Metastannic acid/Tin dioxide as catalyst

[pesco]

Hi guys,
I've made very interesting observation today. As it is very fresh it would have to be tested further to confirm.

The story:
I have whole batch of obsolete 600A fuses, silver plated solid copper studs on both sides, ceramic body filled with sand, 5 silver plated copper/brass/bronze perforated sheets acting as a breaking link between both sides of fuse. As I will need a bit of electrolyte for the copper nitrate cell (thx rusty!) I started to dissolve bits. First the sheets, 100ml of 59% HNO₃, 10ml of 35% H₂O₂ ready to be added as the fuming starts, 700ml beaker with watch glass on top. I wanted to do dissolving slowly to minimize NO₂ fuming, so the intention was to drop one perforated sheet at the time. After first one I got surprised and annoyed - it had Sn solder pads I missed to spot. In effect I got quite a bit of H₂SnO₃/SnO₂ floating around. It did settle nicely after a moment, so I decided to keep dissolving the perforated sheets and after all are done wash the jelly off by decantation. And then it hit me.

Observation:
As I was annoyed with the jelly I haven't added any H₂O₂ and still had no NO₂ fuming, none, even the foam formed on the surface had bubbles clear and not even a hint of brown gas.

Conclusion:
Looks like the metastannic acted as a catalyst or "catalyst" depends what actually took place:
1) metastannic jelly slows the bubbles and allows enough time for them to dissolve before break free on the surface
2) metastannic itself reacts with NO₂ disallowing it to run off

I can't tell now what actually happened, could be that both of the scenarios at once or something else altogether was the cause of lack of brown gas.
Will do online research over next few days and some test at the end of the week coming or during following one (depends time availability :| )


I am not entirely sure that H₂SnO₃/SnO₂ solves the problem of nitric losses due to escape of (toxic!) nitrogen oxides when dissolving Ag/Cu, but if there is a chance it works it might be worth applying in practice.
After all it is not THAT difficult to wash it off by decantation :mrgreen:


It is possible I made a mistake somewhere on the line.
Please make test yourself and confirm or deny this observation.

 

[butcher]

I am more inclined to believe it is water and hydrogen peroxide that is changing Nitrous oxide and nitrous dioxide gas back into nitric acid in solution.

 

[pesco]

butcher, I mentioned that I have forgotten to add any peroxide

I've run quick test and I am pretty confident now what is happening.

Procedure:
1) prepared around 50ml of metastannic clean of HNO₃
a) let the copper/ag scrap sit in a beaker for around 20h, that eliminated free nitric
b) washed metastannic with distilled water by decantation, each time, diluting mixture with around 15 times the distilled water each time. 5 times total
2) test tube A - 5ml of metastannic coloid + 5ml of 59% nitric
3) test tube B - 2ml of metastannic coloid +3ml of distilled water + 5ml of 59% nitric
4) test tube C - 5ml of distilled water + 5ml of 59% nitric
5) dropped similar (identical?) piece of Ag plated Cu piece to each tube

Tube C - a lot of deep red brown gasses, quickly digested
Tube B - less of brown gasses, slower digestion time
Tube C - even less then in Tube B, but still some were generated, slowest digestion time

Conclusion:
metastannic does not eliminate, but it does reduce the amount of NO₂ generated.
As I mentioned in a first post it looks like it actually is "catalyst" - does not react as such, but 2 things aid lower rate of NO₂ generation:
1) I could clearly see in a test tubes the jelly slows down bubbles escape, this allows them to be "in touch" with water for a bit longer what helps to dissolve NO₂ before they escape
2) digestion time and reaction violence in each tube suggests that metastannic jelly holds nitric in and makes the reaction longer, but less violent what in turn generates less and smaller bubbles at any given time, less and smaller bubbles means NO₂ has better chances to dissolve in water before escapes from the liquid


There are easier ways to reduce NO₂ generation like peroxide or electric current.

We can file it as curiosity :mrgreen: