained.
A concentrated solution of 3.2 g KCl in hot water was then added to the hot ‘HSbCl6’ solution, giving a total of about 100 ml which was then
gently simmered away to about 20 ml, upon which at the end some more NOx evolved. For a minute I was thinking the HNO3 excess might have been
oxidising Cl- to Cl2 but that appears impossible according the reduction potentials series. On cooling a hard white/yellow (from Fe3+ in the HCl) was
formed.
To get rid of any remaining HNO3 I redissolved that in 30 ml of hot 22 % HCl, in which it dissolved effortlessly and completely, simmered down some
and allowed to cool: again a hard mass of white/yellow stuff formed. I think I might have been pushing the boiling down process just a tad too hard
and will recrystallise once more. Crystallising behaviour seems distinctly different from the analogues (NH4)2SnCl6 and K2SnCl6.
It should be possible to determine Sb content of the worked up product because SbCl6 (-) theoretically oxidises I- to I2, this allowing back titration
with thiosulphate…
[Edited on 9-12-2010 by blogfast25]
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bfesser
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It would be nice to see a photo of your final crystalline product--something that's not white is always nice to see.
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blogfast25
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Quote: Originally posted by bfesser  | | It would be nice to see a photo of your final crystalline product--something that's not white is always nice to see. |
Hmmm... except that this one will be almost certainly snow white after I finished purifying it.
It's just been redissolved in 20 ml of hot 22 % HCl (forming a solution of just over 2 M) and this will be allowed to cool and hopefully crystallise
overnight. The crystals will treated with ice cold water and/or ice cold acetone to extract the FeCl3 contamination. After that it should be nice and
white.
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blogfast25
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Searching for the high volume search term (  ) ‘KSbCl6’ I found this thread in
the #1 spot and saw there was unfinished business there.
In short, I was unable to crystallise either NH4SbCl6 or KSbCl6 which I believe to be due to extremely high solubilities of these salts. Holleman’s
‘Inorganic Chemistry’ does provide a reference to the acid HSbCl6, that can be isolated as a 4.5 hydrate (acc. this reference). If this (strong)
acid exists, there’s little reason to believe salts of it can’t exist.
I think the way forward is to start from strong solutions of either the NH4 or K salt, then use a water-miscible anti-solvent to force them out of
solution. I’m thinking methanol, ethanol, isopropyl alcohol or acetone so far.
[Edited on 23-2-2014 by blogfast25]
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blogfast25
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Well, well.
Looks like I’ve got a quick and unexpected breakthrough here.
5.0 g of 99.5 % antimony was dissolved in 23 ml of 12 M HCl and 4 ml of 70 % HNO3. To that effect the mixture Sb/HCl was brought to the boil, then
taken off the heat and the nitric acid added in small aliquots, until all metal was dissolved. The oxidation reaction is very exothermic.
To the solution was added 3.1 g of high grade KCl, dissolved in the minimum amount of of water. This urine yellow solution was then boiled in to about
15 ml, or about 2.5 M in Sb.
On cooling nothing crystallised, in line with previous preparations of this compound.
Five test tubes were loaded with about 1.5 ml of this cold solution and various potential anti-solvents were added, about 3 – 4 times the volume of
initial watery solution:
From left to right: methanol, ethanol, isopropanol, acetone, no anti-solvent (control).
So in all cases a white precipitate was obtained. Now it remains to be seen if this precipitate really is KSbCl<sub>6</sub> and not just
plain KCl.
[Edited on 23-2-2014 by blogfast25]
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blogfast25
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I prepared a bit more of this presumed KSbCl6, precipitated it with ethanol, filtered it off on a glass filter and washed the filter cake with small
amounts of ethanol. The product was then dried in a glass dish on the electrical plate. The snow white product was lightly ground.
A few basic tests were carried out.
1. Dissolution:
In cold water it dissolved only with difficulty and with considerable hydrolysis (white precipitate of Sb (V) oxide). This is in line with the
literature information about the heaxchloroantimonates being more prone to hydrolysis than the equivalent hexachlorostannates.
In 50 Celsius concentrated HCl it dissolved completely and effortlessly.
2. Displacement reactions:
A blank copper conductor wire immersed in the acidic solution gets coated in a black metallic coating quickly. Like arsenic, antimony is displaced
from its solutions by copper (tin isn’t).
A blank stainless steel spoon in contact with the powdered product because quickly coated in a dark coating, presumably Sb.
Adding a few chips of aluminium to the solution and some heat caused the antimony to precipitate as a black, fine powder. I may use this as the basis
of a Sb content determination.
3. Pyrolysis:
A test tube was clamped at about 30 degrees and about 1.5 ml of the powdered product loaded into it. Full Bunsen heat was then applied to the bottom
of the test tube. Fumes started to come off almost immediately.
A 250 ml dry conical flask, previously stoppered and cooled at – 18 C, was used as a primitive condenser, by holding its neck over the end of the
heated test tube. Soon something started to condense and after some minutes the conical flask was stoppered again and cooled at – 18 C for a few
minutes. A few small drops of an oily liquid with an unpleasant, meaty smell had been formed.
To the conical flask a few ml of conc. HCl were added and the liquid swirled around the walls of the flask. On adding some aluminium chips a lot of
antimony powder precipitated.
This is strong evidence that SbCl<sub>5</sub> was formed according to:
KSbCl<sub>6</sub> (s) === > KCl (s) + SbCl<sub>5</sub> (g)
This presents an interesting way to prepare anhydrous SbCl<sub>5</sub>... 
[Edited on 26-2-2014 by blogfast25]
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