Goran ! your post hit me like lightning and opened my mind !
Tin chemistry is quite tricky due to its amphoterous character, complexicity, ox. states etc
If the metastannic acid is treated for a long time with concentrated hydrochloric acid, the Sn5O5 group is finally broken down, and the tin goes into solution in the form of ordinary alpha-stannic chloride:
Sn5O5(OH)10+20 HCl=15 H2O + 5SnCl4
this equation changed my point of view...
according to reaction between Sn and nitric acid
Kunkel likewise recorded the fact that to dissolve tin the nitric acid employed must be cold, or calx of tin would be precipitated. The explanation of these observations is that whilst tin dissolves slowly, in very dilute nitric acid to produce stannous nitrate, the stannous nitrate first formed, when hot and more concentrated acid is employed, is very unstable and quickly decomposes, yielding the form of hydrated stannic oxide known as β-stannic acid. Probably α-stannic acid is first produced from stannic nitrate, which then passes into the β-form.
β-stannic acid is also called β-metastannic acid or hydrated stannic oxide
α-stannic acid has different constitutions of hydroxyle groups and oxygen in space, also secondary structure, it is soluble in strong acids / alkalis
for an explanation what happens when you let your tin gel on air
Gelatinous, precipitated β-stannic acid has the empirical composition SnO2.4H2O, when air-dried SnO2.2H2O, and when dried in a vacuum, SnO2.H2O; These formulae do not, however, convey a just idea of the nature of the β-acid, which is gained from a study of its salts and other derivatives.
in my opinion, the most probable structural nature of this compound is a net structure or a ring with oxygens between Sn 4+ and hydroxyles above and under the plane of this twisted ring
Sodium β-stannate, prepared by the action of cold sodium hydroxide solution on β-stannic acid, is a sparingly soluble crystalline powder, having the composition Na2Sn5O11.4H2O. Similarly the potassium salt is K2Sn5O11.4H2O. Thus the molecule of β-stannic acid appears to contain five tin atoms; and the air-dried acid becomes H2Sn5O11.9H2O instead of SnO2.2H2O, whilst the acid dried in a vacuum is H2Sn5O11.4H2O instead of simply SnO2.H2O. Alternative formulae are Sn5O5(OH)10.5H2O and Sn5O5(OH)10 respectively, which suggest that β-stannic acid may possibly contain a ten-membered ring of alternated tin and oxygen atoms. At least an analogy is suggested between β-stannic acid and the polymerised silicic acids.
as i find out, it is impossible to dissolve dryed "tin gel" in HCl ( read higher - β-stannic acid looses some water molecules- this changes its structure - properties )
it is important to let your "tin gel" wet, without changing pH ( to avoid structure changes)- that means let it sit and siphon solution above
Goran, it is not "my aqua regia", it is only reaction of strong acid - HCl with residual nitrates (mostly Cu nitrate ) with a production of nitric acid and subsequent production of nitrosyl chloride and gaseous chlorine in situ
to conclusion, my opinion is, that due to highly hydrated status and large structure which contains at least 5 tin atoms it is more unstable- that means more reactive friendly for reaction with HCl. after drying restructuralisation occurs with a loss of water molecules, this structure is more stable - unable to dissolve
it is possible to cement gold on copper from this mixture, but i dont like this method due to production of very fine particles ( few times i got colloidal particles with a transmitance in purple spectra- it can be find what was particle size )