blogfast25 - 13-7-2011 at 08:14
I’ve been making, on and off, some small quantities of ammonium hexachlorostannate ((NH4)2SnCl6), an interesting Sn (IV) coordination complex. Known
colloquially as ‘pink salt’ it is (was?) used as a dye mordant, more potent than alum, or so I’ve read.
I make the salt by dissolving pewter into a mix of strong HCl and 38 % HNO3 (‘pH down’, from the pot growers) and adding the requisite amount of a
saturated NH4Cl solution. I’ve more or less got it down to a tee now, where the metal dissolves (even quite large amounts) in a matter of minutes to
a perfectly clear solution and adding the NH4Cl solution causes much of the ammonium hexachlorostannate (AHCS) to crystallise immediately from the
still hot solution and more crystals appearing on cooling. Although also NH4SbCl6 forms, this is extremely soluble in water (almost impossible to
crystallise in my experience), and being the minority constituent it’s safe to assume it stays in the mother liquor. Here’s about 33 g of AHCS
(0.09 mol), after washing with methylated spirits and drying at 80 C:
Parts of it are still slightly yellow coloured (FeCl3) and would benefit from washing with acetone, before drying.
Having a bit of a stockpile now I’ve been trying to explain some of the unexpected behaviour I’ve seen this salt exhibit.
Firstly, the solubility of AHCS in 1 M HCl at BP was determined to be about 38 g of AHCS / 100 ml of 1 M HCl (at BP). But on adding some 10 ml of 37 %
HCl to about 73 g of this saturated solution quite a bit of precipitate formed, which didn’t disappear on further simmering. Most of the solute then
crystallised out on cooling to RT. Prior observations showed that icing the supernatant solution yielded very few extra crystals (in 1 M HCl), so that
it can be assumed that the big difference in solubility occurs between BP and RT. I will now test that hypothesis also with a supernatant liquid based
on strong HCl.
Next about 15 ml saturated solution of AHCS in DIW was prepared, then measured out to 2 test tubes. To both about the same volume (as saturated
solution already present) of 37 w% HCl was added and in both cases a fine crystalline powder dropped out. The second test tube was heated for about 15
min on steam bath: no dissolution was apparent and no secondary crystals formed on cooling. So it appears AHCS is poorly soluble in strong HCl (15
to 20 % or more), both hot and cold.
It was further confirmed by combining the supernatant solutions of a recrystallisation in 1 M HCl and one in strong HCl: immediately fine crystals of
AHCS dropped out. Because it’s quite a heavy salt it sediments out very quickly.
Some more information on the synth. of AHCS can be found here:
http://www.sciencemadness.org/talk/viewthread.php?tid=14668#...